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 <i>HIPP</i> genes in the response of soybean (<i>Glycine max</i>) to aluminum (Al) toxicity remains unexplored. This study aimed to comprehensively characterize the <i>GmHIPP</i> gene family and investigate its role in Al toxicity. A total of 76 <i>GmHIPP</i> genes were identified in the soybean genome. Phylogenetic and synteny analyses revealed that HIPP evolution was highly conserved among soybean, Arabidopsis (<i>Arabidopsis thaliana</i>), and rice (<i>Oryza sativa</i>). Cis-element analysis indicated that <i>GmHIPP</i> genes might be involved in phytohormone response, abiotic and biotic stresses, and plant growth. RNA-seq analysis further revealed that the expression of 20 <i>GmHIPP</i>s was up-regulated, and three <i>GmHIPP</i>s were down-regulated under Al stress in roots. Among them, six genes (<i>GmHIPP9</i>/<i>13</i>/<i>29</i>/<i>43</i>/<i>58</i>/<i>73</i>) were highly induced by Al, with <i>GmHIPP29</i> exhibiting particularly high expression in root tips. Subcellular localization demonstrated that GmHIPP29 is a plasma membrane-localized protein. <i>GmHIPP</i>29-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 <i>GmHIPP29</i> acts as a negative regulator of Al tolerance by promoting Al accumulation in soybean roots.