StHEMERA and StPIF4 synergistically regulate thermomorphogenesis in potato (Solanum tuberosum L.)

While high-temperature stress severely inhibits the growth, development, and tuberization of potatoes ( Solanum tuberosum L.), moderate high temperatures can trigger thermomorphogenesis. However, the molecular mechanisms underlying thermomorphogenesis in potato remain largely elusive. In this study, we identified a heat-responsive, nucleus-plastid shuttling transcriptional activator, StHEMERA (StHMR), whose nuclear translocation was enhanced under high-temperature conditions. Transgenic analyses revealed that the high-temperature induced increase in plant height and cell elongation in potato depends on the function of StHMR. The StHMR-silenced lines displayed a phenotype insensitive to elevated temperatures. Biochemical evidence further revealed that StHMR forms a regulatory module with PHYTOCHROME INTERACTING FACTOR 4 (StPIF4), a potato homolog of the central Arabidopsis thermosensory. StPIF4 is indispensable for high-temperature-induced stem elongation and directly binds the promoters of the auxin biosynthesis gene YUCCA8 ( StYUC8 ) and the cell-elongation gene α-EXPANSION A8 ( StEXPA8 ). StPIF4 regulates cell elongation under high-temperature conditions by binding to E-box elements within the promoters of these genes. Notably, StHMR functions as a transcriptional co-activator that enhances the ability of StPIF4 to activate the transcription of StYUC8 , especially under heat conditions. Collectively, this study characterizes the StHMR-StPIF4 module as a key molecular unit of potato thermomorphogenesis and highlights its potential relevance to tuber formation, providing promising molecular targets for breeding heat-tolerant potato varieties.