Role of UeMsb2 in Filamentous Growth and Pathogenicity of Ustilago esculenta

<i>Ustilago esculenta</i> is a dimorphic fungus that specifically infects <i>Zizania latifolia</i>, causing stem swelling and the formation of an edible fleshy stem known as jiaobai. The pathogenicity of <i>U. esculenta</i> 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 <i>UeMsb2</i> gene from <i>U. esculenta</i> (GenBank No. MW768949). The open reading frame of <i>UeMsb2</i> 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 <i>UeMsb2</i> is inducibly expressed during both hyphal growth and infection processes. Deletion of <i>UeMsb2</i> 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 <i>b</i> signaling pathway genes associated with pathogenicity. Notably, the deletion of <i>UeMsb2</i> did not influence the in vitro growth of <i>U. esculenta</i> under hyperosmotic, thermal, or oxidative stress conditions. These findings underscore the critical role of <i>UeMsb2</i> in regulating the pathogenicity of <i>U. esculenta</i>. This study provides insights into the interaction between <i>U. esculenta</i> and <i>Z. latifolia</i>, particularly the mechanisms that drive host stem swelling.