Masahiro Inouhe

The detoxification of lead in Sedum alfredii H. is not related to phytochelatins but the glutathione

Two ecotypes of S. alfredii [Pb accumulating (AE) and Pb non-accumulating (NAE)] differing in their ability in accumulating Pb were exposed to different Pb levels to evaluate the effects on plant length, photosynthetic pigments, antioxidant enzymes (SOD and APX), cysteine, non-protein thiols (NP-SH), phytochelatins (PCs) and glutathione (GSH) vis-à-vis Pb accumulation. Both ecotypes showed significant Pb accumulation in roots, however only the AE showed significant Pb accumulation in shoots. We found that both AE and NAE of S. alfredii-induced biosynthesis of GSH rather than phytochelatins in their tissue upon addition of even high Pb levels (200 microM). Root and shoot length were mostly affected in both ecotypes after addition of higher Pb concentrations and on longer durations, however photosynthetic pigments did not alter upon addition of any Pb treatment. Both superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities of AE were higher than NAE. The levels of cysteine and NP-SH were also higher in AE than in NAE. Hence, the characteristic Pb accumulation of ecotypes differed presumably in relation to their capacity for detoxification of Pb. These results suggest that enzymatic and non-enzymatic antioxidants play a key role in the detoxification of Pb-induced toxic effects in Sedum alfredii. This plant can be used as an indicator species for Pb contamination.

Different characteristics of roots in the cadmium-tolerance and Cd-binding complex formation between mono- and dicotyledonous plants

Effects of Cd2+ on growth and Cd-binding complex formation in roots were examined with various seedlings of mono- and dicotyledonous plants. Maize, oat, barley and rice exhibited the greater tolerance to Cd2+ (100 μM) than did azuki bean, cucumber, lettuce, pea, radish, sesame and tomato (10–30 μM). Azuki bean was the most sensitive to Cd2+ (<10 μM). Under these Cd-treatments, cereal roots accumulated Cd2+ in the cytoplasmic fractions and transported Cd2+ into the same fractions of shoot tissues, to larger extents than did dicotyledonous roots. Cereal roots synthesized a Cd-binding complex containing phytochelatins in the cytoplasmic fractions, depending upon Cd2+ concentrations applied (30–100 μM). Such a complex was not detected from the same fractions of dicotyledonous roots treated with Cd2+. These results suggest that the Cd-binding complex formation has an important role in the tolerance of cereal roots against Cd2+.

Changes in the water status and osmotic solute contents in response to drought and salicylic acid treatments in four different cultivars of wheat (Triticum aestivum)

Salicylic acid (SA) controls growth and stress responses in plants. It also induces drought tolerance in plants. In this paper, four wheat (Triticum aestivum L.) cultivars with different drought responses were treated with SA in three levels of drain (90, 60, 30% of maximum field capacity) to examine its interactive effects on drought responses and contents of osmotic solutes that may be involved in growth and osmotic adjustment. Under drought condition, the cultivars Geza 164 and Sakha 69 had the plant biomass and leaf relative water content (LRWC) greater than the cultivars Gemaza 1 and Gemaza 3. In all cultivars, drought stress decreased the biomass, LRWC, and the contents of inorganic solutes (Ca, K, Mg) and largely increased the contents of organic solutes (soluble sugars and proline). By contrast, SA increased the biomass, LRWC and the inorganic and organic solute contents, except proline. Correlation analysis revealed that the LRWC correlated positively with the inorganic solute contents but negatively with proline in all cultivars. SA caused maximum accumulations of soluble sugars in roots under drought. These results indicated that SA-enhanced tolerance might involve solute accumulations but independently of proline biosynthesis. Drought-sensitive cultivars had a trait lowering Ca and K levels especially in shoots. Possible functions of the ions and different traits of cultivars were discussed.

Nickel resistance mechanisms in yeasts and other fungi

SummaryThis review describes nickel toxicity and nickel resistance mechanisms in fungi. Nickel toxicity in fungi is influenced by environmental factors such as pH, temperature and the existence of organic matter and other ions. We describe resistance mechanisms in nickel-resistant mutants of yeasts and filamentous fungi which were obtained by exposure to a mutagen or by successive culture in media containing increasing concentrations of nickel ion. Nickel resistance may involve: (1) inactivation of nickel toxicity by the production of extracellular nickel-chelating substances such as glutathione; (2) reduced nickel accumulation, probably by modification of a magnesium transport system; (3) sequestration of nickel into a vacuole associated with free histidine and involving Ni-insensitivity of vacuolar membrane H+-ATPase.

Changes in the levels of phytochelatins and related metal-binding peptides in chickpea seedlings exposed to arsenic and different heavy metal ions

Phytochelatin-related peptides were analyzed in chickpea plants exposed to six different heavy-metal ions. Cadmium and arsenic stimulated phytochelatin and homophytochelatin synthesis in roots but other metals did not. These metals, however, caused an overall increase in the precursors, glutathione, homoglutathione and cysteine. These changes may be different biochemical indexes for heavy-metal contamination.

