Resistance to the plant PR-5 protein osmotin in the model fungus Saccharomyces cerevisiae is mediated by the regulatory effects of SSD1 on cell wall composition

The capacity of plants to counter the challenge of pathogenic fungal attack depends in part on the ability of plant defense proteins to overcome funga l resistance by being able to recognize and eradicate the invading fungi. F ungal genes that control resistance to plant defense proteins are therefore important determinants that define the range of fungi from which an induce d defense protein can protect the plant. Resistance of the model fungus Sac charomyces cerevisiae to osmotin, a plant defense PR-5 protein, is strongly dependent on the natural polymorphism of the SSD1 gene. Expression of the SSD1-v allele afforded resistance to the antifungal protein. Conversely, ye ast strains carrying the SSD1-d allele or a null ssd1 Delta mutation displa yed high sensitivity to osmotin. The SSD1-v protein mediates osmotin resist ance in a cell wall-dependent manner. Deletion of SSD1-v or SSD1-d impeded sorting of the PIR proteins (osmotin-resistance factors) to the cell wall w ithout affecting mRNA levels, indicating that SSD1 functions in post-transc riptional regulation of gene expression. The sensitivity of ssd1 Delta cell s to osmotin was only partially suppressed by over-accumulation of PIR prot eins in the cell wall, suggesting an additional function for SSD1 in cell w all-mediated resistance. Accordingly, cells carrying a null ssd1 mutation a lso displayed aberrant cell-wall morphology and lower levels of alkali-inso luble cell-wall glucans. Therefore SSD1 is an important regulator of fungal cell-wall biogenesis and composition, including the deposition of PIR prot eins which block the action of plant antifungal PR-5 proteins.