| Summary: | 14-3-3 proteins are highly conserved regulatory molecules in plants. In grapevine (<i>Vitis vinifera</i> L.), 14-3-3 proteins are studied under abiotic stress. However, the role of 14-3-3 proteins in the interaction between grapevine and downy mildew is yet to be studied. In this study, we identified a highly conserved 14-3-3 protein in grapevine and performed a phylogenetic analysis, revealing a close relationship between one of its homologs, 14-3-3ω proteins from <i>Arabidopsis thaliana</i> and <i>Nicotiana benthamiana</i>. We designated this homolog as Vv14-3-3ω. Subcellular localization studies showed that Vv14-3-3ω resides in the plasma membrane and cytoplasm. Expression analysis revealed a strong induction of Vv14-3-3ω at early time points following <i>Plasmopara viticola</i> infection, correlating with enhanced pathogen sporulation in grapevine. Furthermore, transient overexpression of Vv14-3-3ω in <i>N. benthamiana</i> increased susceptibility to the <i>Phytophthora capsici</i> pathogen and suppressed Flg22-induced pattern-triggered immunity (PTI) responses. Overexpression of Vv14-3-3ω in <i>Nb14-3-3</i>-silenced <i>N. benthamiana</i> plants resulted in increased susceptibility to <i>P. capsici,</i> suggesting functional conservation of this isoform. These findings indicate that Vv14-3-3ω functions as a susceptibility factor, facilitating pathogen infection and disease progression in grapevine, and highlight its potential role for improving resistance against downy mildew.
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