ENDOGENOUS METHYL SALICYLATE IN PATHOGEN-INOCULATED TOBACCO PLANTS

The tobacco (Nicotiana tabacum) cultivar Xanthi-nc (genotype NN) produ ces high levels of salicylic acid (SA) after inoculation with the toba cco mosaic virus (TMV). Gaseous methyl salicylate (MeSA), a major vola tile produced in TMV-inoculated tobacco plants, was recently shown to be an airborne defense signal. Using an assay developed to measure the MeSA present in tissue, we have shown that in TMV-inoculated tobacco plants the level of MeSA increases dramatically, paralleling increases in SA. MeSA accumulation was also observed in upper, noninoculated le aves. In TMV-inoculated tobacco shifted from 32 to 24 degrees C, the M eSA concentration increased from nondetectable levels to 2318 ng/g fre sh weight 12 h after the temperature shift, but subsequently decreased with the onset of the hypersensitive response. Similar results were o bserved in plants inoculated with Pseudomonas syringae pathovar phaseo licola, in which MeSA levels were highest just before the hypersensiti ve response-induced tissue desiccation. Transgenic NahG plants unable to accumulate SA also did not accumulate MeSA after TMV inoculation, a nd did not show increased resistance to TMV following MeSA treatment. Based on the spatial and temporal kinetics of its accumulation, we con clude that tissue MeSA may play a role similar to that of volatile MeS A in the pathogen-induced defense response.