How the right evolved partners in cycads and legumes drive enhanced growth in a harsh environment.

Abstract

Cycads are ancient plants that establish symbiotic associations with plant growth-promoting (PGP) microbes. These ancient associations are rarely contrasted with more recent associations involving PGP microbes and legumes. This study investigated if Vigna unguiculata growing in Encephalartos villosus rhizosphere and non-rhizosphere soils shares similar symbionts with E. villosus and if there is any sanction by plants towards certain soil bacteria. Also, the biomass accumulation and plant nutrition of V. unguiculata growing in these soils was investigated. Vigna unguiculata seeds were grown in E. villosus rhizosphere and non-rhizosphere soils. Thereafter, growth characteristics and plant nutrition were analyzed. Vigna unguiculata plants grown in E. villosus rhizosphere and non-rhizosphere soils were nodulated by Paenibacillus, Bacillus, Peribacillus, Brevibacillus, Alkalihalobacillus, and Lysinibacillus species identified in E. villosus coralloid roots. Bacteria isolated from nodules and coralloid roots were phylogenetically close, regardless of the soil from which these bacteria came. That supports the filter theory by which specific environmental conditions select certain microbial groups to establish symbiotic interactions with plants. No significant differences were observed in the total plant biomass, however, V. unguiculata plants grown in rhizosphere and non-rhizosphere soils invested significantly more resources in belowground biomass that could be related to the extra nitrogen coming from the biological nitrogen fixation that is devoted to roots. This study shows that V. unguiculata and E. villosus growing in similar soil conditions may share the same symbionts promoting plant nutrient assimilation and growth, this opens an idea of a common evolution of the two species and their symbionts.