An evo-devo view of the gynoecium

Copyedited by: OUP Journal of Experimental Botany, Vol. 74, No. 14 pp. 3933–3950, 2023 https://doi.org/10.1093/jxb/erad135 Advance Access Publication 19 April 2023 © The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: [email protected] FLOWERING NEWSLETTER REVIEW An evo-devo view of the gynoecium J. IrepanReyes-Olalde1,2,*,, MitsuhiroAida1,3, and Stefande Folter4,*, 1 International Research Organization for Advanced Science and Technology (IROAST), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan 2 Laboratorio de Genética Molecular, Epigenética, Desarrollo y Evolución de Plantas, Instituto de Ecología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Av. Universidad 3000, Coyoacán, México DF 04510, México 3 Faculty of Advanced Science and Technology (FAST), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan 4 Unidad de Genómica Avanzada (LANGEBIO), Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV-IPN), Irapuato 36824, Guanajuato, México * Correspondence: joseir[email protected] or [email protected] Received 29 November 2022; Editorial decision 5 April 2023; Accepted 11 April 2023 Editor: RainerMelzer, University College Dublin, Ireland Abstract The appearance of the flower marks a key event in the evolutionary history of plants. Among the four types of floral organs, the gynoecium represents the major adaptive advantage of the flower. The gynoecium is an enclosing structure that protects and facilitates the fertilization of the ovules, which then mature as seeds. Upon fertilization, in many species, the gynoecium itself eventually becomes the fruit, which contributes to the dispersal of the seeds. However, despite its importance and the recent advances in our understanding of the genetic regulatory network guiding early gynoecium development, many questions remain to be resolved regarding the extent of the conservation of the molecular mechanisms for gynoecium development among different taxa, and how these mechanisms give origin and diversification to the gynoecium. In this review, we compile the existing knowledge about the evolution, development, and molecular mechanisms involved in the origin and evolution of the gynoecium. Keywords: Carpel, carpel margin meristem, development, evolution, genetic regulatory network, gynoecium. Introduction The flower is the reproductive structure in angiosperms and its origin has contributed to the great success of the taxon (Zahn et al., 2005; Sauquet et al., 2017). However, the origin and evolution of the flower remain among the most difficult and important unresolved topics in evolutionary biology and botany (Darwin, 1860; Darwin and Seward, 1903). The main reasons are the limited number of fossil records, the ambiguous series of morphological intermediate stages, and the fast diversification of the flower (Theissen and Melzer, 2007; Pennisi, 2009; Sauquet et al., 2017; van der Kooi and Ollerton, 2020). Flowers generally have four distinct organs that arise in concentric rings called whorls attached to the tip of a stalk (Fig. 1). The outermost first whorl consists of sepals, which enclose and protect the rest of the flower. The second whorl is composed of petals, which attract insects and animals that help the process of pollination. However, the sister lineages to eudicots and many monocots have undifferentiated outer floral organs (sepals–petals) referred to as tepals (Fig. 1B) (Eames, 1961; Sokoloff et al., Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP 3934 | Reyes-Olalde et al. 2022). The third whorl is the androecium, the male structure that consists of the stamens. Lastly, the innermost fourth whorl is the gynoecium, which is the female reproductive structure of the flower (Krizek and Fletcher, 2005). The gynoecium is a highly complex structure with a great diversity of forms; they range in number from many to one and, in arrangement, from spiral to whorled (Eames, 1961; Igersheim and Endress, 1997; Staedler et al., 2009; Sokoloff et al., 2022). In addition to its central role in reproduction, the gynoecium mostly develops into the fruit, an adaptive structure suitable for seed dispersal (Simpson, 2018). The term ‘gynoecium’ is used to denote the whole female part of the flower. According to the traditional view, the gynoecium is composed of the carpel(s), which are specialized structures responsible for ovule production and protection (Cresens and Smets, 1989; Endress and Igersheim, 2000). The gynoecium is a key innovation in angiosperm evolution because it confers several selective advantages such as the physical protection of the ovules and an efficient collective and selective apparatus for pollination (Lorts et al., 2008; Williams, 2009). The carpels of living basal-most angiosperms, the ANITA grade (or ANA grade; Amborellaceae, Nymphaeales, and Austrobaileyales) and magnoliids, are predominantly free-carpellate (apocarpous). In contrast, in the majority of monocots and eudicots plants, the carpels are fused to various degrees (syncarpous) (Eames, 1961; Remizowa et al., 2010; Endress, 2011; Sokoloff et al., 2022). Despite the importance of the gynoecium, how it originated is still unknown. Several hypotheses have been proposed to explain the origin and evolution of the gynoecium (Arber and Parkin, 1907; Meyen, 1988; Friis and Endress, 1990; Endress and Igersheim, 2000; Frohlich and Parker, 2000; Frohlich, 2003; Doyle, 2008, 2012; Endress, 2019). Recently, massive new datasets and state-of-the-art analytical methods, which use the distribution of gynoecium traits among extant angiosperms, have been employed to reconstruct ancestral gynoecium traits (Endress and Doyle, 2009; Endress, 2015; Sauquet et al., 2017). However, several essential aspects of the ancestral gynoecium remain unresolved due to the variation in the gynoecium structure among the sister lineages to eudicots and monocots. On the other hand, there are plenty of data about the molecular mechanisms that control gynoecium development, especially in the model plant Arabidopsis thaliana (Smyth et al., 1990; Roeder and Yanofsky, 2006; Ferrándiz et al., 2010; Reyes-Olalde et al., 2013; Herrera-Ubaldo and de Folter, 2022). Furthermore, the great success in resolving the phylogenetic relationships within angiosperms and the development of gene regulatory networks (GRNs) (Pfannebecker et al., 2017a; Li et al., 2019; ZúñigaMayo et al., 2019; APG IV, 2020; Herrera-Ubaldo and de Folter, 2022; Herrera-Ubaldo et al., 2023) now allow us to resolve some controversies about the gynoecium origin and suggest new hypotheses about gynoecium evolution with improved background knowledge (Scutt, 2018; Endress, 2019; Becker, 2020; Liu et al., 2023). Nevertheless, despite the recent advances in our understanding of gene regulation in gynoecium development, our knowledge about the molecular mechanisms that regulate its development is limited to several model species, and little is known about how these mechanisms evolved to give rise to a great variety of forms. The focus of this review is two-fold. First, it is to compile the existing knowledge about the evolution, development, and molecular mechanisms involved in the origin and evolution of the gynoecium. Secondly, this study will provide the background information needed for future research. Fig. 1. Flower structure. (A) The flower of Arabidopsis thaliana consists of four whorls of organs: sepals, petals, androecium (male reproductive part; stamens), and gynoecium (female reproductive part; carpels). The two carpels are fused and form the gynoecium, which harbours the ovules. (B) The flower of Nymphaea colorata has undifferentiated outer floral organs called tepals, androecium (purple), and gynoecium (yellow). The gynoecium is ascidiate, and harbours the ovules. