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 wit h a different degree of sensitivity to the fungus Phytophthora capsici, res ponded 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 li gnin-like polymer (as measured by derivatization with thioglycolic acid). T he cultivation medium was optimised for both plant and fungus growth in ord er to avoid any stress during their combination. The resistant pepper varie ty, Smith-5, showed a more intense response to the elicitor preparations th an the sensitive varieties, Americano and Yolo Wonder. This was particularl y evident when the cell suspensions were elicited with the filtrate, After elicitation, the cell walls thickened through the accumulation of lignin, a s can be observed by staining microscope preparations with methylene blue. Elicitation also reduced the level of total peroxidase activity in the susc eptible varieties, while such activity increased in resistant varieties, an d 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 pos sible role of these isoenzymes in lignin biosynthesis, used to reinforce th e cell wails against fungal penetration of the cells, is discussed. These r esults are in accordance with those previously observed in plant stem secti ons.