Isolation and characterization of wheat aluminum-regulated genes: possible involvement of aluminum as a pathogenesis response elicitor

Proteolytic and Structural Changes in Rye and Triticale Roots under Aluminum Stress

Proteolysis and structural adjustments are significant for defense against heavy metals. The purpose of this study was to evaluate whether the Al³⁺ stress alters protease activity and the anatomy of cereale roots. Azocaseinolytic and gelatinolytic measurements, transcript-level analysis of phytocystatins, and observations under microscopes were performed on the roots of Al³⁺-tolerant rye and tolerant and sensitive triticales exposed to Al³⁺. In rye and triticales, the azocaseinolytic activity was higher in treated roots. The gelatinolytic activity in the roots of rye was enhanced between 12 and 24 h in treated roots, and decreased at 48 h. The gelatinolytic activity in treated roots of tolerant triticale was the highest at 24 h and the lowest at 12 h, whereas in treated roots of sensitive triticale it was lowest at 12 h but was enhanced at 24 and 48 h. These changes were accompanied by increased transcript levels of phytocystatins in rye and triticale-treated roots. Light microscope analysis of rye roots revealed disintegration of rhizodermis in treated roots at 48 h and indicated the involvement of root border cells in rye defense against Al³⁺. The ultrastructural analysis showed vacuoles containing electron-dense precipitates. We postulate that proteolytic-antiproteolytic balance and structural acclimation reinforce the fine-tuning to Al³⁺.

Aluminum toxicity and aluminum stress-induced physiological tolerance responses in higher plants

Aluminum (Al) precipitates in acidic soils having a pH < 5.5, in the form of conjugated organic and inorganic ions. Al-containing minerals solubilized in the soil solution cause several negative impacts in plants when taken up along with other nutrients. Moreover, a micromolar concentration of Al present in the soil is enough to induce several irreversible toxicity symptoms such as the rapid and transient over-generation of reactive oxygen species (ROS) such as superoxide anion (O₂•-), hydrogen peroxide (H₂O₂), and hydroxyl radical (•OH), resulting in oxidative bursts. In addition, significant reductions in water and nutrient uptake occur which imposes severe stress in the plants. However, some plants have developed Al-tolerance by stimulating the secretion of organic acids like citrate, malate, and oxalate, from plant roots. Genes responsible for encoding such organic acids, play a critical role in Al tolerance. Several transporters involved in Al resistance mechanisms are members of the Aluminum-activated Malate Transporter (ALMT), Multidrug and Toxic compound Extrusion (MATE), ATP-Binding Cassette (ABC), Natural resistance-associated macrophage protein (Nramp), and aquaporin gene families. Therefore, in the present review, the discussion of the global extension and probable cause of Al in the environment and mechanisms of Al toxicity in plants are followed by detailed emphasis on tolerance mechanisms. We have also identified and categorized the important transporters that secrete organic acids and outlined their role in Al stress tolerance mechanisms in crop plants. The information provided here will be helpful for efficient exploration of the available knowledge to develop Al tolerant crop varieties.

Molecular Mechanisms Underlying Sugarcane Response to Aluminum Stress by RNA-Seq

Some metals are beneficial to plants and contribute to critical physiological processes. Some metals, however, are not. The presence of aluminum ions (Al³⁺) can be very toxic, especially in acidic soils. Considerable parts of the world's arable land are acidic in nature; mechanistically elucidating a plant's response to aluminum stress is critical to mitigating this stress and improving the quality of plants. To identify the genes involved in sugarcane response to aluminum stress, we generated 372 million paired-end RNA sequencing reads from the roots of CTC-2 and RB855453, which are two contrasting cultivars. Data normalization resulted in 162,161 contigs (contiguous sequences) and 97,335 genes from a de novo transcriptome assembly (trinity genes). A total of 4858 and 1307 differently expressed genes (DEGs) for treatment versus control were identified for the CTC-2 and RB855453 cultivars, respectively. The DEGs were annotated into 34 functional categories. The majority of the genes were upregulated in the CTC-2 (tolerant cultivar) and downregulated in RB855453 (sensitive cultivar). Here, we present the first root transcriptome of sugarcane under aluminum stress. The results and conclusions of this study are a crucial launch pad for future genetic and genomic studies of sugarcane. The transcriptome analysis shows that sugarcane tolerance to aluminum may be explained by an efficient detoxification mechanism combined with lateral root formation and activation of redox enzymes. We also present a hypothetical model for aluminum tolerance in the CTC-2 cultivar.

