The Axillary Bud’s Decision Maker: How Strigolactones Dictate Plant Shoot Branching Patterns

Abstract

: Plant morphology is a fundamental horticultural trait that plays a critical role in plant growth and development. As a core determinant of plant morphology, shoot branching significantly influences light capture efficiency, nutrient utilization capacity, and stress tolerance. The developmental process of shoot branching begins with the formation of axillary buds, a process tightly coordinated by the shoot apical meristem (SAM) and axillary meristems (AM). Axillary buds outgrowth is regulated by complex interactions among genetic factors, photoperiod, environmental cues, and phytohormones such as auxin, cytokinin (CK), and strigolactones (SLs). As a class of carotenoid-derived terpenoid lactone phytohormones, SLs have been identified as key negative regulators of axillary buds outgrowth and have remained a prominent research focus in plant developmental biology over the past decades. This review systematically summarizes the core pathways of SLs biosynthesis and signal transduction and dissects the molecular basis underlying SLs-mediated control of axillary bud development and shoot branching patterning. We highlight the evolutionary conservation of core SLs biosynthetic and signaling components across diverse plant species, alongside their lineage-specific functional divergence. We further synthesize current evidence regarding the emerging, and often debated, roles of SLs in woody perennial species. Additionally, we dissect the crosstalk regulatory networks between SLs and other phytohormones, with a focus on their synergistic and antagonistic interactions. Finally, we outline key unresolved molecular questions, including the mechanistic basis of SLs transport, the PAT-independent regulatory pathway, and the tissue-and developmental stage-specific coordination of SLs signaling. We also critically evaluate the inherent experimental limitations of the widely used synthetic SLs analog GR24. Addressing these knowledge gaps is critical to deepening the mechanistic understanding of SLs-mediated branching regulation and unlocking translational applications for crop ideotype breeding.