Asparagine and ureide accumulation in nodules and shoots as feedback inhibitors of N2 fixation in soybean

Inhibition of N2 fixation under water deficits has been hypothesized to result from N feedback within the plant involving ureides and/or asparagine (Asn). This study was undertaken to investigate ureide and Ash accumulation in shoots and nodules in response to treatments inhibiting nodule activity (acetylene reduction assay, ARA) such as Asn, ureide, or polyethylene glycol application to the nutrient solution of plants, boric acid on leaves, and imposition of a water deficit. ARA inhibition and nodule concentration of Ash and ureide were correlated to the ureide treatment applied (3-15 mM applied in the nutrient solution). Supplying Asn (3-9 mM applied in the nutrient solution) also induced an increase in nodule Asn and ureide concentration, which was associated with ARA inhibition. Spraying boric acid on leaves also inhibited ARA in parallel to an increase in shoot ureide and nodule Ash concentration while nodule ureide remained unchanged. By contrast, polyethylene glycol (PEG) inhibited ARA in parallel to an increase in nodule Asn and ureide concentration while shoot ureide was unchanged. The decline in ARA in response to water deficit was associated with an increase in nodule ureide, Asn and aspartate (Asp), although the increases in Asn and Asp were less than for ureides. Altogether, the results of these experiments indicated that Ash cannot be the only compound involved in the feedback inhibition of ARA. Instead ureide and Asn are probably both involved, either directly by accumulation of products that fail to be exported, or by feedback from the shoot due to an N-compound supply that exceeds shoot requirements.