The Predominance of the Apoplasmic Phloem-unloading Pathway is Interrupted by a Symplasmic Pathway During Chinese Jujube Fruit Development

Phloem unloading plays a pivotal part in photoassimilate transport and partitioning into sink organs. However, it remains unclear whether the unloading pathway alters to adapt to developmental transitions in sinks, especially in fleshy fruits accumulating a high level of soluble sugars. Using a combination of electron microscopy, transport of the phloem-mobile symplasmic tracer carboxyfluorescein and assays of acid invertase, the pathway of unloading was investigated in different varieties of Chinese jujube fruit (Zizyphus jujuba Mill. cv Dongzao and Lizao). Structural investigation showed that the sieve element-companion-cell complex of bundles feeding the fruit flesh is symplasmically connected with surrounding parenchyma cells at the middle stage, and isolated during the early and late stages. Numerous plasmodesmata are present between phloem parenchyma cells and flesh cells throughout fruit development. Confocal laser scanning images of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands during the early and late stages of fruit development, whereas it was released from the functional phloem at the middle stage. The levels of both the expression and activities of cell wall acid invertase and soluble acid invertase varied in an inverse pattern relative to fruit development. These results provided clear evidence for the predominance of an apoplasmic phloem unloading pathway interrupted with a symplasmic pathway and simultaneous symplasmic and apoplasmic unloading pathways in post-phloem transport during fruit development. Similar unloading pathways were obtained in different varieties of jujube fruit. The mechanisms and significance of the adaptive switch in the phloem-unloading pathway during fruit development were discussed.