Role of UeMsb2 in Filamentous Growth and Pathogenicity of Ustilago esculenta

Ustilago esculenta is a dimorphic fungus that specifically infects Zizania latifolia, causing stem swelling and the formation of an edible fleshy stem known as jiaobai. The pathogenicity of U. esculenta is closely associated with the development of jiaobai and phenotypic differentiation. Msb2 acts as a key upstream sensor in the MAPK (mitogen-activated protein kinase) signaling pathway, playing critical roles in fungal hyphal growth, osmotic regulation, maintenance of cell wall integrity, temperature adaptation, and pathogenicity. In this study, we cloned the UeMsb2 gene from U. esculenta (GenBank No. MW768949). The open reading frame of UeMsb2 is 3015 bp in length, lacks introns, encodes a 1004-amino-acid protein with a conserved serine-rich domain, and is localized to the vacuole. Expression analysis revealed that UeMsb2 is inducibly expressed during both hyphal growth and infection processes. Deletion of UeMsb2 did not affect haploid morphology or growth rate in vitro but significantly impaired the strain's mating ability, suppressed filamentous growth, slowed host infection progression, and downregulated the expression of b signaling pathway genes associated with pathogenicity. Notably, the deletion of UeMsb2 did not influence the in vitro growth of U. esculenta under hyperosmotic, thermal, or oxidative stress conditions. These findings underscore the critical role of UeMsb2 in regulating the pathogenicity of U. esculenta. This study provides insights into the interaction between U. esculenta and Z. latifolia, particularly the mechanisms that drive host stem swelling.