Published Paper Details:

The small phenolic compound salicylic acid (SA) plays an important regulatory role in multiple physiological processes including plant immune response. The plant recognizes the bacterial pathogen and activates structural and biochemical defences to protect itself. Pathogen infection and recognition occurs in a series of responses. First, plants utilize pattern recognition receptors to recognize pathogen associated molecular patterns (PAMPs) such as flagellin and lipopolysaccharides. In PAMP triggered immunity (PTI), a signal cascade involves reactive oxygen species (ROS) production, activation of mitogen activated protein-kinases, cell wall fortification and pathogen response (PR) gene transcription. However, pathogens can attenuate the PTI responses by injecting effector proteins into the plant cells to inhibit the basal defense response. In recognition of these effector proteins, plants have evolved to express resistance genes encoding R proteins, which are able to recognize effector proteins and signal a hypersensitive response (HR) and pathogen resistance gene expression. This defense response is called effector triggered immunity (ETI), characteristic of an incompatible response. Plants that do not recognize effector proteins are susceptible and have a compatible reaction with the infecting pathogen. Compatible interaction is characterized by increased bacterial growth, increased levels of hormones such as ethylene and jasmonic acid (JA), and necrotic cell death. Incompatible reaction are characterized by HR involving a decrease in bacterial growth, programmed cell death, an increase in salicylic acid (SA) used to induce PR proteins and systemic acquired resistance(SAR) response. SA is a secondary metabolite produced by a wide range of prokaryotes and eukaryotes including plants. During the past two decades, significant progress has been made in understanding SA metabolism, signalling, and its interaction with other defense mechanisms, especially hormones. SA is predominantly associated with resistance against biotrophic and hemibiotrophic pathogens, and triggering SAR. Exogeneous application of SA induces SAR in plants, resulting in resistance to certain pathogens. Conversely, plants expressing the bacterial NahG (encoding salicylate hydroxylase) , which converts SA to catechol , are more susceptible to several pathogens. Direct evidence for the role of SA in plant defense comes from the identification and characterization of an Arabidopsis isochorismate synthase (ICS) mutant that is defective in SA biosynthesis. There are several studies suggesting that endogeneous SA in plants plays a critical role in pathogenesis by its direct effects on the pathogen. This review focuses on effect of salicylic acid on the pathogenesis of Xanthomonas campestris and its role in inducing SAR responses in the host plants.