Viewpoin s
Is plas idic glu amine syn he ase
essen ial o C
3
plan s? A ale o
pho o espi a o y mu an s,
ammonium ole ance and
coni e s
Summa y
Ag icul u e aces he conside able challenge o ha ing o adap o a
p og essi ely changing clima e (including he inc ease in CO
2
le els
and empe a u es); en i onmen al impac mus be educed while a
he same ime c op yields need o be main ained o inc eased o
ensu e oodsecu i y.Unde hisscena io,inc easingplan s’ni ogen
(N) use e iciency and minimizing he ene gy losses associa ed wi h
pho o espi a ion a e wo goalso c op b eeding ha a elongsough
a e .Theplas idicglu aminesyn he ase(GS2)enzymes andsa he
c oss oads o N assimila ion and pho o espi a ion, and is he e o e a
key candida e o he imp o emen o c op pe o mance. The GS2
enzyme has long been conside ed essen ial o angiospe m su i al
unde pho o espi a o y condi ions. Su p isingly, in A abidopsis GS2
is no essen ial o plan su i al, and i s absence con e s ole ance
owa ds ammonium s ess, which is in con lic wi h he idea ha
NH
4
+
accumula ion is one o he main causes o ammonium s ess.
Al oge he , i appea s ha he ‘ ex book’ iew o his enzyme mus
be e isi ed, especially ega ding he deg ee o which i is essen ial
o plan g ow h unde pho o espi a o y condi ions, and he ole o
NH
4
+
assimila ion du ing ammonium s ess. In his a icle we open
he deba e on whe he mo e o less GS2 is a desi able ai o plan
p oduc i i y.
In oduc ion
The glu amine syn he ase/glu ama e syn he ase (GS/GOGAT)
cycle is he pa hway o he inco po a ion o ino ganic ni ogen
(N) in o o ganic molecules. Glu amine syn he ase ca alyzes he
con e sion o glu ama e (Glu) and ammonium (NH
4
+
) in o
glu amine (Gln). Then, GOGAT p oduces wo molecules o Glu
om Gln and 2-oxoglu a a e (Fig. 1). In seed plan s, he GS
amily is composed o he cy osol-localized GS1 and he plas id-
localized GS2. In a ecen s udy, i was p oposed ha GS1 is
di ided in o wo e olu iona y lineages, named acco ding o hei
sequence and unc ional simila i y o he gymnospe m GS1s:
GS1a-like and GS1b-like (Valde ama-Ma ın e al., 2022).
GS1b is ound in all seed plan s and GS1a is p esen exclusi ely
in gymnospe ms and basal angiospe ms. GS2 is p esen in all seed
plan s excep coni e s and gne ales (Valde ama-Ma ın e al.,
2022). In gene al, GS1 is encoded by mul iple genes, whe eas
GS2 is, in diploid species, gene ally encoded by a single gene. Fo
ins ance, he A abidopsis haliana genome ha bo s i e GS1 genes
(GLN1-5) and one gene o GS2 (GLN2). In e es ingly, some
diploid species, such as Medicago unca ula, possess a second
GS2 gene ha is exclusi ely exp essed in seeds (Seab a e al.,
2010). Al hough di e en GS isozymes exhibi speci ic unc ions,
GS1 is gene ally conside ed o go e n p ima y NH
4
+
assimila ion
and i s eassimila ion du ing N emobiliza ion and ansloca ion.
Fo GS2, i s main ole is he eassimila ion o NH
4
+
eleased
du ing pho o espi a ion and he assimila ion o NH
4
+
de i ed
om ni i e educ ion in he plas ids (Be na d & Habash, 2009;
Thomsen e al., 2014; Hi el & K app, 2021). Ni ogen is he
majo nu ien which limi s c op p oduc i i y, and esea ch on
GS has he e o e been ex ensi e, wi h he aim o imp o ing
plan s’ N use e iciency (NUE) ia GS o e exp ession s a egies
(James e al., 2018a; Amiou e al., 2021). Among he di e en
isozymes o GS, he gene ic manipula ion o GS2 has p o ided
con as ing esul s ha ha e always been in e p e ed in e e ence
o i s unc ion in pho o espi a ion (Table 1). In ligh o ecen
esul s ob ained wi h GS2 mu an s, he bes s a egy o he
bio echnological use o GS2 (i.e. inc easing o dec easing i s
exp ession o imp o e c op p oduc i i y) is a opic o ongoing
deba e.
