ENDOGENOUS SALICYLIC-ACID LEVELS CORRELATE WITH ACCUMULATION OF PATHOGENESIS-RELATED PROTEINS AND VIRUS-RESISTANCE IN TOBACCO

Salicylic acid (SA) is hypothesized to be an endogenous regulator of local and systemic disease resistance and an inducer of pathogenesis-related (PR) proteins among plants. High levels of PR proteins have been observed in an uninoculated amphidiploid hybrid of Nicotiana glutinosa X N. debneyi, which is highly resistant to tobacco mosaic virus (TMV). Fluorescence, UV, and mass spectral analysis established that the levels of SA in healthy N. glutinosa X N. debneyi leaves were 30 times greater than in N. tabacum 'Xanthi-nc' tobacco, which does not constitutively express PR proteins and is less resistant to TMV. Upon TMV-inoculation, SA levels increased at least 70-fold in leaves of Xanthinc but rose only slightly in the hybrid. Phloem exudates of N. glutinosa X N. debneyi contained at least 500 times more SA than those of Xanthi-nc. SA treatment caused the appearance of PR-1 protein in Xanthi-nc but did not affect constitutively high levels of PR-1 protein in N. glutinosa X N. debneyi. In contrast to Xanthi-nc tobacco, TMV-inoculated N. glutinosa X N. debneyi kept at 32 C accumulated more than 0.5 mug SA/g fresh weight, maintained high levels of PR proteins, and developed a hypersensitive response to TMV. PR proteins have previously been shown to accumulate in the lower leaves of healthy, flowering Xanthinc tobacco, which exhibited increased resistance to TMV. These developmentally induced increases in resistance and PR-1 proteins positively correlated with tissue levels of SA. These results affirm the regulatory role of SA in disease resistance and PR protein production.