MdKNOX15, a class I knotted-like transcription factor of apple, controls flowering and plant height by regulating GA levels through promoting the MdGA2ox7 transcription

Plant height and flowering characteristics are two main considerations for apple tree breeding. Hormone metabolism and signaling pathways are important for plant development. In this study, an apple KNOX transcription factor gene, MdKNOX15, was more highly expressed in the scion of the Fuji/M9 trees, which exhibit dwarfism and early flowering, than in the self-rooted Fuji trees. Subsequent analyses indicated that MdKNOX15 can bind directly to the MdGA2ox7 promoter to up-regulate transcription. The overexpression of MdKNOX15 in Arabidopsis resulted in dwarfism and delayed flowering due to GA deficiency. Furthermore, MdBLH, a cofactor of MdKNOX15, interacted with the HOX domain, which is a C-terminal sequence in MdKNOX15 with putative DNA-binding activity. The interaction between MdBLH and MdKNOX15 appears to prevent MdKNOX15 from binding to the MdGA2ox7 promoter and up-regulating expression. This study further elucidates the biological function of the apple KNOX15 transcription factor as well as the underlying mechanism involving plant growth and flower induction. The data presented herein may provide researchers with important information applicable for the molecular breeding of new apple varieties.