Ion Homeostasis in NaCl Stress Environments

AtHKT1 is a salt tolerance determinant that controls Na(+) entry into plant roots

Two Arabidopsis thaliana extragenic mutations that suppress NaCl hypersensitivity of the sos3-1 mutant were identified in a screen of a T-DNA insertion population in the genetic background of Col-0 gl1 sos3-1. Analysis of the genome sequence in the region flanking the T-DNA left border indicated that sos3-1 hkt1-1 and sos3-1 hkt1-2 plants have allelic mutations in AtHKT1. AtHKT1 mRNA is more abundant in roots than shoots of wild-type plants but is not detected in plants of either mutant, indicating that this gene is inactivated by the mutations. hkt1-1 and hkt1-2 mutations can suppress to an equivalent extent the Na(+) sensitivity of sos3-1 seedlings and reduce the intracellular accumulation of this cytotoxic ion. Moreover, sos3-1 hkt1-1 and sos3-1 hkt1-2 seedlings are able to maintain [K(+)](int) in medium supplemented with NaCl and exhibit a substantially higher intracellular ratio of K(+)/Na(+) than the sos3-1 mutant. Furthermore, the hkt1 mutations abrogate the growth inhibition of the sos3-1 mutant that is caused by K(+) deficiency on culture medium with low Ca(2+) (0.15 mM) and <200 microM K(+). Interestingly, the capacity of hkt1 mutations to suppress the Na(+) hypersensitivity of the sos3-1 mutant is reduced substantially when seedlings are grown in medium with low Ca(2+) (0.15 mM). These results indicate that AtHKT1 is a salt tolerance determinant that controls Na(+) entry and high affinity K(+) uptake. The hkt1 mutations have revealed the existence of another Na(+) influx system(s) whose activity is reduced by high [Ca(2+)](ext).

Evidence for a role in growth and salt resistance of a plasma membrane H+-ATPase in the root endodermis

The plasma membrane of plant cells is energized by an electrochemical gradient produced by P-type H+-ATPases (proton pumps). These pumps are encoded by at least 12 genes in Arabidopsis. Here we provide evidence that isoform AHA4 contributes to solute transport through the root endodermis. AHA4 is expressed most strongly in the root endodermis and flowers, as suggested by promoter-GUS reporter assays. A disruption of this pump (aha4-1) was identified as a T-DNA insertion in the middle of the gene (after VFP(574)). Truncated aha4-1 transcripts accumulate to approximately 50% of the level observed for AHA4 mRNA in wild-type plants. Plants homozygous for aha4-1 (-/-) show a subtle reduction in root and shoot growth compared with wild-type plants when grown under normal conditions. However, a mutant phenotype is very clear in plants grown under salt stress (e.g., 75 or 110 mM NaCl). In leaves of mutant plants subjected to Na stress, the ratio of Na to K increased 4-5-fold. Interestingly, the aha4-1 mutation appears to be semidominant and was only partially complemented by the introduction of additional wild-type copies of AHA4. These results are consistent with the hypothesis that aha4-1 may produce a dominant negative protein or RNA that partially disrupts the activity of other pumps or functions in the root endodermal tissue, thereby compromising the function of this cell layer in controlling ion homeostasis and nutrient transport.

Genes that are uniquely stress regulated in salt overly sensitive (sos) mutants

Repetitive rounds of differential subtraction screening, followed by nucleotide sequence determination and northern-blot analysis, identified 84 salt-regulated (160 mM NaCl for 4 h) genes in Arabidopsis wild-type (Col-0 gl1) seedlings. Probes corresponding to these 84 genes and ACP1, RD22BP1, MYB2, STZ, and PAL were included in an analysis of salt responsive gene expression profiles in gl1 and the salt-hypersensitive mutant sos3. Six of 89 genes were expressed differentially in wild-type and sos3 seedlings; steady-state mRNA abundance of five genes (AD06C08/unknown, AD05E05/vegetative storage protein 2 [VSP2], AD05B11/S-adenosyl-L-Met:salicylic acid carboxyl methyltransferase [SAMT], AD03D05/cold regulated 6.6/inducible2 [COR6.6/KIN2], and salt tolerance zinc finger [STZ]) was induced and the abundance of one gene (AD05C10/circadian rhythm-RNA binding1 [CCR1]) was reduced in wild-type plants after salt treatment. The expression of CCR1, SAMT, COR6.6/KIN2, and STZ was higher in sos3 than in wild type, and VSP2 and AD06C08/unknown was lower in the mutant. Salt-induced expression of VSP2 in sos1 was similar to wild type, and AD06C08/unknown, CCR1, SAMT, COR6.6/KIN2, and STZ were similar to sos3. VSP2 is regulated presumably by SOS2/3 independent of SOS1, whereas the expression of the others is SOS1 dependent. AD06C08/unknown and VSP2 are postulated to be effectors of salt tolerance whereas CCR1, SAMT, COR6.6/KIN2, and STZ are determinants that must be negatively regulated during salt adaptation. The pivotal function of the SOS signal pathway to mediate ion homeostasis and salt tolerance implicates AD06C08/unknown, VSP2, SAMT, 6.6/KIN2, STZ, and CCR1 as determinates that are involved in salt adaptation.

Plasma membrane H-ATPase activity is involved in adaptation of tomato calli to NaCl

A tomato (Lycopersicon esculentum Mill. cv. Pera) callus culture tolerant to NaCl was obtained by successive subcultures of NaCl-sensitive calli in medium supplemented with 50 mM NaCl. NaCl-tolerant calli grew better than NaCl-sensitive calli in media supplemented with 50 and 100 mM NaCl. Analysis of callus ion content showed a strong increase in Na+ and Cl- both in NaCl-tolerant and -sensitive calli grown in media containing NaCl for one subculture. Cells from NaCl-tolerant calli showed a higher H+ extrusion activity than those from NaCl-sensitive calli grown for one subculture in the presence of NaCl. The inhibition of H+ extrusion by NaCl-sensitive cells was correlated with an inhibition of microsomal vanadate-sensitive H+-ATPase (EC 3.6.1.35) and ATP-dependent H+ transport, while the stimulation of H+ extrusion by cells tolerant to 50 mM NaCl was correlated with an increase in plasma membrane ATP-dependent H+ transport. The increase of ATP-dependent H+ extrusion in plasma membranes isolated from 50 mM NaCl-tolerant calli was not a result of stimulation of a vanadate-sensitive ATP hydrolytic activity or an increase in passive permeability to H+. Relative to NaCl-sensitive calli, plasma membrane H+-ATPase from calli tolerant to 50 mM NaCl showed a lower Km for Mg2+-ATP. Our results indicate that tolerance of tomato calli to 50 mM NaCl increases the affinity of plasma membrane H+-ATPase for the substrate ATP and stimulates the H+-pumping activity of this enzyme without modifying its phosphohydrolytic activity.

