The species-dependent effect of PGPR co-inoculation in legume plants: A global meta-analysis

The microorganisms living in the rhizosphere significantly impact the growth and development of the plants they associate with. For instance, leguminous plants form relationships with both rhizobia and other rhizobacteria, and these partnerships can be influenced by the presence of a third microorganism. Given this context, this meta-analysis aims to assess the effect of co-inoculation of rhizobia and plant growth-promoting rhizobacteria on legumes of agricultural interest under different cultivation conditions. We collected quantitative data from 123 articles and 32 countries, encompassing various regions of the world, where the impact of rhizobia-rhizobacteria co-inoculation on the productivity and symbiotic characteristics of legume crops was evaluated. The moderating variables included the experimental conditions, legume species, rhizobia and rhizobacteria genera, the presence or absence of stress, and the soil's chemical attributes. The field and greenhouse data demonstrate that co-inoculation enhanced the attributes of all the included legume plants. Co-inoculation of legume crops with both rhizobia and plant growth-promoting rhizobacteria increased the yields of the common bean (+34%), chickpea (+19%), and soybean (+2%). The most effective genera in increasing grain yield were Pseudomonas and Bacillus, followed by Azospirillum. Co-inoculation improved plant attributes under all stress conditions, with the best response observed in saline environments (mean increase of 37%). Co-inoculation proved beneficial regardless of the chemical composition of the soil, with a higher yield response observed at pH > 7 and soil organic carbon ≤1%. In summary, this study demonstrates that co-inoculation is superior to single inoculation with rhizobia, resulting in improved productivity of legume plants across various agronomic and environmental conditions.