Is GS2 necessa y o plan su i al? A ale o
pho o espi a ion and pho o espi a o y mu an s
Mu an s lacking GS2 we e i s isola ed in ba ley by sc eening a
la ge e hyl me hanesul ona e (EMS)-mu agenized popula ion in
sea ch o pho o espi a o y mu an s (Blackwell e al., 1987, 1988;
Wallsg o e e al., 1987). GS2 mu an s showed se e e s ess
symp oms, such as chlo osis and nec osis o he lea es, and inally
died when g own unde no mal ai condi ions. Indeed, hese
mu an s lacked he abili y o eassimila e he NH
4
+
los du ing
pho o espi a ion and appa en ly died no because o he oxic
buildup o NH
4
+
bu a he because o he d ain on he o ganic
ni ogen pool (Wallsg o e e al., 1987). In ag eemen wi h his
explana ion, he mu an s g ew no mally unde a CO
2
-en iched
a mosphe e, whe e pho o espi a ion is supp essed, leading he
au ho s o conclude ha GS2 is only necessa y o plan su i al
when pho o espi a ion is ac i e. The inding ha a plas idial
enzyme was esponsible o he eassimila ion o NH
4
+
p oduced in
he mi ochond ia by he b eakdown o glycine was puzzling.
Indeed, due o he high lux o he pho o espi a o y ou e, a
anspo e ha pe mi s he impo o pho o espi a o y NH
4
+
in o
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dis ibu ion and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed and is no used o comme cial pu poses.
Fo um
he chlo oplas is likely needed. Howe e , in spi e o many e o s o
iden i y such a anspo e , i has no ye been ound (Kuhne e al.,
2021).
Ex ensi e sc eening o pho o espi a o y mu an s was also
ca ied ou in A abidopsis, bu mu an s lacking GS2 we e no
isola ed (Some ille & Og en, 1982). Indeed, while mu an s
om o he A abidopsis pho o espi a o y enzymes, like Fd-
GOGAT, we e iden i ied, A abidopsis GLN2 mu an s we e
missing o >35 yea s a e he o iginal sc eenings. This was
especially su p ising since GS2 mu an s wi h a se e e pho o es-
pi a o y pheno ype we e also isola ed in he model legume Lo us
japonicus (O ea e al., 2002). The hypo heses ha ha e been pu
o wa d in a ious a emp s o explain he A abidopsis GS2
enigma included, among o he s, a possible le hal pheno ype o
his mu a ion, and he p esence o compensa o y cy osolic GS
ac i i y ( e iewed by Lam e al., 1996). Finally, an A abidopsis
GLN2 mu an was desc ibed by Fe ei a e al. (2019). The
mu an showed lowe g ow h unde no mal ai (pho o espi a-
o y ac i e condi ions), bu su p isingly did no show he s ong
and inally le hal pheno ype obse ed in o he species, and was
e en able o comple e i s li e cycle. These esul s we e also
con i med by Hachiya e al. (2021). Al oge he , hese obse a-
ions b eak a long-s anding pa adigm and demons a e ha GS2
unc ion is no essen ial o plan su i al in no mal ai , since,
unlike L.japonicus and ba ley, GS2 absence in A abidopsis is
no le hal.
Pho o espi a ion is an ene ge ically expensi e p ocess ha may
ha e a s ong impac on c op yields. A p esen , pho o espi a ion is
es ima ed o educe US soybean and whea p oduc ion by up o
36% and 20%, espec i ely (Walke e al., 2016). In spi e o he
di e ences in he pheno ypes epo ed o GS2 mu an s om
di e en species (Fig. 1; Table 1), he e is no doub ha
his enzyme has a cen al ole in pho o espi a ion. In a u u e
clima e change scena io, ele a ed CO
2
will p obably educe
Ba ley Coni e s and gne alesLo us A abidopsis
Le hal1Le hal1
D ough -sensi i e2
Reduced g ow h1,2
Inc eased GS1 ac i i y1
Ammonium- ole an 1
Salini y- ole an 1
GS1a is in cha ge o
pho o espi a o y
NH4
+ assimila ion1
Ammonium- ole an 1
Enginee ed GS2 knock-ou Na u al GS2 knock-ou
NH4
+NH4
+
NH4
+
Gly
Se Se
Gly
Gln
Gln
Glu
Glu
Glu
Glu
NH4
+
NO3
–
NO3
–
NO2
–
NO2
–
GS1
GS2
NiR
NR
Fd-GOGAT
Pho o espi a ion
Pe oxisome
Mi ochond ion
P o eolysis
and deamina ion
eac ions
Chlo oplas
Fig. 1 Schema ic illus a ing he basics o
ammoniumassimila ioninpho osyn he iccells
and a summa y o he epo ed e ec s o he
absence o GS2 (in mu an s/coni e s). Pu ple
solid lines show pa hways ela ed o GS2
ac i i y. Red dashed lines show pa hways
ela ed o GS1 ac i i y. Black solid lines show
common pa hways. 1 and 2 e e o
pho o espi a o y and nonpho o espi a o y
condi ions, espec i ely.
