Increased expression of a plant actin gene during a biotrophic interactionbetween round-leaved mallow, Malva pusilla, and Colletotrichum gloeosporioides f. sp malvae

Two actin genes, actA from the hemibiotrophic anthracnose fungus, Colletotr ichum gloeosporioides (Penz.) Pent. & Sacc. f. sp. malvae, and act1 from it s host, Malva pusilla (Sm.) were cloned from a cDNA library developed from infected host tissue. The actin gene, actA, of C. gloeosporioides f. sp. ma lvae, which is similar to that of other euascomycetes, appears to be expres sed constitutively. The actin gene of M. pusilla is most similar to one of the actin genes of Arabidopsis thaliana that is unique in being responsive to environmental stimuli such as wounding. Expression of actA was used to f ollow the growth of the fungus in the plant tissue. Low actA expression occ urred until 72-96 h after inoculation and then increased rapidly, correspon ding with the timing of the shift from slower biotrophic fungal growth to m uch more rapid necrotrophic growth. In contrast, expression of act1 approxi mately doubled during the biotrophic phase and then rapidly declined during the necrotrophic phase. Increased host actin expression could be due to ho st cytoskeleton rearrangement in response to biotrophic infection, and the subsequent decrease in host actin expression could be due to host cell disr uption resulting from tissue maceration during necrosis. This is the first report of a host actin gene that can increase in expression during a compat ible plant-pathogen interaction.