Legume-Rhizobium interactions: role of cowpea root exudate in polysaccharide synthesis and infectivity of Rhizobium species

Abstract

Nodulation pattern of cowpea (Vigna sinensis) was markedly influenced by the growth phase of the microsymbiont, Rhizobium sp. strain 1001. Rhizobia from stationary growth phase cultures required much longer time after inoculation to initiate successful infections on cowpea seedlings than the rhizobia from log phase. Preincubation of stationary phase rhizobia in root exudate of cowpea seedlings grown without NH ⁺₄ in the medium enhanced the formation of capsules around a large proportion of the cells. The longer the time of incubation the larger was the proportion of cells with capsules. The increase in cell-surface polysaccharide content showed, up to a limit, a positive relationship with the increase in percentage of plants scored for faster nodulation response. Root exudate of cowpea plants grown with NH ⁺₄ could neither stimulate polysaccharide synthesis by the rhizobia from stationary phase nor promote them to initiate infections without delay, indicating the regulatory role of combined nitrogen in early infection events. Root exudates of Vigna radiata (mung bean) grown without NH ⁺₄ also promoted polysaccharide synthesis by Rhizobium sp. strain 1001 and faster nodulation on cowpea plants, but the exudates of Trigonella foenum-graecum and Medicago sativa were not effective indicating the relative specificity of the early interactions.