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

168j 9 Rice–Rhizobia Association: Evolution of an Alternate Niche
irrigated lowlands. Currently, about 60–70% of the 500 000 ha of land area used for
rice production in Egypt is engaged in rice–clover rotation. Berseem clover s high
yield, protein content and symbiotic nitrogen-fixing capacity enhance its use as a
forage and green manure plant in this region. An interesting enigma for this successful
farming system is that the clover rotation with rice can replace 25–33% of the
recommended amount of nitrogen fertilizer needed for optimal rice production.
However, nitrogen balance data indicate that this benefit of rotation with clover cannot
be explained solely by the increase in available soil nitrogen created by mineralization
of the biologically fixed, nitrogen-rich clover crop residues. So the question asked was
whether there is a natural endophytic Rhizobium–rice association that has evolved,
which contributes to this added benefit of clover–rice rotation. The answer is yes
indeed. Studies have indicated that the well-known clover root-nodule occupant R.
leguminosarum bv. trifolii does indeed participate in such an association with rice,
independent of root nodule formation and biological nitrogen fixation [1,7]. Further
studies showed that inoculation of some varieties of rice with certain strains of
endophytic rhizobia can significantly improve their vegetative growth, grain productivity
and agronomic fertilizer nitrogen use efficiency. Rhizobia can help to produce
higher rice grain yield with less dependence on inorganic fertilizer inputs, which is
fully consistent with sustainable agriculture.
Subsequently, Chaintreuil et al. [24] investigated the natural existence of endophytic
photosynthetic Bradyrhizobium (an endosymbiont of Aeschynomene indica and
A. sensitiva) within the roots of the wetland wild rice Oryza breviligulata from Senegal
and Guinea. In Africa, the wetland rice O. breviligulata, which is the ancestor of the
African cultivated rice O. glaberrima, has been harvested and consumed in the Sahel
and Sudan regions for more than 10 000 years. O. breviligulata grows spontaneously
in temporary ponds, wetland plains and river deltas of the semiarid and semihumid
regions of Africa. This primitive rice species is frequently found growing in association
with several aquatic legumes belonging to the genera Aeschynomene and Sesbania
[25–27]. Among these aquatic species, A. indica and A. sensitiva form stem nodules
with photosynthetic Bradyrhizobium strains [28], which occur as endophytes in O.
breviligulata nodal roots.
Following the same working hypothesis, the possible existence of endophytic
rhizobia from India, a major rice producer of the world. Several diverse strains of
endophytic rhizobia has also been explored in India, identified as R. leguminosarum
bv. phaseoli and Burkholderia cepacia were isolated from rice roots collected at different
locations in India [29,30]. Greenhouse and field experiments revealed that
these legume-nodulating isolates have a great potential to promote rice growth and
productivity.
9.3
Confirmation of Natural Endophytic Association of Rhizobia with Rice
Several laboratory, greenhouse and field experiments were performed using an
ecological approach to detect, enumerate and isolate rhizobial endophytes from