MeSUT1a is essential for sucrose transport and apoplastic phloem loading in cassava ( <i>Manihot esculenta</i> Crantz)

Cassava (Manihot esculenta Crantz) exhibits high photosynthetic efficiency and remarkable starch accumulation in its storage roots. The effective loading of photosynthates into the phloem from mesophyll cells in leaves is a critical determinant of yield; however, this process remains poorly understood. In this study, we propose a theoretical model of apoplastic sucrose phloem loading in cassava based on a multitechnique approach. The concentration of primary photoassimilates in leaf veins, analyzed using a [14C]CO2 tracer, and the existence of few plasmodesmata between bundle sheath cells/phloem parenchyma cells and sieve element-companion cell (SE-CC) complexes in minor veins suggest characteristic apoplastic phloem loading in cassava. We identified five sucrose transporters (MeSUTs) in the cassava genome, among which MeSUT1a exhibited the highest expression and the strongest sucrose intake activity. Subcellular localization analyses showed that MeSUT1a specifically localizes in the plasma membrane of the SE-CC complexes and that sugar transporter (MeSWEET2a) localizes on parenchyma cells, suggesting potential functional synergy. Interference with MeSUT1a expression led to a reduction in sucrose loading efficiency by more than 50%, resulting in abnormal sucrose and transient starch accumulation. This interference subsequently impaired chloroplast development and leaf photosynthesis, ultimately reducing storage root yield and starch content. RNA-seq analysis of MeSUT1a transgenic lines further revealed remarkable transcriptional changes in genes associated with sugar transport, carbohydrate metabolism, and photosynthesis. These results establish that MeSUT1a is essential for driving sucrose phloem loading and plays a key role in the distribution of photosynthetic assimilates and the coordination of source-sink dynamics in cassava.