Effects of Extracellular pH on UV-Induced K⁺ Efflux from Cultured Rose Cells

Abstract

Ultraviolet (UV) light causes a specific leakage of K+ from cultured rose cells (Rosa damascena). During K efflux, there is also an increase in extracellular HC03-and acidification of the cell interior. We hypothesized that the HC03-originated from intracellular hydration of respiratory CO2 and served as a charge balancing mechanism during K+ efflux, the K+ and HC03-being cotransported out of the cell through specific channels. An alternative hypothesis which would yield similar results would be the countertransport of K+ and H+. To test these hypotheses, we studied the effect of a range of extemal pH values (pH 5-9), regulated by various methods (pH-stat, 100 millimolar Tris-Mes buffer, or CO2 partial pressure), on the UV-induced K efflux. Both UV-C (<290 nanometers) and UV-B (290-310 nanometers) induced K+ efflux with a minimum at about pH 6 to 7, and greater efflux at pH values of 5, 8, and 9. Since pH values of 8 and 9 increased instead of reduced the efflux of K+, these data are not consistent with the notion that the efflux of K+ is dependent on an influx of H+, a process that would be sensitive to extemal H+ concentration. We suggest that the effect of pH on K+ efflux may be mediated through the titration of specific K -transporting proteins or channels in the plasma membrane. Since we could not detect the presence of carbonic anhydrase activity in cell extracts, we could not use the location of this enzyme to aid in our interpretation regarding the site of hydration of CO2. Solar UV radiation can stimulate a leakage of ions from plant cells. Ion leakage as a stress response has also been shown to be caused by other factors such as herbicides (14), chilling (7), ozone (5), heavy metals (8), and certain plant pathogens (2). However, while most of these stress factors cause a generalized leakage of a variety of solutes, UV at low fluences causes a specific K+ leakage from cultured rose cells (12). Treatment of suspension-cultured tobacco cells with Pseudomonas syringae, a bacterial plant pathogen, can also cause a specific leakage of K+ (2).The mechanism by which UV induces K+ leakage is not known. At present, it is thought to involve active metabolism, since the leakage is inhibited by anaerobiosis (10), as well as by chemical inhibitors of respiration, protein synthesis, and other biochemical processes (9,12 cellular HC03-as well as an increase in pH (about 0.2-0.3 pH units). There were no comparable net fluxes of other ions. They proposed a mechanism by which the electrical charge of the K+ efflux was balanced by a concomitant efflux of HCO3-, the HCO3-presumably originating from intracellular hydration of respiratory CO2 (Fig. lA). An alternative mechanism involves a counterflux of H+ from the outside to the inside of the cell (Fig. 1 B): in respiring cells, the H+ on the outside would be supplied to a great extent by the hydration of CO2 and dissociation of H2CO3 outside the cell. The second mechanism would be consistent with the conclusions of Atkinson et al. (2) regarding the K+/H+ counterfl...

Involvement of Plasma Membrane Calcium Influx in Bacterial Induction of the K+/H+ and Hypersensitive Responses in Tobacco

“…The K+/H+ response was initially described as a K+ efflux/H+ influx exchange (3), but H+ influx is unproven. Inhibition of this response and HR-associated cell death by increased extracellular pH and ATPase inhibitors (1,27) is consistent with an exchange but does not necessarily exclude K+/HCO3-cotransport as suggested by Murphy and coworkers (14,23) (4). Tn5 resides in a 31 kb gene cluster that has been partially characterized (13 twice by resuspension and subsequent centrifugation in washing medium.…”

Effect of UV-C radiation and hot water on the calcium content and postharvest quality of apples

“…Increase in electrolyte leakage, after irradiation, has been reported in cauliflower (Brassica oleracea var. botrytis) florets (Voisine et al, 1991), tomatoes (El Assi et al, 1997), rose cells (Murphy and Wilson, 1982;Murphy, 1988;Huerta and Murphy, 1989) and tobacco (Nicotiana tabacum) cells (Lawrence et al, 1978). The higher Ca content of irradiated fruit could be due to increased electrolyte leakage after irradiation.…”

Kinetics of efflux of K+ from cultured rose cells treated with ultraviolet light

“…Hypothetically, depolarization could be explained by the opening of an anion-efflux pathway, the opening of a cation-influx pathway, or the inhibition of an electrogenic pump. Evidence suggesting that UV induces an efflux of bicarbonate has been presented by Huerta and Murphy (1989).…”