Ulvan-induced resistance in Arabidopsis thaliana against Alternaria brassicicola requires reactive oxygen species derived from NADPH oxidase

• Defense responses induced by ulvan in Arabidopsis thaliana plants.
• Ulvan protected WT and AtrbohF plants against Alternaria brassicicola.
• Ulvan failed to protect WT plants infiltrated with DPI and the AtrbohD mutant.
• Resistance induced by ulvan requires ROS derived from RBOHD.
• A general model for ulvan-induced defense responses is proposed.

The present work aimed to study the role of reactive oxygen species derived from NADPH oxidase in the ulvan-induced resistance against Alternaria brassicicola in Arabidopsis thaliana. Foliar spraying of ulvan, a water-soluble algal polysaccharide, reduced the colonization of host tissues and, consequently, the severity of A. brassicicola by 90% in both wild type and AtrbohF plants, and it increased NADPH oxidase activity and hydrogen peroxide levels. Ulvan also tended to enhance the activity of enzymes related to the removal of reactive oxygen species (APX, GSR, CAT and SOD) suggesting a tight control of the antioxidant system. Ulvan did not protect the AtrbohD mutant as well as wild type plants previously infiltrated with diphenyleneiodonium, both impaired in NADPH oxidase activity and hydrogen peroxide accumulation. Based on our results and those available in the literature, we propose a general model for ulvan-induced defense responses in plant tissues. Collectively, our results suggest that ulvan-induced resistance in A. thaliana against A. brassicicola requires reactive oxygen species derived from the respiratory burst oxidase homologue D (RBOHD) NADPH oxidase.