Structure of nodules induced by auxotrophic and ineffective mutants of Rhizobium meliloti strain L5-30 requiring cysteine, arginine+uracil and histidine.

Nodules produced by ineffective mutants of R. meliloti strain L5-30 requiring arginine+uracil (arg-55) and cysteine requiring mutants (cys-243, cys-244, cys-246) studied under light microscopy were found to be occupied by bacteria. This indicates on defect in transformation of these mutants into N2 fixing bacteroids. These defects were not associated with auxotrophy. In the nodules induced by histidine requiring mutant (his-240) only few host plant cells were occupied by bacteria. This indicate that his-240 mutant is defective in liberation from the infection thread and its multiplication since supplementation of the plant growth medium with 50 microgram/ml of L-histidine enabled establishment of fully effective association. Prototrophic transductants and revertants were fully effective.