Histological and ultrastructural comparisons of compatible, incompatible and DL-beta-amino-n-butyric acid-induced resistance responses of pepper stems to Phytophthora capsici

The compatible interaction of pepper stems with Phytophthora capsici showed more rapid and severe disease development than did the incompatible intera ction, although pathogen penetration styles of host cells in compatible and incompatible interactions were similar to each other. Treatment with DL-B- amino-n-butyric acid (BABA) protected the pepper plants against P. capsici infection. Reduced hyphal growth and sporangial formation were found after P. capsici infection in BABA-induced resistant and incompatible reactions. One of the most noticeable ultrastructural features of the BABA-induced res istant reaction was the formation of electron-dense wall appositions. The t hick and dense wall appositions that encased the haustoria restricted haust orial development, thus leading to limitation of further pathogen penetrati on into inner plant tissues. A main host response in the incompatible inter action was the occlusion of cortical cells with an amorphous material. Plug ging of the intercellular spaces in the cortical cells with electron opaque material was frequently observed in the incompatible interaction, but not in the compatible interaction. Another common feature of the BABA-induced r esistant and incompatible reactions was degeneration of mitochondrial struc ture within penetrating hyphal cytoplasm. The mitochondrial structure in th e BABA-induced resistant or incompatible reactions had no distinct double m embrane layer and well-shaped cristae. (C) 2000 Academic Press.