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

9.6 Importance of Endophytic Rhizobia–Rice Association in Agroecosystemsj177
(IFS). Rice does not naturally produce isoflavones, but similar kinds of signal
molecules do exist in rice and other nonlegumes [55]. Some studies have analyzed
rice seedlings for possible signal molecules that might interact with rhizobial cells
[55]. Extract made from seedlings of rice cultivars were tested with the reporter strain
ANUgus (PMD1), which contains the NGR 234 nodD gene and an inducible promoter.
A higher level of signals similar to legumes was produced, which could
induce the nodD gene [55]. In a recent effort to develop a rice variety possessing
the ability to induce nodulation (nod) genes in rhizobia, the IFS gene from soybean
was incorporated into rice cultivar Murasaki R86 under the control of the 35S
promoter [56]. The presence of IFS in transgenic rice was confirmed by PCR and
Southern blot analysis. Analyses of the 35S–IFS transgenic lines demonstrated that
the expression of the IFS gene led to the production of the isoflavone genistein in
rice tissues. These results showed that the soybean IFS gene-expressed enzyme is
active in the R86 rice plant and that the naringenin intermediate of the anthocyanin
pathway is available as a substrate for the introduced foreign enzyme. The genistein
produced in rice cells was present in a glycoside form, indicating that endogenous
glycosyltransferases were capable of recognizing genistein as a substrate. Studies
with rhizobia demonstrated that the expression of IFS conferred rice plants with the
ability to produce flavonoids that are able to induce nod gene expression, albeit to
varied degrees, in different rhizobia [56]. Thus, the possibilities of establishing a
more effective type of Rhizobium–nonlegume interaction are potentially available in
rice because rice roots contain many of the plant compounds that can stimulate
rhizobia.
9.6
Importance of Endophytic Rhizobia–Rice Association in Agroecosystems
9.6.1
Plant Growth Promotion by Rhizobium Endophytes
Early studies on endophytic colonization of rice by rhizobia indicated that some
strains promoted the shoot and root growth of certain rice varieties in gnotobiotic
culture [1]. Later, more extensive tests established the range of growth responses of
japonica, indica and hybrid rice varieties from Egypt, Philippines, United States,
India and Australia when these cultivars were inoculated with various rice-adapted
rhizobia. The results indicated that the diverse rhizobial endophytes evoked a full
spectrum of growth responses in rice (positive, neutral and sometimes even negative),
often exhibiting a high level of strain–variety specificity [1,7,29,31,42,57–60].
On the positive side, a chronology of PGP þ responses of rice to rhizobia manifested
as increased seedling vigor (faster seed germination followed by increased root elongation,
shoot height, leaf area, chlorophyll content, photosynthetic capacity, root
length, branching and biomass). This effect carries over into increased yield and
nitrogen content of the straw and grain at maturity. Similar results were obtained
when wheat was inoculated with diverse genotypes of endophytic wheat-adapted