Tolerant Mutants Research Articles

The screening of mutants and construction of mutant library for Oryza sativa cv. Nipponbare via ethyl methane sulphonate inducing

Following the sequencing of rice genome, the functional analysis of unidentified genes is gaining wide importance. Mutant isolation is one of the effective ways to isolate and clone the target genes and analyze their functions. To find the various mutants in the same genetic background, seeds of Oryza sativa cv. Nipponbare were treated with ethyl methane sulphonate (EMS). A total of 1056 mutants were screened for five categories in M2 generation with the seedling frequency of 26.29‰ at three-leaf stage, but only 264 mutants were verified in M3 generation with a frequency of 6.57‰. Among the mutants verified in M3 generation, the frequency of leaf mutation was the highest (2.22‰), followed by seedling height (1.74‰) and the abiotic stress tolerance mutant (1.47‰). Nineteen characteristic mutations, including a big group of abiotic stress tolerant mutants such as herbicide resistant, salt tolerant and drought tolerant were identified at this stage. By observation of rice growth characteristics at different developmental stages, another 220 mutants have been isolated and verified in the M3 generation with the mutant frequency of 53.9‰ covering about 28 mutant traits. Among those identified, the highest frequencies were obtained for appearance of brown rice mutant with 18.37‰, followed by panicle mutant with 13.47‰, and grain mutants with 9.06‰. All the mutants screened above were suitable for gene function analysis and for utilization in agronomy.

Potassium channel α-subunit AtKC1 negatively regulates AKT1-mediated K+ uptake in Arabidopsis roots under low-K+ stress

Potassium transporters play crucial roles in K(+) uptake and translocation in plants. However, so far little is known about the regulatory mechanism of potassium transporters. Here, we show that a Shaker-like potassium channel AtKC1, encoded by the AtLKT1 gene cloned from the Arabidopsis thaliana low-K(+) (LK)-tolerant mutant Atlkt1, significantly regulates AKT1-mediated K(+) uptake under LK conditions. Under LK conditions, the Atkc1 mutants maintained their root growth, whereas wild-type plants stopped their root growth. Lesion of AtKC1 significantly enhanced the tolerance of the Atkc1 mutants to LK stress and markedly increased K(+) uptake and K(+) accumulation in the Atkc1-mutant roots under LK conditions. Electrophysiological results showed that AtKC1 inhibited the AKT1-mediated inward K(+) currents and negatively shifted the voltage dependence of AKT1 channels. These results demonstrate that the 'silent' K(+) channel alpha-subunit AtKC1 negatively regulates the AKT1-mediated K(+) uptake in Arabidopsis roots and consequently alters the ratio of root-to-shoot under LK stress conditions.

Production of l-lactic acid by a thermophilic Bacillus mutant using sodium hydroxide as neutralizing agent

A sodium lactate tolerant mutant strain named Bacillus sp. Na-2 was obtained and applied to sodium hydroxide-based l-lactic acid (LA) production process. The influences of aeration and pH were investigated to further improve the resistance of strain Na-2 against sodium lactate stress and to obtain the most efficient l-LA production process. Although mild aeration was favorable for cell growth and l-LA production, vigorous aeration resulted in a metabolic shift from homolactic to mixed-acid/acetoin fermentation. Therefore, a two-stage aeration control strategy was employed. Optimum pH was found to be 6.0. A total of 106.0 g/l l-LA was produced in 30 h by Bacillus sp. Na-2 using sodium hydroxide as neutralizing agent. Productivity, conversion rate and optical purity were 3.53 g/l/h, 94% and 99.5%, respectively. The remarkable fermentation traits of Bacillus sp. Na-2 and the environment-friendly characteristics of NaOH-based process represent new insight for industrial scale production of l-LA.

Registration of Four Upland Cotton (Gossypium hirsutum L.) Genetic Stock Mutants with Tolerance to Imazamox

