Salicylic acid mediates elicitin-induced systemic acquired resistance, but not necrosis in tobacco

Elicitins are proteinaceous elicitors which are secreted by all Phytophthora species analyzed so far, When applied on tobacco they induce necrosis and systemic acquired resistance (SAR). Elicitin-treated tobacco plants that express the bacterial nahG gene coding for salicylate hydroxylase did not show SAR against infection with the black shank fungus, Phytophthora parasitica var. nicotianae. However, nahG-expressing plants still responded to a treatment with the basic elicitin cryptogein by formation of necrosis, Elicitin-induced expression of genes encoding PR-la, PR-2 PR-5 and basic class III chitinase was suppressed in NahG tobacco, while the induced mRNA accumulation of basic PR-1, PR-3, str-246 and str-319 appeared to be unchanged. NahG plants showed enhanced susceptibility to several Phytophthora species and isolates that produce elicitins, but were not affected in the necrosis response during interaction. While SA is involved in SAR and disease resistance to Phytophthora, our results suggest that it does not mediate the hypersensitive-like necrosis response.