Functional definition of leaf sink-to-source transition by 14C tracing reveals transcriptional reprogramming in soybean.

Immature leaves initially act as sinks that import photosynthates from mature leaves. As leaves develop, they gradually acquire source capacity-a developmental shift known as the sink-to-source transition. Although this transition is critical for increasing source number and yield potential, its molecular basis remains largely unclear. To investigate the underlying mechanisms in soybean, we combined carbon-14 (14C) tracer analysis with RNA-seq. At an early vegetative stage, the second trifoliate leaf was classified into sink, transition, or source states based on its 14C accumulation pattern. Autoradiographic analyses further indicated that regulation of photosynthate transport occurs at the nodal region, prompting parallel RNA-seq analysis of both leaf and node tissues. Transcriptome analysis revealed early induction of cytokinin negative regulators, A-type ARABIDOPSIS RESPONSE REGULATORs (ARRs), in leaf tissue, suggesting attenuation of cytokinin signaling at the onset of the sink-to-source transition. In contrast, genes involved in sugar transport, such as sugars will eventually be exported transporters and sucrose transporter family members, showed continuous upregulation across developmental stages in both leaf and node tissues. Together, these results provide a high-resolution view of transcriptional reprogramming accompanying the leaf sink-to-source transition in soybean.