The dehydration- and ABA-inducible germin-like protein CpGLP1 from Craterostigma plantagineum has SOD activity and may contribute to cell wall integrity during desiccation

MAIN CONCLUSION

CpGLP1 belongs to the large group of germin-like proteins and comprises a cell wall-localized protein which has superoxide dismutase activity and may contribute towards ROS metabolism and cell wall folding during desiccation. The plant cell wall is a dynamic matrix and its plasticity is essential for cell growth and processing of environmental signals to cope with stresses. A few so-called resurrection plants like Craterostigma plantagineum survive desiccation by implementing protection mechanisms. In C. plantagineum, the cell wall shrinks and folds upon desiccation to avoid mechanical and oxidative damage which contributes to cell integrity. Despite the high toxic potential, ROS are important molecules for cell wall remodeling processes as they participate in enzymatic reactions and act as signaling molecules. Here we analyzed the C. plantagineum germin-like protein 1 (CpGLP1) to understand its contribution to cell wall folding and desiccation tolerance. The analysis of the CpGLP1 sequence showed that this protein does not fit into the current GLP classification and forms a new group within the Linderniaceae. CpGLP1 transcripts accumulate in leaves in response to dehydration and ABA, and mannitol treatments transiently induce CpGLP1 transcript accumulation supporting the participation of CpGLP1 in desiccation-related processes. CpGLP1 protein from cell wall protein extracts followed transcript accumulation and protein preparations from bacteria overexpressing CpGLP1 showed SOD activity. In agreement with cell wall localization, CpGLP1 interacts with pectins which have not been reported for GLP proteins. Our data support a role for CpGLP1 in the ROS metabolism related to the control of cell wall plasticity during desiccation in C. plantagineum.

Convergent evolution in Arabidopsis halleri and Arabidopsis arenosa on calamine metalliferous soils

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa, which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Convergent evolution in Arabidopsis halleri and Arabidopsis arenosa on calamine metalliferous soils

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa, which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Convergent evolution in Arabidopsis halleri and Arabidopsis arenosa on calamine metalliferous soils

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa, which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Transcriptional Regulation of Aluminum-Tolerance Genes in Higher Plants: Clarifying the Underlying Molecular Mechanisms

Aluminum (Al) rhizotoxicity is one of the major environmental stresses that decrease global food production. Clarifying the molecular mechanisms underlying Al tolerance may contribute to the breeding of Al-tolerant crops. Recent studies identified various Al-tolerance genes. The expression of these genes is inducible by Al. Studies of the major Arabidopsis thaliana Al-tolerance gene, ARABIDOPSIS THALIANA ALUMINUM-ACTIVATED MALATE TRANSPORTER 1 (AtALMT1), which encodes an Al-activated malate transporter, revealed that the Al-inducible expression is regulated by a SENSITIVE TO PROTON RHIXOTOXICITY 1 (STOP1) zinc-finger transcription factor. This system, which involves STOP1 and organic acid transporters, is conserved in diverse plant species. The expression of AtALMT1 is also upregulated by several phytohormones and hydrogen peroxide, suggesting there is crosstalk among the signals involved in the transcriptional regulation of AtALMT1. Additionally, phytohormones and reactive oxygen species (ROS) activate various transcriptional responses, including the expression of genes related to increased Al tolerance or the suppression of root growth under Al stress conditions. For example, Al suppressed root growth due to abnormal accumulation of auxin and cytokinin. It activates transcription of TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 and other phytohormone responsive genes in distal transition zone, which causes suppression of root elongation. On the other hand, overexpression of Al inducible genes for ROS-detoxifying enzymes such as GLUTATHIONE-S-TRANSFERASE, PEROXIDASE, SUPEROXIDE DISMUTASE enhances Al resistance in several plant species. We herein summarize the complex transcriptional regulation of an Al-inducible genes affected by STOP1, phytohormones, and ROS.

Do Cupins Have a Function Beyond Being Seed Storage Proteins?

