CONTROL OF SUPERNODULATION IN INTERGENERIC GRAFTS OF SOYBEAN AND COMMON BEAN

Understanding how nodulation is controlled should provide the basis for improving symbiotic N2 fixation in legume crops. Intergeneric grafting was carried out between soybean [Gylcine mar (L.) Merr.] and common bean (Phaseolus vulgaris L.) to investigate whether the autoregulation mechanism operates identically in two species which belong to different genera and which host different microsymbionts. Soybean cultivar Enrei and its supernodulating mutant En6500 as well as common bean cultivar OAC Rico (Rico) and its respective mutant R32BS15 (RBS15) were reciprocally grafted. Subsequently, the degree of nodulation of the grafted plants was determined and compared. when I-shaped grafts were used, only plants which were grafted intraspecifically with the mutants expressed supernodulation. Plants which were grafted intergenerically did not supernodulate. Both Y-shaped and X-shaped grafting procedures were used to graft and additional scion to the supernodulating mutant stocks. Suppression of supernodulation was observed only when wild-type scions were of the same species as the stock. These results support the hypothesis that the mechanism controlling nodulation or substances involved in autoregulation are different between soybean and common bean.