Rhizobium Diversity Is the Key to Efficient Interplay with Phaseolus vulgaris. Case of Study of Southern Ecuador

Abstract

Plants acquire different combined forms of nitrogen by addition of ammonia and/or nitrate fertilizer or manure to the soil, during organic matter decomposition, by the conversion of nitrogen into different compounds, or by biological nitrogen fixation (BNF). Diverse soil bacteria collectively called rhizobia are capable to fix N2 from the atmosphere through symbiosis with legume plants. The N2 fixed by the legume crops represents a renewable source of nitrogen for agricultural soils, turning symbiotic nitrogen fixation (SNF) in a natural process of significant importance in world agriculture. Within the legumes carrying out this process, common bean (Phaseolus vulgaris L.) constitutes a staple, being the most important grain legume worldwide, especially for developing countries. However, P. vulgaris is a low nitrogen fixer compared with other legumes, mainly attributed to the presence of high but inefficient diversity of indigenous rhizobia in soil, increasing the promiscuity of bean genotypes and lack of response under field conditions. Rhizobia diversity has been extensively studied. Polyphasic taxonomy and recently the average nucleotide identity approach have allowed to discover about 117 so-called Rhizobium species and the real genetic differences of microsymbionts in ecosystems around the world. Nevertheless, phylogenomic, ecological, and population genetic criteria to delineate biologically meaningful species in interplay with their host are still needed. Therefore, understanding genotypic variabilities between bean genotypes and Rhizobium strains contributes to achieve an efficient interaction, increase plant parameters, nitrogen fixation, and yields of common bean. Here, we discuss about the most recent studies on Rhizobium diversity linked to P. vulgaris in the American continent as the center of origin/diversification and outside this continent. The abiotic and biotic factors mediate the efficiency of the interaction, with special emphasis in the promiscuity of common bean as a constraint to achieve high nitrogen fixation rates and we show a case of study at southern Ecuador where genotypic variability among local bean genotypes and native Rhizobium strains was assessed to seek the efficiency of symbiosis based on its diversity.