An electron probe X-ray microanalysis study during organogenesis from internode-derived nodules of Humulus lupulus var. Nugget

Elemental changes during the induction of organogenesis from internode-derived nodules of Humulus lupulus var. Nugget were studied by electron probe X-ray microanalysis (EPMA) of specimens submitted to physical fixation procedures. X-ray spectra were collected from cambial and cortical cells. Four days after explants inoculation an increase of K and Ca was detected in cells of both regions. Four to twelve days after explants inoculation an increase of Cu, Zn, Fe, S and Mn was therein detected. Values of Cu, Zn, Fe, K and S were lower in control explants than in induced explants 12 days after induction. Although S presented fluctuations it increased throughout the induction period. X-ray spectra collected from organogenic nodules revealed higher levels of Ca, K, Fe, P and S on peripheral regions where regeneration was occurring. Ca was mobilized in several directions, from inner regions of nodules towards their periphery at the onset of plantlet regeneration. Levels of Mg and Na were low or absent. Control explants neither formed nodules nor regenerated plantlets. The results suggest that EPMA can be used to study relative elemental changes during plant morphogenesis induction and enables the early establishment of organogenic regions in nodules.

Identification of two loci in tomato reveals distinct mechanisms for salt tolerance

Salt stress is one of the most serious environmental factors limiting the productivity of crop plants. To understand the molecular basis for salt responses, we used mutagenesis to identify plant genes required for salt tolerance in tomato. As a result, three tomato salt-hypersensitive (tss) mutants were isolated. These mutants defined two loci and were caused by single recessive nuclear mutations. The tss1 mutant is specifically hypersensitive to growth inhibition by Na(+) or Li(+) and is not hypersensitive to general osmotic stress. The tss2 mutant is hypersensitive to growth inhibition by Na(+) or Li(+) but, in contrast to tss1, is also hypersensitive to general osmotic stress. The TSS1 locus is necessary for K(+) nutrition because tss1 mutants are unable to grow on a culture medium containing low concentrations of K(+). Increased Ca(2)+ in the culture medium suppresses the growth defect of tss1 on low K(+). Measurements of membrane potential in apical root cells were made with an intracellular microelectrode to assess the permeability of the membrane to K(+) and Na(+). K(+)-dependent membrane potential measurements indicate impaired K(+) uptake in tss1 but not tss2, whereas no differences in Na(+) uptake were found. The TSS2 locus may be a negative regulator of abscisic acid signaling, because tss2 is hypersensitive to growth inhibition by abscisic acid. Our results demonstrate that the TSS1 locus is essential for K(+) nutrition and NaCl tolerance in tomato. Significantly, the isolation of the tss2 mutant demonstrates that abscisic acid signaling is also important for salt and osmotic tolerance in glycophytic plants.

Salinity-induced inhibition of leaf elongation in maize is not mediated by changes in cell wall acidification capacity

The physiological mechanisms underlying leaf growth inhibition under salt stress are not fully understood. Apoplastic pH is considered to play an important role in cell wall loosening and tissue growth and was demonstrated to be altered by several growth-limiting environmental conditions. In this study we have evaluated the possibility that inhibition of maize (Zea mays) leaf elongation by salinity is mediated by changes in growing cell wall acidification capacity. The kinetics of extended apoplast pH changes by leaf tissue of known expansion rates and extent of growth reduction under stress was investigated (in vivo) and was found similar for non-stressed and salt-stressed tissues at all examined apoplast salinity levels (0.1, 5, 10, or 25 mM NaCl). A similar rate of spontaneous acidification for the salt and control treatments was demonstrated also in in situ experiments. Unlike growing cells that acidified the external medium, mature nongrowing cells caused medium alkalinization. The kinetics of pH changes by mature tissue was also unchanged by salt stress. Fusicoccin, an enhancer of plasmalemma H(+)-ATPase activity level, greatly stimulated elongation growth and acidification rate to a similar extent in the control and salt treatments. That the ability of the growing tissue to acidify the apoplast did not change under same salt stress conditions that induced inhibition of tissue elongation rate suggests that salinity does not inhibit cell growth by impairing the acidification process or reducing the inherent capacity for cell wall acidification.

Tobacco and Arabidiopsis SLT1 mediate salt tolerance of yeast

A tobacco cDNA (NtSLT1, for Nicotiana tabacum sodium- and lithium-tolerant) was isolated by functional complementation of the salt-sensitive phenotype of a calcineurin (CaN)-deficient yeast mutant (cnb delta, regulatory subunit null). CaN is a Ca2+/calmodulin-dependent type 2B protein phosphatase that regulates Na+ homeostasis in yeast. This phosphatase modulates plasma membrane K+/Na+ selectivity through the activation of high-affinity K+ transport, and increaseses extracellular Na+ efflux by activation and transcriptional induction of the Na+/Li+ translocating P-type ATPase encoded by ENA1. Expression of N-terminally truncated NtSLT1 (Met-304), but not full-length protein, suppressed salt sensitivity of cnb1. Truncated NtSLT1 also increased salt tolerance of wild-type yeast, indicating functional sufficiency. NtSLT1 encodes a protein of yet unknown function but experimentation in yeast confirms it as a salt tolerance determinant. The Arabidopsis thaliana orthologue, AtSLT1, also suppressed salt sensitivity of cnb delta but only when expressed without the N-terminus (Met-301), suggesting that this region of the proteins from these evolutionarily diverse plant species contains an autoinhibitory domain. NtSLT1 enhanced transcription of the CaN-dependent ENA1 gene promoter and compensated the salt sensitivity of a mutant deficient in TCN1--a transcription factor that is activated by CaN and then induces ENA1 expression. NtSLT1 partially suppressed the salt sensitivity of ena1-4 indicating that NtSLT1 has both ENA-dependent and independent functions. NtSLT1 suppressed spk1 hal4 (SPK1/HAL4 which encodes a serine-threonine kinase that regulates TRK1-2 transporters to have high K+/Na+ selectivity) but not ena1-4 trk1-2 implicating the ENA-independent function to be through TRK1-2. Together, these results implicate SLT1 as a signal regulatory molecule that mediates salt tolerance by modulating Na+ homeostasis.

