CHEMICALS RESPONSIBLE FOR SYSTEMIC ACQUIRED RESISTANCE IN PLANTS- A CRITICAL REVIEW

Systemic acquired resistance (SAR) refers to a distinct signal transduction pathway that plays an important role in the ability of plants to defend themselves against pathogens. After the formation of a necrotic lesion, either as a part of the hypersensitive response (HR) or as a symptom of disease, the SAR pathway is activated. An understanding of the biochemical changes leading to the resistance state could enable the development of either genetically engineered plants with enhanced disease resistance or novel mode of action plant protection chemicals that act by stimulating the plant's inherent disease resistance mechanisms. In tobacco, SAR activation results in a significant reduction of disease symptoms caused by the fungi Phytophthora, Cercospora, and Peronospora, the viruses tobacco mosaic virus (TMV) and tobacco necrosis virus (TNV), and the bacteria Pseudomonas syringae pv tabaci and Erwinia. Associated with SAR is the expression of a set of genes called SAR genes. However, not all defense related genes are expressed during SAR, and the particular spectrum of gene expression, therefore, distinguishes the SAR response from other resistance responses in plants. SAR was first described as a response to pathogen infection. Subsequently, it has been found that treatment of plants with low molecular weight molecules can also induce SAR. The use of chemicals to activate SAR provides nove1 alternatives for disease control in agronomic systems as well as tools for the elucidation of the SAR signal transduction cascade. Several chemicals or extracts, including DL-8 aminobutanoic acid or probenazole, have been shown to slightly induce either PR-1 gene expression or resistance against one or two pathogens, but activation of bona fide SAR has not been demonstrated. To date, salicylic acid (SA) is the only plant-derived substance that has been demonstrated to be an inducer of SAR. The chemicals 2, 6-dichloroisonicotinic acid and its methyl ester (both referred to as INA) were the first synthetic compounds shown to activate SAR, thus, providing broad-spectrum disease resistance