OXYGEN AS A KEY DEVELOPMENTAL REGULATOR OF RHIZOBIUM-MELILOTI N-2-FIXATION GENE-EXPRESSION WITHIN THE ALFALFA ROOT-NODULE

The symbiotic pattern of expression of Rhizobium meliloti N-2-fixation genes is tightly coupled with the histological organization of the al falfa root nodule and thus is under developmental control. N-2-fixatio n gene expression is induced very sharply at a particular zone of the nodule called interzone II-III that precedes the zone where N-2 fixati on takes place, We show here that this coupling can be disrupted, ther eby resulting in ectopic expression of N-2-fixation genes in the prefi xing zone II of the nodule, Uncoupling was obtained either by using a R. meliloti strain in which a mutation rendered N-2-fixation gene expr ession constitutive with respect to oxygen in free-living bacterial cu ltures or by placing nodules induced by a wild-type R. meliloti strain in a microoxic environment. These results implicate oxygen as a key d eterminant of the symbiotic pattern of N-2-fixation gene expression.