Nitrogen Fixation and Metabolism in Soybean Plants

In the first part, the authors introduce the progress of researches on nitrogen metabolism of soybean nodules and roots. They investigate the fate of nitrogen fixed in soybean nodules by tracer experiment with 15N2 gas. The results indicate that major part of fixed N in bacteroids (a symbiotic state of rhizobia) is excreted rapidly to cytosol of infected cells in the form of ammonia, then the ammonia is assimilated into amino acids via GS/GOGAT pathway. Then the fixed nitrogen is assimilated into ureides, allantoin and allantoic acid, and then transported to the shoots via xylem. A small portion of fixed N was assimilated in the bacteroids directly into glutamate and alanine. On the other hand, nitrate absorbed from the roots are mainly assimilated into asparagine. The characteristics of nitrate absorption and metabolism were studied. It is well known that nitrate is a potent inhibitor to nodulation and nitrogen fixation, although the inhibitory mechanism is not fully understood. The authors recently found that nitrate depresses individual nodule growth and nitrogen fixation activity rapidly and reversibly when nodules were in direct contact with nitrate. The indirect effects of nitrate on nodule growth and nitrogen fixation activity were different among treatment concentration and period of supply. The continuous supply of low levels of nitrate from the lower part of roots promoted the nodulation and nitrogen fixation of the upper part of the roots. Hypernodulation mutant lines of soybean were isolated which have profuse nodulation compared with parents. They also exhibit partial-nitrate tolerant to nodulation. The characteristics of hypernodulation mutant lines were studied in relation to nitrate inhibition. The results suggest that lower nitrate absorption and assimilation activity in hypernodulation mutants may be one reason to milder inhibition by nitrate on hypernodulation mutant lines.