gln2mutants available from legume plants were
isolated fromL. japonicusbelonging to the class
of photorespiratory mutants previously described
in other plant species (Márquez et al. 2005 ; Orea
et al. 2002 ). TheLjgln2mutants were further
characterized at the molecular level and shown to
be affected by single point mutations within the
structural part of theLjGln2gene, which leads to
amino acid replacements that abolish GS2 enzy-
matic activity completely (Betti et al. 2006 ).
These mutants were used to analyze how photo-
respiratory metabolism affects nodule function in
L. japonicusplants (García-Calderón et al. 2012 ).
The results obtained indicated that in this plant,
and, particularly, in photorespiration, GS2 defi-
ciency results in profound limitations in carbon
metabolism that affect the nodulation process and
nitrogen fixation. An anticipated senescence
phenotype linked to an important reduction in
starch and sucrose levels was observed (García-
Calderón et al. 2012 ). In a separate study, a single
LjGln2 locus encoding for GS2 was mapped
together with other symbiotic loci (Sandal et al.
2006 ). More recently, GS2 was also involved in
drought stress, nitrogen nutrition, and photore-
spiratory metabolism transcriptomic responses in
L. japonicusplants (Betti et al. 2012 ;Díaz et al.
2010 ;Pérez-Delgado et al. 2013 ).
11.3.2 Glutamate Synthase (GOGAT)
OneLjGlu1gene encoding for Fd-GOGAT and
two different genes encoding for NADH-GOGAT
(LjGlt1andLjGlt2) were identified in the Kazusa
database. Expression levels ofLjGlu1were very
high in photosynthetic tissues but were also high
in roots and nodules, and similarly, LjGlt2
expression levels were also high in the three types
of tissues. However, this was not the case for the
LjGlt1gene that was poorly expressed in roots,
nodules, and leaves (Table11.1). Early studies
indicated that NADH-GOGAT appeared to play a
major role in legume root nodules, in which the
activity increases dramatically following the onset
of nitrogenfixation. Two different isoforms of
NADH-GOGAT have been described in other
plant species, one of them being clearly associated
with effective nodules (Ireland and Lea 1999, and
references therein). Measurements of mRNA
levels and promoter-GUS fusions of the NADH-
GOGAT genes in alfalfa andLotushave shown
the tight relationship of the regulated expression
of NADH-GOGAT to the nodulation process in
legumes (Vance et al. 1995 ).11.3.3 Glutamate Dehydrogenase
(GDH)Three different genes encoding for the NAD+-
dependent GDH were identified inL. japonicus
(Table11.1). One of them (LjGdh1) showed high
or very high levels of expression in leaves and
roots, respectively, and low level in nodules. The
other two genes (LjGdh2 and LjGdh3) were
poorly expressed in these tissues. The majority of
recent studies performed on NAD+-GDH in
higher plants have been focused on deciphering
the role of theαandβsubunits in the formation
of seven isoenzymes, which are encoded by two
distinct nuclear genes,Gdh2andGdh1, respec-
tively. More recently, it has been found that in
Arabidopsis, there is a third gene (AtGdh3)
encoding a putative NAD+-GDH that is actively
transcribed and perhaps regulated by cytokinin.
Similarly, in rice and soybean, three genes
encoding NAD+-GDH were reported, although it
seems that the physiological functions of the
GDH isoenzymes is a complex issue and may
vary from one species to another (Fontaine et al.
2012 , and references therein).
A NADP+-dependent form of GDH also exists,
which appears to be localized in the chloroplast
(in contrast to NAD+-GDH that appears to be
localized in mitochondria). However, the role of
NADP+-GDH is not clear. Consequently, a fourth
expressed gene (AtGdh4) encoding a putative
NADP+-GDH has been identified inArabidopsis
and rice, which is 50 % longer than the NAD+-
GDH (Fontaine et al. 2012 , and references
therein). This is also the case inL. japonicus,
where aLjGdh4gene was also identified in the
Kazusa database, and is associated with four11 Genes Involved in Ammonium Assimilation 121