Molecular Microbiology Research 2024, Vol.14, No.5, 218-225 http://microbescipublisher.com/index.php/mmr 220 2.2 Key functions of rhizosphere microbes in soil and plant health Rhizosphere microbes perform several key functions that are essential for soil health and plant growth. These functions include nutrient cycling, enhancing plant stress resistance, and acting as a barrier against pathogens. Rhizosphere microbes are integral to the cycling of essential nutrients such as nitrogen, phosphorus, and potassium. For instance, AM fungi facilitate the uptake of phosphorus and other nutrients by plants, while nitrogen-fixing bacteria convert atmospheric nitrogen into forms that plants can use (Fu et al., 2023). This nutrient exchange is crucial for maintaining soil fertility and promoting plant growth. Rhizosphere microbes can significantly enhance plant resistance to various abiotic stresses such as heavy metals, drought, and salinity. For example, AM fungi have been shown to improve plant tolerance to heavy metal stress by altering the rhizosphere microbiome and promoting the enrichment of beneficial microbes. Similarly, co-inoculation with rhizobia and AM fungi can enhance plant resistance to cadmium stress by modifying the rhizosphere microbial community. The rhizosphere microbiome acts as a protective barrier against soil-borne pathogens. Beneficial microbes can outcompete pathogens for resources, produce antimicrobial compounds, and induce systemic resistance in plants (Hu et al., 2020; Lazcano et al., 2021; Li et al., 2021). For instance, certain bacterial genera such as Pseudomonas and Burkholderia are associated with increased resistance to fungal pathogens in strawberry cultivars. 3 Mechanisms of Stress Resistance Enhancement 3.1 Nutrient uptake facilitation by rhizosphere microbes 3.1.1 Enhanced phosphorus solubilization Rhizosphere microbes, particularly arbuscular mycorrhizal fungi (AMF) and phosphate-solubilizing bacteria, play a crucial role in enhancing phosphorus (P) availability to plants. AMF, such as Claroideoglomus etunicatum, have been shown to significantly increase the uptake of phosphorus in maize, even under stress conditions like heavy metal contamination (Hao et al., 2021). Similarly, phosphate-solubilizing bacteria can convert insoluble phosphorus into forms that are readily available for plant uptake, thereby improving plant growth and nutrient content. For instance, co-inoculation with diazotrophic and P-solubilizing bacteria has been demonstrated to enhance phosphorus content and biological nitrogen fixation in wheat. 3.1.2 Nitrogen fixation and mobilization Nitrogen fixation and mobilization are critical processes facilitated by rhizosphere microbes. Plant growth-promoting rhizobacteria (PGPR) such as Bacillus subtilis can enhance nitrogen fixation and mobilization, thereby improving plant growth and stress resilience. The synergistic application of diazotrophic bacteria and P-solubilizing bacteria has been shown to significantly increase nitrogen content in both plant tissues and soil, further promoting plant health and growth. 3.1.3 Iron chelation and availability Iron availability is another essential factor for plant growth, often limited by its low solubility in soil. Rhizosphere microbes, including Bacillus species, produce siderophores that chelate iron, making it more available to plants. This process not only enhances iron uptake but also suppresses the growth of soil-borne pathogens, contributing to overall plant health and stress resistance (Hashem et al., 2019; Li et al., 2020). 3.2 Hormonal modulation and stress signaling Rhizosphere microbes can modulate plant hormonal levels and stress signaling pathways, thereby enhancing plant stress resistance. For example, Bacillus subtilis produces various phytohormones and stress-related metabolites that help plants cope with biotic and abiotic stresses. These microbes can induce systemic resistance in plants, leading to the activation of stress-response genes and the production of antioxidants (Tsotetsi et al., 2022). The co-inoculation of rhizobia and AMF has been shown to increase antioxidant enzyme activities in plants, mitigating stress-induced damage (Wang et al., 2021).
RkJQdWJsaXNoZXIy MjQ4ODYzNA==