Role of cell wall and cell membrane integrity in imparting defense response against Phytophthora capsici in black pepper (Piper nigrum L.)

The structural defense response of a black pepper line (04-P24) showing root resistance to Phytophthora capsici was studied in comparison with a highly susceptible line (Sreekara). Role of cell wall reinforcement and cell membrane integrity was analyzed. Cell membrane integrity was studied under hydroponic system in terms of leakage of electrolytes caused by the cell membrane damage due to pathogen entry. Root cell membrane rupture and resultant phenolic leakage were clearly visible in the form of color change of the liquid phase during the course of infection. Root leachates of Sreekara turned highly dark due to the increased level of phenol leakage which was proportional to the cell membrane damage. The root leachate was analyzed for change in conductivity, total phenols and ortho-dihydroxy (OD) phenols. Cell wall reinforcement of the vascular tissues of black pepper root upon pathogen inoculation was analyzed histochemically. Toluidine blue O and Maule staining differentiated the intensity of lignin deposition in the root cells of both lines and it was comparatively stronger in the resistant line. Scanning electron microscopy revealed that hyphae of P. capsici are not penetrating the root of 04-P24 supporting the finding that roots of this line don’t support Phytophthora infection.