Hydrophobized laminarans as new biocompatible anti-oomycete compounds for grapevine protection

Laminaran, a β-(1→3)-glucan extracted from Laminaria digitata, is a known elicitor of plant defenses, but provides only low level of disease control in vineyard trials. In this context, laminaran was partly hydrophobized by grafting from 1.6 to 7.6 lauryl chains to the native saccharidic chain and the impact of sulfation of the hydrophobized glucans was studied. The activity of the different synthetized laminaran derivatives as antimicrobial agents against Plasmopara viticola, the causal agent of grape downy mildew, and as elicitors of defense reactions in planta, was evaluated. Our results showed that acylation imparts an antimicrobial activity to laminaran which is related to the degree of acylation, AL3, with 7.6 lauryl chains, being the most effective derivative. Sulfation of the acylated laminarans did not further increase the antimicrobial activity. Our results also demonstrated that the efficacy of AL3 against Plasmopara viticola was most likely due to the direct antimicrobial activity of the lauryl chains rather than to an elicitation of plant defenses.