Evidence of the predominance of passive symplastic phloem loading and sugar transport with leaf ageing in Camellia oleifera

Phloem loading and transport of sugar from leaves to sink tissues such as fruits are crucial for yield formation. Camellia oleifera is an evergreen horticultural crop with high value; however, its low production limits the development of the C. oleifera industry. In this study, using a combination of ultrastructural observation, fluorescence loss in photobleaching (FLIP) and inhibitor treatment, we revealed that C. oleifera leaves mainly adopt a symplastic loading route from mesophyll cells to the surrounding vascular bundle cells in minor veins. HPLC assays showed that sucrose is the main sugar transported and only a small amount of raffinose or stachyose was detected in petioles, supporting a passive symplastic loading route in C. oleifera leaves. Compared to leaves grown this year (LT), the carbohydrate synthesis capacity in leaves grown last year (LL) was decreased while LL retained more soluble sugar, suggesting a decrease in transport capacity with leaf ageing. TEM and tissue staining showed that a reduction in plasmodesmata density leads to a decline in the degree of cellular coupling and is responsible for the weakening transport capacity in older leaves. RNA-seq revealed several differentially expressed genes (DEGs) including CoPDCB1-1, CoSUT1 and CoSWEET12, which are likely involved in the regulation of phloem loading and sugar transport. An expression correlation network is constructed between PD-callose binding protein genes, sugar transporter genes and senescence-associated genes. Collectively, this study provides the evidence of the passive symplastic phloem loading pathway in C. oleifera leaves and constructs the correlation between sugar transport and leaf ageing.