Regulation of H+ Extrusion and Cytoplasmic pH in Maize Root Tips Acclimated to a Low-Oxygen Environment

Abstract

We tested the hypothesis that H+ extrusion contributes to cytoplasmic pH regulation and tolerance of anoxia in maize (Zea mays) root tips. We studied root tips of whole seedlings that were acclimated to a low-oxygen environment by pretreatment in 3% (v/v) O,. Acclimated root tips characteristically regolate cytoplasmic pH near neutrality and survive prolonged anoxia, whereas nonacclimated tips undergo severe cytoplasmic acidosis and die much more quickly. We show that the plasma membrane H+-AlPase can operate under anoxia and that net H+ extrusion increases when cytoplasmic pH falls. However, at an external pH near 6.0, H+ extrusion contributes little to cytoplasmic pH regulation. At more acidic external p H values, net H+ flux into root tips increases dramatically, leading to a decrease in cytoplasmic pH and reduced tolerante of anoxia. We present evidence that, under these conditions, H+ pumps are activated to partly offset acidosis due to H+ influx and, thereby, contribute to cytoplasmic pH regulation and tolerante of anoxia. l h e regulation of H+ extrusion under anoxia is discussed with respect to the acclimation response and mechanisms of intracellular p H regulation in aerobic plant cells.Root tips of whole maize (Zea mays) seedlings subjected to HPT with 3% (v/v) O, in N, survive longer under anoxia than NHPT tips (Saglio et al., 1988;Johnson et al., 1989). HPT root tips regulate cytoplasmic pH near neutrality for many hours, in contrast to NHPT root tips, which exhibit severe cytoplasmic acidosis early in anoxia (Xia and Roberts, 1994;Xia et al., 1995). Cytoplasmic acidosis can cause early death in hypoxic maize root tips (Roberts et al., 1984), and the extent of cytoplasmic acidosis in plants under low oxygen has been widely shown to correlate with their intolerance of this stress (reviewed by Kennedy et al., 1992 andRicard et al., 1994). It is therefore important to understand the factors and mechanisms contributing to intracellular pH regulation in plant cells under anoxia.One phenomenon that may be valuable in preventing cytoplasmic acidosis during anoxic stress is lactate efflux, which has been observed to be induced in maize roots during acclimation in 3% O, (Xia and Saglio, 1992;Xia and Roberts, 1994) and also in certain Limonium sp. (Rivoal and Hanson, 1993). Fermentation to cytoplasmic lactate leads to significant cytoplasmic acidosis (reviewed by Ricard et al., 1994). Therefore, it is possible that remova1 of cytoplasmic lactate by efflux might serve to limit acidosis. Such a pos- sibility depends in part on the fate of H+ generated during fermentation of Glc to lactate; H+ generation is stoichiometric with lactate at constant ATP (Hochachka and Mommsen, 1983). It is not currently known if HPT root tips excrete lactate anion or lactic acid or if cytoplasmic H+ produced during fermentation to lactate is pumped into the extracellular space or the vacuole.Saint-Ges et al. (1991) observed a correlation between cytoplasmic pH and ATP levels after the onset of anoxia in excised NHPT maize root tips....

Occurrence and Characteristics of the Mitochondrial Permeability Transition in Plants

“…7A, trace d) confirms the protective effect of protons depicted above (Fig. 5) and can be interpreted in terms of the pH paradox (53); anoxia lowers the cytoplasmic pH of plant cells to values that are low enough (33) to efficiently inhibit MPT, whereas reaeration would tend to restore a more alkaline pH and only then trigger pore opening.…”

“…7B). It is also known that anoxia shifts the cytoplasmic pH of plant cells from about 7.5 under normoxia to as low as 6.2 under anoxia (33). When the anoxia treatment was carried out at pH 6.6 (rather than 7.4), swelling did not occur after the At the end of the light scattering measurement period (see Fig.…”

Altering hemoglobin levels changes energy status in maize cells under hypoxia

“…Maize root tips develop a tolerance to short-term anoxia if they have been acclimated by exposure to hypoxic conditions (19,20). Acclimation is accompanied by increased energy charge (21) resulting from a sustained glycolytic rate compared with nonacclimated root tips (22,23). Similarly, winter cereals show increased survival to hypoxia caused by ice encasement if they have been acclimated by exposure to hypoxic conditions (24).…”

Flooding tolerance: suites of plant traits in variable environments

“…Despite this ATP generation, the 'energy crisis' ensues since the production of ATP by glycolysis is much less than compared with oxidative phosphorylation . Low ATP availability leads to cellular damage, owing to a deterioration of cellular components, such as membranes , and/or cytoplasmic acidosis in sensitive species (Xia and Roberts 1996). Such damage can, however, be avoided (or substantially delayed) in anoxia-tolerant plants (e.g.…”