Few mutants conferring herbicide tolerance in cotton (Gossypium hirsutum L.) have been developed. Imazamox {2‐[(RS)‐4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl]‐5‐methoxymethylnicotinic acid} is a broad spectrum imidazolinone herbicide that could give growers a new tool to control many troublesome annual broadleaf and grass weeds. Our objective was to identify imazamox tolerance in chemical mutation derived cotton lines. Four imazamox tolerant mutant cotton (G. hirsutum) genetic stocks, SCM3‐4‐3 (Reg. No. GS‐3, PI 657941), SCM3‐7‐3 (Reg. No. GS‐4, PI 657942), RM3‐8‐1 (Reg. No. GS‐5, PI 657943), and EM4‐3‐1 (Reg. No. GS‐6, PI 657944) were developed and jointly released by Texas Tech University, Department of Plant and Soil Science, and USDA‐ARS Crop Genetics and Production Research Unit, Stoneville, MS, in July, 2009. These genetic stocks were obtained through ethyl methanesulfonate (EMS) mutagenesis of seeds of three High Plains cotton cultivars (SC 9023, Rocket, and Explorer) and selection with imazamox herbicide. Application of five rates of imazamox (0, 88, 175, 350, and 700 g a.i. ha−1) has shown that these mutants have elevated levels of tolerance to imazamox. These genetic stocks are available for use by the cotton industry and public researchers to create hybrids, parental lines, or germplasm lines with resistance to the imazamox herbicide. The identification of these mutants is expected to increase weed management options available to conventional growers who have become heavily dependent on glyphosate [N‐(phosphonomethyl) glycine] based crop production systems.

Development of AFLP and STS markers linked to a waterlogging tolerance in Korean soybean landraces

Among the 400 soybean (Glycine max) landraces, we selected 3 tolerant (KAS150-9, KAS160-15, and KAS170-9) and 3 susceptible lines (KAS160-14, KAS160-20, and KAS201-6-1) by the survival percentage and injury scores. Susceptible lines showed decrease in chlorophyll content and increase in glucose and malondialdehyde (MDA) contents under waterlogging stress, while tolerant lines did not change significantly. For AFLP analysis, 8 EcoRI (+3) and 8 MseI (+3) primers used in 32 primer combinations generated a total of 2 566 bands with a mean of 80 bands per primer combination, of which 1 117 (43.5 %) were clearly polymorphic between the tolerant and susceptible lines. A genetic similarity coefficient, based on cluster analysis using an unweighted pair grouping method of average (UPGMA), was 0.79 for the tolerant group, while the susceptible landraces were genetically less related, with a genetic similarity coefficient of 0.17. The 10 reproducible polymorphic PCR products present in the 3 tolerant or susceptible lines were sequenced and converted into sequence tagged site (STS) markers. These STS primer sets were designated GmWT01-GmWT06 and GmWS01-GmWS04. Two STS primer sets, GmWT06 and GmWS02, generated a single monomorphic PCR product identical in size to the original AFLP fragments. For the broad application of these STS markers in marker-assisted selection (MAS) for soybean genotypes tolerant to waterlogging stress, two developed STS markers are being evaluated with putative waterlogging tolerant mutant lines induced by γ-radiation in soybean mutation breeding programs.

A salt tolerant Enterobacter cloacae mutant for bioaugmentation of petroleum- and salt-contaminated soil

A NaCl-tolerant Enterobacter cloacae variant (MU-1) was obtained by mutagenesis using atmospheric pressure glow discharge (APGD) plasmas. The variant exhibited regular growth behavior in slurry cultivation and reached a cell density of 5.72 × 10 8 and 6.44 × 10 8 colony-forming units (CFU/mL) in the presence and absence of 7.5% NaCl, respectively, when crude oil was used as the sole carbon source (crude oil/soil = 1.5%). The total petroleum hydrocarbon (TPH) degradation percentage was 7.94% with mutant MU-1 in the presence of 7.5% NaCl whereas that of the wild-type strain was 3.17%. When cultivated in saline medium, MU-1 showed a slight change in membrane permeability but significant increases in both the K + concentration inside the cell membrane (from 234.24 to 1422.88 ppm/g dry cell weight in the first 2 h) and the exopolysaccharide (EPS) level outside the membrane (from 1350 to 1825 mg/g dry cell weight). The rapid increase in K + inside the cell and the simultaneous accumulation of EPS outside the cell may be responsible for maintaining the osmotic balance during saline cultivation, and this could facilitate the microbial growth and TPH degradation of MU-1.

Comparative Protein Modeling, Prediction of Conserved Residue and Active Sites in Cold Resistant Protein Isolated from CRPF1, A Cold Tolerant Mutant of Pseudomonas fluorescens

Proteins interacting with the biological information molecules DNA and RNA play important cellular roles in all organisms. One widespread super family of proteins implicated in such function(s) is cold shock protein (CSP) that contains the cold shock domain (CSD). This work is planned to study the three-dimensional structure, conserved residues, and different active sites in the structure of cold resistant protein (CRP) from CRPF(1), cold tolerant mutant of Pseudomonas fluorescence by comparative homology modeling. Here we tried to identify crucial residues that are involved in active sites or functional sites of the protein. The study reveals that CRP represent the prototype of the CSD and share a highly similar overall fold consisting of five antiparallel beta-sheets forming a beta-barrel structure with surface exposed aromatic and basic residues that were responsible for nucleic acid binding properties of variable binding affinities and sequence selectivity and harbors the nucleic acid binding motifs RNP1 and RNP2 that is highly conserved in CSP family.

