MYB4 and HY5 integrate light and cytokinin signaling pathways during Arabidopsis seedling development

Arabidopsis seedling growth is regulated by integration of various environmental and hormonal signals. While the interactions between light and cytokinin signaling pathways have been studied, the molecular mechanism by which their interaction regulate hypocotyl elongation remain unclear. In this study, we demonstrate that MYB4, a positive regulator of photomorphogenesis, physically interacts with HY5 and attenuates HY5-mediated regulation of MYB4 . The expression of MYB4 is induced by different wavelengths of light and it genetically interacts with HY5 to regulate hypocotyl length during light-mediated seedling development. The MYB4 and HY5 mediated regulation of hypocotyl length is altered upon cytokinin treatment in a light intensity-dependent manner. Furthermore, on contrary to the light signals, cytokinin suppresses MYB4 expression. MYB4 together with HY5 regulates the expression of the key genes such as ABCG14 , ARR4 , and CHS involved in cytokinin signaling pathway. Taken together, this study highlights how the HY5-MYB4 module integrates light and cytokinin signals to fine-tune Arabidopsis seedling development.