DISSECTION OF THE SALICYLIC-ACID SIGNALING PATHWAY IN TOBACCO

Certain chemicals induce the accumulation of pathogenesis-related (PR) proteins and enhance resistance to tobacco mosaic virus (TMV) in toba cco at the application site, These compounds include polyacrylic acid (PAA), thiamine, L-alpha-amino butyric acid, barium chloride, 2-thiour acil, beta-1,4-endoxylanase and 2,6-dichloroisonicotinic acid (INA), B ecause there is considerable evidence that salicylic acid (SA) is an e ndogenous signal leading to PR gene expression and enhanced resistance following TMV infection, we evaluated these chemicals to see if they act via SA, We report here that all of the compounds tested except INA induced stable accumulation of the glucoside of SA (SAG), probably vi a transient or low level production of SA. Thiamine induction of PR-1 gene expression and enhanced resistance to TMV was significantly reduc ed in salicylate hydroxylase-producing transgenic plants which are una ble to accumulate SA. In contrast, induction of PR-1 gene expression a nd enhanced resistance to TMV by PAA was similar in the transgenic and wild-type plants, Furthermore, thiamine induction of SAG accumulation was blocked at elevated temperatures, whereas PAA was still effective , This suggests the presence of multiple pathways that lead to defense responses, one of which is independent of SA, As PAA, thiamine, and I NA enter the pathway(s) leading to PR-1 gene expression and enhanced r esistance via distinct mechanisms, they should provide chemical tools for dissection of these pathways.