Melatonin enhances salt tolerance by inducing <i>StMYB55</i> to improve flavonoid accumulation in potato

Salinity poses a major global threat to agriculture, severely damaging crop production and productivity. Potato (Solanum tuberosum L.) is an important food crop for ensuring food security and, as a moderately salt-sensitive crop, is extremely stressed by salinity. Melatonin (MT) is involved in various abiotic stress responses in plants. However, the detailed regulatory mechanisms by which melatonin contributes to salt stress tolerance in potatoes remain unclear. Here, we determined that an appropriate MT concentration of 10 µM significantly alleviates salt stress damage in potatoes. Transcription-metabolite association analysis identified the core transcription factor genes StMYB55 and StWRKY28, which exhibit both salt- and MT-responsive expression. Overexpressing StMYB55 enhanced salt tolerance in transgenic potato plants by increasing flavonoid accumulation. Importantly, StMYB55 promoted StF3H, StFLS, and StUGT78D2 transcription, while StWRKY28 directly enhanced StCHI expression. Additionally, StMYB55 interacted with StWRKY28 to promote StWRKY28-regulated StCHI expression and flavonoid biosynthesis in potato. Our findings reveal that the MT-mediated StMYB55-StWRKY28 transcriptional complex module regulates flavonoid accumulation, thereby improving salt stress tolerance in potatoes. These results shed light on the molecular mechanism of potato response to salinity stress, providing the basis for the genetic breeding of a melatonin-enriched salt-resistant potato variety.