Update on Legume Evolution Legume Evolution: Where Do Nodules and Mycorrhizas Fit In?1

Recent findings on legume biogeography and the timing of evolution of key legume tribes have sup-ported a newview of the evolution of nodule processes. It is suggested that an initial infection process not involving root hairs led to two branches of legume nodule development, one that subsequently devel-oped transcellular infection threads (ITs) to carry bac-teria to young nodule cells and one in which such ITs were not formed. Two types of nodules, with indeter-minate or determinate growth, evolved from each of these. Knowledge of the diversity of bacteria known to nodulate legumes and their relations with other bac-teria is expanding rapidly, posing new questions about nodulation in the field. Ectomycorrhizas (ECMs) are found in both nodulating and non-nodulating le