Attachment to plant roots and nod gene expression are not affected by pH or calcium in the acid-tolerant alfalfa-nodulating bacteria Rhizobium sp. LPU83.

Soil acidification is one of the environmental factors that more strongly hampers the establishment of an effective symbiotic interaction between rhizobia and leguminous plants. Sinorhizobium meliloti and the acid-tolerant Rhizobium sp. strain LPU83 are able to nodulate alfalfa plants at pH 5.6 but both exhibit a delayed nodulation and a reduction in the number of elicited nodules. We show here that the addition of calcium (Ca) has no positive effect on the nodulation kinetics shown by LPU83 at low pH, but does retrieve the competition capacity of S. meliloti strains in acidic media, likely by improving the ability of these bacteria to attach to plant roots. In contrast, the attachment of the acid-tolerant strain LPU83 to alfalfa roots is not greatly affected by pH or Ca concentration. Media acidification impairs nod gene induction in different S. meliloti strains but not in LPU83. However, the addition of Ca at low pH does not affect neither nod gene expression in alfalfa-nodulating rhizobia (S. meliloti or strain LPU83) nor the quality of nod gene inducers exudated by alfalfa plants, in contrast to what has been reported previously. These data reveal differential features among alfalfa-nodulating rhizobia and point out the adsorption of S. meliloti to alfalfa roots as the major limiting step affecting its symbiotic performance in acidic conditions.