THE UNIQUE ROOT-NODULE SYMBIOSIS BETWEEN RHIZOBIUM AND THE AQUATIC LEGUME, NEPTUNIA-NATANS (L-F) DRUCE

We examined the development of the aquatic N-2-fixing symbiosis betwee n Rhizobium sp. (Neptunia) and roots of Neptunia natans L. f. (Druce) (previously N. oleracea Lour.) under natural and laboratory conditions . When grown in its native marsh habitat, this unusual aquatic legume does not develop root hairs, the primary sites of rhizobial infection for most temperate legumes. Under natural conditions, the aquatic plan t floats and develops nitrogen-fixing nodules at emergence of lateral roots on the primary root and on adventitious roots at stem nodes, but not from the stem itself. Cytological studies using various microscop ies revealed that the mode of root infection involved an intercellular route of entry followed by an intracellular route of dissemination wi thin nodule cells. After colonizing the root surface, the bacteria ent ered the primary root cortex through natural wounds caused by splittin g of the epidermis and emergence of young lateral roots, and then stim ulated early development of nodules at the base of such roots. The bac teria entered the nodule through pockets between separated host cells, then spread deeper in the nodule through a narrower intercellular rou te, and eventually evoked the formation of infection threads that pene trated host cells and spread throughout the nodule tissue. Bacteria we re released from infection droplets at unwalled ends of infection thre ads, became enveloped by peribacteroid membranes, and transformed into enlarged bacteroids within symbiosomes. In older nodules, the bacteri a within symbiosomes were embedded in an unusual, extensive fibrillar matrix. Cross-inoculation tests of 18 isolates of rhizobia from nodule s of N. natans revealed a host specificity enabling effective nodulati on of this aquatic legume, with lesser affinity for Medicago sativa an d Ornithopus sp., and an inability to nodulate several other crop legu me species. Acetylene reduction (N-2 fixation) activity was detected i n nodules of N. natans growing in aquatic habitats under natural condi tions in Southern India. These studies indicate that a specific group of Rhizobium sp. (Neptunia) occupies a unique ecological niche in aqua tic environments by entering into a N-2-fixing root-nodule symbiosis w ith Neptunia natans.