Biochemical and biophysical pH clamp controlling Net H ⁺ efflux across the plasma membrane of plant cells

P-type H+ ATPases mediate active efflux from plant cells. They generate a proton motive force across the plasma membrane, providing free energy to drive transport of other solutes, partly by coupling influx. Wegner & Shabala (2020) recently suggested that passive influx can exceed pump-driven due ‘active buffering’, is, cytosolic scavenging and apoplastic generation metabolism (‘biochemical pH clamp’). Charge balance is provided K+ or anion Here, this hypothesis extended net efflux: even though pumping faster than backflow via symporters antiporters, progressive increase in transmembrane gradient avoided. Cytosolic release associated with bicarbonate formation CO2. Bicarbonate serves as substrate for PEPCase, catalyzing reaction phosphoenolpyruvate oxaloacetate, which subsequently reduced malate. Organic anions such malate citrate are released membrane (partly) protonated apoplast, thus limiting pump-induced acidification. Moreover, ‘biophysical clamp’ introduced, adjustment apoplastic/cytosolic involving fluxes while between compartments maintained. The clamps not mutually exclusive but likely coexist.