The apoplast and its significance for plant mineral nutrition

It has only recently become apparent that the apoplast plays a major role in a diverse range of processes, including intercellular signalling, plant-microbe interactions and both water and nutrient transport. Broadly defined, the apoplast constitutes all compartments beyond the plasmalemma - the interfibrillar and intermicellar space of the cell walls, and the xylem, including its gas- and water-filled intercellular space - extending to the rhizoplane and cuticle of the outer plant surface. The physico-chemical properties of cell walls influence plant mineral nutrition, as nutrients do not simply pass through the apoplast to the plasmalemma but can also be adsorbed or fixed to cell-wall components. Here, current progress in understanding the significance of the apoplast in plant mineral nutrition is reviewed. The contribution of the root apoplast to short-distance transport and nutrient uptakes is examined particularly in relation to Na⁺ toxicity and Al³⁺ tolerance. The review extends to long-distance transport and the role of the apoplast as a habitat for microorganisms. In the leaf, the apoplast might have benefits over the vacuole as a site for short-term nutrient storage and solute exchange with the atmosphere. Contents Summary 167 I. Introduction 168 II. The properties of the apoplast and its implication for solute movement 168 1. The middle lamella 168 2. The primary wall 168 3. The secondary cell wall 169 III. The root apoplast - nutrient uptake and short-distance transport 170 IV. The apoplast as a compartment for long distance transport 174 V. The apoplast - habitat for microorganisms 175 VI. The apoplast of leaves - a compartment of storage and of reactions 177 1. Transport routes in the leaf apoplast 177 2. Methods of studying apoplastic solutes 177 3. Solute relations in the leaf apoplast 178 4. Concentration gradients in the leaf apoplast 179 5. Ion relations in the leaf apoplast and symptoms of deficiency and toxicity 179 6. Ion relations in the leaf apoplast - influence of nutrient supply 180 7. The leaf apoplast - compartment for transient ion storage 180 8. Ion fluxes between apoplast and symplast 181 9. Apoplastic ion balance 181 10. Leaf apoplast - interaction with the atmosphere 183 VII. Conclusions 183 Acknowledgements 183 References 183.

The low-temperature- and salt-induced RCI2A gene of Arabidopsis complements the sodium sensitivity caused by a deletion of the homologous yeast gene SNA1

Two closely related, tandemly arranged, low-temperature- and salt-induced Arabidopsis genes, corresponding to the previously isolated cDNAs RCI2A and RCI2B, were isolated and characterized. The RCI2A transcript accumulated primarily in response to low temperature or high salinity, and to a lesser extent in response to ABA treatment or water deficit stress. The RCI2B transcript was present at much lower levels than RCI2A, and could only be detected by reverse transcription-PCR amplification. The predicted 6 kDa RCI2 proteins are highly hydrophobic and contain two putative membrane-spanning regions. The polypeptides exhibit extensive similarity to deduced low-temperature- and/or salt-induced proteins from barley, wheat grass and strawberry, and to predicted proteins from bacteria, fungi, nematodes and yeast. Interestingly, we found that a deletion of the RCI2 homologous gene, SNA1 (YRD276c), in yeast causes a salt-sensitive phenotype. This effect is specific for sodium, since no growth defect was observed for the sna1 mutant on 1.7 M sorbitol, 1 M KCl or 0.6 M LiCl. Finally, we found that the Arabidopsis RCI2A cDNA can complement the sna1 mutant when expressed in yeast, indicating that the plant and yeast proteins have similar functions during high salt stress.

sas1, an Arabidopsis mutant overaccumulating sodium in the shoot, shows deficiency in the control of the root radial transport of sodium

A recessive mutation of Arabidopsis designated sas1 (for sodium overaccumulation in shoot) that was mapped to the bottom of chromosome III resulted in a two- to sevenfold overaccumulation of Na(+) in shoots compared with wild-type plants. sas1 is a pleiotropic mutation that also caused severe growth reduction. The impact of NaCl stress on growth was similar for sas1 and wild-type plants; however, with regard to survival, sas1 plants displayed increased sensitivity to NaCl and LiCl treatments compared with wild-type plants. sas1 mutants overaccumulated Na(+) and its toxic structural analog Li(+), but not K(+), Mg(2)+, or Ca(2)+. Sodium accumulated preferentially over K(+) in a similar manner for sas1 and wild-type plants. Sodium overaccumulation occurred in all of the aerial organs of intact sas1 plants but not in roots. Sodium-treated leaf fragments or calli displayed similar Na(+) accumulation levels for sas1 and wild-type tissues. This suggested that the sas1 mutation impaired Na(+) long-distance transport from roots to shoots. The transpiration stream was similar in sas1 and wild-type plants, whereas the Na(+) concentration in the xylem sap of sas1 plants was 5.5-fold higher than that of wild-type plants. These results suggest that the sas1 mutation disrupts control of the radial transport of Na(+) from the soil solution to the xylem vessels.

The ACA4 gene of Arabidopsis encodes a vacuolar membrane calcium pump that improves salt tolerance in yeast

Several lines of evidence suggest that regulation of intracellular Ca(2+) levels is crucial for adaptation of plants to environmental stress. We have cloned and characterized Arabidopsis auto-inhibited Ca(2+)-ATPase, isoform 4 (ACA4), a calmodulin-regulated Ca(2+)-ATPase. Confocal laser scanning data of a green fluorescent protein-tagged version of ACA4 as well as western-blot analysis of microsomal fractions obtained from two-phase partitioning and Suc density gradient centrifugation suggest that ACA4 is localized to small vacuoles. The N terminus of ACA4 contains an auto-inhibitory domain with a binding site for calmodulin as demonstrated through calmodulin-binding studies and complementation experiments using the calcium transport yeast mutant K616. ACA4 and PMC1, the yeast vacuolar Ca(2+)-ATPase, conferred protection against osmotic stress such as high NaCl, KCl, and mannitol when expressed in the K616 strain. An N-terminally modified form of ACA4 specifically conferred increased NaCl tolerance, whereas full-length ATPase had less effect.

