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Determining the Relationship Between Salicylic Acid Levels and Systemic Acquired Resistance Induction in Tobacco

Salicylic acid (SA) has been proposed as the systemic signal for the induction of systemic acquired resistance (SAR). It has been suggested that SA is synthesized at the site of pathogen-induced necrosis and is translocated to induce SAR in uninfected leaves. Grafting studies between wild-type tobacco plants and plants that are unable to accumulate significant amounts of SA have shown that the large increase in SA accumulation seen in inoculated leaves is not necessary for SAR induction, suggesting that SA is not the primary systemic signal. However, these studies have not addressed whether decreased levels of SA accumulation in inoculated leaves are sufficient to fully induce SAR. In this study, we have determined the relationship between free SA levels in the inoculated leaf and SAR induction in tobacco. These results support our previous conclusion that SA is not likely to be the systemic signal.