ABA regulates not only root nodule formation
but also nitrogenfixation activity. When mature
nodules ofP. sativumwere treated with ABA,
nitrogen fixation activity decreased (González
et al. 2001 ). Conversely, the nitrogenfixation
activity on a per-plant basis in enf1(reduced
ABA) plants was 1.8 times that in wild-type
plants (Tominaga et al. 2009 ). Nitric oxide (NO)
is a strong inhibitor of nitrogenfixation (Trin-
chant and Rigaud 1982 ; Shimoda et al. 2009 ).
The reduced endogenous ABA concentration in
enf1 plants correlated with elevated nitrogen
fixation activity and reduced NO production in
the root nodule. These results suggest that
endogenous ABA concentration regulates nitro-
genfixation activity by regulating NO production
in nodules.
9.7 Jasmonic Acid (JA)
Jasmonic acid (JA) is involved in defense
responses to pathogens, wound responses, leaf
senescence, and tuber formation. JA has often
been reported to be a negative regulator of root
nodule formation. For example, inL. japonicus,
foliar application of 1 μM or more methyl
jasmonate (MeJA) inhibited nodule development
(Nakagawa and Kawaguchi 2006 ). InM. trun-
catula, Nod factor–inducedENOD11andRIP1
gene expression, calcium spiking, and root nod-
ule development were suppressed by the addition
of JA to the growth medium (Sun et al. 2006 ). In
contrast, the JA concentration in leaves of theG.
max ntshypernodulation mutant was higher than
in the wild-type (Seo et al. 2007 ). The expression
of genes related to JA biosynthesis and JA
responses in the leaves of wild-typeG. maxis
normally suppressed by inoculation with rhizo-
bia, but no suppression was seen in the hyper-
nodulation mutant nts1007 (Kinkema and
Gresshoff 2008 ), consistent with thefindings of
Seo et al. ( 2007 ). Furthermore, Kinkema and
Gresshoff ( 2008 ) showed that spraying the shoots
of hypernodulation mutant plants withn-propyl
gallate, an inhibitor of JA biosynthesis, signifi-
cantly reduced the number of root nodules.
Recently, we reported that root nodule formation
was enhanced by treatment with a low concen-
tration of JA (0.1 μM) (Suzuki et al. 2011 ).
Therefore, JA functions as a positive regulator of
root nodulation over a certain range of concen-
trations in these plant species. Because plants
secrete JA from the roots into the rhizosphere
(Creelman and Mullet 1995 ), and nod gene
expression and Nod factor production are up-
regulated by JA application (Rosas et al. 1998 ;
Mabood et al. 2006 ), it is unclear whether plant
response, rhizobial response, or both cause the
positive effects of JA on root nodulation. We also
found that nodule formation was suppressed on
the roots of aphytochrome B(phyB) mutant ofL.
japonicusthat had not only decreased levels of
photoassimilates but also a reduced concentration
of JA-Ile (the active JA derivative) (Suzuki et al.
2011 ). In fact, the number of root nodules in the
phyBmutant was restored by JA treatment, pro-
viding further evidence that JA can act as a
positive regulator of nodulation.9.8 Salicylic Acid (SA)Salicylic acid (SA) is an inducer of systemic
acquired resistance (SAR) in defense responses
to pathogens. Van Spronsen et al. ( 2003 ) repor-
ted that root nodule formation inVicia sativa
subsp.nigra (vetch, an indeterminate-type no-
dulating plant), but not in L. japonicus, was
suppressed by treatment with SA. In contrast,
when endogenous levels of SA were modulated
through the transgenic expression of salicylate
hydroxylase (NahG)inL. japonicus, the number
of rhizobial infections and nodules increased
(Stacey et al. 2006 ). Thus, endogenous SA con-
centration affects the number of root nodules
regardless of nodule type (i.e., determinate or
indeterminate). In M. sativa, endogenous SA
concentration did not increase after inoculation
with wild-typeS. meliloti, but did increase after
inoculation with an incompatiblenodCmutant of
S. meliloti(Martínez-Abarca et al. 1998 ). These
results suggest that in the presence of a biotic
stress, the plant defense response begins with an90 A. Suzuki