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pho o espi a o y a es (Walke e al., 2016), and i is possible ha
high le els o GS2 migh no be s ic ly necessa y. The in oduc ion
o di e en pho o espi a o y bypasses ha a oid NH
4
+
elease
om glycine b eakdown ha e been success ul in inc easing plan
yield (Shen e al., 2019; Ca anagh e al., 2022). Al hough bypass-
ing GS2 will ce ainly educe pho o espi a o y ATP losses, i will
also p e en he p o ec i e ole o his cycle unde s ess condi ions.
How pho o espi a ion will e ol e in he u u e and whe he mo e
o less GS2 is a desi able ai o plan p oduc i i y a e s ill open
ques ions.
A abidopsis GLN2 mu an s e eal ha plas idic shoo
NH
4
+
assimila ion may be de imen al o plan s’
ammonium ole ance
Al hough NH
4
+
is an essen ial in e media e o N inco po a ion
in o biomolecules, when plan s a e exposed o high soil NH
4
+
concen a ions hey o en display s ess symp oms ha include
g ow h e a da ion and lea chlo osis. When se e e, hese symp-
oms may e en lead o plan dea h (B i o & K onzucke , 2002).
Ob aining c ops ha exhibi op imal pe o mance unde
Table 1 Ca alogue o mu an and ansgenic plan pheno ypes enginee ed o highe o lowe GS2 exp ession.
Species T ansgenic plan /mu an G ow h condi ions Pheno ype Re e ence
Ho deum ulga e c
Ma is Mink
KO (azide) Con ol condi ions Le hal Wallsg o e e al. (1987)
Blackwell e al. (1988)NPC Simila o WT
H. ulga e c Ma is Mink KO (GS2) 9KO (Fd-GOGAT) (azide) Con ol condi ions Le hal Blackwell e al. (1988)
NPC Simila o WT
Nico iana abacum O e exp ession 35S:N GS2 High-in ensi y ligh Inc eased ole ance Kozaki & Takeba (1996)
Co-supp ession 35S:N GS2 Inc eased sensi i i y
O yza sa i a c Kinuhika i O e exp ession 35S:OsGS2 Salini y Inc eased ole ance Hoshida e al. (2000)
Cold s ess Inc eased ole ance
Co-supp ession 35S:OsGS2 Salini y Inc eased sensi i i y
N. abacum line SR1 O e exp ession bcS:N GS2 Con ol condi ions Inc eased g ow h Migge e al. (2000)
Lo us japonicus KO (EMS) Con ol condi ions Le hal O ea e al. (2002)
NPC Simila o WT
B assica napus Reduced exp ession Con ol condi ions Simila o WT Hus ed e al. (2002)
35S: an isense BnGS2
N. abacum line SR1 Co-supp ession 35S:PsGS2 Con ol condi ions Reduced g ow h and chlo osis Oli ei a e al. (2002)
A abidopsis haliana Col-0 O e exp ession 35S:D GS2
1
Con ol condi ions Inc eased g ow h Zhu e al. (2014)
Low N Inc eased g ow h
O. sa i a c Zhongua 11 Co-o e exp ession Phosphino h icin Enhanced esis ance Sun e al. (2005a)
pOsAc 1:PsGS1 +pZmUbi1:PsGS2
O. sa i a Co-o e exp ession N de iciency Enhanced g ow h Sun e al. (2005b)
c . Zhongua 11 pOsAc 1:PsGS1 +pZmUbi1:PsGS2
T i icum aes i um Co-o e exp ession Phosphino h icin Enhanced esis ance Huang e al. (2005)
pOsAc 1:PsGS1 +pZmUbi1:PsGS2
L. japonicus KO (EMS) D ough in NPC Inc eased sensi i i y D
ıaz e al. (2010)
O. sa i a c Zhongua 11 Co-supp ession 35S:OsGS2 Con ol condi ions Reduced g ow h and chlo osis Cai e al. (2010)
