The cellular pathway and enzymatic activity for phloem-unloading transition in developing Camellia oleifera Abel. Fruit

The phloem-unloading pathway of sucrose and mechanism of sugar-to-oil transition are still unknown in Camellia oleiferaFruit. Here, transmission electronic microscopy (TEM) and confocal laser-scanning microscopy(CLSM) were used to observe the cellular structure of vascular bundlesand symplastictracer, carboxyfluorescein (CF), transport in phloemzone. The results showed that sucrose was transported via symplastsystem in the early and late phases, whereas apoplastsystem exerted the function in middle stage. Moreover, enzymatic assays showed that acid invertasehad a higher activity at the transition stage during the whole fruit development. The cell wall bound invertase(CWI) activity reached the highest at the middle stage of fruit development and the switch in phloem-unloading coincided with fruit developmental phase change and oil accumulation. Correlation analysis showed that the oil accumulation was significantly negatively correlated with content of soluble sugar at P < 0.05 level. However, the soluble acid invertase(SAI), CWI, and neutral invertaseshowed a significant positive correlation with oil accumulation at P < 0.01 level. In summary, our data provide new cytological insights into the transition of unloading transfer between symplasmic and apoplasmic patterns in C. oleifera fruit and suggest that invertasesare positively involved in sugar–oil transition process.