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP Evo-devo view of the gynoecium | 3935 Gynoecium evolution Angiospermy is a key innovation in the angiosperms, which means that the ovules are completely enclosed in the gynoecium (Eames, 1961; Endress, 2015). In practically all angiosperms, the gynoecium occupies the centre of the flower and is surrounded by stamens and tepals (petals and sepals), except in the Lacandonia schismatica flower (Martinez and Ramos, 1989; Ambrose et al., 2006). However, it is not clear which group gave rise to the angiosperms, although it is no living gymnosperm group, but an extinct group that is more closely related to the angiosperms. This does not mean that angiosperms and gymnosperms were derived independently from non-seed plants (Doyle, 2006; Soltis, 2021). One of the most popular theories about the origin of the gynoecium is the anthostrobilus (bennettitalean) theory. According to this, the gynoecium is derived from structures similar to the megasporophyll (modified leaves which bear megasporangia) of the pteridosperms (Arber and Parkin, 1907). However, further studies have shown that the megasporophyll of bennettitales does not represent homology with the gynoecium. Furthermore, gnetales show resemblances to flowering plants, but they are not close relatives of flowering plants; they are closer to conifers (Winter et al., 1999; Frohlich, 2003; Wan et al., 2018; Sokoloff et al., 2022). Alternatively, several hypotheses propose that the origin of the gynoecium is from a phyllome (leaf or leaf-derived organ) and not a megasporophyll (Meyen, 1988; Frohlich and Parker, 2000; Frohlich, 2003; Sokoloff et al., 2022). For example, Meyen proposed gamoheterotopy (the transfer of characters from one sex to another) origin of the gynoecium, by the transfer of a bennettitalean phyllome structure to a seed-bearing organ (Meyen, 1988). The mostly male theory proposes that the basic organization of the flower derived largely from the male reproductive structure of the gymnosperm ancestor, with the minimal female structure (the ovule) probably being ectopic on the (formerly) male structure (Frohlich and Parker, 2000; Frohlich, 2003). Recently, Shi and colleagues have provided strong evidence supporting that the outer integument of the angiosperm seeds is derived from the cupule of this extinct gymnosperm (Frohlich, 2003; Doyle, 2008; Shi et al., 2021). However, the exact origin of the gynoecium remains unknown. On the other hand, the phyllome origin is well supported by molecular and transcriptomic studies (Reyes-Olalde et al., 2013; Villarino et al., 2016; Kivivirta et al., 2021). Many genes involved in gynoecium development are also involved in leaf development, which is consistent with a common regulatory molecular mechanism (Reyes-Olalde and de Folter, 2019). Although the gynoecium origin theory from a phyllome is highly accepted, the ancestral states of primitive gynoecia are still a matter of debate (Endress and Igersheim, 2000; Endress, 2019; Becker, 2020). For example, it has long been discussed whether the primitive carpels in angiosperms are plicate or ascidiate (Fig. 2) (Endress and Igersheim, 2000; Doyle, 2006). In the plicate carpel, the folding of the carpels with ovules inside resembles a folded leaf, while, in the ascidiate carpel, the invagination of the carpel forms a tubular or pitcher-like hollow structure with the ovules inside (Endress and Igersheim, 2000; Doyle, 2006; Endress and Doyle, 2009). Based on morphological studies in the ANITA grade, it is generally assumed that the ascidiate character is the ancestral state and the plicate carpel is a derivate character that probably evolved from the progressive elongation of the asymmetric mouth of the sac-like primitive carpel (Fig. 2A) (Doyle, 2006; Hawkins and Liu, 2014; Endress, 2019). However, it is important to take into account that ANITA grade flowers form a true or ‘complete’ gynoecium, because of the presence of a style–stigma region and ovules, leading to the suggestion that the ascidiate carpel also evolved from the ancient plicate carpel form through fusions along the margins (Fig. 2B). The fusion in the gynoecium (i.e. carpel fusion) is a key innovation in angiosperms, and its origin and evolution are closely tied to its role in the development of inner surface tissues of the gynoecium such as ovules, septum, stigma, pollen tube transmitting tissue (PTTT), and style (Endress and Igersheim, 2000; Endress and Doyle, 2009; Williams, 2009; Endress, 2015; Phillips et al., 2020). The fusion of carpels can occur in two ways (Walker, 1975). Congenital fusion occurs in the early primordial differentiation stage, meaning that two carpel primordia arise from the basal meristematic region as a single entity from the earliest stages (Fig. 3A, B). On the other hand, post-genital fusion involves adhesion of epidermal surfaces, resulting in redifferentiation of the epidermal cells into parenchyma cells (Fig. 3A, C). In most angiosperms, both congenital and post-genital gynoecia fusion can co-occur during development; however, generally, there is only one in the basal groups of angiosperms (Endress and Igersheim, 2000; Sokoloff et al., 2017; Phillips et al., 2020). Furthermore, a better understanding of the molecular and developmental mechanisms involved in gynoecia fusion might up open the way to clarify the evolutionary events that originated the angiosperms. Gynoecium ontogeny The gynoecium ontogeny is perhaps one of the most complex plant developmental processes, because a large number of developmental events take place. In apocarpous gynoecium (which probably represents the ancestral state), development is less complex because the carpels are either completely free or post-genitally fused with each other in some species, while, in the syncarpous gynoecium, the situation is more complex since both congenital and post-genital fusion can co-occur during development (Endress and Doyle, 2009; Endress, 2015; Sokoloff et al., 2017). However, gynoecium development can be clearly divided according to a series of highly conserved landmark events. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP 3936 | Reyes-Olalde et al. The gynoecium has a cylindrical shape with reproductive organs inside. According to anatomical and histological features, the gynoecium structure can be described based on two axes: apical–basal and adaxial–abaxial. In addition, the mediolateral axis can additionally be defined in the gynoecium with biradial or bilateral symmetry (e. g. Arabidopsis; Fig. 4) (Reyes-Olalde et al., 2013). Along the apical–basal axis, the gynoecium of Arabidopsis can be divided into four domains: the most apical part is the stigma, consisting of a single layer of elongated cells called papillae; below, there is a solid cylinder called the style; then there is the ovary, which is the most complex part containing the ovules; and at the basal part is the gynophore, which is a short stalk connecting the gynoecium with the rest of the plant (Fig. 4A) (Balanzá et al., 2006; Roeder and Yanofsky, 2006; Zúñiga-Mayo et al., 2014). In the more primitive carpels, the stigma is not clearly set apart—it merges into the style or ovary; in more specialized forms, it is usually distinct. During gynoecium ontogeny, organ identity is established by the formation of the axes (Krizek, 2011; Reyes-Olalde and de Folter, 2019). A cross-section of the ovary reveals the adaxial–abaxial axis, which describes the position relative to the apical–basal