Switch from Apoplasmic to Symplasmic Phloem Unloading during Storage Roots Formation and Bulking of Sweetpotato

To clarify the unloading pathway of assimilates in storage roots of sweetpotato ( Ipomoea batatas L.), a combination of methods including electron microscopy, movement of the phloem–mobile symplasmic tracer carboxyfluorescein, and assays of invertase and sucrose synthase activities were explored to investigate this pathway from storage roots formation to harvest. The sieve element–companion cell complex was symplasmically connected to surrounding parenchyma cells by plasmodesmata and isolated before storage root bulking (40–140 d after planting). Numerous plasmodesmata appeared among phloem parenchyma throughout storage root bulking. Images of carboxyfluorescein movement indicated that the dye was restricted to phloem and released into surrounding tissues before and after storage root formation, respectively. Sucrose synthase activity increased continually during storage root bulking, and it was much higher than that of insoluble acid invertase. Although this remained low and changed little, that of soluble acid invertase increased and remained high. The starch content in storage roots increased during bulking, but sucrose content decreased. Thus, the predominant unloading pathway switched from apoplasmic to symplasmic during storage root formation and bulking. This switch resulted in enhanced sink potential of storage roots, evidencing opportune sink–source relationships in sweetpotato.