Co-domestication of cold tolerance and female flower is determined by CsEIN2 in cucumber

Survive and successful reproduction at cold stress are critical for cucumber adaptation to environmental conditions in high-latitude regions. However, the molecular basis for the co-domestication of cold adaptation and reproductive success in cucumber remains unknown. Here, we demonstrate that CsEIN2 acts as an indispensable hub in regulating both cold tolerance and female flower percentage (FFP) in cucumber. Specifically, we discover that three completely linked natural variations at the C-terminus of CsEIN2, that is, the CsEIN2^CTC and CsEIN2^TCG haplotypes determine the differences in cold tolerance and FFP. These variations affect the interaction of CsEIN2 with the transcription factor CsEIN3, which activates the expression of target genes such as CsCBF2 and CsERF31 to regulate cold tolerance and FFP, respectively. Interestingly, the geographical distribution analyses show that the CsEIN2^CTC haplotype, conferring higher cold tolerance and FFP, underwent artificial selection for adaptation to environmental conditions in high-latitude regions. Further, we find that CsEIN2 is the only sex determination gene for selection of both cold tolerance and FFP. Collectively, our findings not only establish CsEIN2 as a 'hub' regulator that enhances cold tolerance and FFP, but also provides target gene for breeding.