axis running through the centre of the gynoecium cylinder. The cells close to the apical–basal axis comprise the adaxial domain and those far from it make up the abaxial domain, generating a bifacial structure in the carpel. Within the carpel wall, patterning of vascular bundles is also dependent on the adaxial–abaxial axis (Fig. 4B, C) (Doyle, 2006; Krizek, 2011; Wynn et al., 2011; Shi et al., 2021). In some species with unilocular and bilocular gynoecia, a secondary axis called the medio-lateral axis is present, which can be defined in crosssections, with ovules and placenta in the medial region and valves in the lateral region (Fig. 4A, B). However, in the plurilocular gynoecium, which has more locules, it is difficult to define the presence of the medio-lateral axis (Ferrándiz et al., 2010; Krizek, 2011; Wynn et al., 2011). Fig. 2. Gynoecium evolution. Two theories exist about how the gynoecium has evolved: the tube-like (ascidiate) carpel theory (Baum, 1952; Leinfellner, 1965) and the leaf-like (plicate) carpel theory (Bailey and Swamy, 1951). (A) The first theory postulates that the first carpels originated from macrosporophylls in the form of a tubular or pitcher-like hollow structure with the ovules (yellow) inside. After the hollow tube grows apically and develops a monomerous apocarpous gynoecium (has one carpel) with stigma/style region (red), two or more different apocarpous gynoecia begin to unite to form a polymerous apocarpous which are separate or unfused; as more gynoecia come together they begin fusing together (connate) to form syncarpous gynoecia or are united to varying degrees (coenocarpous type). (B) The second theory postulates that the first carpels are formed from macrosporophylls that have folded with the opposite edges sealed together, with the ovules formed adaxially (i.e. the surface usually near its edges). This leaf folded in half fuses, leading to the internalization of the adaxial surface, and results in the formation of a hollow tube with the ovules (yellow) inside (monomerous apocarpous gynoecium). Subsequently, two or more different carpels begin to unite to form a polymerous apocarpous gynoecium; as more carpels come together they begin fusing together (connate) to form syncarpous gynoecia. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP Evo-devo view of the gynoecium | 3937 The great diversity of the gynoecium has been crucial to the success of the group (Endress and Igersheim, 1997; Igersheim and Endress, 1997; Endress, 2011). However, despite great morphological variation of gynoecium development, it can be clearly divided into several key stages. Based on the analysis of gynoecium development in several species, especially Fig. 3. Schematic drawings of congenital and post-genital fusion. (A) Transverse sections of Arabidopsis gynoecia at different developmental stages, depicting congenital and post-genital fusions. At stages 5 and 7, gynoecia are depicted that are composed of two congenitally fused carpels (light green) that emerge from the centre of the flower. At stages 9 and 10, post-genital fusion (dark green area) of the two septa primordia occurs. (B) Longitudinal section of gynoecia depicting the congenital fusion process, where the tissues arise already fused from a basal meristematic zone. (C) Longitudinal section of gynoecia depicting the post-genital fusion process, which involves the uniting of tissues that arise as separate epidermal surfaces that actually contact each other and adhere. Pink, ovule primordia; yellow greenish at stage 10 in (A), epidermis. (B and C) are based on Walker (1975). Fig. 4. Structure of the gynoecium of Arabidopsis. (A) A complete view of the gynoecium components along the apical–basal axis. (B) Cross-section of an ovary with two carpels, showing the abaxial–adaxial and the medio-lateral axes. SE, septum; OV, ovule; FU, funiculus; TT (PTTT), pollen tube transmitting tissue. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP 3938 | Reyes-Olalde et al. in the model plant A. thaliana (Sharman, 1960; Smyth et al., 1990; Schneider et al., 2003; Remizowa et al., 2006; Roeder and Yanofsky, 2006; Endress, 2015), we classified gynoecium development into five main stages. In stage A, gynoecium initiation commences with floral meristem (FM) termination (Alvarez-Buylla et al., 2010; Sun and Ito, 2015). In this stage, the gynoecium domain is initiated from a small number of undifferentiated cells in the centre of the FM; however, the stamens are not completely differentiated and separate from the gynoecium domain (Fig. 5A, B) (Gómez-Mena et al., 2005; AlvarezBuylla et al., 2010; Endress, 2019). In stage B, the stamen and gynoecium domains are clearly distinguishable. Gynoecium initiation usually begins with the formation of a dome-shaped basal meristem called the gynoecium primordium (Sharman, 1960; Roeder and Yanofsky, 2006; Endress, 2015; Simpson, 2018) (Fig. 5A, B). Histologically, the gynoecium primordium does not show vascular tissue differentiation and has an increase in meristematic activity, expressed by darker staining (Smyth et al., 1990; Bowman et al., 1999; Endress, 2019; Chang and Sun, 2020). In stage C, the gynoecium primordium begins to grow apically through a series of cell divisions and expansion largely dictated by hormonal and genetic factors (Larsson et al., 2013; Zúñiga-Mayo et al., 2014; Reyes-Olalde and de Folter, 2019; Herrera-Ubaldo and de Folter, 2022). In stage D, the gynoecium continues to elongate as an open tube or folded structure (Endress, 2011, 2015; Simpson, 2018), which will be closed later (Fig. 5) (Endress, 2015). Stage E defines the onset of organogenesis. Once the three axes have been established, the different organs of the gynoecium are formed, such as ovules, septum, PTTT, style, and stigma. Although placenta and ovule initiation appear to be concomitant with, or just subsequent to, the beginning of carpel closure in most angiosperms (including ANITA grade), there are some examples of very early initiation of ovules in open carpels (Tucker and Kantz, 2001; Staedler et al., 2009; Endress, 2015). However, it is unknown whether there are selective advantages or certain evolutionary patterns related to an open gynoecium during development while ovules are being initiated (Tucker and Kantz, 2001; Endress, 2015). The carpel margin is a meristematic area where placenta and ovules are usually formed (Puri, 1952; Endress, 2019; ReyesOlalde and de Folter, 2019). Unfortunately, the carpel margin has not been clearly defined morphologically and histologically in many species; however, in Arabidopsis, the carpel margin Fig. 5. Schematic illustrations of gynoecium development. Overview of gynoecium development, depicted from the side (A) and from the top (B). Briefly, the gynoecium–stamen domain is formed at stage A. At stage B, the gynoecium primordium is visible and separated from the stamens. At stage C, the gynoecium primordium continues to grow until it transforms into a tubular or folded carpel. At stage D, the closure of the gynoecium starts to become visible as an oval tube or folded structure (B). Organ development continues after fusion at stage E; ovules and internal tissues arise from the CMM. Blue, sepals; purple, petals; yellow, stamens; green, gynoecia; red, ovules. