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ARS Home » Midwest Area » Urbana, Illinois » Soybean/maize Germplasm, Pathology, and Genetics Research » Research » Publications at this Location » Publication #65476

Title: THE FEEDBACK MECHANISM OF NITRATE INHIBITION OF NITROGENASE ACTIVITY IN BRADYRHIZOBIUM JAPONICUM/SOYBEAN SYMBIOSIS INVOLVES PHLOEM-TRANSLOCATED ASPARAGINE

Author
item BACANAMWO, METHODE - U OF ILLINOIS, URBANA
item Harper, James

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Nodulation occurs on legumes such as soybean and results in ability of the plant to obtain nitrogen from the atmosphere, termed symbiotic nitrogen fixation. Understanding the control points in the nodulation process is essential to enable increasing symbiotic nitrogen fixation. This study determined that a specific amino acid (asparagine) is strongly elevated when soybean plants are provided with nitrate in the growth media. At the same time the symbiotic nitrogen fixation process is markedly inhibited. From this it is suggested that asparagine may serve as a signal compound to limit symbiotic nitrogen fixation in the presence of nitrate. Identification of this possible control compound will enable targeting of specific metabolic pathways for possible alteration, in an attempt to overcome the inhibitory effect of nitrate on symbiotic nitrogen fixation.

Technical Abstract: A feedback mechanism which involves sensing of change in phloem N concentration has been proposed to control nodulation and dinitrogen fixation in the presence of external combined N. Whether this control is in response to a change in phloem total N or in some specific signal compound(s) is not known. This study reevaluated the hypothesis that control of nodulation and N2 fixation involves sensing of change in phloem N composition and attempted to identify potential signal molecule(s) involved. Two soybean (Glycine max [L.] Merr) genotypes differing in nodulation tolerance to nitrate were grown for 28 days to develop good nodulation. Half of 28-d-old plants were treated with 15 mM NO3- for 24 h, then control and treated plants were sampled for evaluation of nitrogenase activity and tissue concentration of total N and of each individual free amino acid. The decline in nitrogenase activity following the short-term nitrate treatment was associated with an increase in the concentration of total free amino acids and some individual amino acids in the shoot of both genotypes but not in the nodule. Asparagine was the only amino acid with a dramatic concentration increase in the shoot following nitrate treatment in both genotypes, increasing 35 fold in Williams 82, and more than tripling in NOD1-3. These results indicate that the feedback control of nodulation and nodule activity is located in the shoot and indicate that Asn may be the signal molecule involved in sensing of soybean N status.