Differential sensitivity of nodulation to ethylene in soybean cv. Bragg and a supernodulating mutant

We previously found that the ethylene inhibitor Ag+ could overcome the inhi bitory effect of nitrate on nodulation of soybean (Glycine max) cv. Bragg. The same treatment increased nodulation quantitatively under non-inhibitory conditions, strongly suggesting involvement of ethylene in the control of nodulation in this species. Supernodulation mutants that lack internal auto regulation of nodulation, however, had biosynthesis capacity similar to the wild type. In the present work, the effects of ethylene on nodulation of ' Bragg' and two separate, but allelic, supernodulating mutants (nts382 and n ts1007) were compared. the nodulation process appeared much more sensitive than plant growth and development to ethylene, which reduced the number of nodules per plant, but nearly twofold more in the wild type than in the sup ernodulation mutants. The cause-effect relationship is established by the c ounteracting effect of Ag+ and the fact that the stronger the inhibition by ethylene, the higher the recovery of nodulation ability with the ethylene antagonist. This higher tolerance of or lower sensitivity to ethylene in nt s382 persists even under low inoculum dose, where nodule number and mass co uld be decreased to wild-type levels. Differences between the mutant and th e wild type in the triple response test do not appear to support difference s in ethylene perception on a whole-plant basis. The results suggest that s ensitivity of nodulation to ethylene might have been affected in supernodul ation mutants.