Glucuronan and oligoglucuronans isolated from green algae activate natural defense responses in apple fruit and reduce postharvest blue and gray mold decay

The efficacy of two algal saccharides, glucuronan and oligoglucuronans (average degree of polymerization = 3), against postharvest blue mold (Penicillium expansum) and gray mold (Botrytis cinerea) on apple fruit (Malus domestica Borkh. cv Golden Delicious) and the related-defense responses involved were evaluated. The severity of both blue and gray mold in apple fruits was reduced with the greatest efficacy being achieved using oligoglucuronans. Moreover, glucuronan and its oligomers trigger a rapid and transient accumulation of hydrogen peroxide (H2O2) as well as the activation of antioxidant-related enzymes, namely catalase (CAT) and superoxide dismutase (SOD). These algal saccharides increased also the activities of phenylalanine ammonialyase (PAL), peroxydase (POD), and polyphenoloxydase (PPO) as well as the levels of lignin and phenolic compounds. These results suggest that the protective effects of glucuronan and oligoglucuronans on apple might be due to its ability on activating an onset-related defensive enzymes and metabolites instead of its direct antifungal activity on the pathogens. These findings showed that glucuronan and even more oligoglucuronans treatments could be a promising method to reduce dependency on synthetic fungicides.