Whispers beneath the soil: soybean-microbe communication pathways in the rhizosphere

Abstract

The rhizosphere is a dynamic ecosystem that hosts diverse microbial communities, essential for nutrient cycling, and promoting plant health and resistance to environmental stresses and pathogens. Understanding the communication strategies between plant roots and these microbial communities is vital for sustainable agriculture, as these interactions can enhance crop resilience and productivity while reducing the need for chemical fertilizers. Extensive research has focused on how soybean plants shape the rhizosphere microbiota and the signaling processes that promote these interactions; however, many influencing factors, particularly environmental stresses, remain unexplored. Key elements, including soybean genetics, growth development stages, soil properties, agricultural practices, and environmental conditions, all play crucial roles in shaping microbial symbioses. This review examines the intricate interactions between soybean and their rhizospheric microbiota, emphasizing how various stresses affect these relationships. It also discusses the role of secondary metabolites from both microbes and soybean in facilitating communication, alongside other factors that significantly influence these microbial interactions and soybean productivity.

Keywords: environmental stresses; microbiota assembly; rhizospheric microbiota; secondary metabolites; soybean; sustainable agriculture.

Figure 1

Overview of the abiotic and biotic stresses affecting microbial communities in the soybean rhizosphere. Interactions between plants, microbes and growth conditions shape the rhizosphere. Changes in plant growth and root exudate profiles influence the makeup of the rhizosphere microbial community, attracting specific microbial taxa that can adapt to the new conditions and requirements. This figure was created using BioRender (https://BioRender.com/p1ck2lu).

Figure 2

Proposed mechanisms of how plant growth-promoting rhizobacteria (PGPR) could influence plant-microbe interactions. Some IAA-producing PGPR can convert tryptophan from soybean into indole-3-acetic acid (IAA), a hormone that promotes root growth. IAA can then be converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase and further into ethylene by ACC oxidase in soybean. Ethylene is a phytohormone that inhibits root growth and development while enhancing plant stress responses. Some PGPR possess ACC deaminase activity that converts ACC to ammonia and α-ketobutyrate. The ammonia produced could be used as an essential macronutrient for plant growth. It is proposed that the IAA and ammonia produced from the beneficial interactions between soybean and microbes can foster the growth and development of soybean roots, ultimately enhancing the total surface area for interacting with the soil microbiota. This figure was created using BioRender (https://BioRender.com/u4p3b0o).

Figure 3

Factors affecting the interactions between soybean and soil microbes. Interactions between soybean plants and soil microbes can vary widely among different species due to many factors. Genotypic differences among soybean cultivars can lead to variations in the metabolites released into the rhizosphere, which may attract or deter distinct microbial populations. The soybean-interacting microbes might in turn secrete metabolites that modulate the gene expressions in soybean, either fostering beneficial interactions or causing infections. Soil properties, such as pH and the carbon-to-nitrogen ratio, play a crucial role in determining the types of microbes present in the soil. Furthermore, the agricultural and land-use history could also affect the microbial communities, thereby limiting the types of interactions that could occur between soybean and the soil microbiota. The interactions among microbes could also affect how soybean interacts with other microbial species. This figure was created using BioRender (https://BioRender.com/dttxnvt).