Plants continue to flourish around the site of the Chernobyl Nuclear Power Plant disaster. The ability of plants to transcend the radio-contaminated environment was not anticipated and is not well understood. The aim of this study was to evaluate the proteome of flax (Linum usitatissimum L.) during seed filling by plants grown for a third generation near Chernobyl. For this purpose, seeds were harvested at 2, 4, and 6 weeks after flowering and at maturity, from plants grown in either non-radioactive or radio-contaminated experimental fields. Total proteins were extracted and the two-dimensional gel electrophoresis (2-DE) patterns analyzed. This approach established paired abundance profiles for 130 2-DE spots, e.g., profiles for the same spot across seed filling in non-radioactive and radio-contaminated experimental fields. Based on Analysis of Variance (ANOVA) followed by sequential Bonferroni correction, eight of the paired abundance profiles were discordant. Results from tandem mass spectrometry show that four 2-DE spots are discordant because they contain fragments of the cupin superfamily-proteins. Most of the fragments were derived from the N-terminal half of native cupins. Revisiting previously published data, it was found that cupin-fragments were also involved with discordance in paired abundance profiles of second generation flax seeds. Based on these observations we present an updated working model for the growth and reproductive success of flax in a radio-contaminated Chernobyl environment. This model suggests that the increased abundance of cupin fragments or isoforms and monomers contributes to the successful growth and reproduction of flax in a radio-contaminated environment.

Mitogen-activated protein kinase signal transduction and DNA repair network are involved in aluminum-induced DNA damage and adaptive response in root cells of Allium cepa L

In the current study, we studied the role of signal transduction in aluminum (Al(3+))-induced DNA damage and adaptive response in root cells of Allium cepa L. The root cells in planta were treated with Al(3+) (800 μM) for 3 h without or with 2 h pre-treatment of inhibitors of mitogen-activated protein kinase (MAPK), and protein phosphatase. Also, root cells in planta were conditioned with Al(3+) (10 μM) for 2 h and then subjected to genotoxic challenge of ethyl methane sulfonate (EMS; 5 mM) for 3 h without or with the pre-treatment of the aforementioned inhibitors as well as the inhibitors of translation, transcription, DNA replication and repair. At the end of treatments, roots cells were assayed for cell death and/or DNA damage. The results revealed that Al(3+) (800 μM)-induced significant DNA damage and cell death. On the other hand, conditioning with low dose of Al(3+) induced adaptive response conferring protection of root cells from genotoxic stress caused by EMS-challenge. Pre-treatment of roots cells with the chosen inhibitors prior to Al(3+)-conditioning prevented or reduced the adaptive response to EMS genotoxicity. The results of this study suggested the involvement of MAPK and DNA repair network underlying Al-induced DNA damage and adaptive response to genotoxic stress in root cells of A. cepa.

Aluminum uptake and disease resistance in Nicotiana rustica leaves

The comparative effectiveness of aluminum hydroxide and aluminum chloride has been studied in the development of bacterial wilt infection on leaves of Nicotiana rustica cv. Gansu yellow flower. We have analyzed the changes of foliar H(2)O(2) content, as well as of non-enzymatic and enzymatic antioxidants under aluminum stress. Pretreatment with aluminum hydroxide before pathogen challenge reduced the development of Ralstonia solanacearum infection and decreased the extent of leaf injury. The pretreatment also reduced the Al uptake in comparison to pretreatment with aluminum chloride. H(2)O(2) generation was significantly enhanced by pretreatment with aluminum hydroxide. Increased NADPH oxidase and superoxide dismutase activities were correlated with limited infection. Aluminum hydroxide pretreatment shifted the leaf redox homeostasis of AsA/DHA and GSH/GSSG toward oxidation, yielding higher oxidant levels than aluminum chloride before bacterial inoculation. The results support the idea that aluminum hydroxide induced H(2)O(2) accumulation through non-enzymatic and enzymatic regulation, ultimately resulting in resistance to tobacco wilt disease.

Identification of genes and pathways associated with aluminum stress and tolerance using transcriptome profiling of wheat near-isogenic lines

Background

Aluminum is considered the most limiting factor for plant productivity in acidic soils, which cover large areas of the world's potential arable lands. The inhibition of root growth is recognized as the primary effect of Al toxicity. To identify genes associated with Al stress and tolerance, transcriptome analyses of four different wheat lines (2 Al-tolerant and 2 Al sensitive) that differ in their response to Al were performed.

Results

Microarray expression profiling revealed that 83 candidate genes are associated with Al stress and 25 are associated with tolerance. The stress-associated genes include important enzymes such as pyruvate dehydrogenase, alternative oxidase, and galactonolactone oxidase, ABC transporter and ascorbate oxido-reducatase. The Al tolerance-associated genes include the ALMT-1 malate transporter, glutathione S-transferase, germin/oxalate oxidase, fructose 1,6-bisphosphatase, cysteine-rich proteins, cytochrome P450 monooxygenase, cellulose synthase, zinc finger transcription factor, disease resistance response protein and F-box containing domain protein.

