Phenotypic expression of the gene‐for‐gene interaction in plant pathogenic bacteria: An implication for resistance to bacterial diseases by plants
Plant pathogenic bacteria cause massive losses in agriculture and damage to natural plant habitats. Bacterial pathogens not only reduce crop production, but they also reduce crop quality by releasing toxins into the environment. Pathogenic bacteria that invade plant tissues must overcome the plant's defense mechanisms. Plants possess an innate immune system that defends against pathogens. Their primary immune system detects microbe‐associated molecular patterns of potential pathogens through pattern recognition receptors, initiating a basal defense response. A gene‐for‐gene relationship occurs when the presence of a gene in one population depends on the continued presence of a gene in another population, and the interaction between these genes results in a single phenotypic expression, allowing the recognition of the relevant gene's presence or absence in either organism. The presence of a resistance (R) gene in plants and an avirulence (Avr) gene in bacteria is often utilized to evaluate plant resistance to bacterial infections. Plants' recognition of bacteria is considered the initial critical event in their response. This recognition can occur through physical interactions, such as adhesions, fimbriae, flagella, and Type III and Type IV secretion systems, or through signaling by small molecules. Secretion is a vital function for prokaryotic organisms to interact with their environment. Bacterial pathogens utilize specialized protein secretion systems that play multiple roles in enhancing virulence. These roles include improving attachment to eukaryotic cells, scavenging resources in environmental niches, and directly intoxicating target cells. Toxins and effector proteins are the key virulence strategies of Gram‐negative bacterial pathogens, with characterized effectors acting as enzymes to suppress plant immune perception and promote bacterial colonization. Biofilms also shield bacteria from harmful environments, and biofilm formation appears to be a significant factor in the bacterial pathogen disease cycle in plants. Quorum sensing is a method of bacterial communication that controls virulence. This review will focus on plant immune responses to bacterial pathogens, bacterial protein secretion systems, quorum sensing, and biofilms. It will also explore their roles in enhancing the virulence of bacterial pathogens based on gene‐for‐gene interactions and the implications for plant resistance to bacterial infections.