Role of lectin in enhancing associative nitrogen fixation in rice

In this research, we study three rice cultivars (Oryza sativa subspecies indica: RD7, NMS4 and KDML5) grown in axenic culture under N-deficient condition, with or without inoculation of rhizospheric. Klebsiella oxytoca, strain R15 (10*[9) cell/plant on day 7 after germination). Inoculation in all rice cultivars results in the specific increase about 2-3 folds of internal root lectin content, while there is no effect on shoot lectin. This significant increase in root lectin is independent on total protein content which remains normal, and most likely results from de novo synthesis. Internal root lectins and specifically localized by immunogold-protein A labeling technique. Processed by golgi body into vacuolar lectin and transported to the epidermal cell surface. The function of secretory lectin as a necessary associative factor in the adhesion between klebsiella R15 and epidermal cells of rice root has been confirmed. In control rice seedlings'root and leaf without R15 inoculation, distribution of lectins are similar to those inoculated plants. Both in the root and leaf tissues, beside the vacuoles, specific localization of lectin have been found in the xylem and phloem, especially in the cell wall, middle lamella and intercellular spaces among these cells. One of the physiological function of lection might be a carrier of some metabolites or nutrients according to its sugar-binding specificity, or indirectly as receptors of some macromolecules containing GlcNac or its oligomer as component. Interaction between Klebsiella R15 and rice results in the significant increase of nitrogenase activity (300 nmol/tube/day over the free-living R15 within 2 weeks after inoculation, which has no direct correlation with the 2-3 folds increase of root lectin. The rice plant dry weight, percent total nitrogen and total nitrogen of associative rice-R15 show no significant difference from non-inoculated rice seedlings. These results indicate that fixed-N2 should be assimilated by bacterial GS to form amino acids, and probably transported into plant cells in the form of amino acids. Transamination of fixed-N2 in the leaf of rice has been studied by determination of rice GS specific activity on day 7, 14 and 21 after inoculation, our results show about 20-40 percent increase of leaf GS specific activity in association with R15.