A PERSISTENT MERISTEM IS FORMED IN NODULAR STRUCTURES ELICITED BY NODFACTOR OR BY A RHIZOBIUM-MELILOTI EXOPOLYSACCHARIDE MUTANT IN ALFALFAPLANTS WHICH NODULATE SPONTANEOUSLY

Certain alfalfa (Medicago sativa L.) plants are able to nodulate spont aneously (Nar character) and form well-developed, elongated nodules in the absence of Rhizobium. We selected and selfed Nar(+) and Nar(-) pl ants, obtaining S-1 populations with either a high or a very low frequ ency of Nar(+) plants, respectively. To assess the influence of the Na r character on nodule development, we tested whether Nar(+) and Nar(-) progenies treated by Rhizobium meliloti Nod factor or inoculated with a R. meliloti exopolysaccharide mutant differed in terms of nodule me ristematic activity. Following addition of NodRmIV(Ac,S,C-16:2), only small nodular outgrowths were formed on Nar(-) progenies, whereas an i ncreased number of elongated nodules was observed on Nar(+) progenies. Nodules induced by a R. meliloti exo(-) mutant on Nar(-) progenies la cked a persistent meristem and arrested in development soon after emer gence, whereas on Nar(+) progenies they possessed a persistent meriste m and elongated. From both experiments, we conclude that in Nar(+) pla nts, the capacity to induce the persistent meristem observed during sp ontaneous nodulation is conserved in nodules elicited by Nod factor or exo(-) mutant. (C) 1997 Elsevier Science Ireland Ltd.