Involvement of Ureides in Nitrogen Fixation Inhibition in Soybean1

Abstract

The sensitivity of N 2 fixation to drought stress in soybean (Glycine max Merr.) has been shown to be associated with high ureide accumulation in the shoots, which has led to the hypothesis that N 2 fixation during drought is decreased by a feedback mechanism. The ureide feedback hypothesis was tested directly by measuring the effect of 10 mM ureide applied by stem infusion or in the nutrient solution of hydroponically grown plants on acetylene reduction activity (ARA). An almost complete inhibition of ARA was observed within 4 to 7 d after treatment, accompanied by an increase in ureide concentration in the shoot but not in the nodules. The inhibition of ARA resulting from ureide treatments was dependent on the concentration of applied ureide. Urea also inhibited ARA but asparagine resulted in the greatest inhibition of nodule activity. Because ureides did not accumulate in the nodule upon ureide treatment, it was concluded that they were not directly inhibitory to the nodules but that their influence mediated through a derivative compound, with asparagine being a potential candidate. Ureide treatment resulted in a continual decrease in nodule permeability to O 2 simultaneous with the inhibition of nitrogenase activity during a 5-d treatment period, although it was not clear whether the latter phenomenon was a consequence or a cause of the decrease in the nodule permeability to O 2 .The physiological basis of N 2 fixation inhibition by water deficits in legume nodules is not clearly understood. A potential physiological basis for this water-deficit sensitivity may be that drought stress decreases the P o (Weisz et al., 1985), as has been shown with other stresses such as temperature, salinity, or nitrate (Hunt and Layzell, 1993;Serraj et al., 1994; Denison and Harter, 1995). The role of O 2 limitation in the response of nitrogenase activity to drought stress has been discussed extensively (Diaz del Castillo and Layzell, 1995;Purcell and Sinclair, 1995;Serraj and Sinclair, 1996b;Serraj et al., 1999). However, the mechanisms by which drought affects P o have not been elucidated. It is not clear whether drought stress has a direct effect on P o , or whether the decrease in P o is a consequence of a decrease in nodule activity.An alternative explanation for the decrease in nitrogenase activity under drought could be a feedback mechanism involving the accumulation of N compounds. Pate et al. (1969) proposed that lower rates of water movement out of the nodule during drought stress may restrict export of products of N 2 fixation, and the accumulation of these products would inhibit nitrogenase activity. Others have suggested that N 2 fixation in legumes might be regulated by a feedback mechanism involving N metabolism and the pool of reduced N in the plant (Silsbury et al., 1986;Parsons et al., 1993; Hartwig et al., 1994). Oti-Boateng and Silsbury (1993) reported an inhibition of nitrogenase activity in fava bean after plant uptake of Asn or Gln.Soybean (Glycine max Merr.) usually exports more than 80% of the N comp...

Feedback regulation of symbiotic N2 fixation under drought stress

“…Ureide accumulation was recently compared in nodule, root and shoot tissues in a drought-sensitive soybean cultivar exposed to soil dehydration [22]. Under wellwatered conditions, ureide concentration was very low in nodule, root and leaf tissues [22].…”

Consequences of the water deficit on water relations and symbiosis in Vigna unguiculata cultivars

“…This way, it inhibits the biochemical activity responsible by the nitrogen metabolism (Serraj et al 1999).…”

Ureide Content of Guar under Influence of Hexaconazole and Triazophos

“…Remobilization of nitrogen from senescent tissues was suggested as the most likely alternative source of ureides and involved in the N-feedback regulation of nitrogen fixation [6]. Moreover, ureide accumulation upon drought stress has been hypothesized to be responsible for N 2 fixation inhibition [7]. The ureide composition under pesticidal stress is not been reported in leguminous vegetables plants to ensure the nitrogen fixation and metabolism altering effect.…”