PATHWAY OF SALICYLIC-ACID BIOSYNTHESIS IN HEALTHY AND VIRUS-INOCULATED TOBACCO

Salicylic acid (SA) is a likely endogenous regulator of localized and systemic disease resistance in plants. During the hypersensitive respo nse of Nicotiana tabacum L. cv Xanthi-nc to tobacco mosaic virus (TMV) , SA levels rise dramatically. we studied SA biosynthesis in healthy a nd TMV-inoculated tobacco by monitoring the levels of SA and its likel y precursors in extracts of leaves and cell suspensions. In TMV-inocul ated leaves, stimulation of SA accumulation is accompanied by a corres ponding increase in the levels of benzoic acid. C-14-Tracer studies wi th cell suspensions and mock- or TMV-inoculated leaves indicate that t he label moves from trans-cinnamic acid to SA via benzoic acid. In hea lthy and TMV-inoculated tobacco leaves, benzoic acid induced SA accumu lation. o-Coumaric acid, which was previously reported as a possible p recursor of SA in other species, did not increase SA levels in tobacco . In healthy tobacco tissue, the specific activity of newly formed SA was equal to that of the supplied [C-14]benzoic acid, whereas in TMV-i noculated leaves some isotope dilution was observed, presumably becaus e of the increase in the pool of endogenous benzoic acid. We observed accumulation of pathogenesis-related-1 proteins and increased resistan ce to TMV in benzoic acid- but not in o-coumaric acid-treated tobacco leaves. This is consistent with benzoic acid being the immediate precu rsor of SA. We conclude that in healthy and virus-inoculated tobacco, SA is formed from cinnamic acid via benzoic acid.