REGULATION OF PLANT ELONGATION GROWTH BY SURFACE AND XYLEM PROTON PUMPS

The roles of plasmalemma electrogenic proton pumps in elongation growt h of plant stems are discussed on the basis of growth-electrophysiolog ical studies on hypocotyl segments of Vigna unguiculata. Plant stems u sually have two spatially separated electrogenic proton pumps: the sur face proton pump which is located on the surface membrane of the sympl ast and the xylem proton pump, on the cell membrane of the symplast/xy lem apoplast boundary. The surface proton pump excretes protons into t he surface cell wall layer and causes the loosening of the cell wall. The xylem proton pump excretes protons into the xylem apoplast and dri ves the uptake of solute and water into the symplast via secondary and /or tertiary active mechanisms: the proton cotransport system and the apoplast canal system. Both the surface and the xylem proton pumps are active during elongation growth because both the yielding of cell wal l loosening and the uptake of water are necessary for continued elonga tion growth.