NODULIN GENE-EXPRESSION IN EFFECTIVE ROOT-NODULES OF WHITE SWEETCLOVER (MELILOTUS-ALBA DESR) AND IN INEFFECTIVE NODULES ELICITED BY MUTANT STRAINS OF RHIZOBIUM-MELILOTI

Fifteen nodulins and several nodule-stimulated gene products were expr essed in effective, nitrogen-fixing root nodules of white sweetclover (Melilotus alba Desr, cv, U389), as determined by two-dimensional gel electrophoresis of in vitro translation products, The number and gel p osition of eight leghaemoglobin (Lb) products, as well as a product te ntatively identified as nodule-stimulated glutamine synthetase (Gs), w as similar to previous reports of alfalfa (Medicago sativa L, cv, Iroq uois) nodulins, Three mutants of Rhizobium meliloti, including an exoH mutant, a lipopolysaccharide mutant, and a nifH mutant, elicited inef fective sweetclover nodules blocked at empty (bacteria-free), partiall y infected, or fully infected stages of nodule development, respective ly, In these ineffective nodules, the nodulin Nma30 and nodule-stimula ted NSTma42 were expressed early in development, while a group of four nodulins and two nodule-stimulated products were intermediate in orde r of expression, Lb, as and the late nodulin Nma12a were expressed lat er, following infection, The exoH mutant, Rm7154, appeared to be a lea ky mutant, as a small percentage of the plants developed nitrogen-fixi ng nodules about 4 weeks after inoculation, The sequential expression of a large number of nodulins and nodule-stimulated products, as well as the availability of sweetclover nodulation mutants indicates that s weetclover is a useful diploid system for analysis of host genes essen tial to the Rhizobium/legume symbiosis.