Abscisic acid inhibition of root nodule initiation in Pisum sativum

The effect of exogenous abscisic acid (ABA) on root nodule formation in Pisum sativum cv. Alaska was examined. ABA supplied to the roots at 1.9×10(-6)M reduced the number of nodules/plant 61% without affecting root or shoot growth. The first noticeable inhibition of nodulation occurred at 3.8×10(-7)M ABA. ABA at a concentration of 1.9×10(-6)M inhibited neither root hair formation nor infection of root hairs by Rhizobium leguminosarum. Similar numbers of infection threads penetrated the cortex in both control and treated plants. ABA concentrations of 3.8×10(-6)M had no effect on the doubling time or maximum density of R. leguminosarum in pure cultures. Normal nodule formation involves a polyploid cortical proliferation. This response to rhizobial infection can be imitated by culturing 1-mm pea-root segments on a medium containing 4.7×10(-6)M kinetin. Under these conditions a highly significant reduction in the number of polyploid mitoses after 72 h is produced by 3.8×10(-8)M ABA. A maximum inhibition of 68% was found with 3.8×10(-6)M ABA. A similar range of ABA concentrations also inhibited the cytokinin-induced cell division in soybean callus. It is concluded that ABA reduces the number of root nodules/plant by inhibiting the cortical cell divisions required for root nodule formation.