Sugars Integrate External and Internal Signals in Regulating Shoot Branching

Plant phenotypes exhibit high plasticity, with shoot branching as a prime example and a key factor influencing yield in many species. The availability of photosynthates is a critical determinant of shoot branching (or tillering in monocots). Carbohydrates, primarily in the form of sucrose, are synthesised in actively photosynthetic leaves (sources) and transported to non-photosynthetic tissues (sinks), such as tiller buds. Glucose, fructose, sucrose and their intermediates, including trehalose-6-phosphate (Tre6P), function both as energy sources and signalling molecules. Once sucrose is transported from source to sink tissues, it is rapidly hydrolysed into hexoses, which support starch accumulation, and the formation and elongation of tiller buds (outgrowth into a branch or tiller). This review aims to summarise recent discoveries with the focus on (i) sugar synthesis, metabolism, loading and unloading; (ii) sugars as crucial signals in regulating branching; (iii) roles of sugars in mediating the environment-modulated branching; (iv) the interactions between sugars and phytohormonal pathways that influence bud outgrowth and branching. A comprehensive understanding of sugar synthesis, transport, metabolism and signalling in relation to shoot branching will aid in optimising plant architecture and ultimately contribute to enhanced crop yield.