Molecular Pathogens, 2025, Vol.16, No.4, 159-170 http://microbescipublisher.com/index.php/mp 164 5 Metabolic Pathways and Disease-Resistant Substance Accumulation 5.1 Synthesis and effects of secondary metabolites When wheat is attacked by pathogenic bacteria, multiple secondary metabolic pathways in its body are activated to synthesize a series of compounds with antibacterial activity. These secondary metabolites include phenols, flavonoids, phytoalexins, etc., which play an important role in inhibiting pathogen growth and strengthening host defense. The role of phenolic compounds (such as lignin and its precursors) in disease resistance is particularly prominent (Ma and Li, 2023). After wheat is infected with fungal infection, the phenol propane metabolic pathway accelerates, producing a large number of phenol monomers and polymerizing into lignin and depositing on the cell wall. The rapid accumulation of lignin can enhance the mechanical strength and anti-enzymatic ability of the cell wall, making it difficult for pathogens to invade. Studies have shown that lignin content increases faster and higher when the ears are infected, thus effectively limiting the spread of lesions (Doppler et al., 2019). In addition, phenolic acid substances (such as ferulic acid, chlorogenic acid, etc.) also have antibacterial activity, can accumulate locally in the infected site, and chemically curb the growth of bacteria. Flavonoids are another important secondary metabolites, including anthocyanins, dihydroxystilbene, etc. Disease-resistant varieties tend to have more active flavonoid synthesis pathways, and related synthetase genes are strongly expressed under pathogen stimulation. The newly synthesized antibacterial compound specially used to resist pathogens when plants are infected is called Phytoalexin. Some crops (such as soybeans and rice) can synthesize unique phytopoestins under pathogen attack. 5.2 Cell wall reinforcement and structural defense mechanism The cell wall is the first physical barrier for plants to resist pathogen invasion. In order to break through the cell wall, fungal pathogens usually secrete a variety of cell wall degradation enzymes (such as cellulase, pectinase, etc.) in the early stages of invasion. Wheat can strengthen cell wall structure through a variety of mechanisms, improve its anti-degradation and mechanical barrier capabilities, thereby achieving structural defense. After sensing the pathogen, wheat quickly deposits iodide (mainly carbohydrate polymers) into the inside of the invaded cell wall, forming a locally thickened structure called callose (papilla) (Underwood, 2012). Second, the polysaccharide components in the wheat cell wall will undergo cross-linking and lignification. Studies have found that when wheat powdery white bacteria invade disease-resistant hosts, they are often blocked from the cell wall-mastoid structure, that is, because the cell wall of the disease-resistant host has a high degree of lignification and quickly modified the invading site, it is difficult for the bacteria to expand smoothly. Again, some structural proteins and polysaccharide components in the cell wall play a special role in disease resistance (Ao et al., 2025). For example, hydrolase inhibitor proteins rich in hydrolase binding sites can be embedded in the cell wall, inhibiting the activity of pathogenic enzymes; some Arabin galactoglycan proteins (AGPs) can enhance anti-invasiveness by increasing the crosslinking of the wall. After wheat is infected, the expression of this type of wall-related protein is upregulated, which helps to improve the defense structure (Zhao et al., 2018). 5.3 Regulation of reactive oxygen species (ROS) and defense enzyme systems Reactive Oxygen Species, ROS, such as hydrogen peroxide (H2O2), superoxide anions (O2 -), etc., play a dual role in plant disease resistance: they can not only amplify immune responses as signal molecules, but also directly poison pathogens or limit their expansion. After wheat cells recognize pathogen invasion, an phenomenon called "oxidative burst" occurs, that is, a large amount of ROS is generated and accumulated in a short period of time. These ROS partly come from the oxidation reaction catalyzed by NADPH oxidase in the plasma membrane, and partly come from the action of cell wall peroxidase, etc. The transient accumulation of reactive oxygen species has many effects: First, as a signal molecule, ROS can activate the expression of downstream defense-related genes and induce apoptosis of allergic cells; Second, high concentrations of ROS have direct killing or inhibiting effects on bacteria; Third, ROS can promote the polymerization of some components in the cell wall such as lignin and strengthen physical barriers. Studies have found that wheat with strong disease resistance can quickly produce high levels of hydrogen peroxide at the pathogen-invasive site, while the sensory variety ROS produces delayed and has a low level (Rodrigues et al., 2017).
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