Microbial strategies for the improvement of legume production in hostile environments - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 139

9.4 Biomechanisms regulating
growth and development

(Tanimoto, 2005; Tilak et al., 2006; Jabborova et al.,
2013a). Root-associated bacteria synthesize and release
phytohormones such as auxins and gibberellins as
secondary metabolites because of the rich supplies of
substrates exuded from the roots (Egamberdiyeva &
Hoflich, 2002; Tsavkelova et al., 2007; Shahab et al.,
2009). Nutman (1977) reported that the IAA-producing
abilities of Rhizobium strains have a positive influence on
plant growth and also play an important role in the
formation and development of root nodules. In another
study Sridevi and Mallaiah (2007) reported IAA produc-
tion by 26 Rhizobium strains isolated from the root
nodules of the green manure crop, Sesbania sesban.
The comparative effects of auxin (IAA) and IAA-
producing bacteria on the amelioration of salt stress on
seedling growth of soybean under saline conditions
were studied by Jabborova et al. (2013b). They observed
that seedling growth of soybean was inhibited at
100 mM NaCl concentration and salt stress reduced the
length of root by up to 56%. The plant growth regulator
IAA did reverse the growth-inhibiting effect of salt stress
to a certain extent in both shoot and root, whereas the
IAA-producing bacterial strain Pseudomonas putida
TSAU1 significantly increased seedling root growth by
up to 29% in non-salinated conditions and by up to
86% at 100 mM NaCl compared to control plants. This
study suggests that the application of low concentra-
tions of auxin (IAA) or IAA-producing bacteria may
improve the growth of soybean seedlings, which could
enhance the tolerance of plants to soil salinity.

Although the mechanisms playing in the PGPR stimula-
tion of plant growth are not yet well understood, it is
likely that PGPR can promote plant growth through
several different mechanisms, which might function in
tandem or separately, and depend on the species of
plant and bacteria, the environment, and the nature
and degree of stress (Ashraf et al., 2004; Adesemoye &
Egamberdieva, 2013; Berg et al., 2013). Mechanisms by
which bacteria are able to promote plant growth and
prevent physiological plant disorders caused by salinity
include production of phytohormones like indole acetic
acid (IAA), gibberellic acid and cytokinins (Mishra et al.,
2010), solubilization of phosphates and micronutrients
(Medeot et al., 2010), production of 1-aminocyclopropane-
1-carboxylate (ACC) deaminase to reduce the level of
stress ethylene in the roots (Dey et al., 2004) and symbi-
otic nitrogen fixation (Ardakani et al., 2009). It has also
been reported that PGPR strains can produce exopoly-
saccharides (EPSs), which may bind Na and decrease
the content of Na available for plant uptake. Dardanelli
et al. (2008) suggested that inoculation with Azospirillum
spp. combined with rhizobia in common bean induced
the synthesis of flavonoids by roots.

9.4.1 phytohormone production
Phytohormones play an important role in plant physi-
ology, and regulate many aspects of plant development,
including the differentiation of vascular tissues, elonga-
tion growth, apical dominance, lateral root initiation,
and stress responses and adaptation (Sharma et al.,
2005; Egamberdieva, 2009; Javid et al., 2011). The exog-
enous application of auxins to alfalfa (Gruodien &
Zvironaite, 1971), groundnut (Srinivasan & Gopal, 1977)
and mung bean (Hayat et al., 2008) promoted plant
growth and nodulation. Abiotic stresses inhibit phyto-
hormone synthesis in plants, resulting in decreased
plant growth and development. According to Figueiredo
et al. (2008) drought stress causes a change in the
balance of plant hormones, like cytokinin, zeatin, IAA
and gibberellins in common bean. Phytohormones pro-
duced by root-associated bacteria will be taken up by
plant cells, and can stimulate plant cell proliferation;
this mechanism might be responsible for the enlarged
root system and increased number of infection sites
prior to nodulation, especially under stressed conditions

9.4.2 aCC (1-aminocyclopropane-
1-carboxylate) deaminase
Ethylene is one of the endogenous hormones that play
important roles in plant growth and development. Its
production by plants is also considered as one of the
stress responses, and is closely associated with various
stress factors such as salinity, drought, metal toxicity
and nutrient deficiency (Lynch & Brown, 1997; Schmidt,
2001; Li et al., 2009). It has been reported that in legu-
minous plants, extra ethylene production inhibits
nodulation by rhizobia (Hirsch & Fang, 1994). The level
of ethylene in stressed plants can be reduced by ACC
deaminase enzyme, which can cleave the ethylene pre-
cursor ACC to α-ketobutyrate and ammonium (Hontzeas
et al., 2005; Glick et al., 2007). It has been reported that
many root-colonizing PGPR have the ability to produce
ACC deaminase and may enhance the survival of