N. abacum c Xan hi O e exp ession bcS:A GS2 Low-N condi ion Inc eased g ow h Wang e al. (2013)
Co-o e exp ession
bcS:A GS2 + bcS:Do 1;7
Inc eased g ow h ela i e
o bcS:A GS2
T. aes i um c Ji5265 Exp ession o GS2 allele om Xiaoyan 54
pTaGS2-2Ab:TaGS2-2Ab
High-N ield ial Inc eased g ow h and yield Hu e al. (2018)
Low-N ield ial Inc eased g ow h and yield
O. sa i a c Nipponba e Co-o e exp ession wi h GS1;1
pOsAc 1:OsGS2 +
pOsAc 2 OsGS1;1
Osmo ic s ess Inc eased ole ance James e al. (2018b)
Salini y Inc eased ole ance
D ough Inc eased ole ance
Phosphino h icin Enhanced esis ance
N. abacum c K326 O e exp ession SP: TaGS2 Con ol condi ions Simila o WT Wei e al. (2018)
N s a a ion Simila o WT
A. haliana Col-0 Knocked-ou (T-DNA) No mal ai Reduced g ow h and chlo osis Fe ei a e al. (2019)
Salini y Inc eased ole ance
NPC Reduced g ow h
N. abacum c K326 O e exp ession SP: TaGS2 D ough Inc eased ole ance Yu e al. (2020)
A. haliana Col-0 Co-supp essed 35S:A GS2 Con ol condi ions Reduced g ow h, no chlo osis Hachiya e al. (2021)
Knocked-ou (T-DNA) Con ol condi ions Reduced g ow h, no chlo osis
Ammonium s ess Inc eased ole ance
A , A abidopsis haliana; Bn, B assica napus; D , Dunaliella i idis; EMS, e hyl me hanesul ona e mu agenesis; KO, knockou ; NPC, non-pho o espi a o y
condi ion; N , Nico iana abacum; Os, O yza sa i a; Ps, Pisum sa i um; SP, supe p omo e ; Ta, T i icum aes i um; WT, wild- ype; Zm, Zea mays.
1
D GS2 is a homologue o he GLN2 gene om he g een algae Chlamydomonas einha d ii and hus co esponds o a di e en e olu iona y lineage han GS2
om seed plan s (Valde ama-Ma
ın e al., 2022).
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ammonium nu i ion is o g ea in e es , since boos ing
ammonium-based nu i ion, as opposed o ni a e-based nu i ion,
has he bene i o mi iga ing NO
3
–
leaching and N
2
O g eenhouse
gas emission (Subba ao & Sea chinge , 2021). The cause o
ammonium s ess-de i ed symp oms is mul i ac o ial and includes,
among o he ac o s, oxida i e s ess, pH al e a ions, ene ge ic
ade-o s and ca ion imbalance. In gene al, he excessi e NH
4
+
accumula ion in issues has been con en ionally accep ed o be he
main igge unde lying plan s’ sensi i i y o ammonium s ess. In
ag eemen wi h his idea, he p omo ion o NH
4
+
assimila ion has
been gene ally shown o ac as a ole ance-p omo ing mechanism,
and mu an s de ec i e in NH
4
+
assimila ion showed enhanced
sensi i i y owa ds ammonium nu i ion, such as he A abidopsis
gln1;1:gln1;2, gln1;2, and gln1;2:gln1;3 GS1 mu an s (Guan e al.,
2016; Konishi e al., 2017), he ice OsGS1;1 knockou mu an
(Kusano e al., 2011), and A abidopsis mu an s de ec i e in NADH-
GOGAT (Konishi e al., 2014). Su p isingly, A abidopsis GLN2
mu an plan s, in addi ion o being able o su i e unde no mal ai
condi ions, we e also mo e ole an o ammonium s ess han wild-
ype plan s (Hachiya e al., 2021). In ag eemen wi h ou
unde s anding o GS2 unc ion, GLN2 mu an plan s accumula ed
e y high quan i ies o NH
4
+
. Ob iously, hese obse a ions a e in