Fine and coarse regulation of reactive oxygen species in the salt tolerant mutants of barnyard grass and their wild-type parents under salt stress

The growth of the wild-type and three salt tolerant mutants of barnyard grass (Echinochloa crusgalli L.) under salt stress was investigated in relation to oxidative stress and activities of the antioxidant enzymes superoxide dismutase (SOD: EC 1.15.1.1), catalase (CAT: EC 1.11.1.6), phenol peroxidase (POD: EC 1.11.1.7), glutathione reductase (GR: EC 1.8.1.7) and ascorbate peroxidase (APX: EC 1.11.1.1). The three mutants (fows B17, B19 and B21) grew significantly better than the wild-type under salt stress (200 mM NaCl) but some salt sensitive individuals were still detectable in the populations of the mutants though in smaller numbers compared with the wild-type. The salt sensitive plants had slower growth rates, higher rates of lipid peroxidation and higher levels of reactive oxygen species (ROS) in their leaves compared with the more tolerant plants from the same genotype. These sensitivity responses were maximized when the plants were grown under high light intensity suggesting that the chloroplast could be a main source of ROS under salt stress. However, the salt sensitivity did not correlate with reduced K(+)/Na(+) ratios or enhanced Na(+) uptake indicating that the sensitivity responses may be mainly because of accumulation of ROS rather than ion toxicity. SOD activities did not correlate to salt tolerance. Salt stress resulted in up to 10-fold increase in CAT activity in the sensitive plants but lower activities were found in the tolerant ones. In contrast, the activities of POD, APX and GR were down regulated in the sensitive plants compared with the tolerant ones. A correlation between plant growth, accumulation of ROS and differential modulation of antioxidant enzymes is discussed. We conclude that loss of activities of POD, APX and GR causes loss of fine regulation of ROS levels and hence the plants experience oxidative stress although they have high CAT activities.

Salt tolerant mutant of Anabaena doliolum exhibiting efficient ammonium uptake and assimilation.

Effect of salinity (NaCl, 100 mM) on growth, nitrate reductase (NR) and glutamine synthetase (GS) activities, and uptake of NH4 (+) was studied in the wild type (WT) and the NaCl-tolerant mutant type (MT) of cyanobacterium Anabaena doliolum. Results obtained in the presence of salt showed significant reduction in the growth rate of both WT and MT cells of A. doliolum by about 77.8 and 40 %, respectively over without NaCl. Similarly rate of NR activity in both WT and MT strains was reduced by 45.5 and 44.5 %, respectively. On the contrary, rate of GS activity of both the WT and MT strains in the presence 100 mM of NaCl increased by 34 and 159 %, respectively. The results of this study indicate that tolerance to NaCl in A. doliolum is more dependent on NH4 (+) assimilation rather than on nitrate assimilation in relation to N-metabolism. The increased GS activity in MT cells of the cyanobacterium is possibly because of high rate of energy dependent NH4 (+) uptake.

Imazamox Tolerance in Mutation‐Derived Lines of Upland Cotton

ABSTRACTInduction of genes conferring herbicide resistance by mutagenesis could facilitate use of imidazolinone herbicides in upland cotton (Gossypium hirsutum L.). In 1997 and 1998, seeds of eight High Plains cotton cultivars were treated with 2.45% v/v ethyl methanesulfonate. The resulting M3 and M4 generations were sprayed with imazethapyr, and the M5 and M6 generations were sprayed with imazamox. Four stable M6 to M7 lines with tolerance to imazamox were identified in 2004. During 2005 and 2006, tolerant mutants and their nonmutated parents were treated at the four‐leaf stage with topical applications of imazamox applied at five rates (0, 88, 175, 350, and 700 g a.i. ha−1). Elevated levels of tolerance to imazamox were observed in all mutants. Imazamox did not impact fiber length. Preliminary investigation indicated that tolerance to imazamox was controlled by a partially dominant single gene. An allelism test revealed that the tolerance genes in the four mutants are either alleles in the same locus or are very tightly linked.

