Developmental regulation of nodulation in Arachis hypogea (peanut) and Aeschynomene americana (jointvetch)

Legumes in the tribe Aeschynomeneae develop nodules only at the site of lateral root emergence. Rhizobia infect the host through epidermal cracks rather than utilizing infection threads in root hairs. In this study we have examined the ontological factors that regulate nodule distribution on two members of the tribe, joinvetch (Aeschynomene americana L.) and peanut (Arachis hypogea L.). In both hosts nodulation is generally restricted to young lateral roots, less than or equal to 2.5 cm in length at the time of inoculation. This developmental window is independent of concomitant maturation of the tap root. In peanut, removal of all visible lateral roots from mature tap roots caused new laterals to emerge; these lateral roots nodulated at a frequency similar to that of young laterals on untreated tap roots. Additionally, removal of lateral root meristems two days after inoculation did not alter nodule development. With jointvetch new lateral roots naturally develop from mature regions of the tap root, and these lateral roots are capable of nodulation. Autoregulation, the partial restriction of nodulation in regions of the tap root that develop after inoculation, was evident in both hosts.