Characterization of Salicylic Acid-Binding Proteins in Plant Defense Responses

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Plant disease causes an estimated loss worldwide of $100B annually. A variety of strategies are being developed to protect plants against disease. One approach is to induce the plant's own natural defenses. This strategy is attractive since it can provide protection against a broad spectrum of pathogens. Broad-spectrum resistance to a wide array of pathogens, know as systemic acquired resistance (SAR), can be activated by infection with a pathogen or treatment with SAR-inducing natural chemicals such as salicylic acid (SA) and its synthetic functional analogues, INA or BTH. The development of strategies that control disease by manipulating endogenous plant defense responses will be very important for sustaining agricultural production and improving our environment and health.It is now well established that SA plays a key role in the development of resistance in many, but not all, plant-pathogen systems. While our understanding of SA function in disease resistance is far from complete, it is becoming increasingly clear that its role is complex. This increase in the complexity of SA functions in defense is matched by the discovery in tobacco of multiple proteins with which SA interacts. An understanding of SA functions in defense against microbial pathogens will require the identification and characterization of these proteins, the major objective of this research.The overall objective of this proposal is to characterize two SA-binding proteins, SABP2 and SABP3 from tobacco. SABP2 is a very low abundance lipase whose activity is stimulated by SA binding. Silencing of SABP2 suppresses several disease resistance responses. Major emphasis is placed on defining SABP2's role in defense responses. A second objective is to begin an analysis to determine which member(s) of the 18-member Arabidopsis family of SABP2-like proteins corresponds to the tobacco SABP2. Preliminary characterization of SABP2's enzymatic activity, a third objective, will be continued. The fourth objective involves further characterizing SABP3. This protein is the chloroplastic carbonic anhydrase (CA), and based on yeast complementation analysis, it appears to have antioxidant activity. Initial studies have indicated that silencing SABP3 expression alters the HR and local resistance to several pathogens. These findings will be extended by assessing the effects of SABP3/CA silencing on defense gene expression and resistance to other pathogens in Arabidopsis, as well as in tobacco.