INTERACTIONS OF RHIZOBIA WITH RICE AND WHEAT

Recently, evidence has been obtained that naturally occurring rhizobia , isolated from the nodules of non-legume Parasponia species and from some tropical legumes, are able to enter the roots of rice, wheat and maize at emerging lateral roots by crack entry. We have now investigat ed whether Azorhizobium caulinodans strain ORS571, which induces root and stem nodules on the tropical legume Sesbania rostrata as a result of crack entry invasion of emerging lateral roots, might also enter ri ce and wheat by a similar route. Following inoculation with ORS571 car rying a lacZ reporter gene, azorhizobia were observed microscopically within the cracks associated with emerging lateral roots of rice and w heat. A high proportion of inoculated rice and wheat plants had coloni zed lateral root cracks. The flavanone naringenin at 10(-4) and 10(-5) M stimulated significantly the colonization of lateral root cracks an d also intercellular colonization of wheat roots. Naringenin does not appear to be acting as a carbon source and may act as a signal molecul e for intercellular colonization of rice and wheat by ORS571 by a mech anism which is nod gene-independent, unlike nodule formation in Sesban ia rostrata. The opportunity now arises to compare and to contrast the ability of Azorhizobium caulinodans with that of other rhizobia, such as Parasponia rhizobia, to intercellularly colonize the roots of non- legume crops.