Defence responses in leaves of resistant and susceptible pepper (Capsicum annuum L.) cultivars infected with different inoculum concentrations of Phytophthora capsici Leon

Three pepper cultivars (PM-702: resistant, Demre-8 and KM-hot: susceptible) with different resistances to Phytophthora capsici-22 (P. capsici-22) were inoculated with different concentrations of zoospores to analyze the time course of phenylalanine ammonia lyase (PAL) activity, malondialdehyde (MDA), hydrogen peroxide (H2O2), proline and total protein.Samples were collected from the leaves of three pepper cultivars on the 2nd, 4th, and 6th days after infection; important differences were observed in PAL activity, MDA amount, H2O2, proline, and total protein with respect to zoospore concentration, infection time, and type of pepper cultivars. Activity of phenylalanine ammonia-lyase (PAL), the first enzyme in the phenylpropanoid biosynthetic pathway, was increased in inoculated leaves of all three cultivars. The increase in production of PAL upon challenge was higher in the infected plants compared to the non-infected (control) plants. An early induction of PAL was observed in the leaves of all peppers infected with P. capsici within two days, making it one of the detected earlier responses to infection. Moreover, pathogen stress increased the rate of lipid peroxidation (which is indicated by increasing MDA content). Production of reactive oxygen species, H2O2, and lipid peroxidation increased in pepper leaves by increasing time of infection and inoculum concentration. The findings suggested that PM-702 pepper cultivar was inherently better protected against lipid peroxidation under P. capsici stress than KM-hot and Demre-8. In addition, it was observed that proline content in the leaves of infected susceptible cultivars increased with time, but highest proline and total protein in non-infected (control) leaves were recorded in resistant cultivar.

Research highlights
► Induction of PAL by P. capsici might result in the activation. Pathogen challenge might cause damage and necrosis that decreases the activation of PAL.
► The decrease PAL activity in infected leaves are responsible for the increase in MDA and H2O2. MDA content was a found as a significance indicator of plant defence.
► We describe that proline accumulation, typical plant pathogen stress response.