Conclusion

In this survey, we identified stress- and tolerance-associated genes that may be involved in the detoxification of Al and reactive oxygen species. Alternative pathways could help maintain the supply of important metabolites (H₂O₂, ascorbate, NADH, and phosphate) needed for Al tolerance and root growth. The Al tolerance-associated genes may be key factors that regulate these pathways.

Physiological and molecular characterization of aluminum resistance in Medicago truncatula

BACKGROUND

Aluminum (Al) toxicity is an important factor limiting crop production on acid soils. However, little is known about the mechanisms by which legumes respond to and resist Al stress. To explore the mechanisms of Al toxicity and resistance in legumes, we compared the impact of Al stress in Al-resistant and Al-sensitive lines of the model legume, Medicago truncatula Gaertn.

RESULTS

A screen for Al resistance in 54 M. truncatula accessions identified eight Al-resistant and eight Al-sensitive lines. Comparisons of hydroponic root growth and root tip hematoxylin staining in an Al-resistant line, T32, and an Al-sensitive line, S70, provided evidence that an inducible Al exclusion mechanism occurs in T32. Transcriptional events associated with the Al resistance response were analyzed in T32 and S70 after 12 and 48 h Al treatment using oligonucleotide microarrays. Fewer genes were differentially regulated in response to Al in T32 compared to S70. Expression patterns of oxidative stress-related genes, stress-response genes and microscopic examination of Al-treated root tips suggested a lower degree of Al-induced oxidative damage to T32 root tips compared to S70. Furthermore, genes associated with cell death, senescence, and cell wall degradation were induced in both lines after 12 h of Al treatment but preferentially in S70 after 48 h of Al treatment. A multidrug and toxin efflux (MATE) transporter, previously shown to exude citrate in Arabidopsis, showed differential expression patterns in T32 and S70.

CONCLUSION

Our results identified novel genes induced by Al in Al-resistant and sensitive M. truncatula lines. In T32, transcription levels of genes related to oxidative stress were consistent with reactive oxygen species production, which would be sufficient to initiate cell death of Al-accumulating cells thereby contributing to Al exclusion and root growth recovery. In contrast, transcriptional levels of oxidative stress-related genes were consistent with excessive reactive oxygen species accumulation in S70 potentially resulting in necrosis and irreversible root growth inhibition. In addition, a citrate-exuding MATE transporter could function in Al exclusion and/or internal detoxification in T32 based on Al-induced transcript localization studies. Together, our findings indicate that multiple responses likely contribute to Al resistance in M. truncatula.

Transcriptome profiling identified novel genes associated with aluminum toxicity, resistance and tolerance in Medicago truncatula

Oligonucleotide microarrays corresponding to over 16,000 genes were used to analyze changes in transcript accumulation in root tips of the Al-sensitive Medicago truncatula cultivar Jemalong genotype A17 in response to Al treatment. Out of 2,782 genes with significant changes in transcript accumulation, 324 genes were up-regulated and 267 genes were down-regulated at least twofold by Al. Up-regulated genes were enriched in transcripts involved in cell-wall modification and abiotic and biotic stress responses while down-regulated genes were enriched in transcripts involved in primary metabolism, secondary metabolism, protein synthesis and processing, and the cell cycle. Known markers of Al-induced gene expression including genes associated with oxidative stress and cell wall stiffening were differentially regulated in this study. Transcript profiling identified novel genes associated with processes involved in Al toxicity including cell wall modification, cell cycle arrest and ethylene production. Novel genes potentially associated with Al resistance and tolerance in M. truncatula including organic acid transporters, cell wall loosening enzymes, Ca(2+) homeostasis maintaining genes, and Al-binding were also identified. In addition, expression analysis of nine genes in the mature regions of the root revealed that Al-induced gene expression in these regions may play a role in Al tolerance. Finally, interfering RNA-induced silencing of two Al-induced genes, pectin acetylesterase and annexin, in A17 hairy roots slightly increased the sensitivity of A17 to Al suggesting that these genes may play a role in Al resistance.

