NATURAL ENDOPHYTIC ASSOCIATION BETWEEN RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII AND RICE ROOTS AND ASSESSMENT OF ITS POTENTIAL TO PROMOTE RICE GROWTH

For over 7 centuries, production of rice (Oryza sativa L.) in Egypt ha s benefited from rotation with Egyptian berseem clover (Trifolium alex andrinum). The nitrogen supplied by this rotation replaces 25-33% of t he recommended rate of fertilizer-N application for rice production. T his benefit to the rice cannot be explained solely by an increased ava ilability of fixed N through mineralization of N-rich clover crop resi dues. Since rice normally supports a diverse microbial community of in ternal root colonists, we have examined the possibility that the clove r symbiont, Rhizobiurn leguminosarum bv. trifolii colonizes rice roots endophytically in fields where these crops are rotated, and if so, wh ether this novel plant-microbe association benefits rice growth. MPN p lant infection studies were performed on macerates of surface-steriliz ed rice roots inoculated on T. alexandrinum as the legume trap host. T he results indicated that the root interior of rice grown in fields ro tated with clover in the Nile Delta contained similar to 10(6) clover- nodulating rhizobial endophytes g(-1) fresh weight of root. Plant test s plus microscopical, cultural, biochemical, and molecular structure s tudies indicated that the numerically dominant isolates of clover-nodu lating rice endophytes represent 3-4 authentic strains of R. leguminos arum by. trifolii that were Nod(+) Fix(+) on berseem clover. Pure cult ures of selected strains were able to colonize the interior of rice ro ots grown under gnotobiotic conditions. These rice endophytes were rei solated from surface-sterilized roots and shown by molecular methods t o be the same as the original inoculant strains, thus verifying Koch's postulates. Two endophytic strains of R. leguminosarum by. trifolii s ignificantly increased shoot and root growth of rice in growth chamber experiments, and grain yield plus agronomic fertilizer N-use efficien cy of Giza-175 hybrid rice in a field inoculation experiment conducted in the Nile Delta. Thus, fields where rice has been grown in rotation with clover since antiquity contain Fix(+) strains of R. leguminosaru m bv. trifolii that naturally colonize the rice root interior, and the se true rhizobial endophytes have the potential to promote rice growth and productivity under laboratory and field conditions.