Signalling in plant disease resistance
In the natural environment, plants have evolved mechanisms to perceive and respond effectively to a vast range of biotic stresses. A plant′s survival depends on efficient recognition of microbial and invertebrate pathogens and the timely activation of the local and systemic defence machinery. Unlike multicellular animals, plants lack a circulatory system for the surveillance and destruction of foreign material. However, they possess a functionally equivalent recognition system in the form of a basal resistance machinery and a repertoire of pathogen-specific resistance (R) genes. The cloning of R genes from several dicotyledonous and monocotyledonous species sheds some light on genetic and molecular mechanisms by which a plant may respond to pathogen selection pressures through the evolution of novel recognition molecules. Also, activation of local plant defences leads to the generation of systemic-acquired resistance (SAR), establishing a heightened level of immunity within the whole plant. R gene-mediated disease resistance to pathogens is commonly characterized by rapid programmed plant cell death at the site of pathogen attack, a process known as the hypersensitive response (HR). This is accompanied by a battery of defence-related processes, including ion flux changes, an oxidative burst, the accumulation of potent signalling molecules, such as salicylic acid (SA) or jasmonic acid (JA), and the local and systemic transcriptional activation of sets of genes encoding pathogenesis-related (PR) proteins. Although little is understood about the precise nature of the plant-pathogen recognition events and consequent signalling processes that result in arrest of pathogen infection, the complexities of plant disease resistance signalling are now beginning to be unravelled by a combination of scientific approaches.