Virulent Effector Hasp155 of <i>Puccinia striiformis</i> f. sp. <i>tritici</i> Suppresses Plant Immunity and Promotes Fungus Infection

As a kind of obligate biotrophic fungus, <i>Puccinia striiformis</i> f. sp. <i>tritici</i> (<i>Pst</i>) secretes vast effectors via haustoria to host cells during the infection to inhibit host defense responses and promote fungal invasion. In this study, based on the completion of genome sequencing and haustorial transcriptome sequencing of <i>Pst</i>, we identified a <i>Pst</i> effector (Hasp155) that is significantly induced in the early stage of <i>Pst</i> infection to wheat. The 18 N-terminal amino acids of Hasp155 encoded a signal peptide with a secretory function. Transient expression of <i>Hasp</i>155 in <i>Nicotiana benthamiana</i> inhibited Bax-induced cell death as well as chitin-triggered callose deposition and defense-related gene expression. Moreover, delivery of the Hasp155 protein into wheat cells via type three secretion systems (TTSS) led to reduced plant immunity to nonpathogenic bacteria and to the avirulent <i>Pst</i> race with decreased H₂O₂ accumulation and promoted <i>Pst</i> development. Furthermore, transgenic overexpression of <i>Hasp</i>155 significantly renders wheat resistance susceptible, resulting in a decreased defense response and increased <i>Pst</i> pathogenicity. Overall, these results indicate that Hasp155 is an important effector of <i>Pst</i> pathogenicity by suppressing plant immunity.