Cytokinin depends on GA biosynthesis and signaling to regulate different aspects of vegetative phase change in Arabidopsis

The vegetative juvenile-to-adult transition (vegetative phase change) is a critical phase in plant development, the timing of which is controlled by the highly conserved age pathway, comprising the miR156/miR157-SPL module and the downstream miR172-AP2-like module, and is modulated by exogenous and endogenous cues. The phytohormones cytokinin (CK) and gibberellin (GA) have been described to both alter miR172 levels, most probably by regulating SPL activity. In this study, we establish an epistatic relation between CK and GA, in which CK action depends on GA, contrasting with the antagonistic nature described previously for CK-GA crosstalk. We show that CK positively affects GA biosynthesis during Arabidopsis vegetative development and depends on the GA biosynthetic enzymes GA3ox1 and GA3ox2 to modify the appearance of abaxial trichomes as well as leaf shape, both hallmarks of vegetative phase change. Downstream of CK, epidermal identity is regulated in dependence of SPL transcription factors, the GA signaling repressors GAI and RGA and the miR172-targets TOE1 and TOE2. Notably, genetic analysis revealed that GA regulates this process also CK-independently. Furthermore, our data from genetic analyses suggests that CK affects leaf shape through other GA signaling components and AP2-like transcription factors rather than SPLs. Hence, CK differentially regulates several aspects of vegetative phase change. The work contributes to the understanding of vegetative phase change regulation as well as phytohormone crosstalk in general.