Shift of Phloem Unloading from Symplasmic to Apoplasmic Pathway Is Involved in Developmental Onset of Ripening in Grape Berry

It remains unclear whether the phloem unloading pathway alters to adapt to developmental transition in fleshy fruits that accumulate high level of soluble sugars. Using a combination of electron microscopy, transport of the phloem-mobile symplasmic tracer carboxyfluorescein, movement of the companion cell-expressed and the green fluorescent protein-tagged viral movement protein, and assays of the sucrose cleavage enzymes, the pathway of phloem unloading was studied in the berries of a hybrid grape (Vitis vinifera x Vitis labrusca). Structural investigations showed that the sieve element-companion cell complex is apparently symplasmically connected through plasmodesmata with surrounding parenchyma cells throughout fruit development, though a small portion of plasmodesmata are apparently blocked in the ripening stage. Both carboxyfluorescein and the green fluorescent protein-tagged viral movement protein were released from the functional phloem strands during the early and middle stages of fruit development, whereas the two symplasmic tracers were confined to the phloem strands during the late stage. This reveals a shift of phloem unloading from symplasmic to apoplasmic pathway during fruit development. The turning point of the phloem unloading pathways was further shown to be at or just before onset of ripening, an important developmental checkpoint of grape berry. In addition, the levels of both the expression and activities of cell wall acid invertase increased around the onset of ripening and reached a high level in the late stage, providing further evidence for an operation of the apoplasmic unloading pathway after onset of ripening. These data demonstrate clearly the occurrence of an adaptive shift of phloem unloading pathway to developmental transition from growing phase to ripening in grape berry.