The mechanism of a shoot-induced regulation of N2-fixation in soybean

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A main objective of the proposal is to transfer results obtained on the model legume Medicago truncatula to the economically important crop soybean. The regulation of N2 fixation in Medicago truncatula is clearly shoot-induced and dependent on the N-status of the shoot. Several environmental and nutritional factors induce a consistent, rhythmic time-course of N2 fixation, that decreases the per day amount of fixed nitrogen. Among these diverse factors is the application of ammonium to the roots or the formation of a high number of nodules in supernodulating mutants. The precise time-course of N2-fixation after ammonium application or darkening of the shoots of soybean will be ascertained in the beginning of the project. Based on the hypotheses that the shoot induced effect on nodules is related to miRNA, a comprehensive comparative mRNA and miRNA transcriptome profiling (RNAseq) will subsequently be performed at precisely defined point in time during the rhythmic N2-fixation of the nodules. As a next step, identified candidates will be followed in their expression by qPCR under the influence of various factors that induce the observed rhythmic N2 fixation over a 24 h period. Among these factors is the influence of pod-formation or that of an unusual large number of nodules in a mutant that is defect in the autoregulation of nodulation. The results will be a prerequisite for a possible effort to produce soybean that is independent in its N2 fixation of the ammonium concentration in the soil and might fix nitrogen more efficiently in a state of supernodulation. Both factors could be of significant importance for the productivity of soybean.