Characterization of the Soybean (Glycine max) Heavy-Metal-Associated Isoprenylated Plant Protein (HIPP) Gene Family in Response to Aluminum

Heavy-metal-associated (HMA) isoprenylated plant proteins (HIPPs) play crucial roles in plant responses to biotic/abiotic stresses and heavy-metal homeostasis. However, the involvement of HIPP genes in the response of soybean (Glycine max) to aluminum (Al) toxicity remains unexplored. This study aimed to comprehensively characterize the GmHIPP gene family and investigate its role in Al toxicity. A total of 76 GmHIPP genes were identified in the soybean genome. Phylogenetic and synteny analyses revealed that HIPP evolution was highly conserved among soybean, Arabidopsis (Arabidopsis thaliana), and rice (Oryza sativa). Cis-element analysis indicated that GmHIPP genes might be involved in phytohormone response, abiotic and biotic stresses, and plant growth. RNA-seq analysis further revealed that the expression of 20 GmHIPPs was up-regulated, and three GmHIPPs were down-regulated under Al stress in roots. Among them, six genes (GmHIPP9/13/29/43/58/73) were highly induced by Al, with GmHIPP29 exhibiting particularly high expression in root tips. Subcellular localization demonstrated that GmHIPP29 is a plasma membrane-localized protein. GmHIPP29-overexpression significantly increased Al accumulation in the cell sap of the transgenic soybean hairy root tips, leading to increased Al sensitivity. Collectively, these results demonstrate that GmHIPP29 acts as a negative regulator of Al tolerance by promoting Al accumulation in soybean roots.