A Rhamnogalacturonan Acetylesterase Effector FsRGAE1 Enhances the Virulence of Fusarium sacchari by Localizing to the Nucleus and Suppressing Plant Immunity

Abstract

Fusarium sacchari is one of the major pathogenic fungi that cause sugarcane Pokkah Boeng disease (PBD). Effectors play pivotal roles in F. sacchari–sugarcane interaction; thus, characterizing these effectors is essential for elucidating the molecular mechanisms underlying F. sacchari pathogenicity and for developing effective strategies to control PBD. However, only a limited number of effectors have been functionally validated to date. Here, we report FsRGAE1, a candidate effector protein from F. sacchari predicted to encode a rhamnogalacturonan acetylesterase (RGAE). FsRGAE1 exhibits high expression during the early stages of infection and maintains relatively elevated expression levels throughout the F. sacchari–sugarcane interaction. Targeted deletion of the FsRGAE1 gene in F. sacchari had no discernible impact on mycelial growth, conidiation, or carbon-source utilization, yet it significantly attenuated fungal virulence. FsRGAE1 possesses both a signal peptide conferring secretory capacity and a transit peptide enabling its translocation into the host cytoplasm and nucleus. Using the Agrobacterium tumefaciens-mediated transient expression system in Nicotiana benthamiana, FsRGAE1 was confirmed to suppress cell death induced by Bcl-2-associated X protein (BAX), as well as ROS accumulation and callose deposition, and its nuclear localization is indispensable for this immunosuppressive activity. Collectively, these findings indicate that FsRGAE1 promotes F. sacchari virulence by suppressing host immune responses in a nuclear localization-dependent manner, providing new insights into effector-mediated F. sacchari pathogenesis and potential target for resistance breeding in sugarcane.