Effects of ABA on the Distribution of Sucrose and Protons Across the Plasmalemma of Pea Mesophyll Protoplasts. — Suggesting a Sucrose/Proton Symport

Abstract

Summary The effects of abscisic acid (10 −9 −10 −6 mol/L) were studied on light induced CO 2 -dependent O 2 evolution, sucrose synthesis, efflux of sucrose and changes in cytosolic proton concentration in mesophyll protoplasts of Pisum sativum L. cv. Fenomen. Photosynthesis did not change with increased apoplastic ABA or sucrose concentrations although sucrose synthesis decreased in the presence of ABA. In the absence of apoplastic sucrose the sucrose effiux increased and was most pronounced at 10 −7 mol/L ABA. The sucrose efflux also increased in the presence of ABA and sucrose together and was most pronounced at 10 −9 mol/L ABA and 20 mmol/L apoplastic sucrose. The cytosolic proton concentration decreased upon addition of ABA with or without sucrose, but increased upon addition of p -chloromercuribenzenesulfonic acid, a sucrose transport inhibitor. The cytosolic proton concentration decreased in the presence of 10 -9 mol/L ABA in combination with various apoplastic sucrose concentrations. In the presence of 10 -7 -10 -6 mol/L ABA in combination with apoplastic sucrose levels higher than 1 mmol/L, the cyrosolic concentration of proton increases. The results indicate that there is a relationship between the external environment and the sucrose effiux and cytosolic proton concentration in pea mesophyll protoplasts and that there is a sucrose/proton symport at the plasmalemma of pea mesophyll cells. They also show that sucrose increases the sensitivity to ABA in its action on sucrose effiux and cytosolic proton concentration.