NODULE ORGANOGENESIS AND SYMBIOTIC MUTANTS OF THE MODEL LEGUME LOTUS-JAPONICUS

A detailed microscopical analysis of the morphological features that d istinguish different developmental stages of nodule organogenesis in w ild-type Lotus japonicus ecotype Gifu B-129-S9 plants was performed, t o provide the necessary framework for the evaluation of altered phenot ypes of L. japonicus symbiotic mutants, Subsequently, chemical ethyl m ethanesulfonate (EMS) mutagenesis of L. japonicus was carried out. The analysis of approximately 3,000 M-1 plants and their progeny yielded 20 stable L. japonicus symbiotic variants, consisting of at least 14 d ifferent symbiosis-associated loci or complementation groups. Moreover , a mutation affecting L. japonicus root development was identified th at also conferred a hypernodulation response when a line carrying the corresponding allele (LjEMS102) was inoculated with rhizobia, The phen otype of the LjEMS102 line was characterized by the presence of nodule structures covering almost the entire root length (Nod(++)), and by a concomitant inhibition of both root and stem growth, A mutation in a single nuclear gene was shown to be responsible for both root and symb iotic phenotypes observed in the L, japonicus LjEMS102, line, suggesti ng that (a) common mechanism(s) regulating root development and nodule formation exists in legumes.