Endogenous salicylic acid (SA) levels increase and several families of pathogenesis-related genes (including PR-1 and PR-2) are induced during the resistance response of tobacco to tobacco mosaic virus (TMV) infection. We have found that at a temperature (32 degrees C) that prevents the induction of PR genes and resistance, the increases in SA levels were eliminated. However, when the resistance response was restored by shifting inoculated plants to lower temperatures, SA levels increased dramatically and preceded PR-1 gene expression and necrotic lesion formation associated with resistance. SA was also found in a conjugated form whose levels increased in parallel with the free SA levels. This SA beta-glucoside (SAG) was as active as SA in inducing PR-1 gene expression. PR-1 gene induction by SAG was preceded by a transient release of SA. The existence of a mechanism that releases SA from SAG suggests a possible role for SAG in the maintenance of systemic acquired resistance. Previously, we identified a soluble salicylic acid-binding protein (SABP) in tobacco whose properties suggest that it may play a role in transmitting the SA signal during plant defence responses. This SABP has been purified 250-fold by sequential chromatography on DEAE-Sephacel, Sephacryl S-300, Blue Dextran-Agarose and Superose 6. Several monoclonal antibodies (mAbs) raised against the highly purified SABP immunoprecipitated the SA-binding activity and a 280 kDa protein. This 280 kDa protein also co-purified with the SA-binding activity during the various chromatography steps, suggesting that it was responsible for binding SA. Immunoblot analysis with the SABP-specific mAbs also detected the 280 kDa protein in highly purified preparations of SABP. However, in crude homogenates these mAbs only recognized a 57 kDa protein. These and other results suggest that SABP is a multimeric complex which contains, at least, a 57 kDa protein and whose components are readily cross-linked during purification.