Interaction of nod and exo Rhizobium meliloti in Alfalfa Nodulation. Shoshana Klein. Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Ann M. Hirsch(2), Carol A. Smith(2), and Ethan R. Signer(1). (1) Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and (2) Department of Biological Sciences, Wellesley College, Wellesley, Massachusetts 02181, U.S.A.. MPMI 1:94-100. Accepted 20 January 1988. Copyright 1988 The American Phytopathological Society.

Among the genes of Rhizobium meliloti SU47 that affect nitrogen-fixing symbiosis with alfalfa are nod genes, in which mutations block nondule induction, and exo genes, in which mutations allow nodule formation but block rhizobial exopolysaccharide production as well as nodule invasion and nitrogen fixation. To investigate whether an exo⁺ bacterium can “help” (that is, reverse the symbiotic defect of) an exo mutant in trans, we have coinoculated alfalfa with pairs of rhizobia of different genotypes. Coinoculant genotypes were chosen so that the exo⁺ helper strain was nif while the exo “indicator” strain was nif⁺, and thus any fixation observed was carried out by the exo coinoculant. We find that a nod exo⁺ coinoculant can help an exo mutant both to invade nodules and to fix nitrogen. However, a nod⁺ exo⁺ coinoculant cannot help an exo mutant: Few exo bacteria are recovered from nodules, some bacteroids differentiate into bizarre aberrant forms, and the nodules fail to fix nitrogen. In a triple coinoculation, the effect of nod⁺ helper supersedes that of nod helper. Implications of these results for interaction of nod and exo gene products are discussed.