Structure-activity relationships of oligoagar elicitors toward Gracilaria conferta (Rhodophyta)

Agar oligosaccharides in the neoagarobiose series were prepared by partial enzyme hydrolysis, separated on Biogel P2 and P4, and analyzed by high-perf ormance anion exchange chromatography with pulsed amperometric detection, y ielding neoagarosaccharide fractions with a disaccharide repetition degree ranging from 1 (neoagarobiose) to more than 8 (neoagarohexadecaose). These fractions were analyzed for their biological activity toward the marine red alga Gracilaria conferta (Schousboe ex Montagne) J. et G. Feldmann in term s of increase of oxygen consumption, release of hydrogen peroxide, eliminat ion of epiphytic bacteria, and induction of thallus tip bleaching. The stru cture-activity and dose-response relationships of neoagarosaccharides were very similar in the respiratory and oxidative burst responses and in their bactericidal properties, with neoagarosaccharides consisting of 6 to 8 disa ccharide repeating units being the most active. Ah these responses were com petitively inhibited by the reduced form of neoagarohexaose, neoagarohexait ol, In contrast, the tip-bleaching response was light dependent, required m uch higher concentrations of neoagarosaccharides, and was not inhibited by neoagarohexaitol, suggesting that it is an unspecific oxidative stress reac tion. Putative structural effects on the recognition of endogenous agar-oli gosaccharide elicitors by G, conferta are discussed.