Determination of the phloem transport pathways and destination of photosynthates in soybean using autoradiography and fluorescent tracer imaging

Vascular tissues transport water and nutrients in plants, with the phloem distributing photosynthates from source to sink. The direction of phloem transport is determined by the positional relationship between sources and sinks and by vascular connections. Although aspects of phloem transport have been studied, a comprehensive understanding remains lacking. Here, we used soybean as a model system to investigate the translocation pathways and destinations of photosynthates using autoradiography with ¹⁴C-labeled sucrose and fluorescent imaging with carboxyfluorescein (CF), a known phloem tracer. Soybean exhibits simple phyllotaxy, with alternate trifoliate leaves arranged oppositely along the stem. Applying ¹⁴C-sucrose to mature leaves revealed that young developing leaves received photosynthates from source leaves on both sides of the stem. To visualize pathways, ¹⁴C-sucrose and carboxyfluorescein diacetate (CFDA) were applied to sequential source leaves. Signals from ¹⁴C and CF in the stem's vascular bundles showed no overlap, indicating distinct transport pathways. Additionally, when ¹⁴C-sucrose was applied separately to the left and right halves of a single mature leaf, it was followed corresponding sides to the sink leaves. These findings demonstrate that photosynthates are delivered to sink tissues via multiple, well-compartmentalized phloem pathways, providing new insight into the spatial organization of phloem transport.