con lic wi h he idea ha NH
4
+
is a oxic molecule when p esen a
high concen a ions, and hey indica e ha NH
4
+
assimila ion,
a he han NH
4
+
accumula ion, may be esponsible o he
sensi i i y o A abidopsis o ammonium nu i ion. Likewise,
Pouce e al. (2021) also epo ed ha NH
4
+
accumula ion in he
lea es o oma o plan s g own unde ammonium nu i ion was
dependen on lea phenological s age and was no co ela ed o hei
educed g ow h compa ed o lea es o plan s g own wi h ni a e
(NO
3
). Hachiya e al. (2021) desc ibed he pheno ype o GLN2
mu an s in ela ion o he p e en ion o shoo acidi ica ion
associa ed wi h plas idic p o on (H
+
) eleasedu ing excessi eNH
4
+
assimila ion by GS2. Indeed, his acidi ica ion did no occu in
gln1;2:gln1;3 mu an plan s (Hachiya e al., 2021). These obse -
a ions lead o a numbe o key ques ions which need o be
answe ed in o de o ad ance ou unde s anding o plan s’
me abolic adap a ion o ammonium s ess. Fi s , i appea s ha
he localiza ion o NH
4
+
assimila ion –plas idic in he shoo s
cy osolic in he oo –is associa ed wi h di e en unc ions, in
e ms o coping wi h ammonium s ess. Among o he obse a-
ions, he con as ing esponse be ween GLN1 and GLN2
mu an s (i.e. GLN1 mu an s a e sensi i e o ammonium s ess)
implies he exis ence o a oo -speci ic mechanism o e icien ly
deal wi h H
+
elease ha is ye o be elucida ed. In addi ion, i
migh be possible ha he oxic e ec o NH
4
+
o e accumula ion
is dependen on i s subcellula localiza ion, a hypo hesis ha
needs o be explo ed. Finally, i should no be o go en ha
NO
3
–
assimila ion, bu no NH
4
+
, depends on he educing
powe expo ed om he chlo oplas h ough he mala e al e
d i en by he lux h ough he pho o espi a o y pa hway (Shi &
Bloom, 2021). I pho o espi a o y le els a e educed unde a
u u e clima e scena io, plan s ha use NH
4
+
as a p ima y N
sou ce migh ha e an ad an age o e plan s ha depend on
NO
3
. Should his be he case, a educ ion in GS2 le els (less
need o pho o espi a o y capaci y and p obably inc eased
ammonium ole ance) po en ially appea s o be a win–win
app oach.
Can gymnospe ms shed ligh on he need o GS2?
Coni e s and gne ales lack GS2 (Valde ama-Ma ın e al.,
2022) bu hey ha e high pho o espi a o y a es (Hanawa e al.,
2017). As an al e na i e o GS2, gymnospe ms possess GS1a,
which, al hough cy osolic, pe o ms a GS2-like unc ion,
namely pho o espi a o y NH
4
+
eassimila ion (Cano as e al.,
2007). Indeed, as is he case wi h GS2, GS1a is mainly
exp essed in pho osyn he ic issues, and i s gene exp ession is
ligh -dependen (Can on e al., 1999; Valde ama-Ma ın e al.,
2022). This ac , oge he wi h he indings ega ding GLN2
mu an s in A abidopsis (Fe ei a e al., 2019; Hachiya e al.,
2021), demons a e ha GS subcellula loca ion is no essen ial
o plan su i al, bu p obably o he coo dina ion o GS gene
exp ession wi h pho osyn hesis and pho o espi a ion, as is he
case o GS1a in coni e s. In ac , A abidopsis GLN2 mu an s
exhibi inc eased exp ession o GLN1;2 and GLN1;3 (Fe ei a
e al., 2019).
