Molecular Pathogens, 2025, Vol.16, No.2, 77-86 http://microbescipublisher.com/index.php/mp 80 4 Biochemical and Cellular Mechanisms of Defense 4.1 Production of phytoalexins and other antimicrobial compounds When grapes are attacked by pathogens, they will create some defensive substances, such as plant antitoxins. Among them, stilbene is an important antitoxin that can help prevent bacteria from growing. These substances usually begin to be synthesized after the invasion of bacteria. Sometimes beneficial bacteria (such as Pseudomonas fluorescens) can help. They allow grapes to produce more antitoxins, thus better resisting pathogens like Botrytis ale (Gruau et al., 2015). In addition, the cyclic lipopeptide released by Bacillus subtilis can also stimulate the secretion of antibacterial substances and enhance local disease resistance (Farace et al., 2015). 4.2 Role of reactive oxygen species (ROS) and oxidative stress Reactive oxygen species (ROS) is important in the defensive reaction of grapes. It is not only a signal molecule, but also can directly attack bacteria. When pathogens invade, grape cells quickly release a large amount of reactive oxygen species, which is called an "oxidation outbreak." This step can help destroy bacteria, or make the cell wall stronger and less likely to be broken by bacteria (Vatsa-Portugal et al., 2017; Castro et al., 2023). Good microorganisms, such as Streptomyces, will also make grapes more susceptible to this oxidative outbreak, thereby reducing the degree of infection of pathogens. 4.3 Structural reinforcements: callose deposition and lignification 4.3.1 Role of callose deposition in reinforcing cell walls Grapes deposit a substance called callose on the cell wall. This can block the invasion of bacteria like a wall. Studies have found that callose synthase genes such as CalS1 and CalS10 are significantly elevated when grapes are challenged by pathogens, especially in disease-resistant varieties (Ogrinc et al., 2024). This defense reaction belongs to the grape's natural immune system and starts up very quickly. 4.3.2 Contribution of lignification to preventing pathogen intrusion Grapes can also harden the cell walls, and this process is called timberization. It is done by depositing lignin in the cell wall. This way, the cell wall will be stronger and it will not be easy for bacteria to penetrate. The formation of lignin is related to ROS. Reactive oxygen species can promote the polymerization of lignin and make the cell wall stronger (Vatsa-Portugal et al., 2017). 4.3.3 Coordination between callose and lignin synthesis during infection During bacterial infection, both callose and lignin synthesis may occur together. The combination of the two can make the cell wall both elastic and strong enough. At this time, grapes will turn on a batch of related genes to raise the defense response to a higher level. This "joint combat" method is a common response to biological stress by grapes (Yu et al., 2016), and also shows that their defense mechanisms work together in many aspects. 5 Role of resistance (R) genes in grapevine defense 5.1 Identification and characterization of key Rgenes Scientists have found many important R genes in grapes. They help grapes fight against different bacteria. For example, the two genes RUN1 and REN1 have been used in grape breeding to prevent and treat powdery mildew caused by powdery mildew bacteria. They allow grapes to initiate defensive reactions, such as releasing reactive oxygen species or triggering programmed death in cells, which effectively suppress pathogens (Agurto et al., 2017; Wang et al., 2024). There are also some Rgenes from Asian mountain grapes, such as Rpv10 andRpv12. They can enable grapes to activate defense pathways when the bacteria first invade and upregulate a series of related genes, thereby enhancing resistance to downy mildew (Frommer et al., 2023). Another gene locus is called REN12, which also comes from Shanxi Grape. It can quickly prevent pathogen development and effectively prevent and treat powdery mildew. 5.2 Mechanisms of action of R proteins against specific pathogens The R gene will produce R proteins, and the structure of these proteins often contains domains such as “NBS-LRR” (that is, parts that can bind nucleotides and contain leucine repeats). This structure allows them to
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