ULTRAVIOLET-LIGHT AND OZONE STIMULATE ACCUMULATION OF SALICYLIC-ACID,PATHOGENESIS-RELATED PROTEINS AND VIRUS-RESISTANCE IN TOBACCO

In tobacco (Nicotiana tabacum L. cv. Xanthinc), salicylic acid (SA) le vels increase in leaves inoculated by necrotizing pathogens and in hea lthy leaves located above the inoculated site. Systemic SA increase ma y trigger disease resistance and synthesis of pathogenesis-related pro teins (PR proteins). Here we report that ultraviolet (UV)-C light or o zone induced biochemical responses similar to those induced by necroti zing pathogens. Exposure of leaves to UV-C light or ozone resulted in a transient ninefold increase in SA compared to controls. In addition, in UV-light-irradiated plants, SA increased nearly fourfold to 0.77 m ug.g-1 fresh weight in leaves that were shielded from UV light. Increa sed SA levels were accompanied by accumulation of an SA conjugate and by an increase in the activity of benzoic acid 2-hydroxylase which cat alyzes SA biosynthesis. In irradiated and in unirradiated leaves of pl ants treated with UV light, as well as in plants fumigated with ozone, PR proteins 1a and 1b accumulated. This was paralleled by the appeara nce of induced resistance to a subsequent challenge with tobacco mosai c virus. The results suggest that UV light, ozone fumigation and tobac co mosaic virus can activate a common signal-transduction pathway that leads to SA and PR-protein accumulation and increased disease resista nce.