By Takuji Ohyama, Norikuni Ohtake, Kuni Sueyoshi, Kaushal Tewari, Yoshihiko Takahashi, Sayuri Ito, Toshikazu Nishiwaki, Yoshifumi Nagumo, Satomi Ishii, Takashi Sato
Within the first half, the authors introduce the development of researches on nitrogen metabolism of soybean nodules and roots. They examine the destiny of nitrogen mounted in soybean nodules via tracer test with 15N2 gasoline. the consequences point out that significant a part of fastened N in bacteroids (a symbiotic kingdom of rhizobia) is excreted speedily to cytosol of contaminated cells within the kind of ammonia, then the ammonia is assimilated into amino acids through GS/GOGAT pathway. Then the fastened nitrogen is assimilated into ureides, allantoin and allantoic acid, after which transported to the shoots through xylem. A small element of fastened N was once assimilated within the bacteroids at once into glutamate and alanine. nonetheless, nitrate absorbed from the roots are often assimilated into asparagine. The features of nitrate absorption and metabolism have been studied. it really is popular that nitrate is a powerful inhibitor to nodulation and nitrogen fixation, even supposing the inhibitory mechanism isn't totally understood. The authors lately came upon that nitrate depresses person nodule progress and nitrogen fixation job swiftly and reversibly while nodules have been in direct touch with nitrate. The oblique results of nitrate on nodule progress and nitrogen fixation task have been varied between remedy focus and interval of offer. the continual offer of low degrees of nitrate from the reduce a part of roots promoted the nodulation and nitrogen fixation of the higher a part of the roots. Hypernodulation mutant traces of soybean have been remoted that have profuse nodulation in comparison with mom and dad. additionally they convey partial-nitrate tolerant to nodulation. The features of hypernodulation mutant strains have been studied relating to nitrate inhibition. the consequences recommend that reduce nitrate absorption and assimilation job in hypernodulation mutants should be one cause to milder inhibition via nitrate on hypernodulation mutant strains.
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Additional resources for Nitrogen Fixation and Metabolism in Soybean Plants
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1989b Characteristics of Nitrate Absorption and Transport in Soybean Plants 39 Concerning to nitrate assimilation and transport in the roots, we proposed the hypothetic scheme as shown in Figure 35 (Ohyama et al. 1989a). Some part of NO3- absorbed in the roots is immediately exported to the shoots, whereas another part of NO3- is temporarily stored in the vacuoles of root cells then gradually released to the xylem. On the other hand, some other part of NO3- is reduced and assimilated in the roots and synthesized in asparagine.
25. Ammonium which is most reduced form of nitrogen and nitrate which is most oxidized form of nitrogen are two major inorganic nitrogen compounds in soil. The NH4+ ion is absorbed through the membrane bound protein, ammonium transporter. The NO3- ion is absorbed through the nitrate transporter with 2H+ co-transport. There are two types of nitrate transporter, a high affinity nitrate transporter system (HATS) and a low affinity nitrate transporter system (LATS) (Crawford and Glass 1998). 2. CHARACTERISTICS OF NITRATE ABSORPTION IN SOYBEAN ROOTS The changes in NO3- concentration in solution were monitored by nutrient solution circulation system in which the UV absorbance was detected by UV detector (Figure 26).
Figure 46. 11C traaanslocation in the split root systems of soybean plants. (A) Real time images of the distribution of 11C in soybean using Imaging Plant (BAS-1500). (B) Time course for the accumulation of radioacrivity as monitored by PETIS (Positoron emitting tracer imaging system), (C) The accumulation of radioactivity for the various point of roots. From Fujikake et al. 2003 These results indicate that the decrease in photoassimilate supply to nodules may be involved in the quick and reversible nitrate inhibition of soybean nodule growth and N2 fixation activity (Figure 48).