Maize ZmVPP5 is a truncated Vacuole H+-PPase that confers hypersensitivity to salt stress

In plants, Vacuole H+-PPases (VPPs) are important proton pumps and encoded by multiple genes. In addition to full-length VPPs, several truncated forms are expressed, but their biological functions are unknown. In this study, we functionally characterized maize vacuole H+-PPase 5 (ZmVPP5), a truncated VPP in the maize genome. Although ZmVPP5 shares high sequence similarity with ZmVPP1, ZmVPP5 lacks the complete structure of the conserved proton transport and the inorganic pyrophosphatase-related domain. Phylogenetic analysis suggests that ZmVPP5 might be derived from an incomplete gene duplication event. ZmVPP5 is expressed in multiple tissues, and ZmVPP5 was detected in the plasma membrane, vacuole membrane and nuclei of maize cells. The overexpression of ZmVPP5 in yeast cells caused a hypersensitivity to salt stress. Transgenic maize lines with overexpressed ZmVPP5 also exhibited the salt hypersensitivity phenotype. A yeast two-hybrid analysis identified the ZmBag6-like protein as a putative ZmVPP5-interacting protein. The results of bimolecular luminescence complementation (BiLC) assay suggest an interaction between ZmBag6-like protein and ZmVPP5 in vivo. Overall, this study suggests that ZmVPP5 might act as a VPP antagonist and participate in the cellular response to salt stress. Our study of ZmVPP5 has expanded the understanding of the origin and functions of truncated forms of plant VPPs.