Ultrastructural investigation on plasmodesmata in phloems in developing fruits of persimmon ‘Fuyu'

【Objective】Phloem unloading pathways are categorized into symplasmic and apoplasmic types.The types of phloem unloading pathways are mainly determined by the distribution of plasmodesmata in the phloem.The existence of plasmodesmata at the companion cell-vascular parenchyma cell interface may indicate symplasmic pathway of phloem unloading,and the absence of plasmodesmata signifies that the pathway of phloem unloading is apoplasmic.In order to understand the distribution of plasmodesmata in the phloem in developing young fruits of persimmon,【Method】transmission electron microscopy was applied to characterize the phloem in enlarging fruits of 'Fuyu',and calculate the plasmodesma frequency at interfaces between different cell types.【Result】Results showed that vascular bundles in young fruits did not have bundle sheath,so vascular parenchyma cells directly contacted flesh cells.The phloem consisted of sieve elements,companion cells and vascular parenchyma cells.Plasmodesmata occurrd at all interfaces including sieve element-companion cell interface,companion cell-vascular parenchyma cell interface,and vascular parenchyma cell-flesh cell interface.The plasmodesma frequency at the companion cell-vascular parenchyma cell interface was half of that at the vascular parenchyma-flesh cell interface,indicating a 1-2a type of companion cell.【Conclusion】It is concluded that there is a moderate number of plasmodesmata at companion cell-vascular parenchyma cell interface and therefore phloem unloading pathway is symplasmic type in enlarging fruits of persimmon 'Fuyu'.