LsMAPK6 Phosphorylates the LsCO Protein to Enhance Its Stability and Transcriptional Activity, Promoting Floral Transition Upon High Temperatures in Lettuce.

High temperatures significantly accelerate the timing of floral transition, namely, bolting and flowering, in lettuce, which results in severe loss of marketable yield. Thus, understanding the genetic regulation of floral transition is of great interest to plant biologists and lettuce breeders. Here, we show that mitogen-activated protein kinase (LsMAPK6), whose expression and phosphorylation are stimulated by elevated temperatures, plays a positive role in the floral transition. The lsmapk6 mutants exhibit delayed bolting and flowering, whereas LsMAPK6 overexpression accelerates lettuce bolting. LsMAPK6 physically interacts with LsCO. Knockout of LsCO, to a large extent, phenocopies lsmapk6 mutants, strongly indicating that they function in the same genetic pathway. Mechanistically, LsMAPK6 phosphorylates LsCO at residue serine-258, resulting in the enhanced transcriptional activity and protein stability of LsCO and consequently the activation of LsFT-mediated flowering signalling pathways, which was confirmed by phospho-inactive and mimic analyses. Collectively, these findings reveal that the LsMAPK6-LsCO signalling module fine-tunes the timing of floral transition upon high temperatures, demonstrating that LsMAPK6 is a potential target for breeding lettuce cultivars adapted to global warming.