The beneficial plant growth-promoting association of Rhizobium leguminosarum bv. trifolii with rice roots

Yanni et al., 'The beneficial plant growth-promoting association of Rhizobium leguminosarum bv. trifolii with rice roots.', Australian Journal Of Plant Physiology 28 (2001): 845-870.

<jats:p>This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000This paper summarizes a multinational collaborative project to search fornatural, intimate associations between rhizobia and rice(Oryza sativa L.), assess their impact on plant growth,and exploit those combinations that can enhance grain yield with lessdependence on inputs of nitrogen (N) fertilizer. Diverse, indigenouspopulations of Rhizobium leguminosarum bv. trifolii (theclover root-nodule endosymbiont) intimately colonize rice roots in theEgyptian Nile delta where this cereal has been rotated successfully withberseem clover (Trifolium alexandrinum L.) sinceantiquity. Laboratory and greenhouse studies have shown with certain rhizobialstrainârice variety combinations that the association promotes root andshoot growth thereby significantly improving seedling vigour that carries overto significant increases in grain yield at maturity. Three field inoculationtrials in the Nile delta indicated that a few strainâvarietycombinations significantly increased rice grain yield, agronomic fertilizerN-use efficiency and harvest index. The benefits of this association leadingto greater production of vegetative and reproductive biomass more likelyinvolve rhizobial modulation of the plantâs root architecture for moreefficient acquisition of certain soil nutrients [e.g. N, phosphorus (P),potassium (K), magnesium (Mg), calcium (Ca), zinc (Zn), sodium (Na) andmolybdenum (Mo)] rather than biological N2 fixation. Inoculationincreased total protein quantity per hectare in field-grown grain, therebyincreasing its nutritional value without altering the ratios of nutritionallyimportant proteins. Studies using a selected rhizobial strain (E11) indicated that it produced auxin (indoleacetic acid) and gibberellin [tentatively identified as gibberellin (GA 7 )]phytohormones representing two major classes of plant growth regulators. Axenically collected rice root exudate significantly enhanced E11âs production of this auxin. This strain extensively colonized the rice root surface under gnotobiotic culture conditions, producing distributions of spatial patchiness that would favour their localized erosion of the epidermal surface, colonization of small crevices at epidermal junctions as a possible portal to enter into the root, and quorum sensing of diffusible signal molecules indicating that their nearest bacterial neighbours are in close proximity in situ. Studies of selected rhizobial endophytes of rice indicated that they produced cell-bound cellulase and polygalacturonase enzymes that can hydrolyze glycosidic bonds in plant cell walls, and non-trifolitoxin bacteriocin(s) that can inhibit other strains of clover rhizobia. Strain E11 was able to endophytically colonize rice roots of varieties commonly used by Filipino peasant farmers, and also to stimulate genotype-specificgrowth-promotion of corn (Zea mays, maize) under field conditions. An amalgam of these results indicate somerhizobia have evolved an additional ecological niche enabling them to form a three-component life cycle including a free-living heterotrophic phase in soil, a N2-fixing endosymbiont phase within legume root nodules, and a beneficial growth-promoting endocolonizer phase within cereal roots in the same crop rotation. Our results further indicate the potential opportunity to exploit this newly described, plant�rhizobia association by developing biofertilizer inoculants that may assist low-income farmers in increasing cereal production (especially rice) with less fertilizer N inputs, fully consistent with both sustainable agriculture and environmental safety.</jats:p>