Plant cell wall and mechanisms of resistance to pathogens

A huge variety of phytopathogens (viruses, bacteria, and fungi) are potentially able to infect plant tissues and cause diseases. Numerous plant genes control a complex network of defense mechanisms based on both constitutive and inducible processes. The cell wall is a primary barrier the pathogens have to penetrate to start the infection process. Resistance at the level of the cell wall can prevent invasion of most potential pathogens. The cell wall structure may differ in various plant species. It is based on the network of cellulose microfibrils linked by hemicellulose molecules. In the growing parts of the plant, this network is integrated into the matrix of pectin polysaccharides. In the formed tissues, the cell wall is reinforced with lignin. In addition to polysaccharides, the cell wall contains a significant number of proteins implementing structural and enzymatic functions. The WallProtDB database stores information on many proteins of the cell wall of different plant species. Each of the cell wall components contributes to the formation of resistance to pathogens. The sites of contact with potential pathogens are characterized by the additional strengthening of the cell wall and the accumulation of antimicrobial secondary metabolites. The pathogens secrete enzymes that can break down the components of the cell wall. In response to the attack of microbes the plant produces inhibitors of microbial hydrolytic enzymes. The plant is also able to assess the number of components of the cell wall. For instance, mutants deficient for cellulose normally have an increased level of lignification and an increased defense response. Emerging after the action of microbial enzymes, low-molecular-weight cell wall fragments perform the signal function, enhancing the protective response of the plant. Thus, the cell wall is a dynamic structure, which can prevent the invasion of the majority of potential pathogens and initiate different variants of the immune response. Reconstruction of gene networks that control the structural and functional organization of the cell wall during the growth and under biotic and abiotic stress is crucial for understanding the molecular mechanisms of development and stress resistance. The review deals with the mechanisms of specific and nonspecific resistance of plants to pathogens of different nature, which are related to the cell wall. The cell wall structure and the role of various components in the detecting the invasion of plant pathogens and the induction of defense mechanisms are discussed.