Arabidopsis thaliana and Saccharomyces cerevisiae NHX1 genes encode amiloride sensitive electroneutral Na+/H+ exchangers

Sodium at high millimolar levels in the cytoplasm is toxic to plant and yeast cells. Sequestration of Na(+) ions into the vacuole is one mechanism to confer Na(+)-tolerance on these organisms. In the present study we provide direct evidence that the Arabidopsis thaliana At-NHX1 gene and the yeast NHX1 gene encode low-affinity electroneutral Na(+)/H(+) exchangers. We took advantage of the ability of heterologously expressed At-NHX1 to functionally complement the yeast nhx1-null mutant. Experiments on vacuolar vesicles isolated from yeast expressing At-NHX1 or NHX1 provided direct evidence for pH-gradient-energized Na(+) accumulation into the vacuole. A major difference between NHX1 and At-NHX1 is the presence of a cleavable N-terminal signal peptide (SP) in the former gene. Fusion of the SP to At-NHX1 resulted in an increase in the magnitude of Na(+)/H(+) exchange, indicating a role for the SP in protein targeting or regulation. Another distinguishing feature between the plant and yeast antiporters is their sensitivity to the diuretic compound amiloride. Whereas At-NHX1 was completely inhibited by amiloride, NHX1 activity was reduced by only 20-40%. These results show that yeast as a heterologous expression system provides a convenient model to analyse structural and regulatory features of plant Na(+)/H(+) antiporters.

Leaf gas exchange and solute accumulation in the halophyte Salvadora persica grown at moderate salinity

The domestication of halophytes has been proposed as a strategy to expand cultivation onto unfavorable land. However, halophytes mainly have been considered for their performance in extremely saline environments, and only a few species have been characterized in terms of their tolerance and physiological responses to moderately high levels of salinity. Salvadora persica is an evergreen perennial halophyte capable of growing under extreme conditions, from very dry environments to highly saline soils. It possesses high potential economic value as a source of oil and medicinal compounds. To quantify its response to salinity, S. persica seedlings were exposed to 200 mM NaCl for 3 weeks, and growth, leaf gas exchange and solute accumulation were measured. The presence of NaCl induced a 100% increase in fresh weight and a 30% increase in dry weight, relative to non-salinized controls. Increases in fresh weight and dry weight were not associated with higher rates of net CO(2) assimilation, however. Analysis of ion accumulation revealed that S. persica leaves accumulated Na(+) as a primary osmoticum. The concentration of Na(+) in leaves of salinized plants was approximately 40-fold greater than that measured in non-salinized controls, and this was associated with significant reductions in leaf K(+) and Ca(2+) concentrations. In addition, a significant accumulation of proline, probably associated with osmotic adjustment and protection of membrane stability, occurred in roots of salinized plants.

Two-dimensional electrophoresis of soluble leaf proteins, isolated from two wheat species (Triticum durum and Triticum aestivum) differing in sensitivity towards NaCl

Plants of two wheat species (Triticum aestivum cv. Tanit and T. durum cv. Ben Bachir), differing in their sensitivity to NaCl were cultivated in the presence or absence of 100 mM NaCl for 21 days. Soluble proteins extracted from leaves were analyzed by two-dimensional electrophoresis in order to detect NaCl-induced changes in the polypeptide patterns. In all, 500 spots were detected. Results showed species-dependent differences. The greatest alterations in the polypeptide profiles following salt stress were found in the most sensitive cultivar: among the 12 spots (molecular mass, 15-31 kDa) specifically considered in the acidic region of the gel, 11 declined, even disappeared in the NaCl-sensitive leaf profiles, while in the tolerant species only five spots were affected by the salt treatment and five remained untouched; moreover in the latter, two new polypeptides were shown to be induced by NaCl.

The Arabidopsis thaliana salt tolerance gene SOS1 encodes a putative Na+/H+ antiporter

In Arabidopsis thaliana, the SOS1 (Salt Overly Sensitive 1) locus is essential for Na(+) and K(+) homeostasis, and sos1 mutations render plants more sensitive to growth inhibition by high Na(+) and low K(+) environments. SOS1 is cloned and predicted to encode a 127-kDa protein with 12 transmembrane domains in the N-terminal part and a long hydrophilic cytoplasmic tail in the C-terminal part. The transmembrane region of SOS1 has significant sequence similarities to plasma membrane Na(+)/H(+) antiporters from bacteria and fungi. Sequence analysis of various sos1 mutant alleles reveals several residues and regions in the transmembrane as well as the tail parts that are critical for SOS1 function in plant salt tolerance. SOS1 gene expression in plants is up-regulated in response to NaCl stress. This up-regulation is abated in sos3 or sos2 mutant plants, suggesting that it is controlled by the SOS3/SOS2 regulatory pathway.

Membrane hyperpolarization and salt sensitivity induced by deletion of PMP3, a highly conserved small protein of yeast plasma membrane

Yeast plasma membranes contain a small 55 amino acid hydrophobic polypeptide, Pmp3p, which has high sequence similarity to a novel family of plant polypeptides that are overexpressed under high salt concentration or low temperature treatment. The PMP3 gene is not essential under normal growth conditions. However, its deletion increases the plasma membrane potential and confers sensitivity to cytotoxic cations, such as Na(+) and hygromycin B. Interestingly, the disruption of PMP3 exacerbates the NaCl sensitivity phenotype of a mutant strain lacking the Pmr2p/Enap Na(+)-ATPases and the Nha1p Na(+)/H(+) antiporter, and suppresses the potassium dependency of a strain lacking the K(+) transporters, Trk1p and Trk2p. All these phenotypes could be reversed by the addition of high Ca(2+) concentration to the medium. These genetic interactions indicate that the major effect of the PMP3 deletion is a hyperpolarization of the plasma membrane potential that probably promotes a non-specific influx of monovalent cations. Expression of plant RCI2A in yeast could substitute for the loss of Pmp3p, indicating a common role for Pmp3p and the plant homologue.

PLANTCELLULAR ANDMOLECULARRESPONSES TOHIGHSALINITY

Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to those that regulate osmotic stress responses of yeast. There is evidence also of signaling cascades that are not known to exist in the unicellular eukaryote, some that presumably function in intercellular coordination or regulation of effector genes in a cell-/tissue-specific context required for tolerance of plants. A complex set of stress-responsive transcription factors is emerging. The imminent availability of genomic DNA sequences and global and cell-specific transcript expression data, combined with determinant identification based on gain- and loss-of-function molecular genetics, will provide the infrastructure for functional physiological dissection of salt tolerance determinants in an organismal context. Furthermore, protein interaction analysis and evaluation of allelism, additivity, and epistasis allow determination of ordered relationships between stress signaling components. Finally, genetic activation and suppression screens will lead inevitably to an understanding of the interrelationships of the multiple signaling systems that control stress-adaptive responses in plants.

Molecular aspects of higher plant P-type Ca(2+)-ATPases

Recent genomic data in the model plant Arabidopsis thaliana reveal the existence of at least 11 Ca(2+)-ATPase genes, and an analysis of expressed sequence tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca(2+)-ATPases clearly form distinct groups, type IIA (or ECA for ER-type Ca(2+)-ATPase) and type IIB (ACA for autoinhibited Ca(2+)-ATPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclusively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on the high molecular masses of the encoded proteins, are good candidates for plasma membrane bound Ca(2+)-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore it appears that the activity of type IIB Ca(2+)-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respectively. Possible functions of plant calcium pumps are described and - beside second messenger functions directly linked to calcium homeostasis - new data on a putative involvement in secretory and salt stress functions are discussed.