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP Evo-devo view of the gynoecium | 3939 meristem (CMM) has been extensively studied both morphologically and genetically due to its key role for the production of the internal tissues (Wynn et al., 2011; Reyes-Olalde et al., 2013, 2017b; Reyes-Olalde and de Folter, 2019). The carpel margin is visualized in most species analysed as a thickness in the internal walls of the carpel (adaxially), with an inner and an outer rim where the margins meet. Furthermore, in most cases, the carpel margin develops close to a vascular bundle and the placenta (Eames, 1961; Endress, 2019). Interestingly, in several species, ovules do not always develop from the carpel margin (Endress, 2019). For example, in some members of the ANITA grade, the carpel margin tissue is only in a small region encompassing the placenta and ovules. There are diverse placentation patterns in the ANITA grade: (i) linear placentae, where the ovules are arranged in a line on each side of the carpel or (ii) diffuse placentae, laminar-diffuse, where the ovules are dispersed over a broad region of the carpel flanks; or protruding-diffuse, with the ovules dispersed over a protruding part of parietal or axile placentation. Another example is in Polygonaceae or Cyperaceae, where the central ovule develops far from the visible carpel margins (Mahmoudi et al., 2020; Reutemann et al., 2022). The carpel margin in many angiosperm species is also involved in the formation of other important internal structures such as the septum, PTTT, and the style–stigma region (Endress and Igersheim, 2000; Endress, 2019). In the inner surface, where post-genital fusion occurs, a canal filled with secretion is formed, which serves as a PTTT (Igersheim and Endress, 1997). The septum is a wall that separates the locules (cavity) of a carpel; for example, the septum in Arabidopsis is the central tissue running from the apex to the base of the ovary inside the carpel (Crawford and Yanofsky, 2008). The septum evolved in numerous groups of angiosperms, especially in monocots and core eudicots (Endress, 2011). It plays an important role in plant reproduction because it is the area where the PTTT is differentiated in many species (Endress, 2015; Vialette-Guiraud et al., 2016). The septum occurs only in plants with syncarpous gynoecia with two or more carpels. In syncarpous gynoecia of Brassicaceae (e.g. Arabidopsis), the septum develops from the CMM at a relatively late developmental stage, but other angiosperms possessing septa in their gynoecia show a different developmental pattern; for example, in Koelreuteria (Sapindaceae), the septum is the direct result of the post-genital fusion of the independent gynoecia (Endress and Igersheim, 1997; Igersheim and Endress, 1997; Endress, 2015, 2019; Cao et al., 2018). The PTTT is the tissue through or upon which pollen tubes grow on their way to effect fertilization. It functions as a molecular screen for species-specific recognition and male gametophytic competition for ovule fertilization (Erbar, 2003; Sage et al., 2009; Herrera-Ubaldo et al., 2019; Pereira et al., 2021). The PTTT is usually derived from the cells of the internal surface of the carpel or from the cell layers located close to the internal surface (Endress, 2011; Sokoloff et al., 2017). There are two ways in which a PTTT occurs between families, as well as between different regions of a given gynoecium (Endress and Igersheim, 1997, 2000; Endress, 2015). In the first case, the PTTT is a secretory material that fills the ovary cavity, either along the internal carpel surface or inside the mucilage that fills the carpel cavity; this occurs in several taxa mainly in the ANITA grade (Endress and Igersheim, 2000; Endress, 2011; Sokoloff et al., 2017). In the second case, the PTTT is formed by several cell layers that lie below the epidermis of the septum or grow inside the ventral slit (i.e. the longitudinal area where the carpel closed) (Endress and Igersheim, 1997, 2000; Endress, 2015). In an apocarpous gynoecium such as in magnolias, the PTTT is located in the ventral slit of the plicate zone of the carpel, while, in some syncarpous gynoecia species (e.g. Arabidopsis), the PTTT is developed from the septum (Roeder and Yanofsky, 2006). Furthermore, in other cases, for example, in Apiales, the septa do not have any transmitting tract tissue (Weber and Frosch, 1995). Furthermore, in members of the ANITA grade, the PTTT tissues are mainly formed by a secretory canal (Endress and Igersheim, 2000; Endress, 2015). Morphologically, the stigma–style region is the most distal portion of the gynoecium (Endress and Igersheim, 2000; Crang et al., 2018; Simpson, 2018). The stigma–style region facilitates the rapid adhesion, hydration, germination, and pollen tube growth, and it also functions as a molecular screen for speciesspecific recognition (Crang et al., 2018). Several families show a gradual transition from the ovary to the stigma–style region. The style usually has a terminal position on the ovary, but, in some taxa, the style appears to be lateral. The style is classified, based on the presence or absence of a PTTT and its degree of development, into three categories. The open type has a stylar canal filled with secretion; the half-closed type has a canal partially filled with solid tissues; and the closed or solid type has a canal completely filled with solid tissues (Endress and Igersheim, 1997, 2000; Igersheim et al., 2001; Endress, 2015). The uppermost part of the gynoecium is the stigma, which is mainly composed of stigmatic papillae, the cells which are receptive to pollen. Histologically, the most common form of stigmatic papilla cells is unicellular. However, several taxa in sister lineages to eudicots and other groups, for example in grasses, have stigmas with multicellular protrusions (Endress and Igersheim, 2000). Molecular mechanisms of gynoecium development The molecular mechanisms that establish the pattern of differentiation of the gynoecium have not been determined in all angiosperm plants yet. However, the molecular mechanisms are well studied in the plant model A. thaliana (Eudicot-Brassicales) (Bowman et al., 1999; Ferrándiz et al., 2010; Reyes-Olalde et al., 2013; Simonini and Østergaard, 2019; Zúñiga-Mayo et al., Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP 3940 | Reyes-Olalde et al. 2019; Herrera-Ubaldo and de Folter, 2022). Gynoecium development has also been studied in other plant models such as Solanum lycopersicum (Eudicot-Solanales), Oryza sativa, Zea mays (Monocot-Poaceae), and Cucumis sativus L (Cucurbitaceae), and in other non-model plants such as Eschscholzia californica (Eudicot-Papaveraceae), Persea americana (Laurales-Lauraceae), and Magnolia wufengensis (Magnoliales-Magnoliaceae) (Becker et al., 2005; Jiang et al., 2005; Buzgo et al., 2007; Chanderbali et al., 2009; Orashakova et al., 2009; Silva et al., 2014; Deng et al., 2019; Yan et al., 2020; Cheng et al., 2022; Wang et al., 2022). In the next section, we describe the molecular mechanisms involved in gynoecium development based on the Arabidopsis model and compare them with those described in other plant models (Box 1). For an evolutionary perspective of these gene regulatory networks, we recommend reading the review by Becker (2020). In response to floral inductive cues, the FM passes through a series of transformations culminating in an individual flower; an important step in this process is FM termination and, at the same time, the initiation of gynoecium development. In eudicot species, gynoecium initiation is regulated by AGAMOUS (AG). AG encodes a MIKC-type MADS domain transcription factor (TF) required for the development of stamens and carpels (Coen and Meyerowitz, 1991; Dreni et al., 2013; Serwatowska et al., 2014). Furthermore, orthologues of AG have also been reported from gymnosperms. Remarkably, heterologous ectopic expression of AG orthologues from conifers in Arabidopsis produced a phenotype