| Abstract: | Fifteen nodulins and several nodule-stimulated gene productswere expressed in effective, nitrogen-fixing root nodules ofwhite sweetclover (Melilotus alba Desr. cv. U389), as determinedby two-dimensional gel electrophoresis of in vitro translationproducts. The number and gel position of eight leghaemoglobin(Lb) products, as well as a product tentatively identified asnodule-stimulated glutamine synthetase (GS), was similar toprevious reports of alfalfa (Medicago sativa L. cv. Iroquois)nodulins. Three mutants of Rhizobium meliloti, including anexoH 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 Nma30and nodule-stimulated NSTma42 were expressed early in development,while a group of four nodulins and two nodule-stimulated productswere intermediate in order of expression. Lb, GS and the latenodulin Nmal2a were expressed later, following infection. TheexoH mutant, Rm7154, appeared to be a leaky mutant, as a smallpercentage of the plants developed nitrogen-fixing nodules about4 weeks after inoculation. The sequential expression of a largenumber of nodulins and nodule-stimulated products, as well asthe availability of sweetclover nodulation mutants indicatesthat sweetclover is a useful diploid system for analysis ofhost genes essential to the Rhizobium/legume symbiosis. Key words: Nitrogen fixation, nodulation mutants, nodulins |
