Hormone Action on Transmembrane Electron and H⁺ Transport

Abstract

A possible involvement of two different systems in proton translocation was investigated by simultaneous measurement of transmembrane electron flow and proton secretion in a pH-stat combined with a redoxstat. The pH gradient between cytoplasm and apoplast is probably maintained by an H+-pumping ATPase and by a second proton extrusion system, which seems to be linked to a redox chain with NAD(P)H as electron donor. Indole acetic acid inhibits both e-and H+ efflux, but only if the 'electron draw' from the outside is not too high. The electron draw depends on the hexacyanoferrate level at the plasmalemma surface and on the Ca2+ concentration. The inhibiting effect of auxin on e-and H+ efux in the presence of hexacyanoferrate can be only detected at low levels of bivalent cations and of the artificial electron acceptor. The inhibition of e-and H+ efflux by auxin requires high oxygen levels. idants like HCF III or HCI IV will very likely impair the functions of a variety of plasmalemma proteins, including channels, cotransporters, pumps, and auxin-receptors by oxidizing sulfhydryl groups of the proteins (6). In other words, a number of important transport functions may be impaired which would alter electron and/or proton movement. (c) Alcohols can be used to increase the level of intracellular NADH through the action of the cytoplasmic alcohol dehydrogenase (6, 10, 11). Alcohols, known to be substrates for alcohol dehydrogenase such as propan-1-ol, ethanol, and butan-1-ol increase net H+ efflux immediately; propan-2-ol and methanol have no effect. This stereoselectivity and the requirement of high 02 supply for alcohol action exclude an explanation as a membrane effect. Exogenous NADH was found to increase HCF III reduction of protoplasts (21) and of intact roots (29). Komor et al. (20) have demonstrated that reduction of externally applied NAD(P)H with concomitant acidificaton in the apoplast is without involvement of transmembrane steps. An independence of transplasma membrane proton gradient from NAD(P)H-HCF III oxidoredtction in maize root microsoms was described (24). We therefore avoided addition of NAD(P)H and tried to distinguish between H+-pumping ATPase and redoxchain-linked proton extrusion by the use of different 02 pressures as the ATP generating system and the plasmalemma NAD(P)H oxidase have very different Km values for 02.The aim of the present paper is to investigate the influence of hormones and other effectors on the proton extrusion and its possible linkage to transmembrane electron transfer. MATERIALS AND METHODS

Relationship between Electron Transport across the Plasmalemma and a pH Decrease in the Bulk Medium

“…02) might also lead to H+ excretion. Evidence for 02 as a native acceptor has been presented (3,4). It must still be determined, however, whether the rate of transplasmalemma e-transport under native conditions can be of sufficient magnitude to accelerate the acidification process (2).…”

Hexachloroiridate IV as an Electron Acceptor for a Plasmalemma Redox System in Maize Roots

“…It has been recently found that HCF III reduction by maize roots was dependent on 02 concentration (5). A reversible rise in electron transfer was observed if 02 partial pressure was reduced to below 2 kPa.…”

“…High calcium concentrations greatly enhance proton extrusion and transmembrane electron transfer in the presence of HCF III (1,5). It has been argued that the divalent cation screens the charge on the membrane surface and facilitates the access of the acceptor to the membrane.…”

“…Following the pioneering work of Craig and Crane (6,7), plant plasma membrane redox systems have been investigated in vivo (2)(3)(4)(5)(6)(7)(8)(9)13) and in vitro (11,12) by several groups. Two types of redox activity have been distinguished (2,13).…”

“…The investigation of the constitutive system has been restricted to the use of HCF III as reduced substrate. This appeared to us to be a serious limitation in our eyes, since it could be argued that evidence for the postulation of a general electron transfer system (4,5,8) The seedlings were grown on moist paper in the dark at 26°C. Twelve intact seedlings were selected and the seeds mounted on plastic trays to allow roots to dip into a 16 ml plastic tube containing nutrient solution containing 10 (10), HCI has a much higher oxidizing power than HCF III (En = 0.36 V).…”