The plant plasma membrane H(+)-ATPase: structure, function and regulation

The proton-pumping ATPase (H(+)-ATPase) of the plant plasma membrane generates the proton motive force across the plasma membrane that is necessary to activate most of the ion and metabolite transport. In recent years, important progress has been made concerning the identification and organization of H(+)-ATPase genes, their expression, and also the kinetics and regulation of individual H(+)-ATPase isoforms. At the gene level, it is now clear that H(+)-ATPase is encoded by a family of approximately 10 genes. Expression, monitored by in situ techniques, has revealed a specific distribution pattern for each gene; however, this seems to differ between species. In the near future, we can expect regulatory aspects of gene expression to be elucidated. Already the expression of individual plant H(+)-ATPases in yeast has shown them to have distinct enzymatic properties. It has also allowed regulatory aspects of this enzyme to be studied through random and site-directed mutagenesis, notably its carboxy-terminal region. Studies performed with both plant and yeast material have converged towards deciphering the way phosphorylation and binding of regulatory 14-3-3 proteins intervene in the modification of H(+)-ATPase activity. The production of high quantities of individual functional H(+)-ATPases in yeast constitutes an important step towards crystallization studies to derive structural information. Understanding the specific roles of H(+)-ATPase isoforms in whole plant physiology is another challenge that has been approached recently through the phenotypic analysis of the first transgenic plants in which the expression of single H(+)-ATPases has been up- or down-regulated. In conclusion, the progress made recently concerning the H(+)-ATPase family, at both the gene and protein level, has come to a point where we can now expect a more integrated investigation of the expression, function and regulation of individual H(+)-ATPases in the whole plant context.

Functional conservation between yeast and plant endosomal Na(+)/H(+) antiporters

Vacuolar compartmentation of Na(+) is an essential mechanism for salinity tolerance since it lowers cytosolic Na(+) levels while contributing to osmotic adjustment for cell turgor and expansion. The AtNHX1 protein of Arabidopsis thaliana substituted functionally for ScNHX1, the endosomal Na(+)/H(+) antiporter of yeast. Ion tolerance conferred by AtNHX1 and ScNHX1 correlated with ion uptake into an intracellular pool that was energetically dependent on the vacuolar (H(+))ATPase. AtNHX1 localized to vacuolar membrane fractions of yeast. Hence, both transporters share an evolutionarily conserved function in Na(+) compartmentation. AtNHX1 mRNA levels were upregulated by ABA and NaCl treatment in leaf but not in root tissue.

The Arabidopsis SOS2 protein kinase physically interacts with and is activated by the calcium-binding protein SOS3

The Arabidopsis thaliana SOS2 and SOS3 genes are required for intracellular Na(+) and K(+) homeostasis and plant tolerance to high Na(+) and low K(+) environments. SOS3 is an EF hand type calcium-binding protein having sequence similarities with animal neuronal calcium sensors and the yeast calcineurin B. SOS2 is a serine/threonine protein kinase in the SNF1/AMPK family. We report here that SOS3 physically interacts with and activates SOS2 protein kinase. Genetically, sos2sos3 double mutant analysis indicates that SOS2 and SOS3 function in the same pathway. Biochemically, SOS2 interacts with SOS3 in the yeast two-hybrid system and in vitro binding assays. The interaction is mediated by the C-terminal regulatory domain of SOS2. SOS3 activates SOS2 protein kinase activity in a Ca(2+)-dependent manner. Therefore, SOS3 and SOS2 define a novel regulatory pathway important for the control of intracellular ion homeostasis and salt tolerance in plants.

The Arabidopsis SOS2 protein kinase physically interacts with and is activated by the calcium-binding protein SOS3

The Arabidopsis thaliana SOS2 and SOS3 genes are required for intracellular Na(+) and K(+) homeostasis and plant tolerance to high Na(+) and low K(+) environments. SOS3 is an EF hand type calcium-binding protein having sequence similarities with animal neuronal calcium sensors and the yeast calcineurin B. SOS2 is a serine/threonine protein kinase in the SNF1/AMPK family. We report here that SOS3 physically interacts with and activates SOS2 protein kinase. Genetically, sos2sos3 double mutant analysis indicates that SOS2 and SOS3 function in the same pathway. Biochemically, SOS2 interacts with SOS3 in the yeast two-hybrid system and in vitro binding assays. The interaction is mediated by the C-terminal regulatory domain of SOS2. SOS3 activates SOS2 protein kinase activity in a Ca(2+)-dependent manner. Therefore, SOS3 and SOS2 define a novel regulatory pathway important for the control of intracellular ion homeostasis and salt tolerance in plants.

The Arabidopsis thaliana SOS2 gene encodes a protein kinase that is required for salt tolerance

In Arabidopsis thaliana, the Salt Overly Sensitive 2 (SOS2) gene is required for intracellular Na(+) and K(+) homeostasis. Mutations in SOS2 cause Na(+) and K(+) imbalance and render plants more sensitive toward growth inhibition by high Na(+) and low K(+) environments. We isolated the SOS2 gene through positional cloning. SOS2 is predicted to encode a serine/threonine type protein kinase with an N-terminal catalytic domain similar to that of the yeast SNF1 kinase. Sequence analyses of sos2 mutant alleles reveal that both the N-terminal catalytic domain and the C-terminal regulatory domain of SOS2 are functionally essential. The steady-state level of SOS2 transcript is up-regulated by salt stress in the root. Autophosphorylation assays show that SOS2 is an active protein kinase. In the recessive sos2-5 allele, a conserved glycine residue in the kinase catalytic domain is changed to glutamate. This mutation abolishes SOS2 autophosphorylation, indicating that SOS2 protein kinase activity is required for salt tolerance.