Aluminium induced oxidative stress and DNA damage in root cells of Allium cepa L

Aluminium (Al) was evaluated for induction of oxidative stress and DNA damage employing the growing roots of Allium cepa L. as the assay system. Intact roots of A. cepa were treated with different concentrations, 0, 1, 10, 50, 100, or 200 microM of aluminium chloride, at pH 4.5 for 4 h (or 2 h for comet assay) at room temperature, 25+/-1 degrees C. Following treatment the parameters investigated in root tissue were Al-uptake, cell death, extra cellular generation of reactive oxygen intermediates (ROI), viz. O(2)(*-), H(2)O(2) and (*)OH, lipid peroxidation, protein oxidation, activities of antioxidant enzymes namely catalase (CAT), superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX); and DNA damage, assessed by comet assay. The findings indicated that Al triggered generation of extra-cellular ROI following a dose-response. Through application of specific enzyme inhibitors it was demonstrated that extra-cellular generation of ROI was primarily due to the activity of cell wall bound NADH-PX. Generation of ROI in root tissue as well as cell death was better correlated to the levels of root Al-uptake rather than to the concentrations of Al in ambient experimental solutions. Induction of lipid peroxidation and protein oxidation by Al were statistically significant. Whereas Al inhibited CAT activity, enhanced SOD, GPX and APX activities significantly; that followed dose-response. Comet assay provided evidence that Al induced DNA damage in a range of concentrations 50-200 microM, which was comparable to that induced by ethylmethane sulfonate (EMS), an alkylating mutagen served as the positive control. The findings provided evidence that Al comparable to biotic stress induced oxidative burst at the cell surface through up- or down-regulation of some of the key enzymes of oxidative metabolism ultimately resulting in oxidative stress leading to DNA damage and cell death in root cells of A. cepa.

Transcriptomic responses to aluminum stress in roots of Arabidopsis thaliana

To help characterize the cellular mechanisms underlying the toxicity of Al to plants, we present the first large-scale, transcriptomic analysis of root responses to Al, using a microarray representing approximately 93% of the predicted genes in the genome of Arabidopsis. More transcripts were responsive to Al (25 microM) during long (48 h, 1,114 genes), as compared to short (6 h, 401 genes) exposures, which contrasts with previous microarray analyses of plant responses to other types of abiotic stress. Exposure to Al triggered changes in the transcript levels for several genes related to oxidative stress pathway, membrane transporters, cell wall, energy, and polysaccharide metabolism. Interestingly, lack of abundance of transcripts encoding TCA cycle enzymes, except for malate dehydrogenase, suggested that synthesis of organic anions in response to Al may not be transcriptionally regulated. Al exposures induced differential abundance of transcripts for several ribosomal proteins, peptidases and protein phosphatases mostly after 48 h. We also detected increased abundance of transcripts for several membrane receptor kinases and non-membrane calcium response kinases, which could play a role in transmission of Al-stress signals. Among Al responsive transcription factors, the most predominant families identified were AP2/EREBP, MYB and bHLH. Further, we studied the kinetics of Al stress responses for class III peroxidases using Q-RT-PCR. Our results indicated that Al triggered dynamic changes in transcript abundance of various peroxidases within 1 h. The results of this screen contribute to the identification of candidate genes for the generation of Al-tolerant transgenic plants.

Transcriptional profiling of aluminum toxicity and tolerance responses in maize roots

Aluminum (Al) toxicity is a major factor limiting crop yields on acid soils. There is considerable genotypic variation for Al tolerance in most common plant species. In maize (Zea mays), Al tolerance is a complex phenomenon involving multiple genes and physiological mechanisms yet uncharacterized. To begin elucidating the molecular basis of maize Al toxicity and tolerance, a detailed temporal analysis of root gene expression under Al stress was performed using microarrays with Al-tolerant and Al-sensitive genotypes. Al altered the expression of significantly more genes in the Al-sensitive genotype, presumably as a result of more severe Al toxicity. Nevertheless, several Al-regulated genes exhibited higher expression in the Al-tolerant genotype. Cell wall-related genes, as well as low phosphate-responsive genes, were found to be regulated by Al. In addition, the expression patterns of genes related to Al-activated citrate release indicated that in maize this mechanism is probably regulated by the expression level and/or function of the citrate transporter. This study is the first comprehensive survey of global transcriptional regulation under Al stress. The results described here will help to further our understanding of how mechanisms of Al toxicity and tolerance in maize are regulated at the transcriptional level.

Oxalate oxidase and non-enzymatic compounds of the antioxidative system in young Serbian spruce plants exposed to cadmium stress

We studied changes in the concentrations of ascorbate and glutathione, composition of soluble phenolics, and activity of oxalate oxidase in 75-day-old Serbian spruce plants after exposure to 5 ?M and 50?M cadmium for 6-48 h. The presence of OxOxactivity in a conifer species is here demonstrated for the first time. Both Cd concentrations induced a decrease of OxOxactivity in treated plants in comparison with the control at all sampling dates. The concentrations of reduced glutathione, its oxidized form, and reduced ascorbate in the plants decreased during 48-h treatment with cadmium. Among simple phenolics, only catechin increased significantly during Cd treatment.

