A Comprehensive Review of Mechanisms of Plant Defence Against Selected Categories of Plant Pathogens

Pathogens pose significant threats to plants, and over time, they have evolved strategies to obtain nourishment. These pathogens are broadly categorized into biotrophs, necrotrophs, and hemi-biotrophs. These terms, derived from the Greek "troph" (nourishment), helps clarify their nature: biotrophs ("bio" meaning life) feed on living plant cells, while necrotrophs ("necro" meaning death) nourishment from dead or decaying cells. Hemi-biotrophs are a hybrid category, living a phased life that begins with biotrophy before switching to necrotrophy. Distinctive Pathogen Strategies: Biotrophs establish a long-term, symbiotic relationship with their host to extract nutrients without causing immediate cell death. They employ phased infection strategies, using effector proteins to suppress the plant's defense mechanisms. Such as black sigatoka fungus in bananas and rust fungi in sorghum. These pathogens often develop specialized structures like the appressorium for anchorage and the haustoria for nutrient absorption. Necrotrophs, on the other hand, are aggressive killers. They overwhelm plant defenses by releasing a large number of toxins and lytic enzymes simultaneously, causing rapid tissue necrosis and cell death. They then consume the dead tissue for nourishment. A common example is Botrytis cinerea , which secretes oxalic acid to create an acidic environment optimal for its destructive enzymes. Host-Pathogen Specificity and Plant Defense: Pathogen specificity, the ability of a microbe to infect a particular plant species, is a key aspect of plant pathology. This is often a result of coevolution, leading to generalists (pathogens with a broad host range, like Botrytis cinerea ) and specialists (those that infect only a few related species, like Puccinia graminis ). Plants have evolved a two-layered defense system: PAMP-triggered immunity (PTI): activated when a plant's pattern recognition receptors (PRRs) detect conserved pathogen-associated molecular patterns (PAMPs). Effector-triggered immunity (ETI): activated when plants with specific resistance (R) genes recognize virulence factors (effectors) released by adapted pathogens. This often leads to a hypersensitive response (HR), which is a localized cell death that limits pathogen spread. The "gene-for-gene" model explains this interaction, where a dominant resistance gene in the plant matches a corresponding dominant avirulence ( Avr ) gene in the pathogen, leading to disease resistance. Systemic Acquired Resistance (SAR): Beyond local defense, plants can also develop systemic acquired resistance. This broad-spectrum immunity is triggered by a localized infection and protects the entire plant from future attacks. Mobile signals, such as methyl salicylic acid, are transported from the infection site to other parts of the plant, where they are converted to salicylic acid, leading to systemic protection.