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

The main characteristics of the symbiosis of tagasaste (Chamaecytisus proliferus ssp. proliferus var. palmensis), a woody legume forming N-2-fixing indeterminate nodules in response to infection by strains of Bradyrhizobium sp. (Chamaecytisus), are reported here. The infection process in this legume was examined by bright field, phase contrast and transmission electron microscopy, and was found to be unlike any other previously described. First steps in the infection process involve initiation of infection threads within deformed root hairs and induction of foci of host-cell divisions in 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 intercellular route, eventually infecting host nodule cells directly through altered cell walls. No successful infection threads were detected at any stage of primary-host infection or nodule invasion. Indeterminate nodules were mainly formed on unbranched areas of lateral roots. This is the first description of such a combination of events in an infection process in the Rhizobium-legume root-nodule symbiosis.