Roles for Light, Energy and Oxygen in the Fate of Quiescent Axillary Buds

The hierarchy of events governing the resumption of growth of a quiescent axillary bud are poorly understood. During quiescence, a homeostasis exists in phytohormone and source/sink regulation, which represses the metabolic and mitotic progression of the bud. Environmental change and shoot development can alter the homeostasis, leading to a binary state change and the commitment to growth. Within this context, light and oxygen availability can serve both metabolic and signaling functions. However, the question of substrate versus signal has proven challenging to resolve; in the case of sugars, there are disparities in the data from apical and axillary buds in juvenile shoots, while in postdormant perennial buds, light has only a facultative role in the decision, but signaling may still be essential for bud fate. We briefly update the roles and hierarchies of light-, energy-, and oxygen-dependent functions in axillary bud outgrowth of annual shoots, before focusing discussion on the role of chloroplast-to-nucleus retrograde signaling genes such as GENOMES UNCOUPLED4 (GUN4) and ELONGATED HYPOCOTYL5 (HY5) in bud burst responses to light, examining available transcriptome data from postdormant grapevine (Vitis vinifera) buds. We discuss the evidence implicating cryptochromes (CRY) in the activation of HY5 expression in grapevine, leading to chloroplast biogenesis in the buds, and that this occurs via a biogenic rather than an adaptive developmental process. The cytokinin (CK) signaling pathways and the light-regulated expression of chloroplast processes, especially those involved in carbon and oxygen metabolism, may also play an important role in bud burst.