Preinfection events in legume-Rhizobium symbiosis were analyzed by studying the different nodulation behaviors of 2 rhizobial strains in cowpeas (V. sinensis). Log-phase cultures of Rhizobium sp. strain 1001, an isolate from the plant nodule, initiated host responses leading to infection within 2 h after inoculation, whereas log-phase cultures of Rhizobium sp. strain 32H1 took at least 7 h to trigger a discernible response. The delay observed with strain 32H1 could be eliminated by incubating the rhizobial suspension, before inoculation, for 4.5 h either in the cowpea rhizosphere/rhizoplane condition or in the root exudate of cowpea plants, grown without NH4+ in the rooting medium. The delay could not be eliminated by incubating the rhizobial suspension in the rooting medium of plants grown in the presence of 5 mM NH4+, indicating that there is a regulatory role of combined N in triggering preinfection events by the legume. The sustance(s) in the root exudate which elicited the faster nodulation response by Rhizobium sp. strain 32H1 could be separated into a high-MW fraction by Sephadex G-100 gel filtration. Legume roots release substances that favor the development of rhizobial features essential for infection and nodulation.
