Fusarium oxysporum gas1 Encodes a Putative β-1, 3-Glucanosyltransferase Required for Virulence on Tomato Plants

Abstract

Glycosylphosphatidylinositol-anchored (β)-1,3-glucanosyltransferases play active roles in fungal cell wall biosynthesis and morphogenesis and have been implicated in virulence on mammals. The role of β-1,3-glucanosyltransferases in pathogenesis to plants has not been explored so far. Here, we report the cloning and mutational analysis of the gas1 gene encoding a putative β-1,3-glucanosyltransferase from the vascular wilt fungus Fusarium oxysporum. In contrast to Candida albicans, expression of gas1 in F. oxysporum was independent of ambient pH and of the pH response transcription factor PacC. Gene knockout mutants lacking a functional gas1 allele grew in a way similar to the wild-type strain in submerged culture but exhibited restricted colony growth on solid substrates. The restricted growth phenotype was relieved by the osmotic stabilizer sorbitol, indicating that it may be related to structural alterations in the cell wall. Consistent with this hypothesis, Δgas1 mutants exhibited enhanced resistance to cell wall-degrading enzymes and increased transcript levels of chsV and rho1, encoding a class V chitin synthase and a small monomeric G protein, respectively. The Δgas1 mutants showed dramatically reduced virulence on tomato, both in a root infection assay and in a fruit tissue-invasion model, thus providing the first evidence for an essential role of fungal β-1,3-glucanosyltransferases during plant infection.

Keywords

• glucan