Molecular Pathogens, 2025, Vol.16, No.5, 246-256 http://microbescipublisher.com/index.php/mp 249 4 The Role of Root Exudates in Inhibiting Pathogenic Microorganisms 4.1 Allelopathy and antibacterial compounds Plant root exudates contain a variety of biologically active compounds, which can inhibit the activity of pathogenic microorganisms in the soil through allelopathy and reduce the risk of plant disease. Allelopathy refers to the phenomenon that secondary metabolites released by plants have inhibitory or promotional effects on surrounding organisms. In the wheat rhizosphere, certain secretions act as natural “antibiotics” and directly inhibit the growth and reproduction of pathogenic bacteria (Usyskin-Tonne et al., 2021). For example, wheat roots can secrete phenolic acids, which have antifungal and antibacterial activities and can inhibit mycelial growth and spore germination of a variety of soil-borne pathogens at low concentrations. Research has found that phenolic acids accumulated in the rhizosphere during continuous wheat planting will selectively reduce the number of antagonistic actinomycetes and increase the infection probability of pathogenic Fusarium. However, on the other hand, phenolic acids themselves also have a certain inhibitory effect on Fusarium, and its mechanism of action is quite complex (Dimkić et al., 2021). Wheat roots also release some low-molecular-weight antibacterial compounds, such as peroxidase, plant defense proteins, etc., which can directly attack pathogenic microbial cells. 4.2 Interference of secretions on the behavior of pathogenic bacteria In addition to directly inhibiting the growth of pathogenic microorganisms, root exudates can also indirectly protect plants from damage by interfering with the behavior and pathogenesis of pathogenic bacteria. One typical mechanism is to interfere with the quorum sensing signals of pathogenic microorganisms. Many soil-borne pathogenic bacteria require quorum sensing regulation during their colonization and pathogenesis. For example, Burkholderia and Ralstonia solanacearumin the soil secrete signaling molecules to coordinate attack behaviors. Plants can release some compounds or degradative enzymes similar to signal structures to interfere with the quorum sensing communication of pathogenic bacteria. Some studies have found that root exudates of tomatoes and other plants contain flavonoids and dipyrromethanes (Agnihotri et al., 2021), which can act as false signals to bind to pathogen receptors, causing the pathogen quorum sensing system to malfunction. Another interference mechanism is to alter the chemotaxis and attachment behavior of pathogenic bacteria. Pathogens usually find host roots and attach and infect through chemotaxis, and wheat roots can secrete some compounds to mislead the chemotaxis of pathogenic bacteria (Singh and Singh, 2024). For example, excessive amounts of amino acid mixtures released by wheat roots may induce nematodes or pathogens to move in the wrong direction, reducing their chances of infection. 5 Regulation of Interactions between Wheat Root Secretions and Microorganisms by Environmental Factors 5.1 Nutrient levels and rhizosphere metabolic response Soil nutrient status is an important environmental factor affecting wheat root secretion and rhizosphere microbial interaction. When nutrients are sufficient or scarce, plants adapt to the environment by adjusting their root secretion patterns and regulate their microbiota to help them obtain nutrients. Under conditions of nutrient deficiency, wheat roots tend to increase the secretion of certain metabolites to enhance their ability to acquire nutrients. Under conditions of excess nutrients, plants may reduce the discharge of corresponding secretions to avoid wasting resources. Under high phosphorus environment, the flux of organic acid secretion from wheat roots will be reduced; under high nitrogen conditions, the rate of soluble carbon root secretion will be reduced. This regulation will also affect the rhizosphere microbial community: in nitrogen-rich soils, as plants provide fewer carbon sources, the overall number and functional activity of rhizosphere microorganisms may decrease, while autotrophic and low-nutrient-preferential bacterial communities increase. Nutrient levels also affect root secretion by regulating endogenous plant hormones. When nutrients are lacking, plant hormone signals are enhanced, which will stimulate the roots to actively release H+ and various metabolites to the rhizosphere to change the microenvironment and recruit microorganisms (Sorty et al., 2023). 5.2 Regulation of rhizosphere metabolism under water and salt stress Water conditions and salt stress are key environmental factors affecting wheat rhizosphere metabolism and
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