The Arabidopsis thaliana SOS2 gene encodes a protein kinase that is required for salt tolerance

In Arabidopsis thaliana, the Salt Overly Sensitive 2 (SOS2) gene is required for intracellular Na(+) and K(+) homeostasis. Mutations in SOS2 cause Na(+) and K(+) imbalance and render plants more sensitive toward growth inhibition by high Na(+) and low K(+) environments. We isolated the SOS2 gene through positional cloning. SOS2 is predicted to encode a serine/threonine type protein kinase with an N-terminal catalytic domain similar to that of the yeast SNF1 kinase. Sequence analyses of sos2 mutant alleles reveal that both the N-terminal catalytic domain and the C-terminal regulatory domain of SOS2 are functionally essential. The steady-state level of SOS2 transcript is up-regulated by salt stress in the root. Autophosphorylation assays show that SOS2 is an active protein kinase. In the recessive sos2-5 allele, a conserved glycine residue in the kinase catalytic domain is changed to glutamate. This mutation abolishes SOS2 autophosphorylation, indicating that SOS2 protein kinase activity is required for salt tolerance.

Salinity and hyperosmotic stress induce rapid increases in phosphatidylinositol 4,5-bisphosphate, diacylglycerol pyrophosphate, and phosphatidylcholine in Arabidopsis thaliana cells

In animal cells, phosphoinositides are key components of the inositol 1,4,5-trisphosphate/diacylglycerol-based signaling pathway, but also have many other cellular functions. These lipids are also believed to fulfill similar functions in plant cells, although many details concerning the components of a plant phosphoinositide system, and their regulation are still missing. Only recently have the different phosphoinositide isomers been unambiguously identified in plant cells. Another problem that hinders the study of the function of phosphoinositides and their derivatives, as well as the regulation of their metabolism, in plant cells is the need for a homogenous, easily obtainable material, from which the extraction and purification of phospholipids is relatively easy and quantitatively reproducible. We present here a thorough characterization of the phospholipids purified from [(32)P]orthophosphate- and myo-[2-(3)H]inositol-radiolabeled Arabidopsis thaliana suspension-cultured cells. We then show that NaCl treatment induces dramatic increases in the levels of phosphatidylinositol 4,5-bisphosphate and diacylglycerol pyrophosphate and also affects the turnover of phosphatidylcholine. The increase in phosphatidylinositol 4,5-bisphosphate was also observed with a non-ionic hyperosmotic shock. In contrast, the increase in diacylglycerol pyrophosphate and the turnover of phosphatidylcholine were relatively specific to salt treatments as only minor changes in the metabolism of these two phospholipids were detected when the cells were treated with sorbitol instead of NaCl.

Calcium signaling during abiotic stress in plants

Plants experience a wide array of environmental stimuli, not all of which are favorable, and, unlike animals, are unable to move away from stressful environments. They therefore require a mechanism with which to recognize and respond to abiotic stresses of many different types. Frequently this mechanism involves intracellular calcium. Stress-induced changes in the cytosolic concentration of Ca2+ ([Ca2+]cyt) occur as a result of influx of Ca2+ from outside the cell, or release of Ca2+ from intracellular stores. These alterations in [Ca2+]cyt constitute a signal that is transduced via calmodulin, calcium-dependent protein kinases, and other Ca(2+)-controlled proteins to effect a wide array of downstream responses involved in the protection of the plant and adjustment to the new environmental conditions. Ca2+ signaling has been implicated in plant responses to a number of abiotic stresses including low temperature, osmotic stress, heat, oxidative stress, anoxia, and mechanical perturbation, which are reviewed in this article.

Molecular cloning and expression of the Na+/H+ exchanger gene in Oryza sativa

Na+/H+ exchanger catalyzes the countertransport of Na+ and H+ across membranes. We isolated a rice cDNA clone the deduced amino acid sequence of which had homology with a putative Na+/H+ exchanger in Saccharomyces cerevisiae, NHX1. The sequence contains 2330 bp with an open reading frame of 1608 bp. The deduced amino acid sequence is similar to that of NHX1 and NHE isoforms in mammals, and shares high similarity with the sequences within predicted transmembrane segments and an amiloride-binding domain. The expression of the gene was increased by salt stress. These results suggest that the product of the novel gene, OsNHX1, functions as a Na+/H+ exchanger, and plays important roles in salt tolerance of rice.

Salt-specific regulation of the cytosolic O-acetylserine(thiol)lyase gene from Arabidopsis thaliana is dependent on abscisic acid

The expression of Atcys-3A gene coding for cytosolic O-acetylserine(thiol)lyase, a key enzyme in cysteine biosynthesis, from Arabidopsis thaliana is significantly induced by exposure to salt and heavy-metal stresses. Addition of NaCl to mature plants induced a rapid accumulation of the mRNA throughout the leaf lamina and roots, and later on in stems, being mainly restricted to vascular tissues. The salt-specific regulation of Atcys-3A was also mediated by abscisic acid (ABA) since: (1) exogenous addition of ABA to the culture medium mimicked the salt-induced plant response by raising the level of Atcys-3A transcript, and (2) Arabidopsis mutants aba-1 and abi2-1 were not able to respond to NaCl. Our results suggest that a high rate of cysteine biosynthesis is required in Arabidopsis under salt stress necessary for a plant protection or adaptation mechanism. This hypothesis was supported by the observation that intracellular levels of cysteine and glutathione increased up to 3-fold after salt treatment.

An Arabidopsis GSK3/shaggy-like gene that complements yeast salt stress-sensitive mutants is induced by NaCl and abscisic acid

GSK3/shaggy-like genes encode kinases that are involved in a variety of biological processes. By functional complementation of the yeast calcineurin mutant strain DHT22-1a with a NaCl stress-sensitive phenotype, we isolated the Arabidopsis cDNA AtGSK1, which encodes a GSK3/shaggy-like protein kinase. AtGSK1 rescued the yeast calcineurin mutant cells from the effects of high NaCl. Also, the AtGSK1 gene turned on the transcription of the NaCl stress-inducible PMR2A gene in the calcineurin mutant cells under NaCl stress. To further define the role of AtGSK1 in the yeast cells we introduced a deletion mutation at the MCK1 gene, a yeast homolog of GSK3, and examined the phenotype of the mutant. The mck1 mutant exhibited a NaCl stress-sensitive phenotype that was rescued by AtGSK1. Also, constitutive expression of MCK1 complemented the NaCl-sensitive phenotype of the calcineurin mutants. Therefore, these results suggest that Mck1p is involved in the NaCl stress signaling in yeast and that AtGSK1 may functionally replace Mck1p in the NaCl stress response in the calcineurin mutant. To investigate the biological function of AtGSK1 in Arabidopsis we examined the expression of AtGSK1. Northern-blot analysis revealed that the expression is differentially regulated in various tissues with a high level expression in flower tissues. In addition, the AtGSK1 expression was induced by NaCl and exogenously applied ABA but not by KCl. Taken together, these results suggest that AtGSK1 is involved in the osmotic stress response in Arabidopsis.

