Novel infection process in the indeterminate root nodule symbiosis betweenChamaecytisus proliferus (tagasaste) and Bradyrhizobium sp.

The main characteristics of the symbiosis of tagasaste (Chamaecytisus proli ferus ssp. proliferus var. palmensis), a woody legume forming N-2-fixing in determinate nodules in response to infection by strains of Bradyrhizobium s p. (Chamaecytisus), are reported here. The infection process in this legume was examined by bright field, phase co ntrast and transmission electron microscopy, and was found to be unlike any other previously described. First steps in the infection process involve initiation of infection thread s within deformed root hairs and induction of foci of host-cell divisions i n the inner root cortex. However, infection of root hairs aborts early, and instead, the bacteria use the crack-entry mode of host infection, whereby they penetrate the periphery of the emerging nodule through an intercellula r route, eventually infecting host nodule cells directly through altered ce ll walls. No successful infection threads were detected at any stage of pri mary-host infection or nodule invasion. Indeterminate nodules were mainly f ormed on unbranched areas of lateral roots. This is the first description of such a combination of events in an infecti on process in the Rhizobium-legume root-nodule symbiosis.