Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

188j 9 Rice–Rhizobia Association: Evolution of an Alternate Niche
the Fe–siderophore complex. However, none of the PGP þ genotypes of rhizobial
endophyte strains isolated from the Nile delta produced siderophores detectable
on CAS differential medium [7]. Nonetheless, production of rhizobial siderophores
in the rhizosphere of inoculated plants remains to be examined before reaching
any conclusions about the possible contribution of this mechanism to their commonly
found growth-promoting benefit to rice.
9.7.6
Induction of Systemic Disease Resistance
Recent studies showed that rhizobial inoculation of rice may trigger the biochemical
pathways (particularly enhanced production of phenolic acids) involved in defense
reactions during pathogenic ingress [71]. An HPLC analysis of the different rice
plant parts after inoculation with two Rhizobium sp. (R. leguminosarum bv. phaseoli
RRE6 and R. leguminosarum bv. trifolii ANU 843) as well as Rhizoctonia solani
(which causes blast disease of rice) revealed the induction of phenolic acids such
as gallic, tannic, ferulic and cinnamic acids [71]. These phenolics mediate defense
responses of crop plants against phytopathogens that cause various devastating
diseases [72].
The exact mechanism used by Rhizobium endophytic strains to alter the phenolic
profiles is still not very clear. However, bacterial endophytic biocontrol agents are
reported to benefit crop plants via disease resistance by two possible ways: (i) by
extensive colonization of internal plant tissues and suppression of invading pathogens
by niche occupation, antibiosis or both; and (ii) by colonization of the root
cortex, where they stimulate general plant systemic defenses/resistances [73]. It is
quite possible that endophytic rhizobia employ one or more of these mechanisms
to protect plants and promote their growth while colonizing their root tissues.
9.8
Summary and Conclusion
Studies completed thus far indicate that superior candidate strains of rhizobial
endophytes suitable for use as biofertilizers for rice under field conditions have
been widely developed and are now being used in cereal production worldwide.
Information on the spatial distribution of candidate strains at scales relevant to the
rice farmer is currently under examination to fully exploit their benefits for sustainable
agriculture. The rationale for these spatial ecology studies is that a thorough
understanding of their natural spatial distribution within rice agroecosystems
should assist our biofertilization strategy program by helping to predict and interpret
results of tests to evaluate their efficacy as inoculants [50].
The cumulative information derived from the studies described here indicates
that rhizobia have evolved an alternate ecological niche that enables them to maintain
a three-component life cycle that includes a free-living heterotrophic phase in
soil, a nitrogen-fixing endosymbiont phase within the root nodules of legumes and a