Elicitation of peroxidase activity and lignin biosynthesis in pepper suspension cells by Phytophthora capsici

Cell suspension cultures of three varieties of Capsicum annuum L., each with a different degree of sensitivity to the fungus Phytophthora capsici, responded to elicitation by both lyophilized mycelium and fungus filtrate with a hypersensitive reaction. They showed the synthesis or accumulation of PR-proteins with peroxidase (EC 1.11.1.7) activity and the accumulation of lignin-like polymer (as measured by derivatization with thioglycolic acid). The cultivation medium was optimised for both plant and fungus growth in order to avoid any stress during their combination. The resistant pepper variety, Smith-5, showed a more intense response to the elicitor preparations than the sensitive varieties, Americano and Yolo Wonder. This was particularly evident when the cell suspensions were elicited with the filtrate. After elicitation, the cell walls thickened through the accumulation of lignin, as can be observed by staining microscope preparations with methylene blue. Elicitation also reduced the level of total peroxidase activity in the susceptible varieties, while such activity increased in resistant varieties, and was accompanied by de novo expression of acidic peroxidase isoenzymes in the extracellular and cell wall fractions. Of note was the PR protein of pI 5.7 showing peroxidase activity, which was induced by both elicitor types in the elicited cell suspensions of the resistant variety alone, making it a marker of resistance. The increases in the activity of these peroxidases in the resistant variety are in concordance with the accumulation of lignin observed 24 h after inoculation by both elicitors from the fungus. The possible role of these isoenzymes in lignin biosynthesis, used to reinforce the cell walls against fungal penetration of the cells, is discussed. These results are in accordance with those previously observed in plant stem sections.