ARS5 is a component of the 26S proteasome complex, and negatively regulates thiol biosynthesis and arsenic tolerance in Arabidopsis.

A forward-genetic screen in Arabidopsis led to the isolation of several arsenic tolerance mutants. ars5 was the strongest arsenate- and arsenite-resistant mutant identified in this genetic screen. Here, we report the characterization and cloning of the ars5 mutant gene. ars5 is shown to exhibit an increased accumulation of arsenic and thiol compounds during arsenic stress. Rough mapping together with microarray-based expression mapping identified the ars5 mutation in the alpha subunit F (PAF1) of the 26S proteasome complex. Characterization of an independent paf1 T-DNA insertion allele and complementation by PAF1 confirmed that paf1 mutation is responsible for the enhanced thiol accumulation and arsenic tolerance phenotypes. Arsenic tolerance was not observed in a knock-out mutant of the highly homologous PAF2 gene. However, genetic complementation of ars5 by the overexpression of PAF2 suggests that the PAF2 protein is functionally equivalent to PAF1 when expressed at high levels. No detectible difference was observed in total ubiquitinylated protein profiles between ars5 and wild-type (WT) Arabidopsis, suggesting that the arsenic tolerance observed in ars5 is not derived from a general impairment in proteasome-mediated protein degradation. Quantitative RT-PCR showed that arsenic induces the enhanced transcriptional activation of several key genes that function in glutathione and phytochelatin biosynthesis in the WT, and this arsenic induction of gene expression is more dramatic in ars5. The enhanced transcriptional response to arsenic and the increased accumulation of thiol compounds in ars5, compared with WT, suggest the presence of a positive regulation pathway for thiol biosynthesis that is enhanced in the ars5 background.

Improved metribuzin tolerance in narrow-leafed lupin ( Lupinus angustifolius L.) by induced mutation and field selection

Narrow-leafed lupin ( Lupinus angustifolius L.) is a grain legume well adapted to sandy soils of a Mediterranean climate and is the major grain legume in Australia. Improved herbicide tolerance of lupin cultivars is now considered as top priority by the industry for effective weed management. This paper reports induced mutation by seed mutagenesis in metribuzin-sensitive but anthracnose resistant cv. Tanjil, selection for metribuzin tolerant mutants in the field and evaluation of the tolerant mutants to metribuzin and anthracnose resistance. Two highly tolerant mutants Tanjil-AZ-33 and Tanjil-AZ-55 were identified and they were the first reported metribuzin tolerant mutants. The estimated mutation rate was 2.5 × 10 −5. Tanjil-AZ-33 was six times and Tanjil-AZ-55 was four times more tolerant to metribuzin than the original cultivar Tanjil and they were also more tolerant than the tolerant cv. Mandelup. Tanjil-AZ-33 was the most tolerant lupin germplasm. Disease nursery assessment over 2 years revealed that both mutants also retained anthracnose resistance. Seed yield under irrigation was 4.2 t/ha for Tanjil-AZ-33 and 1.9 t/ha for cv. Tanjil when the seedlings were treated with metribuzin at 300 g/ha at the six-leaf stage. Clearly the mutation process has created new tolerance to metribuzin, but did not alter the anthracnose resistance. The mutant Tanjil-AZ-33 has been used as a parent in the lupin breeding program as a source of both metribuzin tolerance and anthracnose resistance. Seed mutagenesis followed by field selection in M 2 proved to be an effective method to develop new tolerance to herbicides.

Establishment of Tolerance to Both Cadmium and Copper Stress by Expressing Arabidopsis Phytochelatin Synthase in Cu Tolerant Yeast Mutant

Phytochelatins (PCs) production by heterologous expression of Arabidopsis PC synthase (PCS) confers tolerance to Cd but sensitivity to Cu in PCS-deficient yeast, Saccharomycea cerevisiae. Due to its sensitivity to Cu, the PCS cannot be used in bioremediation of Cd in Cu contaminated sites, we attempted to solve this problem in the present study and did so by expressing PCS in copper resistant yeast mutants.

