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

172j 9 Rice–Rhizobia Association: Evolution of an Alternate Niche
invasion of rice roots is through cracks in the epidermis and fissures created during
emergence of lateral roots [41,42]. Further, several workers have reported that there
may be three main portals for rhizobial entry in rice and other nonlegume roots,
which include cracks created during emergence of the lateral root, lysed root hairs and
cracks created between undamaged epidermal cells on the root surface [7,16,42–45].
The main site of rice epidermal root colonization by rhizobia is a collar ring surrounding
the emergence of lateral roots (Figure 9.3a) [40–42]. The main site of internal root
colonization is the intercellular space within root tissue, but rhizobia were also
observed on the root surface, at root tips, in lateral root cracks and even in the cortex
and vascular system [40,42–46]. More recently, it has been shown that rhizobia are
able to achieve ascending migration from within roots, through the stem and into
lower leaves of rice [41] Figure 9.3. There, they are able to grow to high local densities
of up to 10 10 bacteria per cm 3 of rice plant tissue [41]. Similar results were obtained
for the invasion of wheat roots by Azorhizobium caulinodans when the cultures
were supplemented with the flavonoid naringenin [45,47].
In the case of photosynthetic Bradyrhizobium colonization of rice roots, the bacteria
colonize the root surface where numerous lateral roots emerge and produce
fissures in the root epidermis and underlying cortex. These fissures are sites of
intercellular bacterial proliferation where bacteria invade the fissure via disjoining
epidermal cells, forming packets of proliferating bacteria in-between [24]. Interestingly,
in O. breviligulata, photosynthetic bradyrhizobia are also present intracellularly
in cortical cells. However, unlike in Aeschynomene, the number of invaded cells
remained limited in O. breviligulata, and no division of these infected host cells
was observed. The intracellular invasion could thus be the ultimate stage of rice
infection by Bradyrhizobium. Nevertheless, the infection process in O. breviligulata
by some photosynthetic bradyrhizobia is very similar to the first stages of infection in
the leguminous plant Aeschynomene. In both, O. breviligulata and A. sensitiva, expression
of nod genes is not necessary for the first steps of infection involving primitive
crack entry and direct intercellular invasion [24]. Most recently, the crack entry
invasion and intercellular colonization (within cortical cells, stele and aerenchyma)
of rice by Methylobacterium sp. and Burkholderia vietnamiensis strains have been
verified [48,49].
Several technologies, based on reporter gene assay, fluorescence confocal
microscopy and scanning and transmission electron microscopy have been used
to visualize and evaluate intercellular bacterial colonization, entry, spread and the
establishment of internal colonization in this bacteria–plant association [40–42,50].
It has been seen that after inoculation of rice, rhizobia proliferate at the lateral root
emergence site (Figure 9.3a). It is very likely that some type of signal communication/nutritional
stimulation occurs in order to account for the preferential attraction
of rhizobial cells toward root cracks and colonization of those sites. Rhizobial entry
through the root epidermis is thought to be facilitated by cell wall degrading enzymes
such as cellulase and pectinase that are produced by the rhizobial endophytes
[7]. The current model is that rhizobial endophytes use those cell-bound enzymes to
hydrolyze glycosidic bonds in adhesive polymers between epidermal cells, thereby
allowing them ingress into and dissemination within cereal host roots [7].