Co-inoculations with rhizobia and arbuscular mycorrhizal fungi alters mycorrhizal composition and lead to synergistic growth effects in cowpea that are fungal species-dependent. Cowpea symbiotic nitrogen fixing bacteria

Cowpea can effectively form tripartite symbiotic associations with nitrogen-fixing bacteria (NFB) and arbuscular mycorrhizal fungi (AMF) but the selection of compatible AMF species and rhizobial strains that are efficiently promote cowpea growth remains a challenge. The aims of the current study were 1) to evaluate the response of cowpea plants to a symbiotic NFB and a multi-AMF inocula and 2) to explore any interaction between the symbiotic NFB. In a pot gnotobiotic trial, cowpea plants grown under limited N supply, were inoculated with or without a symbiotic nitrogen-fixing bacterium, Sinorhizobium meliloti, and combinations of three different AMF species namely Dominikia disticha (3), Claroideoglomus etunicatum (5) and Rhizophagus irregularis (21). Experimental evaluation was determined through the measurement of above ground biomass, nutrient content and AMF root colonization. The presence of AMF species on cowpea roots was also determined with cloning and sequencing.ResultsCowpea inoculation with both AMF and S. meliloti led to increased cowpea biomass production compared to inoculation with AMF only, but the presence of a positive effect depended on the specific AMF partners used. Inoculation with AMF alone had a highly positive impact on the growth and P uptake of cowpea, but the NFB inoculation was needed to address N deficiency in planta. Co-inoculation with both AMF and the NFB generally led to increased AMF colonization of the cowpea roots, however, cowpea response depended on the AMF species, and became even negative, when all three AMF inocula were used together. The AMF composition in plant roots was also altered in the presence of the S. meliloti. Plant nitrogen content of cowpea plants significantly increased under the presence of both symbionts compared to AMF alone, while phosphorus content was hardly affected by dual inoculations. ConclusionOverall, our results show positive synergistic effects derived from combined AMF species and S.meliloti inoculation on cowpea growth and N acquisition, but the efficiency of synergism depends on the specific AMF partners used and it is not related to their colonization levels.