Arachidonic Acid Induces Local but not Systemic Synthesis of Salicylic Acid and Confers Systemic Resistance in Potato Plants toPhytophthora infestansandAlternaria solani

Abstract

When the lower leaves of young potato plants (cv. Bintje) were treated with an aqueous suspension of the polyunsaturated fatty acid arachidonic acid (AA), a threefold increase in the amount of free salicylic acid (SA) in the treated leaves was observed 3 h after treatment. The highest level of free SA was measured 6 h after treatment, reaching 3.21 g/g fresh weight compared to 0.15 g/g fresh weight in controls. The levels of conjugated SA in the treated leaves increased up to 51 g/g fresh weight 3 days after treatment compared to 0.83 g/g fresh weight in controls. Four days after treatment with AA the upper leaves became resistant to the potato late blight pathogen, Phytophthora infestans, but no accumulation of SA was observed. This systemic protection was observed in plants treated with AA at concentrations as low as 10 ppm and is unlikely to have arisen from AA translocated in the plant. In addition, treatment with AA protected potato plants against the early blight pathogen, Alternaria solani, indicating a broader spectrum of protection. Local but not systemic accumulation of a pathogenesis-related (PR)1-like protein was observed 24 h after treatment with AA. Application of SA on potato leaves did not confer resistance against P. infestans, but a local accumulation of a PR1-like protein was observed 24 h after treatment. Younger leaves of healthy susceptible and resistant potato plants contained higher levels of SA than older leaves, and this gradient correlated with their natural resistance to P. infestans. In general, cultivars showing field resistance contained higher amounts of conjugated SA than did susceptible cultivars.