Insights into the regulation of nitrogen fixation in pea nodules: lessons from drought, abscisic acid and increased photoassimilate availability

Nitrogen fixation in legume nodules has been shown to be very sensitive to drought and other environmental constraints. It has been widely assumed tha t this decline in nitrogen fixation was a consequence of an increase in the so-called oxygen diffusion barrier and a subsequent impairment to bacteroi d respiration. However, it has been recently shown that nitrogen fixation i s highly correlated with nodule sucrose synthase (SS) activity under drough t and other environmental stresses. Whether this correlation reflects a cau sative relationship or not has not been proven yet. The evidence presented here suggests that SS controls nitrogen fixation under mild drought conditi ons. However, nitrogen fixation cannot be enhanced only by increasing glyco lytic flux, as under these conditions nodules become oxygen limited. Abscis ic acid also induces a decline in nitrogen fixation that is independent of SS. The overall results suggest the occurrence of a complex regulation of n odule nitrogen fixation involving, at least, both carbohydrate and oxygen f luxes within the nodule.