Unveiling the Role of Soil Microbes in Herbicide Degradation and Crop Protection

Soil microbes play an important role in promoting plant growth. The correct mix of microbes in the soil can be vital in nutrient cycling, nitrogen fixation, stress tolerance, and enhanced nutrient uptake. Crop rotation is a common agricultural practice of alternating leguminous plants like soybean with maize in the same field over different growing seasons. This strategy offers several benefits, with the most important being an increase in the soil's nitrogen content by biological nitrogen fixation provided by the unique ability of legumes to form symbiotic associations with soil bacteria. However, legume crops can be a double-edged sword, since the residue of many herbicides used in legume production is phytotoxic to sensitive crops. These herbicides may also disrupt the soil community, ultimately neutralizing the beneficial effects of crop rotation. Interestingly, soil microbes themselves come to the rescue with their ability to degrade herbicides (Zhang et al. 2022) in a process called rhizoremediation. Identifying and studying the mechanisms of these degradation processes will greatly help harness the potential of soil microbes in decreasing the harmful effects of herbicides on other beneficial microbes.

Diphenyl ether is a potent herbicide used to control many broad-leaf weeds of soybeans and peanuts (Liang et al. 2021). Diphenyl ether residue is phytotoxic to subsequent crops like maize, an important cereal in many countries of the world, leading to serious agricultural losses (Li et al. 2022). Existing research primarily emphasizes the isolation of bacterial strains that can degrade these herbicides (Zhang et al. 2018). However, the relationship between these strains and the crops during degradation is not understood.

In this issue of Molecular Plant-Microbe Interactions, Tian et al. (2024) have demonstrated that Bacillus sp. strain Za effectively degrades various diphenyl ether herbicides, including fluoroglycofen and lactofen, with significantly enhanced degradation rates in the presence of root exudates. Moreover, the presence of organic acids in maize root exudates, such as oxalic, malic, succinic, fumaric, and trans-aconitic acids, further enhanced strain Za's degradation ability. The authors’ detailed mechanistic investigation showed that root exudates positively influence strain Za colonization and biofilm formation on maize roots, promoting its ability to degrade herbicides. Furthermore, the biofilm formed by strain Za reduced the phytotoxicity of herbicides to maize, highlighting the importance of biofilm formation in strain Za's remediation capabilities (Fig. 1).

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Overall, the findings suggest a mutually beneficial interaction between strain Za and maize roots, providing valuable insights into improving phytoremediation strategies for herbicide-contaminated soils. Mechanistic studies like this are needed not only to further optimize biofilm formation but also to explore genetic engineering options to enhance herbicide degradation by strain Za. Further studies are also needed to explore broader microbial communities in the rhizosphere that interact with strain Za and maize roots. Understanding the dynamics of these microbial communities and their roles in herbicide degradation and biofilm formation could lead to the development of more robust and resilient phytoremediation systems.