Genome-wide characterization of the HIPP gene family in Panicum virgatum reveals PvHIPP23 to be a negative regulator of cadmium tolerance

The heavy metal-associated isoprenylated plant protein ( HIPP ) gene family contributes to metal homeostasis regulation and stress adaptation; however, its biological significance in the bioenergy crop switchgrass ( Panicum virgatum L.) remains largely unexplored. In this investigation, 79 PvHIPP genes were systematically identified at the genome-wide level and phylogenetically assigned into five groups. Among these genes, PvHIPP23 exhibited substantial induction following cadmium (Cd) exposure in both roots and shoots. Suppression of PvHIPP23 through RNA interference improved Cd tolerance in switchgrass, accompanied by diminished Cd accumulation in roots and increased Cd translocation factors. In contrast, ectopic expression of PvHIPP23 in Arabidopsis thaliana reduced Cd tolerance, as indicated by impaired root elongation, decreased chlorophyll levels, and reduced biomass production. Furthermore, PvHIPP23 negatively regulated PvHMA2.1 expression and interacted with the heavy-metal transporter PvHMA2.1 at the plasma membrane. Independent overexpression of PvHMA2.1 enhanced Cd tolerance in switchgrass, producing phenotypic outcomes comparable to those observed following PvHIPP23 silencing. These findings identify PvHIPP23 as a negative regulator of Cd tolerance that can attenuate PvHMA2.1 transcription and that interacts with PvHMA2.1, as suggested by bimolecular fluorescence complementation (BiFC) assays. However, additional evidence is required to fully characterize the PvHIPP23–PvHMA2.1 regulatory module. The preliminary identification of this module expands current knowledge of the molecular mechanisms controlling Cd responses in switchgrass and provides a potential genetic target for developing switchgrass lines with enhanced Cd phytoextraction potential.