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

Figure 9.2 Widespread natural occurrence of three ecological
niches for Rhizobium in legume–cereal rotations. From Yanni
et al. [7] and reprinted with permission from CSIRO Publishing
(http://www.publish.csiro.au/journals/fpb).
within the scientific community, there is now no longer any scientific basis on which
to doubt the existence and potential benefits of this plant–microbe association. In
more recent work, a variety of legumes (berseem clover, alfalfa, soybean, lentil, faba
bean and bean) normally cultivated in rotation with wheat as trap hosts has been
used in an attempt to better reveal the species/biovar diversity of the numerically
dominant rhizobial endophytes of field-grown wheat in the Nile delta. The result
of that study was quite interesting in that the clover symbiont, R. leguminosarum
bv. trifolii, is the dominant Rhizobium endophyte within wheat roots in fields of
the Nile delta, while none of the other rhizobia represented by the other legume
cross-inoculation groups occupied this ecological niche in this same habitat [37].
9.5
Mechanism of Interaction of Rhizobia with Rice Plants
9.5.1
Mode of Entry and Site of Endophytic Colonization in Rice
9.5 Mechanism of Interaction of Rhizobia with Rice Plantsj171
The establishment of the Rhizobium–legume symbiosis and its formation of effective
(i.e. nitrogen fixing) root nodules require a coordinated temporal and spatial
expression of both plant and bacterial genes [38]. A highly specialized and intricately
evolved interaction between these soil microorganisms and legume plants requires
the functions of the nod genes/Nod factors. Primary infection in most Rhizobium–
legume symbioses involves a coordinated development of wall-bound infection
threads within host target cells (most often root hairs) [39]. In contrast, rhizobial
interaction with rice and other cereals is nod gene/Nod factor independent and does
not involve the formation of infection threads [40]. A primary mode of rhizobial