Feedback regulation of nitrogenase and response of a hypernodulating soybean mutant to increased photosynthate supply

Abstract

The hypothesis that nitrogenase activity may be related to N and C levels in plant tissues was evaluated. Soybean (Glycine max L. Merr.) plants were grown in growth chambers using a hydroponic system. A wide range of C:N ratios in various plant tissues was generated through a combination of light, CO\sb2, and N levels, using two soybean genotypes differing in C and N acquisition rates and in tolerance of nodulation to external N. Shoot N concentration was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO\sb3\sp-. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoots and nodules. Nitrogenase inhibition by NO\sb3\sp- was negatively correlated to TNC and C:N ratio in shoots, but not in nodules. Modulation of the nitrogenase activity appeared to occur through sensing changes in plant N status. Changes in nitrogenase activity and in levels of various amino acids in shoots and nodules were investigated 24 h following NO\sb3\sp- treatment. Phenylisothiocyanate- (PITC) amino acid derivatives were separated and quantified using HPLC. The decline in nitrogenase activity following the short-term NO\sb3\sp- treatment was associated with a dramatic asparagine concentration increase in the shoot, not in the nodule. Asparagine concentration increased 35 times from a barely detectable level of 95 to 3327 nmol g\sp{-1} FW in Williams 82 (a normally nodulating control cultivar), and more than tripled from 509 to 1753 nmol g\sp{-1} FW in NOD1-3 (a hypernodulating mutant). These results are consistent with the idea that the feedback control of nodulation and nodule activity is located in the shoot and indicated that asparagine may be the signal molecule involved in sensing of soybean N status. These results also indicated that partial-NO\sb3\sp- tolerance of nodulation in the NOD1-3 hyper-nodulated mutant is associated with a lesser change in tissue N following NO\sb3\sp- treatment. Experiments were also conducted (i) to investigate if increased photoassimilate supply through light enhancement and CO\sb2 enrichment could reverse the deleterious growth and nodule function traits of NOD1-3 and (ii) to gain insight into the process(es) involved in the response of the mutant. Both light enhancement and CO\sb2 enrichment increased nodule number, nodule activity (per plant, not specific activity), and dry matter accumulation in all tissues of both genotypes. While Williams 82 responded more to light enhancement than the hypernodulating mutant, the opposite was observed under CO\sb2 enrichment, indicating that growth of the mutant may be more limited by respiration and/or photorespiration than is Williams 82. However, total biomass and specific nitrogenase activity were always less in the mutant than in Williams 82. The hypernodulated mutant appeared to inherently possess other mutations affecting root and shoot growth, possibly respiration among other things. It is suggested that the greater nodule number observed under light enhancement and CO\sb2 enrichment resulted from the increased photosynthate supply, independent of the nodulation autoregulatory signal, as treatment effects on nodule number were related to their effects on dry matter accumulation. (Abstract shortened by UMI.)U of I OnlyETDs are only available to UIUC Users without author permissio