P ima y o es soils a e gene ally acidic, wi h low ni i ica ion
a es, and NH
4
+
is he e o e he dominan o m o N. In
ag eemen , mos coni e s ake up NH
4
+
p e e en ially and a e
ole an o ammonium nu i ion (Cui & Song, 2007; K onzucke
e al., 1997). Howe e , ecen indings ha e demons a ed ha
ma u e coni e ees can also assimila e NO
3
–
e icien ly in na u al
condi ions (Zhou e al., 2021). As desc ibed in he p e ious sec ion,
he absence o GS2 con e s upon A abidopsis ole ance o
ammonium s ess. Since he educ ion o NO
3
–
akes place in he
plas ids, we can hypo hesize ha when NO
3
–
educ ion is low, a
plas idic GS2 is no essen ial. This hypo hesis is in ag eemen wi h
he enhanced ammonium ole ance obse ed in A abidopsis GLN2
mu an s (Hachiya e al., 2021). Thus, he absence o GS2 in
gymnospe ms migh be an e olu iona y mechanism o p omo e
g ow h in NH
4
+
- ich habi a s. This hypo hesis mus be s udied
h ough he analysis o chlo oplas pH con ol in coni e s and he
p oduc ion o coni e ansgenic lines o e exp essing a plas idic GS
in di e en pho o espi a o y and nu i ional condi ions.
Is GS2 o e exp ession a s a egy o inc ease yield?
Inc easing plas idic GS exp ession has been shown o gene ally be
bene icial o plan g ow h (Table 1) by imp o ing, among o he
ai s, hose ela ed o N use (Zhu e al., 2014; Hu e al., 2018). In
addi ion, GS ac i i y and exp ession a e known o be enhanced by a
numbe o abio ic s esses (Be na d & Habash, 2009; James e al.,
2018b). In pa icula , GS2 o e exp ession lines ha e gene ally
shown inc eased ole ance o abio ic s esses (Table 1). One
hypo hesis ha may explain he bene icial e ec o GS2 o e -
exp ession is ela ed o pho o espi a ion ac ing as an elec on-sink
ha would dissipa e excessi e educing powe , hus p o ec ing
agains he p oduc ion o eac i e oxygen species and con e ing
pho op o ec ion (Kozaki & Takeba, 1996; Be i e al., 2016; James
e al., 2018b). Al e na i ely, abio ic s esses a e known o inc ease
p ocesses ha p omo e NH
4
+
elease, such as p o ein deg ada ion.
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Thus, a highe GS ac i i y would be bene icial o he syn hesis o
osmoly es such as p oline o polyamines. In ag eemen wi h his
idea, L.japonicus GS2 mu an plan s showed low p oline syn hesis
and d ough sensi i i y (Dıaz e al., 2010), and ice plan s co-
o e exp essing OsGS1;1/OsGS2 accumula ed osmoly es and
showed enhanced ole ance o d ough and salini y (James e al.,
2018b). Again, A abidopsis appea s o be an excep ion, and in a
s udy by Fe ei a e al. (2019), A abidopsis GLN2 mu an s
displayed enhanced ole ance o salini y, bu he me abolic
adjus men associa ed wi h GS2 absence was no s udied in his
wo k. Howe e , Hachiya e al. (2021) obse ed p oline accumu-
la ion in A abidopsis GS2 mu an s unde ammonium s ess, which
migh explain he pheno ypes epo ed by Fe ei a e al. (2019).
Howe e , i emains unclea why p oline le els in he con ex o
GS2 exp ession le els a e species-dependen .
Wha o b eed o ? Mo e o less GS2 ac i i y?
Clima e- esilien c ops a e needed o main ain ag icul u al
p oduc i i y, meaning ha c ops adap ed o cons an ly inc easing
a mosphe ic CO
2
condi ions ha a e also able o deal wi h highe
empe a u es and wa e sca ci y a e equi ed. In addi ion, he
impac o ag icul u e on he en i onmen should be minimized,
and one aim should be a educ ion in he loss o N, which is
o en >50% o he amoun o applied N (Socolow, 1999; Coskun
e al., 2017). Thus, b eeding c ops wi h a highe NUE is desi able.
In addi ion, he use o ammonium-based nu i ion combined wi h
ni i ica ion inhibi ion is o g ea in e es , bu his app oach
demands ha c op plan s a e be e adap ed o he use o NH
4
+
as
hei main sou ce o N (Coskun e al., 2017; Ma ino & Mo an,
2019; Subba ao & Sea chinge , 2021). GS2 s ands a a c oss oads
o di e en p ocesses, such as pho o espi a ion, N assimila ion and
s ess ole ance; he e o e, b eeding o al e GS2 exp ession le els
ep esen s a p omising bu also deba able s a egy, especially in
ligh o he con lic ing da a ob ained o GS2 mu an s and
o e exp ession lines ha ha e been summa ized in his a icle
(Table 1). Indeed, GS2 o e exp ession has gene ally been consid-
e ed o be a means by which highe NUE can be achie ed (Zhu
e al., 2014; Hu e al., 2018). Howe e , ecen epo s in
A abidopsis showed enhanced g ow h o GS2 mu an s unde
ammonium nu i ion, and his inding, oge he wi h he ac ha
ammonium- ole an coni e s lack GS2, sugges s ha plas idic GS2
absence may ep esen a bene i o plan pe o mance when g own
unde ammonium nu i ion. The u u e ise in a mosphe ic CO
2
concen a ions should educe pho o espi a ion, which is assumed
o be posi i e o plan p oduc i i y (Walke e al., 2016).
