Legume–Rhizobia Symbiosis Under Herbicide Stress: Insights and Agronomic Implications for Sustainable Agriculture

Leguminous crops contribute approximately 40–60 million tonnes of biologically fixed nitrogen annually to agricultural soils through symbiotic associations with rhizobia, underpinning the sustainability of diverse cropping systems. However, rising herbicide use, driven by the adoption of herbicide-tolerant crops and intensifying weed resistance, poses significant threats to the integrity of the legume–rhizobia symbiosis, depending on the herbicide type, exposure level, soil conditions, and microbial variability. This review synthesizes possible herbicide-induced disruptions of symbiotic nitrogen fixation, including herbicide impairment of symbiosis through multiple interconnected pathways, such as cytotoxicity to rhizobia and soil microbial communities, inhibition of nod-gene-inducing flavonoid biosynthesis by the host plant, suppression of Nod factor production and signalling by rhizobial disruption of cytoskeletal functioning that enhances the progression of infection thread formation, oxidative stress induction, alterations of auxin–cytokinin homeostasis, and modification of soil physicochemical parameters that can affect rhizobial survival and competitiveness. Though still unclear, laboratory (in vitro), field and glasshouse studies suggest that symbiotic impairment is dose- and time-dependent, species-specific, and strongly influenced by the rhizobial strain and probably the host plant species variety. Mitigation strategies, such as the use of herbicide-tolerant rhizobial strains, organic soil amendments, optimized herbicide application, and integrated weed management frameworks, have shown considerable promise. However, many research gaps exist regarding tropical legume systems and the long-term consequences of herbicide use on the legume–rhizobia symbiosis. Little is known about the direct effects of herbicides on the bacterial symbionts and host plants, as well as how soil type, climate and management/agronomic practices influence herbicide exposure and the effect of herbicide degradation products on rhizobial survival, competitiveness, and symbiotic N2-fixation. The existence of herbicide-tolerant soybean suggests that not all herbicides are lethally phytotoxic, an aspect that can be exploited for sustainable agriculture.