Chitosan induces tomato basal resistance against Phytophthora nicotianae and inhibits pathogen development

Chitosan is a polymer of glucosamine with antimicrobial properties and an elicitor of plant defence responses. In this study, different chitosan concentrations were assessed for impact on the life cycle of Phytophthora nicotianae Breda de Haan, an oomycete pathogen of tomato and tobacco crops. Reduced mycelial growth was observed when chitosan was added to solid media, and this inhibitory activity was amplified with increasing chitosan concentrations from 0.5 g L-1 to 2.5 g L-1. Colony morphology and hyphal diameter were unaffected by the polymer; however, the number of oospores (sexual reproductive structures) was reduced at the lowest chitosan concentration (0.5 g L-1) tested. Mycelia plugs imbibed with 0.5 g L-1 chitosan were prone to increased sporangium formation, but the effect was subdued with higher chitosan concentrations. The length/width ratio of the sporangia did not change in the presence of the polymer. Indirect germination of the sporangia was particularly sensitive to this compound, and 1.5 g L-1 completely abolished this process. The zoospores treated with chitosan formed cysts that did not germinate and were unable to infect tomato plant roots. Chitosan was also found to prime tomato plantlets for P. nicotianae resistance; a foliar application of 0.1 and 1 g L-1 of chitosan 72 h before inoculation induced host resistance, which was concurrent with increased activity of phenylpropanoid pathway markers. The results presented herein show the potential of chitosan as a tool to control P. nicotianae inoculum and increase tomato crop resistance against this pathogen.