A Novel Effector FoUpe9 Enhances the Virulence of Fusarium oxysporum f. sp. cubense Tropical Race 4 by Inhibiting Plant Immunity

Fusarium wilt caused by <i>Fusarium oxysporum</i> f. sp. <i>cubense</i> tropical race 4 (Foc TR4) is the most destructive disease of the banana. Effectors play a crucial role in Foc TR4-banana interaction; however, only a few effectors have been functionally characterized. Our previous secretome studies on Foc TR4 highlighted an uncharacterized protein without any conserved domains (named FoUpe9), which was predicted to be a candidate effector. Herein, bioinformatics analysis showed that FoUpe9 was highly conserved among <i>Fusarium</i> species. <i>FoUpe9</i> was highly induced during the early infection stages in the banana. A yeast signal sequence trap assay showed that FoUpe9 is a secretory protein. FoUpe9 could inhibit cell death and ROS accumulation triggered by BAX through the <i>Agrobacterium</i>-mediated <i>Nicotiana benthamiana</i> expression system. Subcellular location showed that FoUpe9 was located in the nucleus and cytoplasm of <i>N. benthamiana</i> cells. Deletion of the <i>FoUpe9</i> gene did not affect mycelial growth, conidiation, sensitivity to cell-wall integrity, or osmotic and oxidative stress, but significantly attenuated fungal virulence. <i>FoUpe9</i> deletion diminished fungal colonization and induced ROS production and expression of SA-related defense genes in banana plants. These results suggest that FoUpe9 enhances Foc TR4 virulence by inhibiting host immune responses and provide new insights into the functions of the uncharacterized proteins, further enhancing our understanding of effector-mediated Foc TR4 pathogenesis.