ETHYLENE-MEDIATED PHENOTYPIC PLASTICITY IN ROOT-NODULE DEVELOPMENT ONSESBANIA-ROSTRATA

Leguminous plants in symbiosis with rhizobia form either indeterminate nodules with a persistent meristem or determinate nodules with a tran sient meristematic region. Sesbania rostrata was thought to possess de terminate stem and root nodules, However, the nature of nodule develop ment is hybrid, and the early stages resemble those of indeterminate n odules. Here we show that, depending on the environmental conditions, mature root nodules can be of the indeterminate type. In situ hybridiz ations with molecular markers for plant cell division, as well as the patterns of bacterial nod and nif gene expression, confirmed the indet erminate nature of 30-day-old functional root nodules. Experimental da ta provide evidence that the switch in nodule type is mediated by the plant hormone ethylene.