Seed treatment with salicylic acid invokes defence mechanism of Helianthus annuus against Orobanche cumana

The root holoparasitic angiosperm sunflower broomrape ( Orobanche cumana ) specifically affects sunflower ( Helianthus annuus ) growth and causes severe damage all over the world. This investigation was designed to examine the protective effects of salicylic acid ( SA ) treatment to the seeds of an O. cumana ‐susceptible cultivar of sunflower ( TK0409 ). Sunflower seeds were pretreated with different concentrations (0, 0.5 and 1 mM) of SA and inoculated with O. cumana for 4 weeks. O. cumana infection resulted in reduction in plant biomass, endogenous SA level, and the expression of SA ‐related genes including pal , chs and NPR1 . By contrast, O. cumana infection enhanced the production of reactive oxygen species, activities of antioxidant enzymes as well as contents of phenolics and lignin. Seed treatment with 1 mM SA increased sunflower biomass in terms of plant height, fresh weight and dry weight by 10%, 13% and 26%, respectively, via reducing the number and biomass of established O. cumana . The increase of hydrogen peroxide contents by 14% in the 1 mM SA treated sunflower plants appeared to be because of the inhibition of ascorbate peroxidase and catalase by exogenous SA . The enhanced expression of pathogenesis‐related genes ( PR3 and PR12 , encoding chitinase and defensin, respectively) after 4 weeks of inoculation indicated that systemic acquired resistance was induced in the SA treated sunflower in which the level of endogenous SA was also elevated in a dose‐dependent manner. The increased expression of a hypersensitive‐responsive (HR) gene hsr indicated that the resistance of sunflowers might be associated with a hypersensitive reaction which was activated by exogenous SA treatment.