similar to that produced by the ectopic expression of the Arabidopsis AG gene (Rutledge et al., 1998; Winter et al., 1999; Zhang et al., 2004). Furthermore, an orthologue of AG from Cycas edentata (CyAG) complemented the AG function in Arabidopsis (Zhang et al., 2004). In Arabidopsis, the ag mutant has normal sepals and petals but lacks stamens and carpels (Yanofsky et al., 1990). In rice, the two paralogous MADS13 and MADS21 genes are orthologous to AG of Arabidopsis (Arora et al., 2007). The mads13 mads21 double mutant shows a very similar phototype to the ag mutant in Arabidopsis with a complete loss of the sexual organ identity and FM determinacy (Dreni et al., 2011). An AG orthologue gene was isolated in the angiosperm M. wufengensis (MAwuAG). Interestingly, the overexpression of the MAwuAG gene in Arabidopsis was unable to convert the sepals and petals into carpels, which is usually observed with AG orthologues from other eudicots, suggesting the possibility that MAwuAG has not developed activities for repressing A-class genes like its homologues from eudicots (Wu et al., 2012). SHATTERPROOF1 and 2 (SHP1/2) are other key MADS domain TFs involved in gynoecium and fruit development (Ferrándiz et al., 2000; Liljegren et al., 2000). The AG and SHP genes are phylogenetically related and are traditionally grouped into the C lineage; however, SHP genes are present exclusively in core eudicots (Kramer et al., 2004; Carvalho et al., 2018). In Arabidopsis and Brassicaceae, there are two paralogous SHP (SHP1/2) genes and they are regulators of dehiscence zone formation (Liljegren et al., 2000; Carvalho et al., 2018). In the Medicago truncatula genome, there is only one SHP gene, MtruSHP, which is strongly expressed in ovules, at a lower level in the inner epidermal layer of the ovary, and the carpel margins—an expression pattern resembling that in Arabidopsis (Fourquin et al., 2013). Another central TF-encoding gene involved in gynoecium determination is CRABS CLAW (CRC). CRC encodes a TF of the YABBY protein family and plays an important role in carpel and nectary development (Bowman et al., 1999; Lee et al., 2005). In Arabidopsis, CRC specifies carpel identity in parallel with AG (Alvarez and Smyth, 1999). Furthermore, the crc mutant has several alterations such as a wider and shorter gynoecium, a reduced amount of style tissue, and loss of post-genital fusion at the apex due to the unfused septum, and, occasionally, crc mutant flowers show extra carpels, indicating that it influences meristem determinacy (Alvarez and Smyth, 1999; Bowman and Smyth, 1999). In monocots, the DROOPING LEAF (DL) gene encodes a functional orthologue of CRC (Ishikawa et al., 2009; Strable et al., 2017). The expression pattern of DL is the most similar to CRC, with a high expression in gynoecium primordium. However, in contrast to eudicots, the rice DL has only a partial role in carpel identity, with a more essential function in carpel specification, since the lossof-function mutation in DL caused a complete homeotic transformation of carpels into stamens (Yamaguchi et al., 2004; Strable et al., 2017). In addition, the CRC-DL gene shows a highly conserved carpel expression pattern in Amborella trichopoda of the ANITA grade (Fourquin et al., 2005). Moreover, the AmbCRC gene in Arabidopsis was able to complement the crc-1 mutation, suggesting that CRC plays an ancestral role in gynoecium development (Fourquin et al., 2007). The KNOX [Knotted1(KN1)-like homeobox] faily are homeodomain TFs found in all green plant lineages that are involved in several plant development processes (Gao et al., 2015). The KNOX family is divided into three classes based on sequence similarity and gene expression pattern (Hake et al., 2004; Magnani and Hake, 2008; Arnaud and Pautot, 2014). Class II and III KNOX genes are differentially expressed in all plant organs and have few known functions in gynoecium development (Avivi et al., 2000; Hay and Tsiantis, 2010). The class I KNOX genes are mainly expressed in meristems and are involved in shoot apical meristem (SAM) and gynoecium function in both monocot and eudicot plants (Hay and Tsiantis, 2010). In Arabidopsis, the class I genes SHOOT MERISTEMLESS (STM) and BREVIPEDICELLUS (BP) are involved in gynoecium development. For example, the loss-offunction stm mutation shows severe alterations in CMM development and its derived tissues, characterized by gynoecium fusion problems with partial or complete loss of the carpels (Scofield et al., 2007), while the bp mutant is affected in replum development (Alonso-Cantabrana et al., 2007). In addition, the overexpression of STM causes ectopic carpels and most of the ovules formed from placental tissues develop into carpel-like Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP Evo-devo view of the gynoecium | 3941 structures (Scofield et al., 2007). A similar phenotype has been described in tomato (S. lycopersicum), in which the overexpression of KNOX-like class I genes Tomato knotted 1 (Tkn1) and Tkn2/LeT6 altered flower and gynoecium development, resembling those observed in STM overexpression (Parnis et al., 1997; Janssen et al., 1998; Shani et al., 2009). Furthermore, in maize, the KNOX-like class I gene knotted1 (kn1) and gnarley (gn1) dominant mutations affect the formation of the gynoecium and lead to ectopic carpels in flowers (Kerstetter et al., 1997; Foster et al., 1999; Scofield et al., 2007). In rice, the orthologue OSH1 displays a conserved gynoecium determination phenotype (Tsuda et al., 2011). In this regard, the KNOX-like Box 1. Gene homologues of the A. thaliana gene regulatory networks (GRNs) controlling gynoecium development (A) Illustrations of gynoecium development in Arabidopsis (stages indicated in parentheses). At stage A (3–4), sepal primordia are visible and the gynoecium domain is initiated from the centre of the floral meristem (FM); stamens are not completely differentiated yet and are separated from the gynoecium domain. At stage B (5), developing sepals cover the FM. The stamen and gynoecium primordia will be formed and clearly distinguishable. The gynoecium begins with the formation of a dome-shaped primordium (two carpel primordia). At stage C (6), the gynoecium begins to grow apically through a series of cell divisions. At stage D (7–8), the gynoecium grows vertically as a hollow tube, which will be closed posteriorly. At stage E (9–10), organogenesis begins inside the gynoecium. In this stage, the three axes are defined and organs such as ovules, septum, style, and stigma begin to form. (B) The GRNs that control gynoecium development of A. thaliana are based on Zúñiga-Mayo et al. (2019), Becker (2020), and Herrera-Ubaldo and de Folter (2022). The GRNs include genes and elements of the auxin and cytokinin pathway required for gynoecium ontogeny specification in Arabidopsis that exist in the four main angiosperm lineages: ANITA (blue), Magnolia (yellow), monocotyledons (red), and dicotyledons (green). In Arabidopsis, in stage A, loss of pluripotent capacity of the FM occurs by the AGAMOUS (AG)–WUSCHEL (WUS)–CLAVATA (CLV) feedback module. The CLV–WUS pathway regulates floral stem cells mainly in the early floral stages through a spatial restriction of WUS. The AG–WUS pathway is involved in the determination of the FM at floral stage C through repression of WUS (Zik and Irish, 2003; Alvarez-Buylla et al., 2010; Sun and Ito, 2015). The AG and WUS homologues have been identified in the four main angiosperm lineages (Kramer et al., 2004; Lian et al., 2014; Pfannebecker et al., 2017a), but CLV homologues have been little explored outside of dicotyledons (RodriguezLeal