NIP1;1, an Aquaporin Homolog, Determines the Arsenite Sensitivity of Arabidopsis thaliana

Arsenite [As(III)] is highly toxic to organisms, including plants. Very recently, transporters in rice responsible for As(III) transport have been described (Ma, J. F., Yamaji, N., Mitani, N., Xu, X. Y., Su, Y. H., McGrath, S. P., and Zhao, F. J. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 9931-9935), but little is known about As(III) tolerance. In this study, three independent As(III)-tolerant mutants were isolated from ethyl methanesulfonate-mutagenized M2 seeds of Arabidopsis thaliana. All three mutants carried independent mutations in Nodulin 26-like intrinsic protein 1;1 (NIP1;1), a homolog of an aquaporin. Two independent transgenic lines carrying T-DNA in NIP1;1 were highly tolerant to As(III), establishing that NIP1;1 is the causal gene of As(III) tolerance. Because an aquaglyceroporin is able to transport As(III), we measured As(III) transport activity. When expressed in Xenopus oocytes, NIP1;1 was capable of transporting As(III). As content in the mutant plants was 30% lower than in wild-type plants. Promoter beta-glucuronidase and real-time PCR analysis showed that NIP1;1 is highly expressed in roots, and GFP-NIP1;1 is localized to the plasma membrane. These data show that NIP1;1 is involved in As(III) uptake into roots and that disruption of NIP1;1 function confers As(III) tolerance to plants. NIP1;2 and NIP5;1, closely related homologs of NIP1;1, were also permeable to As(III). Although the disruption of these genes reduced the As content in plants, As(III) tolerance was not observed in nip1;2 and nip5;1 mutants. This indicates that As(III) tolerance cannot be simply explained by decreased As contents in plants.

Physiological and morphological characterization of tert ‐butylhydroperoxide tolerant Candida albicans mutants

tert -Butylhydroperoxide (t BOOH) tolerant Candida albicans mutants developed from clinical isolates were characterized with increased tolerance of the oxidative stress generating agents t BOOH and H2O2, continuous induction of the antioxidative defence system, reduced pseudohypha and hypha-forming capabilities, decreased phospholipase secretion and delayed growth in Sabouraud dextrose agar and broth media. Changes in antimycotic (fluconazole, voriconazole, amphotericin B, 5-fluorocytosine) tolerances as well as in total and cytochrome c-dependent respirations showed versatile patterns, meanwhile the intensified alternative oxidase-dependent respiration of the mutants indicated that this respiratory pathway was an important element of the antioxidative defence in general. Because the phenotypes of increased oxidative stress tolerance and reduced virulence attribute production always emerged concomitantly in t BOOH-tolerant mutants the natural selection of C. albicans strains more tolerant of oxidative stress is unlikely. Not surprisingly, a screening study failed to detect any C. albicans strains with increased oxidative stress tolerance among 46 randomly selected clinical isolates.

Relationship Between Proline and Hg2+-Induced Oxidative Stress in a Tolerant Rice Mutant

There has been little agreement regarding the mechanism by which proline reduces heavy metal stress. The present work examines the relationship between Hg(2+)-induced oxidative stress and proline accumulation in rice and explores the possible mechanisms through which proline protects against Hg(2+) stress. The effect of proline on alleviation of Hg(2+) toxicity was studied by spectrophotography and enzymatic methods. Hg(2+) induced oxidative stress in rice by increasing lipid peroxidation. Pretreatment of the rice with 2 mM proline for 12 h profoundly alleviated Hg(2+)-induced lipid peroxidation and minimized H(2)O(2) accumulation. Proline pretreatment significantly reduced (p < 0.01) the Hg(2+) content in rice leaves. A comparison of the effects of proline pretreatment on H(2)O(2) accumulation by Hg(2+) and aminotrazole suggested that proline protected cells from Hg(2+)-induced oxidative stress by scavenging reactive oxygen species. The present work demonstrates a protective effect of proline on Hg(2+) toxicity through detoxifying reactive oxygen species, rather than chelating metal ions or maintaining the water balance under Hg(2+) stress.

Effects of decreased specific glutathione reductase activity in a chromate-tolerant mutant of Schizosaccharomyces pombe

A chromate-tolerant mutant chr1-663T bearing a stable one-gene mutation and its parental strain 6chr(+) were used to investigate the background of Cr(VI) tolerance in the fission yeast Schizosaccharomyces pombe. The mutant chr1-663T displayed a significantly decreased specific glutathione reductase (GR) activity coded by the pgr1 (+) gene compared with its parental strain. Transformants of the mutant chr1-663T with a nonintegrative pUR18N vector expressing the pgr1 (+) gene exhibited the same Cr(VI) sensitivity and specific GR activity as their parental strain, demonstrating the importance of the GR-NADPH system in Cr(VI) tolerance. Transformants, nevertheless, exhibited an increased intracellular peroxide concentration, a decreased Cr(VI)-reducing and HO*-producing ability, which suggested an unbalanced oxidoreduction state of cells and partial complementation of the GR function. No mutation was found in the sequences of the pgr1 (+) and the pap1 (+) (transcriptional regulatory gene of GR) genes of the Cr(VI)-tolerant mutant by sequence analysis.

