A pH-stating mechanism in isolated wheat (Triticum aestivum) aleurone layers involves malic acid transport

Abstract

Acidification of the starchy endosperm by the aleurone layer following germination has been established; however, the physiological and metabolic responses of this tissue to external pH have been incompletely investigated. In this investigation, isolated wheat (Triticum aestivum) aleurone layers were incubated in different solutions at initial pH values of 3, 4 and 6 in the absence of phytohormones. After 24 h of incubation, the initial pH of all malate and succinate buffers shifted towards a value close to 4.2. These results suggest the existence of a pH-stating mechanism, instead of the simple acidification process reported previously. The rise of initial pH 3 by aleurone layers was accompanied by a high net uptake of external malic- or succinic acid. In contrast, incubation in glycyl-glycine buffer (a supposedly non-permeating cation at pH 3) partially prevented that pH rise in a pH-3 solution. The 14C-malate taken up from media at pH 3 was mostly broken down to CO2, indicating that an effective metabolic control of the intracellular malate level was operating. At pH 6, an uptake of 14C-malate and 14CO2 production occurred as well, but at slower rates than at pH 3. When buffer concentration was increased, at initial pH values of 3 or 6, a higher uptake or secretion of malic acid, respectively, was carried out by the aleurone layers. The pH of these buffers varied less than that of dilute ones, but always showed a tendency toward a pH near 4. These results suggest that a balance between secretion and uptake of malic acid, accompanied by the corresponding biosynthesis or degradation, is the basis of this pH-stating mechanism.