Signaling network regulating plant branching: Recent advances and new challenges

Auxin alone or supplemented with cytokinins and strigolactones were long considered as the main player(s) in the control of apical dominance (AD) and correlative inhibition of the lateral bud outgrowth, the processes that shape the plant phenotype. However, past decade data indicate a more sophisticated pathways of AD regulation, with the involvement of mobile carbohydrates which perform both signal and trophic functions. Here we provide a critical comprehensive overview of the current status of the AD problem. This includes insight into intimate mechanisms regulating directed auxin transport in axillary buds with participation of phytohormones and sugars. Also roles of auxin, cytokinin and sugars in the dormancy or sustained growth of the lateral meristems were assigned. This review not only provides the latest data on implicated phytohormone crosstalk and its relationship with the signaling of sugars and abscisic acid, new AD players, but also focuses on the emerging biochemical mechanisms, at first positive feedback loops involving both sugars and hormones, that ensure the sustained bud growth. Data show that sugars act in concert with cytokinins but antagonistically to strigolactone signaling. A complex bud growth regulating network is demonstrated and unresolved issues regarding the hormone–carbohydrate regulation of AD are highlighted.