The genetic basic and fine-mapping of a stable quantitative-trait loci for aluminium tolerance in rice

Aluminium (Al) toxicity is a primary cause of low rice productivity in acid soils. We have mapped a number of quantitative-trait loci (QTL) controlling Al tolerance in a recombinant inbred line population derived from a cross between the tolerant japonica cultivar Asominori and the sensitive indica cultivar IR24. Tolerance was assessed on the basis of relative root elongation. QTL were detected on chromosomes 1, 9, and 11, with the percentages of phenotypic variance explained ranging from 13.5 to 17.7%. Alleles from Asominori at all three QTL were associated with increased Al tolerance. qRRE-9 is expressed both in the genetic background of IR24 and in an Asominori/IR24-mixed background. qRRE-9 was reduced to the single recessive Mendelian factor Alt-9. High-resolution genetic and physical maps were constructed for Alt-9 in a BC(3)F(2) population of 1,043 individuals. Alt-9 maps between RM24702 and ID47-2 on chromosome 9, and co-segregates with RM5765.

Candidate gene identification of an aluminum-activated organic acid transporter gene at the Alt4 locus for aluminum tolerance in rye (Secale cereale L.)

Among cereal crops, rye is one of the most tolerant species to aluminum. A candidate gene approach was used to determine the likely molecular identity of an Al tolerance locus (Alt4). Using PCR primers designed from a wheat aluminum tolerance gene encoding an aluminum-activated malate transporter (TaALMT1), a rye gene (ScALMT1) was amplified, cloned and sequenced. Subsequently, the ScALMT1 gene of rye was found to be located on 7RS by PCR amplification using the wheat-rye addition lines. SNP polymorphisms for this gene were detected among the parents of three F(2) populations that segregate for the Alt4 locus. A map of the rye chromosome 7R, including the Alt4 locus ScALMT1 and several molecular markers, was constructed showing a complete co-segregation between Alt4 and ScALMT1. Furthermore, expression experiments were carried out to clarify the function of this candidate gene. Briefly, the ScALMT1 gene was found to be primarily expressed in the root apex and upregulated when aluminum was present in the medium. Five-fold differences in the expression were found between the Al tolerant and the Al non-tolerant genotypes. Additionally, much higher expression was detected in the rye genotypes than the moderately tolerant "Chinese Spring" wheat cultivar. These results suggest that the Alt4 locus encodes an aluminum-activated organic acid transporter gene that could be utilized to increase Al tolerance in Al sensitive plant species. Finally, TaALMT1 homologous sequences were identified in different grasses and in the dicotyledonous plant Phaseolus vulgaris. Our data support the hypothesis of the existence of a common mechanism of Al tolerance encoded by a gene located in the homoeologous group four of cereals.

Use of a large-scale Triticeae expressed sequence tag resource to reveal gene expression profiles in hexaploid wheat (Triticum aestivum L.)

The US Wheat Genome Project, funded by the National Science Foundation, developed the first large public Triticeae expressed sequence tag (EST) resource. Altogether, 116,272 ESTs were produced, comprising 100,674 5' ESTs and 15 598 3' ESTs. These ESTs were derived from 42 cDNA libraries, which were created from hexaploid bread wheat (Triticum aestivum L.) and its close relatives, including diploid wheat (T. monococcum L. and Aegilops speltoides L.), tetraploid wheat (T. turgidum L.), and rye (Secale cereale L.), using tissues collected from various stages of plant growth and development and under diverse regimes of abiotic and biotic stress treatments. ESTs were assembled into 18,876 contigs and 23,034 singletons, or 41,910 wheat unigenes. Over 90% of the contigs contained fewer than 10 EST members, implying that the ESTs represented a diverse selection of genes and that genes expressed at low and moderate to high levels were well sampled. Statistical methods were used to study the correlation of gene expression patterns, based on the ESTs clustered in the 1536 contigs that contained at least 10 5' EST members and thus representing the most abundant genes expressed in wheat. Analysis further identified genes in wheat that were significantly upregulated (p < 0.05) in tissues under various abiotic stresses when compared with control tissues. Though the function annotation cannot be assigned for many of these genes, it is likely that they play a role associated with the stress response. This study predicted the possible functionality for 4% of total wheat unigenes, which leaves the remaining 96% with their functional roles and expression patterns largely unknown. Nonetheless, the EST data generated in this project provide a diverse and rich source for gene discovery in wheat.

