Induction of natural defense and protection against Penicillium expansum and Botrytis cinerea in apple fruit in response to bioelicitors isolated from green algae

- Stimulation of natural defense (burst oxidative, hypersensitive reaction) in apple fruit by bioelicitors (ulvan and oligoulvan) isolated from green algae Ulva lactuca.
- The treatments of apple fruit ulvan with these bioelicitors reduced significantly the disease incidence caused by Penicillium expansum and Botrytis cinerea.
- Oligoulvans (oligomer) were more effective than ulvan (polymer).

The effects of two algal saccharides, ulvan and oligoulvans (average degree of polymerization = 2), on defense-related responses and decay development in apple fruit (Malus domestica Borkh. cv Golden Delicious) were investigated. Our results show that both ulvan and oligoulvans reduced significantly (P < 0.05) lesion diameter in inoculated fruit. Differently, blue and grey mold decays were inhibited completely (P < 0.01) in oligoulvan-treated fruit. Moreover, ulvan and oligoulvans 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, all of which were involved in phenylpropanoid metabolism. Considering all the defense-tested parameters, oligoulvans were more effective than ulvan. The obtained results highlight the efficiency of oligosaccharides with low degree of polymerization (DP) on inducing an onset of defense-related enzymes and metabolites. Together, the data showed that ulvan and even more oligoulvan treatments could be promising method to reduce dependency on synthetic fungicides.