No abstract

osmotica (3, 11, 14). Because cytosolic physiological and biochemical processes apparently are inhibited by high ion concentrations, mechanisms that permit compartmentation of these ions in the vacuole and regulate the concentrations of these ions in the cytosol are of substantial importance for adaptation and growth (11,14,15 for the uphill transport of ions and other solutes into the vacuole. The A/H. across the tonoplast is produced by electrogenic H+ pumps, the H+-ATPase (22), and the H+ translocating pyrophosphatase (17, 25). However, the significance of the pyrophosphatase H+ pump to the generation of the ,AH+ across the tonoplast under physiological conditions is not clear (17,25).H+-ATPases that have been categorized on the basis of similar biochemical properties as V-type enzymes act as H+ pumps in the membranes of a wide variety of acidic compartments in eukaryotes (22). The similarities of the protein sequences of these enzymes have recently been confirmed by analysis of cDNA and genomic clones encoding the major ATPase subunits (12,22). The V-type H+-ATPase is a multimeric enzyme of about 400 to 600 kD that consists of up to nine subunits (22,23). The putative catalytic 70-kD subunit forms a hydrophilic complex with a 60-kD subunit that is the principal component of ATPase activity. The 16-kD proteolipid subunits form a hydrophobic complex that functions in H+ conductance. Additional subunits are postulated to form a stalk connecting these two complexes or to be associated with the proton channel (22).It is reasonable to assume that the V-type H+-ATPase is an enzyme that has to be functional at all times during cell growth. Disruption of the genes encoding the 60-or 70-kD subunits of yeast vacuolar H+-ATPase caused conditional lethality. The mutants grew more slowly than the wild-type cells, failed to grow at neutral pH, and lacked the ability to acidify their vacuoles ( 18,28

A cDNA clone was isolated that encodes the partial sequence of a putative endoplasmic reticulum Ca2+-ATPase of tobacco. The 1.497-kb insert had an open reading frame of 1.149 kb. The deduced peptide had the greatest homology to the endoplasmic reticulum Ca2+-ATPases of Drosophila and Artemia, followed by the mammalian and avian enzymes (SERCA2 and 3). The cDNA insert hybridized to a single mRNA of 4.4 kb from tobacco cultured cells or plant tissues. The level of this transcript was induced about 2-fold by NaCI shock in 428 mm NaCI-deadapted tobacco cells that were maintained in medium without salt, but not in unadapted cells. The level of this transcript was 3-to 4-fold higher in 428 mm NaCI-adapted cells growing in salt than in unadapted cells growing without salt.

A cDNA clone encoding an isoform of the plasma membrane H+‐ATPase was isolated from Nicotiana tabacum. The steady‐state plasma membrane H+‐ATPase message levels were the same in unadapted tobacco cells and tobacco cells adapted to 428 mol m−3 NaCl. When cells adapted to 428 mol m−3 NaCl maintained in the absence of NaCl (deadapted) for an excess of 100 passages were exposed to 400 mol m−3 NaCl for 24 h, there was an increased accumulation of plasma membrane H+‐ATPase message. The NaCl responsiveness of the deadapted cells was dependent upon the growth cycle stage. Alterations in the levels of plasma membrane FT‐ATPase message during the growth cycle support a role for the H+‐ATPase in cell growth. These results document the induction by NaCl of plasma membrane FT‐ATPase message accumulation in tobacco cells, and suggest that enhanced expression of the plasma membrane FT‐ATPase has a role in the short term response of cells of NaCl, but is not necessarily involved in long‐term adaptation.