The cadmium-resistant gene, CAD2, which is a mutated putative copper-transporter gene (PCA1), controls the intracellular cadmium-level in the yeast S. cerevisiae

Abstract Yeast cells carrying the CAD2 gene exhibit a resistance to cadmium. We cloned this gene and demonstrated that it was a mutated form derived from the gene of a putative copper-transporting ATPase (PCA1). By site-directed mutagenesis, it appeared that the mutation conferring cadmium resistance was a R970G-substitution in the C-terminal region of Pca1 protein. The intracellular cadmium level of cells carrying CAD2 was lower than that of cells carrying either PCA1 or Δcad2. Furthermore, cells with overexpression of CAD2 showed a much lower intracellular cadmium level than that of cells with a single-copy CAD2. From these results, we conclude that the Cad2 protein controls the intracellular cadmium level through an enhanced cadmium efflux system.

Oxidative stress and arsenic toxicity: Role of NADPH oxidases

The effect of arsenic (25 and 50 μM As for 1 and 5d) was analysed in wild type (WT) and Arabidopsis thaliana (L.) Heynh plants deficient in NADPH oxidase C (AtrbohC). The content of H(2)O(2) and malondialdehyde (MDA) increased with the As concentration, while the opposite effect was found for NO in WT and AtrbohC plants. The As treatment reduced catalase and increased glutathione reductase activities to the same extent in WT and AtrbohC plants, although the induction of all SOD isoforms (mainly CuZn-SODs) was observed in WT plants, the opposite effects being found in AtrbohC plants. Glycolate oxidase (H(2)O(2) producers) considerably increased with the concentration and time of treatment with As in WT and AtrbohC mutants. Arsenic induced the uptake and translocation of P, S, Cu, Zn, and Fe in WT plants, while in AtrbohC plants the opposite trend was noted and the uptake of As became considerably lower than in WT plants. These results suggest that As causes oxidative stress by inducing glycolate oxidase, while NADPH oxidase does not appear to participate in ROS overproduction but could be critical in regulating antioxidant defences as well as the transport and translocation of As and macro/micronutrients.

Endo-1,3;1,4-β-glucanase from coleoptiles of rice and maize: role in the regulation of plant growth

The Matrix Polymer Hydrolysis Model for regulation of growth in plants is based on the simultaneous hydrolysis and incorporation of new glucans into the cell wall observed in growing plant tissues. The inhibition of growth in rice coleoptile tissues treated with glucanase antibodies confirms similar results observed previously in maize coleoptiles and provides direct evidence for a role of glucanase in control of plant growth. Analysis of two-maize coleoptile endo-glucanase ESTs shows that these sequences are not related to any other previously known family of glycosyl hydrolase. Thus, the coleoptile endo-glucanase enzyme should be classified as a new enzyme group (E.C. 3.2.1.xx). These discoveries enable new initiatives for further investigation of the glucanase role in control of plant growth.

Production of metallothionein in copper- and cadmium-resistant strains ofSaccharomyces cerevisiae

SummaryCertain mutants of the yeastSaccharomyces cerevisiae show copper or cadmium resistance. Both copper- and cadmium-resistant strains produce the same metallothionein with 53 amino acid residues which causes metal detoxification by chelating copper or cadmium. The metal detoxification role is the only known function of the metallothionein in yeast. The MT is encoded by theCUP1 gene on chromosome VIII which is expressed by induction with metals. TheCUP1 is amplified to 3–14 copies with 2 kb-tandem-repeat units in the metal-resistant strains, whereas the wild-type strain contains only a single copy of theCUP1. Although transcription ofCUP1 is inducible by metals, the ACE1 protein serves a dual function as a sensor for copper and an inducer forCUP1 transcription in the copper-resistant strain. In the cadmium-resistant strain, the heat-shock factor having a point mutation may be the regulator forCUP1 transcription. Therefore, it has been clarified that production of MT in yeast in controlled by two systems, the amplification ofCUP1 and its transcriptional regulation.

Cadmium-binding protein in a cadmium-resistant strain of Saccharomyces cerevisiae

A Cd-binding protein in the Cd2+-resistant strain 301N of Saccharomyces cerevisiae was induced by administration to 0.5 mM CdSO4. The protein was purified by a gel-permeation and subsequent ion-exchange column chromatographies. The purified Cd-binding protein had the characteristics of metallothioneins: (1) low molecular weight (9.0 kDa), (2) high Cd content (63 micrograms/mg protein), (3) amino-acid composition rich in cysteine (18%), basic and acidic amino acids and free from aromatic amino acids, and (4) an absorption shoulder at near 250 nm. Acid pH or EDTA treatments abolished 250 nm absorption of the Cd-binding protein, and the formed apoprotein was capable of binding Cd2+, Cu2+ and Zn2+, respectively. Heat treatment (75 degrees C) little affected the ultraviolet absorption or sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles of Cd-binding protein. These results suggest that metallothionein generally found in animals also occurs in Cd-adapted yeast cells and thus has a role in its Cd-resistance.