From Microns to Meters: Exploring Advances in Legume Microbiome Diversity for Agroecosystem Benefits

Abstract

Legumes are of primary importance for agroecosystems because they provide protein-rich foods and enhance soil fertility through fixed atmospheric nitrogen. The legume-rhizobia symbiosis that makes this possible has been extensively studied, from basic research on biochemical signaling to practical applications in cropping systems. While rhizobia are the most-studied group of associated microorganisms, the functional benefit they confer to their legume hosts by fixing nitrogen is not performed in isolation. Indeed, non-rhizobia members of the rhizosphere and nodule microbiome are now understood to contribute in multiple ways to nodule formation, legume fitness, and other agroecosystem services. In this review, we summarize advances contributing to our understanding of the diversity and composition of bacterial members of the belowground legume microbiome. We also highlight applied work in legume food and forage crops that link microbial community composition with plant functional benefits. Ultimately, further research will assist in the development of multi-species microbial inoculants and cropping systems that maximize plant nutrient benefits, while reducing sources of agricultural pollution.

Microbiome of Nodules and Roots of Soybean and Common Bean: Searching for Differences Associated with Contrasting Performances in Symbiotic Nitrogen Fixation

“…However, despite evolving as unique ecological niches for nitrogen fixation through the symbiotic relationship between the host legumes and rhizobia, the biological implications of sharing the nodule environment with other bacteria are not well understood yet. One hypothesis is that other bacteria living inside the nodules and that probably entered the nodule together with rhizobia are endophytes that can help in plant-growth promotion by other microbial processes, such as the synthesis of phytohormones, antimicrobial molecules, siderophores, mineral solubilization capacity, among others [ 27 , 28 , 29 , 30 ]. As an example of benefit, an interesting study of nodule endophytes performed with Lotus burtii pointed out that in healthy nodules of this legume, Pseudomonas species were the prevalent non-rhizobia, and when used as inoculum infected the plant together with a beneficial Mesorhizobium , but not with an ineffective Rhizobium , benefiting the symbiosis by decreasing the number of ineffective nodules [ 31 ].…”

More than a meat- or synthetic nitrogen fertiliser-substitute: a review of legume phytochemicals as drivers of ‘One Health’ via their influence on the functional diversity of soil- and gut-microbes

“…Besides phenolics, organic acids are key determinants on soil microbiome structure and function. It was reported in a metagenomic approach that organic acid exudation by legume roots, when compared to wheat, explained 15% of the observed variance in rhizobacterial abundance in a microcosm ( Schaedel et al., 2021 ). Small molecules like malic or oxalic acid, impact not only microbial activity, and are also capable of improving soil fertility too such as via mobilizing less-labile forms of P ( Yu et al., 2021 ), promoting nutrient cycling, soil structure, and overall ecosystem resilience.…”

The Potential of Cold Plasma-Based Seed Treatments in Legume–Rhizobia Symbiotic Nitrogen Fixation: A Review

“…Legume roots encounter diverse microorganisms in the rhizosphere [70]. To safeguard against potential invasion by these microorganisms, host plants deploy both chemical and mechanical barriers [71].…”