Hypermodulation of soybean, mung bean, and hyacinth bean is controlled by a common shoot signal

Identification of commonality of nodulation control among legumes will facilitate progress in improving symbiotic N-2 fixation in agricultural systems. Interspecies grafts between soybean [Glycine max (L.) Merr] and mung bean (Vigna radiata L.), or soybean and hyacinth bean (Lablab purpureus L.), were made to evaluate whether a common translocatable signal controls expression of hypernodulation among legume species, Grafting of a 'NOD1-3' (hypernodulating soybean) shoot to roots of mung bean or hyacinth bean resulted in hypernodulation, a 4.1-fold increase in nodule number for mung bean, and a 2.7- to 5.5-fold increase (depending on bacterial strain) in nodule number for hyacinth bean, relative to self-grafted mung bean or hyacinth bean, respectively. 'Williams 82' shoots doubled nodule number when grafted to mung bean roots, but had no effect on nodulation when grafted to hyacinth bean roots, Symbiotic N-2 fixation (as measured by nitrogen accumulation in the plant) by nodulated mung bean roots was enhanced by 67% when grafted to Williams 82 and by 78% when grafted to NOD1-3 soybean shoots, compared with self-grafted mung bean plants, Grafting soybean shoots to hyacinth bean roots positively affected symbiotic N-2 fixation (9-40% depending on bacterial strain), but the magnitude was less than observed when mung bean roots were grafted to soybean shoots. It was concluded that control of hypernodulation expression by a shoot-transmissible factor is common among soybean, mung bean, and hyacinth bean, Seed were obtained from reciprocal grafts between soybean and mung bean to test for bacterial specificity for infection and nodule development. Seedling progeny from the reciprocal mung bean-soybean grafts were not altered in terms of bacterial specificity for nodulation.