Aluminium tolerance in barley (Hordeum vulgare L.): physiological mechanisms, genetics and screening methods

Aluminium (Al) toxicity is one of the major limiting factors for barley production on acid soils. It inhibits root cell division and elongation, thus reducing water and nutrient uptake, consequently resulting in poor plant growth and yield. Plants tolerate Al either through external resistance mechanisms, by which Al is excluded from plant tissues or internal tolerance mechanisms, conferring the ability of plants to tolerate Al ion in the plant symplasm where Al that has permeated the plasmalemma is sequestered or converted into an innocuous form. Barley is considered to be most sensitive to Al toxicity among cereal species. Al tolerance in barley has been assessed by several methods, such as nutrient solution culture, soil bioassay and field screening. Genetic and molecular mapping research has shown that Al tolerance in barley is controlled by a single locus which is located on chromosome 4H. Molecular markers linked with Al tolerance loci have been identified and validated in a range of diverse populations. This paper reviews the (1) screening methods for evaluating Al tolerance, (2) genetics and (3) mechanisms underlying Al tolerance in barley.

Aluminium Chloride Enhances Colchicine Production in Root Cultures of Gloriosa superba

Root cultures of Gloriosa superba were treated with 5 mM: methyl jasmonate and 125 microM: AlCl3 which enhanced the intracellular colchicine content of the roots by 50-fold and 63-fold, respectively. Ten millimolar of CaCl2 and 1 mM: CdCl2 enhanced biomass significantly (7- to 8.6-fold, respectively) while maximum release of colchicine into the medium was obtained with 10 mM: CdCl2. Casein hydrolysate, yeast extract and silver nitrate had no significant effect on growth and colchicine accumulation in root cultures.

Isolation and characterization of a rice mutant hypersensitive to Al

Rice (Oryza sativa L.) is a highly Al-resistant species among small grain crops, but the mechanism responsible for the high Al resistance has not been elucidated. In this study, rice mutants sensitive to Al were isolated from M(3) lines derived from an Al-resistant cultivar, Koshihikari, irradiated with gamma-rays. Relative root elongation was used as a parameter for evaluating Al resistance. After initial screening plus two rounds of confirmatory testing, a mutant (als1) was isolated from a total of 560 lines. This mutant showed a phenotype similar to the wild-type plant in the absence of Al. However, in the presence of 10 microM Al, root elongation was inhibited 70% in the mutant, but only 8% in the wild-type plant. The mutant also showed poorer root growth in acid soil. The Al content of root apices (0-1 cm) was much lower in the wild-type plant. The sensitivity to other metals including Cd and La did not differ between the mutant and the wild-type plants. A small amount of citrate was secreted from the roots of the mutant in response to Al stress, but there was no difference from that secreted by the wild-type plant. Genetic analysis of F(2) populations between als1 and wild-type plants showed that the Al-resistant seedlings and Al-sensitive seedlings segregated at a 3 : 1 ratio, indicating that the high sensitivity to Al in als1 is controlled by a single recessive gene. The gene was mapped to the long arm of chromosome 6, flanked by InDel markers MaOs0619 and MaOs0615.

The enzymes of oxalate metabolism: unexpected structures and mechanisms

Oxalate degrading enzymes have a number of potential applications, including medical diagnosis and treatments for hyperoxaluria and other oxalate-related diseases, the production of transgenic plants for human consumption, and bioremediation of the environment. This review seeks to provide a brief overview of current knowledge regarding the major classes of enzymes and related proteins that are employed in plants, fungi, and bacteria to convert oxalate into CO(2) and/or formate. Not only do these enzymes employ intriguing chemical strategies for cleaving the chemically unreactive C-C bond in oxalate, but they also offer the prospect of providing new insights into the molecular processes that underpin the evolution of biological catalysts.

Cytological and enzymatic responses to aluminium stress in root tips of Norway spruce seedlings

•  Aluminium (Al) stress reduces plant growth. However, some species such as Norway spruce (Picea abies) seem to tolerate high Al concentrations. The aim of this study was to investigate characteristics possibly involved in Al tolerance in Norway spruce seedlings. •  Seedlings (10-d-old) were exposed to Al³⁺ concentrations of 0.5 and 5 mm for up to 168 h. The effect of Al stress on root growth, cell morphology and Al distribution, callose production, and peroxidase and chitinase activity was analysed. •  Root growth decreased after 1 d and 2 d with 5 and 0.5 mm Al, respectively. Callose concentration increased strongly after 6 h treatment with 5 mm Al. The activity of many peroxidase and chitinase isoforms decreased after 1-24 h exposure of both treatments. Several isoforms increased after 48-168 h exposure to 5 mm Al. •  We postulate that, with external Al concentrations 0.5 mm or lower, an increased production above constitutive levels of peroxidase or chitinase is not required for Al tolerance in young Norway spruce seedlings. High constitutive levels of peroxidase and chitinase in this species may be part of this Al tolerance.