Expression of Batis maritima methyl chloride transferase in Escherichia coli

Methyl chloride transferase, a novel enzyme found in several fungi, marine algae, and halophytic plants, is a biological catalyst responsible for the production of atmospheric methyl chloride. A previous paper reports the purification of this methylase from Batis maritima and the isolation of a cDNA clone of the gene for this enzyme. In this paper, we describe the isolation of a genomic clone of the methylase gene and the expression of recombinant methyl chloride transferase in Escherichia coli and compare the kinetic behavior of the wild-type and recombinant enzyme. The recombinant enzyme is active and promotes the production of methyl chloride by E. coli under in vivo conditions. The kinetic data indicate that the recombinant and wild-type enzymes have similar halide (Cl-, Br-, and I-)-binding capacities. Both the recombinant and wild-type enzymes were found to function well in high NaCl concentrations. This high salt tolerance resembles the activity of halobacterial enzymes rather than halophytic plant enzymes. These findings support the hypothesis that this enzyme functions in the control and regulation of the internal concentration of chloride ions in halophytic plant cells.

Stress signaling through Ca2+/calmodulin-dependent protein phosphatase calcineurin mediates salt adaptation in plants

Calcineurin (CaN) is a Ca2+- and calmodulin-dependent protein phosphatase (PP2B) that, in yeast, is an integral intermediate of a salt-stress signal transduction pathway that effects NaCl tolerance through the regulation of Na+ influx and efflux. A truncated form of the catalytic subunit and the regulatory subunit of yeast CaN were coexpressed in transgenic tobacco plants to reconstitute a constitutively activated phosphatase in vivo. Several different transgenic lines that expressed activated CaN also exhibited substantial NaCl tolerance, and this trait was linked to the genetic inheritance of the CaN transgenes. Enhanced capacity of plants expressing CaN to survive NaCl shock was similar when evaluation was conducted on seedlings in tissue culture raft vessels or plants in hydroponic culture that were transpiring actively. Root growth was less perturbed than shoot growth by NaCl in plants expressing CaN. Also, NaCl stress survival of control shoots was enhanced substantially when grafted onto roots of plants expressing CaN, further implicating a significant function of the phosphatase in the preservation of root integrity during salt shock. Together, these results indicate that in plants, like in yeast, a Ca2+- and calmodulin-dependent CaN signal pathway regulates determinants of salt tolerance required for stress adaptation. Furthermore, modulation of this pathway by expression of an activated regulatory intermediate substantially enhanced salt tolerance.

Genetic analysis of salt tolerance in arabidopsis. Evidence for a critical role of potassium nutrition

A large genetic screen for sos (for salt overly sensitive) mutants was performed in an attempt to isolate mutations in any gene with an sos phenotype. Our search yielded 28 new alleles of sos1, nine mutant alleles of a newly identified locus, SOS2, and one allele of a third salt tolerance locus, SOS3. The sos2 mutations, which are recessive, were mapped to the lower arm of chromosome V, approximately 2.3 centimorgans away from the marker PHYC. Growth measurements demonstrated that sos2 mutants are specifically hypersensitive to inhibition by Na+ or Li+ and not hypersensitive to general osmotic stresses. Interestingly, the SOS2 locus is also necessary for K+ nutrition because sos2 mutants were unable to grow on a culture medium with a low level of K+. The expression of several salt-inducible genes was superinduced in sos2 plants. The salt tolerance of sos1, sos2, and sos3 mutants correlated with their K+ tissue content but not their Na+ tissue content. Double mutant analysis indicated that the SOS genes function in the same pathway. Based on these results, a genetic model for salt tolerance mechanisms in Arabidopsis is presented in which SOS1, SOS2, and SOS3 are postulated to encode regulatory components controlling plant K+ nutrition that in turn is essential for salt tolerance.

Molecular cloning and expression characterization of a rice K+ channel beta subunit

K+ channel proteins native to animal membranes have been shown to be composed of two different types of polypeptides: the pore-forming alpha subunit and the beta subunit which may be involved in either modulation of conductance through the channel, or stabilization and surface expression of the channel complex. Several cDNAs encoding animal K+ channel beta subunits have been recently cloned and sequenced. We report the molecular cloning of a rice plant homolog of these animal beta subunits. The rice cDNA (KOB1) described in this report encodes a 36 kDa polypeptide which shares 45% sequence identity with these animal K+ channel beta subunits. and 72% identity with the only other cloned plant (Arabidopsis thaliana) K+ channel beta subunit (KAB1). The KOB1 translation product was demonstrated to form a tight physical association with a plant K+ channel alpha subunit. These results are consistent with the conclusion that the KOB1 cDNA encodes a K+ channel beta subunit. Expression studies indicated that KOB1 protein is more abundant in leaves than in either reproductive structures or roots. Later-developing leaves on a rice plant were found to contain increasing levels of the protein with the flag leaf having the highest titer of KOB1. Leaf sheaths are known to accumulate excess K+ and act as reserve sources of this cation when new growth requires remobilization of K+. Leaf sheaths were found to contain higher levels of KOB1 protein than the blade portions of leaves. It was further determined that when K+ was lost from older leaves of plants grown on K+-deficient fertilizer, the loss of cellular K+ was associated with a decline in both KOB1 mRNA and protein. This finding represents the first demonstration (in either plants or animals) that changes in cellular K+ status may specifically alter expression of a gene encoding a K+ channel subunit.

Molecular cloning of a novel Ca2+-binding protein that is induced by NaCl stress

Plant responses to high salt stress have been studied for several decades. However, the molecular mechanisms underlying these responses still elude us. In order to understand better the molecular mechanism related to NaCl stress in plants, we initiated the cloning of a large number of NaCl-induced genes in Arabidopsis. Here, we report the cloning of a cDNA encoding a novel Ca2+-binding protein, named AtCP1, which shares sequence similarities with calmodulins. AtCP1 exhibits, in particular, a high degree of amino acid sequence homology to the Ca2+-binding loops of the EF hands of calmodulin. However, unlike calmodulin, AtCP1 appears to have only three Ca2+-binding loops. We examined Ca2+ binding of the protein by a Ca2+-dependent electrophoretic mobility shift assay. A recombinant AtCP1 protein that was expressed in Escherichia coli did show a Ca2+-dependent electrophoretic mobility shift. To gain insight into the expression of the AtCP1 gene, northern blot analysis was carried out. The AtCP1 gene had a tissue-specific expression pattern: high levels of expression in flower and root tissues and nearly undetectable levels in leaves and siliques. Also, the expression of the AtCP1 gene was induced by NaCl treatment but not by ABA treatment. Finally, subcellular localization experiments using an AtCP1:smGFP fusion gene in soybean suspension culture cells and tobacco leaf protoplasts indicate that AtCP1 is most likely a cytosolic protein.

