HghlightWe provide the first evidence for a mechanism of growth inhibition by ABA during germination and seedling establishment based on inhibition of PM H+-ATPase and altered pH, K+, and anion homeostasis.

SUMMARYIntracellular pH must be kept close to neutrality to be compatible with cellular functions, but the mechanisms of pH homeostasis and the responses to intracellular acidification are mostly unknown. In the plant Arabidopsis thaliana, we found that intracellular acid stress generated by weak organic acids at normal external pH induces expression of several chaperone genes, including ROF2, which encodes a peptidyl-prolyl cis-trans isomerase of the FK506-binding protein class. Loss of function of ROF2, and especially double mutation of ROF2 and the closely related gene ROF1, results in acid sensitivity. Over-expression of ROF2 confers tolerance to intracellular acidification by increasing proton extrusion from cells. The activation of the plasma membrane proton pump (H + -ATPase) is indirect: over-expression of ROF2 activates K + uptake, causing depolarization of the plasma membrane, which activates the electrogenic H + pump. The depolarization of ROF2 over-expressing plants explains their tolerance to toxic cations such as lithium, norspermidine and hygromycin B, whose uptake is driven by the membrane potential. As ROF2 induction and intracellular acidification are common consequences of many stresses, this mechanism of pH homeostasis may be of general importance for stress tolerance.

Zostera marina L. is an angiosperm that grows in a medium in which inorganic phosphate (P(i)) and nitrate (NO(3)(-)) are present in micromolar concentrations and must be absorbed against a steep electrochemical potential gradient. The operation of a Na(+)-dependent NO(3)(-) transport was previously demonstrated in leaf cells of this plant, suggesting that other Na(+)-coupled systems could mediate the uptake of anions. To address this question, P(i) transport was studied in leaves and roots of Z. marina, as well as NO(3)(-) uptake in roots. Electrophysiological studies demonstrated that micromolar concentrations of P(i) induced depolarizations of the plasma membrane of root cells. However, this effect was not observed in leaf cells. P(i)-induced depolarizations showed Michaelis-Menten kinetics (K(m)=1.5+/-0.6 microM P(i); D(max)=7.8+/-0.8 mV), and were not observed in the absence of Na(+). However, depolarizations were restored when Na(+) was resupplied. NO(3)(-) additions also evoked depolarizations of the plasma membrane of root cells only in the presence of Na(+). Both NO(3)(-)- and P(i)-induced depolarizations were accompanied by an increase in cytoplasmic Na(+) activity, detected by Na(+)-sensitive microelectrodes. P(i) net uptake (measured in depletion experiments) was stimulated by Na(+). These results strongly suggest that P(i) uptake in roots of Z. marina is mediated by a high-affinity Na(+)-dependent transport system. Both NO(3)(-) and P(i) transport systems exploit the steep inwardly directed electrochemical potential gradient for Na(+), considering the low cytoplasmic Na(+) activity (10.7+/-3.3 mM Na(+)) and the high external Na(+) concentration (500 mM Na(+)).

NO 3؊ is present at micromolar concentrations in seawater and must be absorbed by marine plants against a steep electrochemical potential difference across the plasma membrane. We studied NO Zostera marina L. is an aquatic angiosperm that grows in a medium with a high salinity-seawater itself-with NaCl concentrations in the region of 0.5 m. Leaf cells of this plant exhibit a plasma membrane potential (E m ) of around Ϫ160 mV in natural seawater (Fernández et al., 1999). Molecular and physiological evidence indicates that the E m in this halophyte is maintained by the activity of a H ϩ -pump (Fukuhara et al., 1996; Fernández et al., 1999). With this highly negative E m , the uptake of essential anions such as NO 3 Ϫ , which usually occurs in seawater at concentrations of 1 to 500 m (Riley and Chester, 1971) and is probably below 10 m in the close environment of the plant (Hernández et al., 1993), must be energized.

Abstract. Gracilaria tenuistipitata Zhang et Xia was cultured for 15 d at low, normal and high inorganic carbon concentrations under constant light, temperature and nutrient conditons. Carbonic anhydrase (CA; EC 4.2.1.1.) activity, ribulose-l,5-bisphosphate carboxylase/ oxygenase (Rubisco; EC 4.1.1.39) content, pigment content and C/N ratio were measured, and also the photosynthesis and growth rates. Both Rubisco content and CA activity increased under conditions of low inorganic carbon (CO but decreased at high C~ with respect to the control. The amount of pigments declined considerably at high Ci and was slightly higher at low C~. The maximum rate of photosynthesis and the photosynthetic efficiency increased in low Ci and the opposite was found at high C~ concentration. The effects of Ci concentration on maximum rate of photosynthesis and photosynthetic efficiency are discussed in relation to the variation in pigment and Rubisco contents and CA activity. The data indicate that Ci may be an important factor controlling the photosynthetic physiology of G. tenuistipitata with regard, not only to the enzymes of C~ metabolism, but also to the pigment content.