Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Abstract

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N2 fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO3− may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO2 levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO3−. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO3− was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO3−, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).