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

180j 9 Rice–Rhizobia Association: Evolution of an Alternate Niche
These results reflect the potential benefits of rhizobial biofertilizer inoculants for
rice and wheat production in the Nile delta. This knowledge should improve agricultural
production by advancing basic scientific knowledge on beneficial plant–
microbe associations and also by assisting low-income farmers to increase cereal
production on marginally fertile soils using biofertilizers consistent with environmental
soundness and sustainable agriculture. In this regard, it must be considered
that the differences between the yields of the best experimental treatment(s) and
those obtained in farmers adjacent fields cannot be solely attributed to inoculation
with the effective endophytes, since they include the whole research package of
recommendations extending beyond inoculation.
9.6.2
Extensions of Rhizobial Endophyte Effects
9.6.2.1 Use of Rhizobial Endophytes from Rice with Certain Maize Genotypes
It is of obvious interest to know whether superior rhizobial endophyte strains that
are PGP þ with rice can also promote the growth of other cereal crops. Field tests
conducted in Wisconsin, USA, have provided preliminary evidence for this, at least
for certain genotypes of maize [7]. In that study, inoculation with one of the rice
endophyte strains isolated from the Nile delta resulted in statistically significant
increase in dry weight in three of the six tested maize genotypes in the greenhouse
and one of the seven maize genotypes in experimental field plots that received no
nitrogen fertilizer [7]. In addition, effects of rice rhizobial endophytic strains (RRE-2,
RRE-5 and RRE-6) isolated from India [30] were also tested on some maize cultivars
and a significant increase in root proliferation and overall plant growth were
observed (R.K. Singh, unpublished data).
A cross between the high-responding maize genotype and a different nonresponding
genotype resulted in a hybrid maize genotype with an intermediate level
of growth responsiveness to inoculation with the rhizobial endophyte strain from
Egypt (Figure 9.5) [7]. This suggests the possibility of genetic transmissibility and
inheritance in corn of the ability to respond to selected endophytic rhizobia. This
result reinforces the earlier finding that induction of positive growth responses in
cereals by rhizobia is genotype specific (also in maize). Such experimental results
may help to identify the genes in cereals necessary for expression of these growth
responses to rhizobia. Also, since recent work indicates that one of our rice endophyte
strains of rhizobia can promote the growth of shoots and roots, root architecture
and uptake of N and Ca 2þ for certain cotton varieties under growth-room
conditions [61], a thorough screen of their potential benefit to a wide variety of
nonlegume crop plants should be made.
9.6.2.2 Rhizobia–Rice Associations in Different Rice Varieties
For practical reasons, it is important to know whether the various desirable interactions
of these rhizobial endophytes can be extended to cereal varieties that are
preferred by farmers in cropping systems worldwide. The number of varieties tested