An Endoglucanase Secreted by Ustilago esculenta Promotes Fungal Proliferation

Ustilago esculenta is a fungus of two morphological forms, among the filamentous dikaryon that can induce the plant stem to expand to form fleshy stem. In order to establish biotrophy with Zizania latifolia which belongs to the tribe Oryzeae (Poaceae), U. esculenta firstly needs to secrete a bunch of effectors, among them being cell wall degrading enzymes (CWDEs). We have isolated a gene, UeEgl1, which was differentially expressed in MT-type and T-type U. esculenta at an early stage of infection, and specifically induced in the filamentous growth of the T-type. Bioinformatics analysis and enzyme activity assay indicated that UeEgl1 functions outside the cell as a β-1,4-endoglucanase with a conserved domain of the glycosyl hydrolase family 45 (GH45) which targets the main component of the plant cell wall β-1,4 linked glycosidic bonds. The phenotype analysis of UeEgl1 deletion mutants and UeEgl1 over-expression transformants showed that UeEgl1 had no significant effect on the budding, cell fusion, and filamentous growth of U. esculenta in vitro. Further study found that over-expression of UeEgl1 promoted the proliferation of mycelia inside Z. latifolia, and raised plant defense responses. The above results show that the UeEgl1 gene may play an important role in the early stage of infection through the decomposition of the plant cell wall.