Nodulin gene expression in effective root nodules of white sweetclover (Melilotus albaDesr.) and in ineffective nodules elicited by mutant strains ofRhizobium meliloti

Fifteen nodulins and several nodule-stimulated gene products were expressed 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 position of eight leghaemoglobin (Lb) products, as well as a product tentatively identified as nodule-stimulated glutamine synthetase (GS), was similar to previous reports of alfalfa (Medicago saliva L. cv. Iroquois) nodulins. Three mutants of Rhizobium meliloti, including an exoH mutant, a lipopolysaccharide mutant, and a nifH mutant, elicited ineffective sweetclover nodules blocked at empty (bacteria-free), partially infected, or fully infected stages of nodule development, respectively. In these ineffective nodules, the nodulin Nma30 and nodule-stimulated NSTma42 were expressed early in development, while a group of four nodulins and two nodule-stimulated products were intermediate in order of expression. Lb, GS and the late nodulin Nma12a were expressed later, following infection. The exoH mutant, Rm7154, appeared to be a leaky mutant, as a small percentage of the plants developed nitrogen-fixing 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 sweetclover is a useful diploid system for analysis of host genes essential to the Rhizobium/legume symbiosis.