Chitosan induced resistance to downy mildew in sunflower caused by Plasmopara halstedii

Induction of resistance to downy mildew caused by Plasmopara halstedii in sunflower was studied after treatment with chitosan. Treatment of sunflower seeds with 5% chitosan resulted in decreased disease severity and offered 46 and 52% protection under greenhouse and field conditions respectively. The induction of resistance to P. halstedii by chitosan was accompanied by the accumulation of various host defense-related enzymes in susceptible sunflower seedlings. Enhanced activation of catalase (CAT) and phenylalanine ammonia lyase (PAL) was observed at 6 h and 9 h post-inoculation respectively, while peroxidase (POX), polyphenol oxidase (PPO) and chitinase (CHI) levels were high at 12 h post-inoculation in sunflower seedlings raised from seeds treated with chitosan. Northern hybridization analysis revealed increased levels of transcripts for five known defense response genes viz., Pr-1a, β-1,3-glucanase, chitinase, peroxidase and chalcone synthase in these seedlings. This enhanced and early activation of defense-related responses in the susceptible cultivar Morden treated with chitosan was comparable to that in the resistant cultivar. The results indicate that chitosan induced resistance against P. halstedii in sunflower is mediated through the enhanced expression of genes for defense-related proteins.