Functional Characterization and Potential Regulatory Role of MdWRKY31 in Cold Tolerance

Identifying cold-resistance genes is essential for improving the ability of apples (Malus × domestica) to tolerate low temperatures, as cold stress significantly limits their growth and productivity. The MdWRKY31 gene was cloned from apple, and its sequence characteristics, expression pattern, and biological function were systematically investigated. Bioinformatic analysis indicated that MdWRKY31 belongs to the group II WRKY transcription factors and is localized in the nucleus. Expression analysis revealed that MdWRKY31 transcript levels were markedly upregulated under low-temperature stress. To further explore its function, MdWRKY31 was heterologously overexpressed in tomato (Solanum lycopersicum). Following low-temperature treatment, transgenic tomato plants exhibited significantly reduced accumulation of reactive oxygen species, markedly enhanced activities of antioxidant enzymes (SOD, POD, and CAT), increased contents of proline and soluble protein, and a notable decrease in malondialdehyde levels. Additionally, transcript levels of SlCBF1, SlCBF2, SlCBF3, SlICE1, along with the ABA signaling-related genes SlNCED1 and SlABI5, were markedly elevated. Further molecular docking showed that the MdWRKY31 protein has strong binding affinity to the W-box elements in the promoters of SlCBF1 suggesting that it may regulate the expression of these genes through direct protein–DNA interactions. These findings indicate that MdWRKY31 improves plant cold tolerance by CBF-dependent pathways to modulate antioxidant defenses and osmotic balance. These findings identify candidate genetic resources for breeding cold-resistant apple cultivation.