Riboflavin (Vitamin B₂) induces defence responses and resistance to Plasmopara viticola in grapevine

Grapevine (Vitis vinifera) is susceptible to a variety of pathogenic fungi that affect yield and wine quality. Their control is generally achieved by widespread application of fungicides in the vineyards. The economic costs and negative environmental impact associated with these applications has led to a quest for alternative strategies, focusing on manipulation of host defence mechanisms. Here, we evaluated the ability of riboflavin (i.e. Vitamin B₂) to induce resistance against downy mildew in grapevine. Our results showed that 2 mM riboflavin applied 1–3 days before pathogen inoculation provided a disease reduction efficiency of 86 %. A microscopic analysis of the time course of P. viticola colonization in riboflavin-treated leaf discs suggests early inhibition of hyphae spreading in the intercellular space. This resistance does not result from a direct fungitoxic effect of riboflavin. However, this vitamin activates host-defence responses including H₂O₂ generation, upregulation of an array of defence-related genes and synthesis of callose in stomata cells, while stilbene synthesis is not affected. Using a pharmacological approach, we deduced that both jasmonic acid and callose biosynthesis pathways are significantly involved in riboflavin-induced resistance against downy mildew in grapevine.