Biochemical and cytochemical responses of <i>Stylosanthes guianensis</i> to infection by <i>Colletotrichum gloeosporioides</i>: association of callose deposition with resistance

Abstract

Biochemical and cytochemical responses of leaves of the tropical pasture legume Stylosanthes guianensis cv. Graham to infection by a compatible and incompatible race of Colletotrichum gloeosporioides have been investigated. No evidence for the synthesis of phytoalexins and induction of the enzyme phenylalanine ammonia-lyase was obtained. Peroxidase was induced but with little difference between the two interactions. Cytochemical staining and radiolabelling with [ 14 C]cinnamic acid indicated that infection was not associated with cell-wall lignification. In the compatible interaction, several pathogenesis-related proteins and chitinase were induced at 4 days after inoculation, but these proteins were not induced in the incompatible interaction. The deposition of callose in host cells associated with fungal appressoria was monitored by staining with aniline blue. The appressoria of the incompatible race were associated with a greater number of host callose deposits than those of the compatible race between 1 and 3 days after inoculation. In addition, the aniline blue staining deposits associated with the appressoria of the incompatible race showed a greater intensity of fluorescence when compared with those of the compatible race. Histological studies showed low levels of penetration (1–6%) for both interactions. In the incompatible interaction, fungal development did not extend past vesicle formation in the penetrated cell, whereas in compatible interactions the fungus had extended to greater than 20 cells by 96 h. Thus, among a number of possible host responses to infection, the deposition of callose associates most closely with race-specific resistance. Key words: anthracnose, phytoalexins, peroxidase, papillae, chitinase.