Positive Regulation of Soybean Seed Vigor and Quality by the Zinc Finger Transcription Factor GmPHD3 Under High-Temperature and High-Humidity Stress

Field high-temperature and high-humidity (HTH) stress causes soybean seed deterioration, including shrinkage, moldiness, browning, and reduced germination rate, thereby decreasing seed vigor, emergence performance, and commercial value. Clarifying the molecular mechanisms of seed vigor formation under HTH stress is critical for identifying key genes and improving spring soybean seed quality. Plant homeodomain (PHD) proteins are conserved zinc-finger transcription factors involved in chromatin remodeling and stress responses. GmPHD3, a soybean PHD family member, has only been studied via heterologous expression in Arabidopsis; its authentic function in soybean remains unclear. In this study, we functionally characterized GmPHD3 in soybean. GmPHD3 is a nuclear-localized transcription factor containing Alfin and PHD domains, and is evolutionarily conserved across legumes and other plant species. Expression analysis showed that GmPHD3 was strongly induced by HTH stress, with rapid induction at 6 h in the tolerant cultivar Xiangdou No. 3 but delayed induction at 48 h in the sensitive Ningzhen No. 1. The GmPHD3 promoter harbors abscisic acid (ABA) and stress-responsive elements, and GUS assays confirmed its response to ABA and HTH. Overexpression of GmPHD3 in soybean significantly enhanced seed vigor under artificial accelerated aging by increasing the germination speed and activities of SOD, POD, while reducing TBARS content. Meanwhile, GmPHD3 negatively regulated seed longitudinal elongation, leading to a short and round seed shape without changing single-seed weight. It also significantly decreased the proportion of palmitic acid, a major saturated fatty acid. These results demonstrate that GmPHD3 positively regulates soybean seed vigor and stress resistance under HTH stress by modulating the antioxidant system and modifying seed morphology and fatty acid composition, providing a valuable target for soybean molecular breeding.