Genetic analysis of salt tolerance in arabidopsis. Evidence for a critical role of potassium nutrition

A large genetic screen for sos (for salt overly sensitive) mutants was performed in an attempt to isolate mutations in any gene with an sos phenotype. Our search yielded 28 new alleles of sos1, nine mutant alleles of a newly identified locus, SOS2, and one allele of a third salt tolerance locus, SOS3. The sos2 mutations, which are recessive, were mapped to the lower arm of chromosome V, approximately 2.3 centimorgans away from the marker PHYC. Growth measurements demonstrated that sos2 mutants are specifically hypersensitive to inhibition by Na+ or Li+ and not hypersensitive to general osmotic stresses. Interestingly, the SOS2 locus is also necessary for K+ nutrition because sos2 mutants were unable to grow on a culture medium with a low level of K+. The expression of several salt-inducible genes was superinduced in sos2 plants. The salt tolerance of sos1, sos2, and sos3 mutants correlated with their K+ tissue content but not their Na+ tissue content. Double mutant analysis indicated that the SOS genes function in the same pathway. Based on these results, a genetic model for salt tolerance mechanisms in Arabidopsis is presented in which SOS1, SOS2, and SOS3 are postulated to encode regulatory components controlling plant K+ nutrition that in turn is essential for salt tolerance.

A calcium sensor homolog required for plant salt tolerance

Excessive sodium (Na+) in salinized soils inhibits plant growth and development. A mutation in the SOS3 gene renders Arabidopsis thaliana plants hypersensitive to Na+-induced growth inhibition. SOS3 encodes a protein that shares significant sequence similarity with the calcineurin B subunit from yeast and neuronal calcium sensors from animals. The results suggest that intracellular calcium signaling through a calcineurin-like pathway mediates the beneficial effect of calcium on plant salt tolerance.

Characterization of Phaseolus vulgaris cDNA clones responsive to water deficit: identification of a novel late embryogenesis abundant-like protein

Six cDNA clones from Phaseolus vulgaris, whose expression is induced by water deficit and ABA treatment (rsP cDNAs) were identified and characterized. The sequence analyses of the isolated clones suggest that they encode two types of late-embryogenesis abundant (LEA) proteins, a class-1 cytoplasmic low-molecular-weight heat shock protein (lmw-HSP), a lipid transfer protein (LTP), and two different proline-rich proteins (PRP). One of the putative LEA proteins identified corresponds to a novel 9.3 kDa LEA-like protein. During the plant response to a mild water deficit (psi w = -0.35 MPa) all genes identified present a maximal expression at around 16 or 24 h of treatment, followed by a decline in expression levels. Rehydration experiments revealed that those genes encoding PRPs and LTP transiently re-induce or maintain their expression when water is added to the soil after a dehydration period. This is not the case for the lea genes whose transcripts rapidly decrease, reaching basal levels a few hours after rehydration (4 h). Under water deficit and ABA treatments, the highest levels of expression for most of the genes occur in the root, excluding the ltp gene whose maximum expression levels are found in the aerial regions of the plant. This indicates that for these genes, both water deficit and ABA-dependent expression are under organ-specific control. The data presented here support the importance of these proteins during the plant response to water deficit.

NaCl-Dependent growth, ion content and regeneration of calluses initiated from the mangrove plant,Bruguiera sexangula

Calluses initiated from leaves and seedlings of the mangrove,Bruguiera sexangula, were isolated from the original tissues and subcultured. Effects of NaCl on growth and ion content of each callus were measured. The growth rate of calluses derived from leaves (leaf callus) gradually decreased as the NaCl concentration in the medium increased, while that of calluses derived from seedlings (seedling callus) was highest in the medium containing 100 mM NaCl. Concentrations of Na and Cl in both calluses increased with increasing the NaCl concentration in the culture medium. The concentration of K of leaf calluses greatly decreased at 300 mM NaCl, while the K concentration of seedling calluses decreased only slightly and remained relatively high even in the presence of 300 mM NaCl.Transient treatment of leaf calluses with media containing high concentrations of NaCl frequently induced regeneration of adventitious tissues.

Salt-Sensitive Mutants of Chlamydomonas reinhardtii Isolated after Insertional Tagging

We describe the isolation of salt-sensitive Chlamydomonas reinhardtii mutants by insertional mutagenesis using the nitrate reductase (Nit1) gene. The plasmid pMN24, containing Nit1, was used for transformation of 305CW15 (nit1 cw15 mt+), and transformants were selected for complementation of the nit- phenotype. From 6875 nit+ colonies, four transformants (S4, S18, S46, and S66) were isolated that exhibited both Na+ and Li+ sensitivity (sod-), and another transformant (S33) was selected that exhibited sensitivity to Li+ but not Na+ (lit-) based on relative growth comparisons with the wild-type strain. S33, S46, and S66 were no more growth inhibited by sorbitol than was 305CW15. In comparison, S4 and S18 exhibited substantial growth inhibition in medium supplemented with sorbitol. Genetic analyses indicated that the salt-sensitive mutants were each defective in a single recessive gene. The mutant genes in S4 (sod1), S33 (lit1), and S66 (sod3) are linked to a functional copy of Nit1 and are presumably tagged with a pMN24 insertion.

NaCl-Induced Alterations in Both Cell Structure and Tissue-Specific Plasma Membrane H+ -ATPase Gene Expression

NaCl-induced plasma membrane H+-ATPase gene expression, which occurs in roots and fully expanded leaves of the halophyte Atriplex nummularia L. (X. Niu, M.L. Narasimhan, R.A. Salzman, R.A. Bressan, P.M. Hasegawa [1993] Plant Physiol 103: 713-718), has been differentially localized to specific tissues using in situ RNA hybridization techniques. Twenty-four-hour exposure of plants to 400 mM NaCl resulted in substantial accumulation of H+ pump message in the epidermis of the root tip and the endodermis of the root elongation/differentiation zone. In expanded leaves, NaCl induction of plasma membrane H+-ATPase message accumulation was localized to bundle-sheath cells. Ultrastructural analyses indicated that significant cytological adaptations in root cells included plasmolysis that is accompanied by plasma membrane invaginations, formation of Hechtian strands and vesiculation, and vacuolation. These results identify specific tissues that are involved in the regulation of Na+ and Cl- uptake into different organs of the halophyte A. nummularia and provide evidence of the intercellular and interorgan coordination that occurs in the mediation of NaCl adaptation.