Matching population diversity of rhizobial A and legume genes in plant-microbe symbiosis.

We hypothesized that population diversities of partners in nitrogen-fixing rhizobium-legume symbiosis can be matched for "interplaying" genes. We tested this hypothesis using data on nucleotide polymorphism of symbiotic genes encoding two components of the plant-bacteria signaling system: (a) the rhizobial A acyltransferase involved in the fatty acid tail decoration of the Nod factor (signaling molecule); (b) the plant receptor required for Nod factor binding. We collected three wild-growing legume species together with soil samples adjacent to the roots from one large 25-year fallow: , , and nodulated by one of the two biovars ( and ). For each plant species, we prepared three pools for DNA extraction and further sequencing: the plant pool (30 plant indiv.), the nodule pool (90 nodules), and the soil pool (30 samples). We observed the following statistically significant conclusions: (a) a monotonic relationship between the diversity in the plant gene pools and the nodule rhizobial A gene pools; (b) higher topological similarity of the gene tree with the A gene tree of the nodule pool, than with the A gene tree of the soil pool. Both nonsynonymous diversity and Tajima's were increased in the nodule pools compared with the soil pools, consistent with relaxation of negative selection and/or admixture of balancing selection. We propose that the observed genetic concordance between gene pools and nodule A gene pools arises from the selection of particular genotypes of the A gene by the host plant.