STRUCTURE OF NODULES FORMED BY RHIZOBIUM STRAIN ANU289 IN THE NONLEGUME PARASPONIA AND THE LEGUME SIRATRO (MACROPTILIUM-ATROPURPUREUM)

A study of root nodules produced by Rhizobium strain ANU289 in the legume siratro (M. atropurpureum) and the nonlegume Parasponia rigida showed that the legume nodule has a "typical" tropical legume nodule morphology: spherical determinate nodules with a peripheral vascular system. The invaded zone contains both infected hypertrophied cells and small uninfected starch-containing interstitial cells. All infected cells are at a comparable developmental stage. The bacteroids are packaged in the peribacteroid membrane that is of plant origin. Typically 3-4 bacteroids are found per enveloping membrane. The nonlegume nodule is a modified lateral root with multiple regions of apical meristematic activity; the nodule is indeterminate and has a centralized vascular system. Developmentally, the infected cells vary; the mature, fully infected cells are most distant from the meristem. The bacteroids are not released from the membrane-bound infection thread, which appears to develop into a lightly staining, thin-walled thread (the "fixation" thread) that contains numerous, loosely packed bacteroids, often with significant deposits of polybetahydroxybutyrate. The morphology of the bacteroids is similar in both nodule types, which are capable of high rates of acetylene reduction.