Molecular Pathogens, 2025, Vol.16, No.5, 246-256 http://microbescipublisher.com/index.php/mp 247 regulate the structure and diversity of rhizosphere microbial communities, and the process of secretions as chemical signals mediating the communication between plants and microorganisms; the selective enrichment effect of wheat root exudates on plant growth-promoting bacteria (PGPR) is analyzed through specific cases. 2 Composition and Characteristics of Wheat Root Exudates 2.1 Main types of root exudates Wheat roots can secrete a wide variety of compounds, and these root exudates can be roughly divided into two categories: low molecular weight and high molecular weight. Low molecular weight root exudates include primary metabolites and secondary metabolites, such as sugars, organic acids, amino acids, phenolic acids, flavonoids and other secondary metabolites. These compounds are usually easily soluble in water and can quickly diffuse into the rhizosphere environment, providing carbon, nitrogen and energy sources for microorganisms (Lang et al., 2025). High molecular weight root exudates mainly include polysaccharide glue and high molecular weight proteins. Wheat roots secrete a large amount of polysaccharide mucilage (root mucilage). These polysaccharide substances are composed of polymers such as fructan and galacturonic acid, which can enhance the retention of water and nutrients in the soil and provide a stable habitat for soil microorganisms. In addition, plant roots also release cell wall components, nucleic acids and signaling molecules into the rhizosphere through root cap cell shedding, which can also be regarded as root secretions in a broad sense. 2.2 Dynamic changes in secretions The release of wheat root exudates is not constant, but is a dynamic process that is significantly affected by plant growth and development stages and environmental conditions. At different stages of wheat growth, the composition and quantity of root exudates show phased changes. Generally speaking, the root metabolism of wheat is vigorous during the seedling stage, and it will secrete more primary metabolites such as soluble sugars and amino acids to provide rapid energy for microorganisms that germinate in the rhizosphere (Álvarez et al., 2023); during reproductive growth stages such as jointing and heading, plants allocate more carbon to the ground, and the total amount of root exudates may decrease, but the proportion of certain secondary metabolites (such as phenolic acids and flavonoids) increases. There are also spatial differences in root secretion. The root tips and lateral root bifurcations are hot spots for secretion release, and the concentration of secretions gradually decreases upward along the root axis. The root tips secrete a large amount of mucus and organic acids, which help to bind soil particles and activate nutrients in micro-zones, while the root hairs in mature areas secrete less secretions. 2.3 Comparison of root secretion profiles of different wheat varieties Wheat cultivars with different genetic backgrounds differ significantly in the quantity and composition of root exudates. These differences originate from differences in metabolic pathways, root structure, and nutrient utilization strategies among varieties, which in turn affect the assembly patterns of their rhizosphere microbial communities. Studies have shown that the roots of some highly nutrient-efficient wheat varieties secrete more organic acids, thereby improving their ability to utilize refractory phosphorus (Campos et al., 2022). For example, an experiment compared different phosphorus-efficient wheat varieties and found that the content of six organic acids in the roots of high-efficiency varieties, including malic acid, citric acid, and oxalic acid, was significantly higher than that of low-efficiency varieties under conditions of sufficient phosphorus and phosphorus deficiency. For another example, when the wheat variety "Yunmai 42" was intercropped with broad beans, it successfully enriched more functional rhizosphere bacteria by increasing the secretion of organic acids from the roots and reducing the secretion of sugars and amino acids, thus effectively inhibiting the occurrence of broad bean fusarium wilt. Some studies compared the rhizosphere microbiomes of wild wheat and domesticated wheat and found that domestication reduced the diversity of wheat rhizosphere microorganisms and the abundance of some beneficial functional bacteria (Yue et al., 2023). This may be related to the relatively single spectrum of root exudates of modern varieties and the lack of certain compounds in wild species that can recruit diverse symbiotic flora.
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