et al., 2019). Recently, Gómez-Felipe et al. (2021) described that AG and cytokinin signalling work together during gynoecium specification. At stage B, the gynoecium primordium begins to separate from the stamen primordia. In this stage, CUC genes are involved in the gynoecium–stamen separation (Ishida et al., 2000). Furthermore, the presence of a positive feedback loop between CUC and STM TF genes has been reported (Reyes-Olalde and de Folter, 2019). Cytokinin and STM play essential roles in the maintenance and generation of a de novo stem cell system in the shoot apical meristem (SAM). STM induces cytokinin synthesis through activation of the ISOPENTENYL TRANSFERASE (IPT) genes (Yanai et al., 2005). It is possible that similar mechanisms occur in the gynoecium. The CUC, STM, and IPT gene homologues have been identified in the four main angiosperm lineages (Hake et al., 2004; Phillips et al., 2020; Wang et al., 2020), but their functional role in other species has not been thoroughly tested yet. At stage C (6), the three main axes are established, and FM termination is complete. The AG–CRC–ETT module regulates the FM termination through the direct and indirect regulation of WUS expression (Zúñiga-Mayo et al., 2019; Herrera-Ubaldo and de Folter, 2022), for example ETT and AG indirectly repress WUS expression through the repression of the cytokinin pathway (Liu et al., 2014; Zúñiga-Mayo et al., 2019; Herrera-Ubaldo and de Folter, 2022). Moreover, CRC is necessary for WUS repression through regulation of the auxin maxima (Yamaguchi et al., 2017; Zúñiga-Mayo et al., 2019; Herrera-Ubaldo and de Folter, 2022). Furthermore, ETT and CRC are well characterized genes involved in development of axes and gynoecium (Reyes-Olalde and de Folter, 2019; Zúñiga-Mayo et al., 2019). Both CRC and ETT are highly conserved genes in the four angiosperm lineages (Pfannebecker et al., 2017a, b). At stages C and D, gynoecium organogenesis takes place. In these stages the SPT–IND–HECs module is involved in the development of CMM-derived tissues (Schuster et al., 2015; Reyes-Olalde et al., 2017b; Zúñiga-Mayo et al., 2019; Herrera-Ubaldo and de Folter, 2022). The SPT–IND–HECs module positively regulates some components of the auxin pathway in the CMM. HEC and SPT directly regulate the expression of genes involved in auxin biosynthesis and transport, triggering de novo auxin synthesis and their subsequent flux away from the CMM region towards the adjacent tissues (Schuster et al., 2015; Reyes-Olalde et al., 2017a). Furthermore, SPT and HEC are positive regulators of stem cell proliferation via cytokinin signalling regulation (Schuster et al., 2015; Gaillochet et al., 2017; Reyes-Olalde et al., 2017a). The SPT–IND–HECs module is highly conserved throughout all angiosperm lineages (Pfannebecker et al., 2017a, b; Becker, 2020). Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP 3948 | Reyes-Olalde et al. Li X, Duan X, Jiang H, et al. 2006. Genome-wide analysis of basic/ helix–loop–helix transcription factor family in rice and Arabidopsis. Plant Physiology 141, 1167–1184. Lian G, Ding Z, Wang Q, Zhang D, Xu J. 2014. Origins and evolution of WUSCHEL-related homeobox protein family in plant kingdom. The Scientific World Journal 2014, 1–12. Liljegren SJ, Ditta GS, Eshed Y, Savidge B, Bowmant JL, Yanofsky MF. 2000. SHATTERPROOF MADS-box genes control dispersal in Arabidopsis. Nature 404, 766–770. Liljegren SJ, Roeder AHK, Kempin SA, Gremski K, Østergaard L, Guimil S, Reyes DK, Yanofsky MF. 2004. Control of fruit patterning in Arabidopsis by INDEHISCENT. Cell 116, 843–853. Liu H, Li J, Gong P, He C. 2023. The origin and evolution of carpels and fruits from an evo‐devo perspective. Journal of Integrative Plant Biology 65, 283–298. Liu S, Zhang Y, Feng Q, Qin L, Pan C, Lamin-Samu AT, Lu G. 2018. Tomato AUXIN RESPONSE FACTOR 5 regulates fruit set and development via the mediation of auxin and gibberellin signaling. Scientific Reports 8, 2971. Liu X, Dinh TT, Li D, Shi B, Li Y, Cao X, Guo L, Pan Y, Jiao Y, Chen X. 2014. AUXIN RESPONSE FACTOR 3 integrates the functions of AGAMOUS and APETALA2 in floral meristem determinacy. The Plant Journal 80, 629–641. Lorts CM, Briggeman T, Sang T. 2008. Evolution of fruit types and seed dispersal: a phylogenetic and ecological snapshot. Journal of Systematics and Evolution 46, 396–404. Magnani E, Hake S. 2008. KNOX lost the OX: the Arabidopsis KNATM gene defines a novel class of KNOX transcriptional regulators missing the homeodomain. The Plant Cell 20, 875–887. Mahmoudi M, Boughalleb F, Pellegrino G, Abdellaoui R, Nasri N. 2020. Flower, seed, and fruit development in three Tunisian species of Polygonum: implications for their taxonomy and evolution of distyly in Polygonaceae. PLoS One 15, e0227099. Marsch-Martínez N, de Folter S. 2016. Hormonal control of the development of the gynoecium. Current Opinion in Plant Biology 29, 104–114. Marsch-Martínez N, Irepan Reyes-Olalde J, Ramos-Cruz D, LozanoSotomayor P, Zúñiga-Mayo VM, de Folter S. 2012a. Hormones talking: does hormonal cross-talk shape the Arabidopsis gynoecium? Plant Signaling & Behavior 7, 1698–1701. Marsch-Martínez N, Ramos-Cruz D, Irepan Reyes-Olalde J, LozanoSotomayor P, Zúñiga-Mayo VM, De Folter S. 2012b. The role of cytokinin during Arabidopsis gynoecia and fruit morphogenesis and patterning. The Plant Journal 72, 222–234. Martinez CC, Koenig D, Chitwood DH, Sinha NR. 2016. A sister of PIN1 gene in tomato (Solanum lycopersicum) defines leaf and flower organ initiation patterns by maintaining epidermal auxin flux. Developmental Biology 419, 85–98. Martinez E, Ramos CH. 1989. Lacandoniaceae (Triuridales): una nueva familia de México. Annals of the Missouri Botanical Garden 76, 128–135. Matsuo S, Kikuchi K, Fukuda M, Honda I, Imanishi S. 2012. Roles and regulation of cytokinins in tomato fruit development. Journal of Experimental Botany 63, 5569–5579. McSteen P, Malcomber S, Skirpan A, Lunde C, Wu X, Kellogg E, Hake S. 2007. barren inflorescence2 encodes a co-ortholog of the Pinoid serine/ threonine kinase and is required for organogenesis during inflorescence and vegetative development in maize. Plant Physiology 144, 1000–1011. Meyen SV. 1988. Origin of the angiosperm gynoecium by gamoheterotopy. Botanical Journal of the Linnean Society 97, 171–178. Orashakova S, Lange M, Lange S, Wege S, Becker A. 2009. The CRABS CLAW ortholog from California poppy (Eschscholzia californica, Papaveraceae), EcCRC, is involved in floral meristem termination, gynoecium differentiation and ovule initiation. The Plant Journal 58, 682–693. Ortiz-Ramírez CI, Giraldo MA, Ferrándiz C, Pabón-Mora N. 2019. Expression and function of the bHLH genes ALCATRAZ and SPATULA in selected Solanaceae species. The Plant Journal 99, 686–702. Ortiz-Ramírez CI, Plata-Arboleda S, Pabón-Mora N. 2018. Evolution of genes associated with gynoecium patterning and fruit development in Solanaceae. Annals of Botany 121, 1211–1230. Pabón-Mora N, Wong GKS, Ambrose BA. 2014. Evolution of fruit development genes in flowering plants. Frontiers in Plant Science 5, 300. Parnis A, Cohen O, Gutfinger T, Hareven D, Zamir D, Lifschitz E. 1997. The dominant developmental mutants of tomato, Mouse-ear and Curl, are associated with distinct modes of abnormal transcriptional regulation of a Knotted gene. The Plant Cell 9, 2143–2158. Pennisi E. 2009. On the origin of flowering plants. Science 324, 28–31. Pereira AM, Moreira D, Coimbra S, Masiero S. 2021. Paving the way for fertilization: the role of the transmitting tract. International Journal of Molecular Sciences 22, 2603. Petrášek J, Mravec J, Bouchard R, et al. 2006. PIN proteins perform a rate-limiting function in cellular auxin efflux. Science 312, 914–918. Pfannebecker KC, Lange M, Rupp O, Becker A. 2017a. An evolutionary framework for carpel developmental control genes. Molecular Biology and Evolution 34, 330–348. Pfannebecker KC, Lange M, Rupp O, Becker A, Purugganan M. 2017b. Seed plant-specific gene lineages involved in carpel development. Molecular Biology and Evolution 34, 925–942. Phillips HR, Landis JB, Specht CD. 2020. Revisiting floral fusion: the evolution and molecular basis of a developmental innovation. Journal of Experimental Botany 71, 3390–3404. Phillips KA, Skirpan AL, Liu X, Christensen A, Slewinski TL, Hudson C, Barazesh S, Cohen JD, Malcomber S, McSteen P. 2011. vanishing tassel2 encodes a grass-specific tryptophan aminotransferase required for vegetative and reproductive development in maize. The Plant Cell 23, 550–566. Przemeck GKH, Mattsson J, Hardtke CS, Sung ZR, Berleth T. 1996. Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization. Planta 200. Puri V. 1952. Placentation in angiosperms. Botanical Review 18, 603–651. Rajani S, Sundaresan V. 2001. The Arabidopsis myc/bHLH gene alcatraz enables cell separation in fruit dehiscence. Current Biology 11, 1914–1922. Reinhardt D, Pesce ER, Stieger P, Mandel T, Baltensperger K, Bennett M, Traas J, Friml J, Kuhlemeier C. 2003. Regulation of phyllotaxis by polar auxin transport. Nature 426, 255–260. Remizowa M, Sokoloff D, Rudall PJ. 2006. Evolution of the monocot gynoecium: evidence from comparative morphology and development in Tofieldia, Japonolirion, Petrosavia and Narthecium. Plant Systematics and Evolution 258, 183–209. Remizowa MV, Sokoloff DD, Rudall PJ. 2010. Evolutionary history of the monocot flower. Annals of the Missouri Botanical Garden 97, 617–645. Reutemann AG, Muchut SE, Manassero NGU, Vanzela ALL, López MG, Vegetti AC, Gonzalez AM. 2022. A comparative approach to understanding the ovule, seed, and fruit development in Bulbostylis (Cyperaceae: Cyperoideae: Abildgaardieae). Protoplasma 259, 141–153. Reyes-Olalde JI, de Folter S. 2019. Control of stem cell activity in the carpel margin meristem (CMM) in Arabidopsis. Plant Reproduction 32, 123–136. Reyes-Olalde JI, Zuñiga-Mayo VM, Chávez Montes RA, MarschMartínez N, de Folter S. 2013. Inside the gynoecium: at the carpel margin. Trends in Plant Science 18, 644–655. Reyes-Olalde JI, Zúñiga-Mayo VM, Marsch-Martínez N, de Folter S. 2017a. Synergistic relationship between auxin and cytokinin in the ovary and the participation of the transcription factor SPATULA. Plant Signaling & Behavior 12, e1376158. Reyes-Olalde JI, Zúñiga-Mayo VM, Serwatowska J, et al. 2017b. The bHLH transcription factor SPATULA enables cytokinin signaling, and both activate auxin biosynthesis and transport genes at the medial domain of the gynoecium. PLoS Genetics 13, e1006726. Reymond MC, Brunoud G, Chauvet A, Martínez-Garcia JF, MartinMagniette ML, Monéger F, Scutt CP. 2012. A light-regulated genetic Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP Evo-devo view of the gynoecium | 3949 module was recruited to carpel development in arabidopsis following a structural change to SPATULA. The Plant Cell 24, 2812–2825. Rodriguez-Leal D, Xu C, Kwon CT, et al. 2019. Evolution of buffering in a genetic circuit controlling plant stem cell proliferation. Nature Genetics 51, 786–792. Roeder AHK, Ferrándiz C, Yanofsky MF. 2003. The role of the REPLUMLESS homeodomain protein in patterning the Arabidopsis fruit. Current Biology 13, 1630–1635. Roeder AHK, Yanofsky MF. 2006. Fruit development in Arabidopsis. The Arabidopsis Book 4, e0075. Rutledge R, Regan S, Nicolas O, Fobert P, Côté C, Bosnich W, Kauffeldt C, Sunohara G, Séguin A, Stewart D. 1998. Characterization of an AGAMOUS homologue from the conifer black spruce (Picea mariana) that produces floral homeotic conversions when expressed in Arabidopsis. The Plant Journal 15, 625–634. Sage TL, Hristova-Sarkovski K, Koehl V, Lyew J, Pontieri V, Bernhardt P, Weston P, Bagha S, Chiu G. 2009. Transmitting tissue architecture in basal–relictual angiosperms: implications for transmitting tissue origins. American Journal of Botany 96, 183–206. Sauquet H, Von Balthazar M, Magallón S, et al. 2017. The ancestral flower of angiosperms and its early diversification. Nature Communications 8, 16047. Schaller GE, Bishopp A, Kieber JJ. 2015. The yin–yang of hormones: cytokinin and auxin interactions in plant development. The Plant Cell 27, 44–63. Schneider EL, Tucker SC, Williamson PS. 2003. Floral development in the Nymphaeales. International Journal of Plant Sciences 164, S279–S292. Schuster C, Gaillochet C, Lohmann JU. 2015. Arabidopsis HECATE genes function in phytohormone control during gynoecium development. Development 142, 3343–3350. Scofield S, Dewitte W, Murray JAH. 2007. The KNOX gene SHOOT MERISTEMLESS is required for the development of reproductive meristematic tissues in Arabidopsis. The Plant Journal 50, 767–781. Scutt CP. 2018. The origin of angiosperms. In: Nuno de la Rosa L, Müller G, eds. Evolutionary developmental biology. Cham: Springer, 1–20. Serwatowska J, Roque E, Gómez-Mena C, Constantin GD, Wen J, Mysore KS, Lund OS, Johansen E, Pío Beltrán J, Cañas LA. 2014. Two euAGAMOUS genes control C-function in Medicago truncatula. PLoS One 9, e103770. Sessions A, Nemhauser JL, McColl A, Roe JL, Feldmann KA, Zambryski PC. 1997. ETTIN patterns the Arabidopsis floral meristem and reproductive organs. Development 124, 4481–4491. Sessions RA, Zambryski PC. 1995. Arabidopsis gynoecium structure in the wild type and in ettin mutants. Development 121, 1519–1532. Shani E, Burko Y, Lilach BY, Berger Y, Amsellem Z, Goldshmidt A, Sharon E, Ori N. 2009. Stage-specific regulation of Solanum lycopersicum leaf maturation by class 1 KNOTTED1-LIKE HOMEOBOX proteins. The Plant Cell 21, 3078–3092. Sharman BC. 1960. Developmental anatomy of the stamen and carpel primordia in Anthoxanthum odoratum. Botanical Gazette 121, 192–198. Shi G, Herrera F, Herendeen PS, Clark EG, Crane PR. 2021. Mesozoic cupules and the origin of the angiosperm second integument. Nature 594, 223–226. Silva GFFE, Silva EM, Da Silva Azevedo M, Guivin MAC, Ramiro DA, Figueiredo CR, Carrer H, Peres LEP, Nogueira FTS. 2014. MicroRNA156-targeted SPL/SBP box transcription factors regulate tomato ovary and fruit development. The Plant Journal 78, 604–618. Simonini S, Østergaard L. 2019. Female reproductive organ formation: a multitasking endeavor. Current Topics in Developmental Biology 131, 337–371. Simpson M. 2018. Evolution of flowering plants. In: Plant systematics. Elsevier, 163–180. Smyth DR, Bowman JL, Meyerowitz EM. 1990. Early flower development in Arabidopsis. The Plant Cell 2, 755–767. Sohlberg JJ, Myrenås M, Kuusk S, Lagercrantz U, Kowalczyk M, Sandberg G, Sundberg E. 2006. STY1 regulates auxin homeostasis and affects apical–basal patterning of the Arabidopsis gynoecium. The Plant Journal 47, 112–123. Sokoloff DD, Fomichev CI, Rudall PJ, Macfarlane TD, Remizowa Mv. 2022. Evolutionary history of the grass gynoecium. Journal of Experimental Botany 73, 4637–4661. Sokoloff DD, Nuraliev MS, Oskolski AA, Remizowa MV. 2017. Gynoecium evolution in angiosperms: monomery, pseudomonomery, and mixomery. Moscow University Biological Sciences Bulletin 72, 972028–972108. Soltis DE. 2021. Ancient seeds spill secrets about the evolution of flowering plants. Nature 594, 185–186. Sorefan K, Girin T, Liljegren SJ, Ljung K, Robles P, Galván-Ampudia CS, Offringa R, Friml J, Yanofsky MF, Østergaard L. 2009. A regulated auxin minimum is required for seed dispersal in Arabidopsis. Nature 459, 583–586. Staedler YM, Weston PH, Endress PK. 