Signal and Supply: Phytohormonal networks regulating source sink dynamics and assimilate partitioning

Plants are complex living organisms that require coordinated physiological processes to maintain their growth and development. One of the fundamental relationships within plants is the source sink relationship, which plays a crucial role in the distribution and allocation of assimilates throughout the plant's structure controlled not only by physical transport processes but also strongly influenced by hormonal signaling. Plant hormones act as modulators of source sink dynamics, regulating carbon assimilation in source tissues and directing its efficient partitioning to physiologically active sink organs such as fruits, seeds and roots. Auxin and ethylene primarily enhance assimilate production by stimulating photosynthetic capacity and sucrose metabolism, whereas gibberellins and cytokinins strengthen sink demand by promoting cellular expansion, sink organ growth and transporter expression. In contrast, Abscisic acid, Strigolactones and Salicylic acid support resource allocation toward plant defense mechanisms and brassinosteroids act as important regulators of assimilate partitioning, particularly under stress conditions. Despite these established roles, the precise regulatory mechanisms by which hormones coordinate assimilate management remain insufficiently understood. This review synthesizes current knowledge on hormonal control of assimilate production in source leaves, phloem loading and transport mechanisms and sucrose unloading and utilization in sink tissues. It also evaluates plant growth substances based on their biosynthetic origins and mobility, highlighting their integration within broader assimilate partitioning frameworks. By integrating recent transcriptomic, physiological and molecular insights, it proposes a clear framework to better understand hormone mediated assimilate partitioning for improved crop productivity and stress tolerance.