Howe e , ele a ed empe a u es and inc easingly equen episodes
o d ough will also in luence pho o espi a ion (Be i e al., 2016),
making i di icul o p edic how pho o espi a o y ac i i y will
change in he u u e. These puzzling esul s open a deba e ega ding
he in e es o b eeding o mo e o o less GS2 in o de o op imize
c op yield and quali y. In e es ingly, i appea s ha in coni e s
and A abidopsis pho o espi a o y NH
4
+
could be assimila ed
by al e na i e enzymes, such as cy osolic GS. In his sense,
enginee ing c ops wi h sligh ly modi ied le els o his enzyme
may be a p omising app oach. Besides, ano he p omising s a egy
o be explo ed is b eeding o inc eased GS1 exp ession while
minimizing GS2 exp ession. Al oge he , mo e esea ch is needed o
ully unde s and GS2 unc ion in di e en plan species. Fo
example, he ole o GS2 unde ammonium nu i ion mus be
s udied in species o he han A abidopsis in o de o obse e
whe he his pheno ype is exclusi e o A abidopsis o conse ed in
o he species om he B assicaceae and in o he plan amilies.
Ob iously, a apid app oach could be o g ow he a ailable GS2
mu an s o co-supp essed lines unde nonpho o espi a o y condi-
ions wi h NH
4
+
as he main sou ce o N. While i is e iden ha
he e a e se e al laye s o complexi y o he GS2 puzzle due o he
in ol emen o his enzyme in se e al key p ocesses, we belie e ha
he ecen ad ances pa e he way o newly p omising s a egies o
c op imp o emen by enginee ing GS2 exp ession.
Acknowledgemen s
This esea ch was unded by he Basque Go e nmen (IT932-16),
he Spanish S a e Resea ch Agency (AEI) (PID2020-113385RB-
I00 and RTI2018-093571-B-100 co- unded by FEDER, EU),
Jun a de Andalucıa (P20_00036 PAIDI 2020/FEDER, UE) and
he p ojec US-1256179 g an om Jun a de Andalucıa, FEDER
and Uni e sidad de Se illa.
Au ho con ibu ions
DM concei ed he a icle; DM, RAC and MB w o e he
manusc ip .
ORCID
Ma co Be i h ps://o cid.o g/0000-0002-7334-5734
Ra ael A. Ca~nas h ps://o cid.o g/0000-0001-9727-5585
Daniel Ma ino h ps://o cid.o g/0000-0002-8788-6646
Daniel Ma ino
1,2
*, Ra ael A.Ca~nas
3
and Ma co Be i
4
1
Depa men o Plan Biology and Ecology, Uni e si y o he
Basque Coun y (UPV/EHU), E-48940 Leioa, Spain;
2
Ike basque, Basque Founda ion o Science,
E-48011 Bilbao, Spain;
3
In eg a i e Molecula Biology Lab, Uni e sidad de Malaga,
Campus Uni e si a io de Tea inos, 29071 Malaga, Spain;
4
Depa amen o de Bioquımica Vege al y Biologıa Molecula ,
Facul ad de Quımica, Uni e sidad de Se illa, 41012 Se illa, Spain
(*Au ho o co espondence: email daniel.ma [email p o ec ed])
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Key wo ds: abio ic s ess, bio echnology, clima e change, glu amine syn he ase,
GS/GOGAT cycle, ni ogen me abolism, ni ogen use e iciency (NUE),
pho o espi a ion.
Recei ed, 21 Decembe 2021; accep ed, 23 Feb ua y 2022.
Ó2022 The Au ho s
New Phy ologis Ó2022 New Phy ologis Founda ion
New Phy ologis (2022) 234: 1559–1565
www.newphy ologis .com
New
Phy ologis Viewpoin s Fo um 1565