2009. Comparative gynoecium structure and development in Calycanthaceae (Laurales). International Journal of Plant Sciences 170, 21–41. Stepanova AN, Robertson-Hoyt J, Yun J, Benavente LM, Xie DY, Doležal K, Schlereth A, Jürgens G, Alonso JM. 2008. TAA1-mediated auxin biosynthesis is essential for hormone crosstalk and plant development. Cell 133, 177–191. Strable J, Wallace JG, Unger-Wallace E, Briggs S, Bradbury PJ, Buckler ES, Vollbrecht E. 2017. Maize YABBY genes drooping leaf1 and drooping leaf2 regulate plant architecture. The Plant Cell 29, 1622–1641. Su YH, Liu YB, Zhang XS. 2011. Auxin–cytokinin interaction regulates meristem development. Molecular Plant 4, 616–625. Sun B, Ito T. 2015. Regulation of floral stem cell termination in Arabidopsis. Frontiers in Plant Science 6, 17. Theissen G, Melzer R. 2007. Molecular mechanisms underlying origin and diversification of the angiosperm flower. Annals of Botany 100, 603–619. Tsuda K, Ito Y, Sato Y, Kurata N. 2011. Positive autoregulation of a KNOX gene is essential for shoot apical meristem maintenance in rice. The Plant Cell 23, 4368–4381. Tucker SC, Kantz KE. 2001. Open carpels with ovules in Fabaceae. International Journal of Plant Sciences 162, 1065–1073. van der Kooi CJ, Ollerton J. 2020. The origins of flowering plants and pollinators. Science 368, 1306–1308. Vernoux T, Brunoud G, Farcot E, et al. 2011. The auxin signalling network translates dynamic input into robust patterning at the shoot apex. Molecular Systems Biology 7, 508. Vialette-Guiraud ACM, Chauvet A, Gutierrez-Mazariegos J, Eschstruth A, Ratet P, Scutt CP. 2016. A conserved role for the NAM/ miR164 developmental module reveals a common mechanism underlying carpel margin fusion in monocarpous and syncarpous eurosids. Frontiers in Plant Science 6, 1239. Villarino GH, Hu Q, Manrique S, et al. 2016. Transcriptomic signature of the SHATTERPROOF2 expression domain reveals the meristematic nature of Arabidopsis gynoecial medial domain. Plant Physiology 171, 42–61. Walker DB. 1975. Postgenital carpel fusion in Catharanthus roseus (Apocynaceae). I. Light and scanning electron microscopic study of gynoecial ontogeny. American Journal of Botany 62, 457–467. Wan T, Liu ZM, Li LF, et al. 2018. A genome for gnetophytes and early evolution of seed plants. Nature Plants 4, 82–89. Wang X, Lin S, Liu D, Gan L, McAvoy R, Ding J, Li Y. 2020. Evolution and roles of cytokinin genes in angiosperms 1: do ancient IPTs play housekeeping while non-ancient IPTs play regulatory roles? Horticulture Research 7, 28. Wang Z, Jiao Z, Xu P, Chen L, Ai J, Liu X, Yang Y. 2013. Bisexual flower ontogeny after chemical induction and berry characteristics evaluation in male Vitis amurensis Rupr. Scientia Horticulturae 162, 11–19. Wang Z, Wang L, Han L, et al. 2021. HECATE2 acts with GLABROUS3 and Tu to boost cytokinin biosynthesis and regulate cucumber fruit wart formation. Plant Physiology 187, 1619–1635. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025 Copyedited by: OUP 3950 | Reyes-Olalde et al. Wang Z, Zhou Z, Wang L, et al. 2022. The CsHEC1–CsOVATE module contributes to fruit neck length variation via modulating auxin biosynthesis in cucumber. Proceedings of the National Academy of Sciences, USA 119, e2209717119. Waseem M, Li Z. 2019. Overexpression of tomato SlbHLH22 transcription factor gene enhances fruit sensitivity to exogenous phytohormones and shortens fruit shelf-life. Journal of Biotechnology 299, 50–56. Weber M, Frosch A. 1995. The development of the transmitting tract in the pistil of Hacquetia epipactis (Apiaceae). International Journal of Plant Sciences 156, 615–621. Wei K, Chen H. 2018. Comparative functional genomics analysis of bHLH gene family in rice, maize and wheat. BMC Plant Biology 18, 309. Williams JH. 2009. Amborella trichopoda (Amborellaceae. and the evolutionary developmental origins of the angiosperm progamic phase. American Journal of Botany 96, 144–165. Winter KU, Becker A, Münster T, Kim JT, Saedler H, Theissen G. 1999. MADS-box genes reveal that gnetophytes are more closely related to conifers than to flowering plants. Proceedings of the National Academy of Sciences, USA 96, 7342–7347. Worthen JM, Yamburenko MV, Lim J, Nimchuk ZL, Kieber JJ, Schaller GE. 2019. Type-B response regulators of rice play key roles in growth, development and cytokinin signaling. Development 146, 174870. Wu W, Chen F, Jing D, Liu Z, Ma L. 2012. Isolation and characterization of an AGAMOUS-like gene from Magnolia wufengensis (Magnoliaceae). Plant Molecular Biology Reporter 30, 690–698. Wynn AN, Rueschhoff EE, Franks RG. 2011. Transcriptomic characterization of a synergistic genetic interaction during carpel margin meristem development in Arabidopsis thaliana. PLoS One 6, e26231. Yamaguchi N, Huang J, Xu Y, Tanoi K, Ito T. 2017. Fine-tuning of auxin homeostasis governs the transition from floral stem cell maintenance to gynoecium formation. Nature Communications 8, 1125. Yamaguchi T, Nagasawa N, Kawasaki S, Matsuoka M, Nagato Y, Hirano HY. 2004. The YABBY gene DROOPING LEAF regulates carpel specification and midrib development in Oryza sativa. The Plant Cell 16, 500–509. Yan S, Ning K, Wang Z, et al. 2020. CsIVP functions in vasculature development and downy mildew resistance in cucumber. PLoS Biology 18, e3000671. Yanai O, Shani E, Dolezal K, Tarkowski P, Sablowski R, Sandberg G, Samach A, Ori N. 2005. Arabidopsis KNOXI proteins activate cytokinin biosynthesis. Current Biology 15, 1566–1571. Yanofsky MF, Ma H, Bowman JL, Drews GN, Feldmann KA, Meyerowitz EM. 1990. The protein encoded by the Arabidopsis homeotic gene agamous resembles transcription factors. Nature 346, 35–39. Yoshida S, Mandel T, Kuhlemeier C. 2011. Stem cell activation by light guides plant organogenesis. Genes and Development 25, 1439–1450. Zahn LM, Leebens-Mack J, DePamphilis CW, Ma H, Theissen G. 2005. To B or not to B a flower: the role of DEFICIENS and GLOBOSA orthologs in the evolution of the angiosperms. Journal of Heredity 96, 225–240. Zhang P, Tan HTW, Pwee KH, Kumar PP. 2004. Conservation of class C function of floral organ development during 300 million years of evolution from gymnosperms to angiosperms. The Plant Journal 37, 566–577. Zik M, Irish VF. 2003. Flower development: initiation, differentiation, and diversification. Annual Review of Cell and Developmental Biology 19, 119–140. Zimmermann R, Werr W. 2005. Pattern formation in the monocot embryo as revealed by NAM and CUC3 orthologues from Zea mays L. Plant Molecular Biology 58, 669–685. Zumajo-Cardona C, Ambrose BA, Pabón-Mora N. 2017. Evolution of the SPATULA/ALCATRAZ gene lineage and expression analyses in the basal eudicot, Bocconia frutescens L. (Papaveraceae). EvoDevo 8, 5. Zumajo-Cardona C, Pabón-Mora N, Ambrose BA. 2018. Duplication and diversification of REPLUMLESS— a case study in the papaveraceae. Frontiers in Plant Science 9, 1833. Zuñiga-Mayo VM, Baños-Bayardo CR, Díaz-Ramírez D, MarschMartínez N, de Folter S. 2018. Conserved and novel responses to cytokinin treatments during flower and fruit development in Brassica napus and Arabidopsis thaliana. Scientific Reports 8, 6836. Zúñiga-Mayo VM, Gómez-Felipe A, Herrera-Ubaldo H, de Folter S. 2019. Gynoecium development: networks in Arabidopsis and beyond. Journal of Experimental Botany 70, 1447–1460. Zúñiga-Mayo VM, Reyes-Olalde JI, Marsch-Martinez N, de Folter S. 2014. Cytokinin treatments affect the apical–basal patterning of the Arabidopsis gynoecium and resemble the effects of polar auxin transport inhibition. Frontiers in Plant Science 5, 191. Downloaded from https://academic.oup.com/jxb/article/74/14/3933/7131409 by guest on 20 October 2025