Effect of Ascorbate-Glutathione (AsA-GSH) Cycle on Hg 2+-Tolerance in Rice Mutant

The rice ( Oryza sativa L.) cultivar Zhonghua 11 and an Hg 2+-tolerant rice mutant were cultured in solution for investigating the reactive oxygen species (ROS) metabolism and the dynamic change of ascorbate-glutathione (AsA-GSH) cycle in response to 0.4 mmol L −1 Hg 2+ treatment. The evolution rate of superoxide radical (O 2) and the contents of H 2O 2, malondialdehyde (MDA), oxidized glutathione (GSSG), and docosahexaenoic acid (DHA) of the leaves were higher in the wild type than in the mutant. The ratio of GSH to GSSG and the ratio of AsA to DHA were higher in the mutant than in the wild type, while the accumulation of Hg 2+ in roots and stems of the mutant was more than that in the wild type. These results indicated that the AsA-GSH cycle was less inhibited in the mutant than in the wild type, and the mutant was thus able to scavenge ROS more. An effective AsA-GSH cycle is important for the mercury resistance of mutant.

Ethyl methane sulfonate(EMS)에 의해 변이된 애기장대 종자 집단으로부터 염 내성 돌연변이체 선발 및 특성 분석

염 내성 돌연변이체를 선발하기 위하여, ethyl methane sulfonate(EMS)로 처리된 돌연변이 종자 집단을 사용하였다. 150 mM NaCl 고염 스트레스 하에서 종자 발아 내성을 보이는 세 종류의 EMS 돌연변이체를 선발하였다. 세 종류의 EMS 돌연변이체들 중, salt tolerance 42-14(sto42-14) 돌연변이체는 175 mM NaCl 고농도에서 종자 발아율이 대조구(WT)에 비해 7배 이상의 높은 발아율을 보였다. 또한 내염성 sto42-14 돌연변이체는 glucose(Glc)에 대해서도 비감수성을 갖고 있음을 관찰되었고, 흥미롭게도 sto42-14돌연변이체에 <TEX>$20{\mu}M$</TEX> gibberellin(GA)을 처리한 결과, 대조구에 비해 하배축과 뿌리의 생장이 억제됨을 관찰할 수 있었다. 이러한 결과를 바탕으로, 고염 내성 sto42-14 돌연변이체는 Glc 뿐만 아니라 GA호르몬 반응에도 관련되어져 있음을 알 수 있다. We conducted a seed germination screening under saline conditions to identify salt tolerance(sto) mutants with ethyl methane sulfonate(EMS) mutagenesis seed pool. During the screening, we identified three mutant lines that seemed to confer elevated salt tolerance in high concentrations of NaCl. At 175 mM NaCl, germination rate of sto42-14 mutant(one of the EMS salt tolerance mutants) was 7-fold higher than that of wild-type plants. Interestingly, sto42-14 mutant exhibited insensitivity to high glucose concentration and growth inhibition to gibberellin. Our results suggest that sto42-14 is involved in salt stress tolerance as well as in glucose and gibberellin response in Arabidopsis.

A Mutant of the Arabidopsis Phosphate Transporter PHT1;1 Displays Enhanced Arsenic Accumulation

The exceptional toxicity of arsenate [As(V)] is derived from its close chemical similarity to phosphate (Pi), which allows the metalloid to be easily incorporated into plant cells through the high-affinity Pi transport system. In this study, we identified an As(V)-tolerant mutant of Arabidopsis thaliana named pht1;1-3, which harbors a semidominant allele coding for the high-affinity Pi transporter PHT1;1. pht1;1-3 displays a slow rate of As(V) uptake that ultimately enables the mutant to accumulate double the arsenic found in wild-type plants. Overexpression of the mutant protein in wild-type plants provokes phenotypic effects similar to pht1;1-3 with regard to As(V) uptake and accumulation. In addition, gene expression analysis of wild-type and mutant plants revealed that, in Arabidopsis, As(V) represses the activation of genes specifically involved in Pi uptake, while inducing others transcriptionally regulated by As(V), suggesting that converse signaling pathways are involved in plant responses to As(V) and low Pi availability. Furthermore, the repression effect of As(V) on Pi starvation responses may reflect a regulatory mechanism to protect plants from the extreme toxicity of arsenic.