Microbial strategies for the improvement of legume production in hostile environments - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 140

seedlings under stressed conditions (Glick et al., 1998).
We have also observed in our previous work that
Pseudomonas trivialis 3Re27 was able to utilize ACC as an
N source, indicating the presence of ACC deaminase,
and thereby increase the salt tolerance of goat's rue
under salinated soil conditions (Egamberdieva et al.,
2013b). Similar results were observed by Shaharoona
et al. (2006), who reported that co-inoculation of
Bradyrhizobium with PGPR isolates possessing ACC
deaminase activity enhanced nodulation in mung bean
compared with inoculation with Bradyrhizobium alone.
In another study Ma et al. (2003) reported that ACC
deaminase-producing R. leguminosarum could lower
ethylene production in pea roots and improve nodula-
tion. Similar results were observed for lentil (Shahzad
et al., 2010; Zahir et al., 2011) and chickpea (Aslam et al.,
2010). Arshad et al. (2008) studied the effect of ACC
deaminase-producing PGPR strains on plant growth of
pea under water stress conditions, and found that plant
inoculation with PGPR decreased the effects of water
stress and improved plant growth and yield. PGPR
strains that produce ACC deaminase showed positive
effects on plant growth of chickpea (Roopa et al., 2012)
and lentil (Zafar-ul-Hye et al., 2013), resulting in
increased number of nodules, root and shoot growth,
and yield of plant under stressful conditions.

strain P. trivialis 3Re27 with R. galegae HAMBI 540
significantly increased nodulation and nitrogen content
of fodder galega, whereas cellulase-negative P. extremo-
rientalis TSAU20 showed no significant stimulation
(Egamberdieva et al., 2010).

9.5 Conclusions and future prospects

The negative effect of abiotic factors such as salinity on
plant growth, development and yield has been described
by many studies. Agricultural biotechnology, particu-
larly the use of rhizobia or consortia of plant-beneficial
microbes, can be an effective approach for enhancing a
plant's tolerance to adverse environmental stresses,
increasing legume productivity and the supply of biologi-
cally fixed N at low cost under stressed conditions. Several
mechanisms of action are used by PGPR to alleviate salt
stress, improve symbiotic performance of legumes and
stimulate plant growth; however, more detailed studies
are needed on the induction of salt stress tolerance at the
plant tissue, cell and molecular levels. Recent studies
have demonstrated that tripartite bacterial-legume sym-
bioses represent the best microbial strategy for arid and
saline regions, and are of great interest as the subject of
future research. Revelations about the mechanisms of
PGPR action on improving legume symbioses open new
doors for improving the efficacy of microbial strategies
under harsh environments.

9.4.3 Cell wall degrading enzymes
The complete erosion of the plant cell wall through
which the bacterial symbiont penetrates to establish an
intracellular endosymbiotic relationship with the host is
a key event of the infection process (Robledo et al.,
2008). Cellulase enzyme may help rhizobia penetrate
more easily into intercellular spaces of root cells, which
may result in development of more nodules. Sindhu
and Dadarwal (2001) explained the improvement of
nodulation and symbiotic performance of rhizobia in
legumes through production of hydrolytic enzymes
such as cellulases by root-colonizing Pseudomonas strains.
In their study, co-inoculations of five cellulase-produc-
ing Pseudomonas strains with Mesorhizobium increased
the number of nodules and especially nodule biomass in
chickpea. Robledo et al. (2008) showed that both the
cell-bound cellulase enzyme from R. leguminosarum bv.
trifolii and the purified enzyme could erode cell walls at
the tip of the root hair of the host, white clover, making
a localized hole of sufficient size to allow rhizobial cell
penetration. Co-inoculation of the cellulase-producing

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