Efflux of Sucrose from Mesophyll Cells during Apoplastic Phloem Loading in Pea

Abstract: Efflux of sucrose from photosynthesizing mesophyll cells into the apoplastic space, during apoplastic phloem loading, was investigated using suspension of protoplasts and inside-out plasma membrane vesicles isolated from mature leaves of in Pisum sativum L. cv. Fenomen. Addition of exogenous sucrose to the suspension medium, affected the carbohydrate allocation but neither the photosynthesis nor respiration of the mesophyll cells. Furthermore, addition of low concentrations of sucrose and/or ABA (below 5 mmol/L and 10-7 mol/L, respectively) to the medium, caused an increase in sucrose efflux from the protoplasts. However, sucrose efflux decreased when the protoplasts were incubated in higher concentrations sucrose and/or ABA (above 15 mmol/L and 10-7 mol/L, respectively). During the sucrose efflux, there was an increase in the cytosolic pH of the mesophyll protoplasts which, however, decreased when PCMBS, an inhibitor of sucrose/H+ symport, was added to the protoplasts. On the other hand, cytosolic [K+] decreased during the sucrose efflux but increased when the release of sucrose from the mesophyll cells was inhibited by PCMBS. The uptake of sucrose into inside-out plasma membrane vesicles was dependent on sucrose content and pH, inhibited by PCMBS, and did not involve MgATPase activity. This work suggests that during the apoplastic phloem loading in pea, sucrose efflux from the photosynthesizing mesophyll cells into the apoplastic space occurs via facilitated-diffusion using a sucrose/H+ symporter on the plasma membrane. The process does not involve MgATPase activity but leads to a simultaneous decrease in cytosolic [K+]. Changes in the apoplastic environment, such as sucrose and/or ABA levels in the apoplast, have a significant effect on the sucrose efflux.