Oxidative burst and expression of germin/oxo genes during wounding of ryegrass leaf blades: comparison with senescence of leaf sheaths

Two bursts of H(2)O(2) production have been detected by in situ 3,3'-diaminobenzidine (DAB) staining after cutting of Lolium perenne L. leaf blades. The first burst, which occurred immediately after wounding was inhibited by Na-diethydithiocarbamate (DIECA), a Cu/Zn-superoxide dismutase (SOD) inhibitor. The second burst, which was initiated several hours later, coincided with the induction of oxalate oxidase (G-OXO) activity detected in vitro or visualized in situ by the alpha-chloronaphtol assay. Four genes encoding G-OXO have been identified from cDNA obtained from wounded L. perenne L. leaf blades. Comparison of protein sequences revealed more than 91% homology in the coding region between G-OXOs of the true cereals and G-OXOs of ryegrass, which is a Gramineae belonging to the tribe of Festucaceae. The wound-dependent increase of G-OXO activity in floated cut leaf blades was the result of differential induction of the four g-oxo genes. The involvement of G-OXOs in wound-induced H(2)O(2) production coincided with the presence in leaf tissues of oxalate throughout the period of increase of G-OXO synthesis. Moreover, expression of g-oxo genes was enhanced by an exogenous supply of H(2)O(2) or methyljasmonate (MeJa). Expression of the four g-oxo genes was also induced after in planta stinging of leaf blades. The pattern of their expression in planta was identical to that occuring in senescing leaf sheaths. These results emphasize the importance of G-OXOs in H(2)O(2) production in oxalate-producing plant species such as ryegrass. G-OXOs might be crucial during critical events in the life of plants such as cutting and senescence by initiating H(2)O(2)-mediated defences against pathogens and foraging animals.

How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorous efficiency

Acid soils significantly limit crop production worldwide because approximately 50% of the world's potentially arable soils are acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring tolerance to acid soil stress has been a focus of intense research interest over the past decade. The primary limitations on acid soils are toxic levels of aluminum (Al) and manganese (Mn), as well as suboptimal levels of phosphorous (P). This review examines our current understanding of the physiological, genetic, and molecular basis for crop Al tolerance, as well as reviews the emerging area of P efficiency, which involves the genetically based ability of some crop genotypes to tolerate P deficiency stress on acid soils. These are interesting times for this field because researchers are on the verge of identifying some of the genes that confer Al tolerance in crop plants; these discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of these tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving crop Al tolerance via both molecular-assisted breeding and biotechnology.

Aluminum-induced gene expression and protein localization of a cell wall-associated receptor kinase in Arabidopsis

Here, we report the aluminum (Al)-induced organ-specific expression of a WAK1 (cell wall-associated receptor kinase 1) gene and cell type-specific localization of WAK proteins in Arabidopsis. WAK1-specific reverse transcriptase-polymerase chain reaction analysis revealed an Al-induced WAK1 gene expression in roots. Short- and long-term analysis of gene expression in root fractions showed a typical "on" and "off" pattern with a first peak at 3 h of Al exposure followed by a sharp decline at 6 h and a complete disappearance after 9 h of Al exposure, suggesting the WAK1 is a further representative of Al-induced early genes. In shoots, upon root Al exposure, an increased but stable WAK1 expression was observed. Using confocal microscopy, we visualized Al-induced closure of leaf stomata, consistent with previous suggestions that the Al stress primarily experienced in roots associated with the transfer of root-shoot signals. Elevated levels of WAK protein in root cells were observed through western blots after 6 h of Al exposure, indicating a lag time between the Al-induced WAK transcription and translation. WAK proteins are localized abundantly to peripheries of cortex cells within the elongation zone of the root apex. In these root cells, disintegration of cortical microtubules was observed after Al treatment but not after the Al analog lanthanum treatments. Tip-growing control root hairs, stem stomata, and leaf stomatal pores are characterized with high amounts of WAKs, suggesting WAKs are accumulating at plasma membrane domains, which suffer from mechanical stress and lack dense arrays of supporting cortical microtubules. Further, transgenic plants overexpressing WAK1 showed an enhanced Al tolerance in terms of root growth when compared with the wild-type plants, making the WAK1 one of the important candidates for plant defense against Al toxicity.

Expressed sequence tag-based gene expression analysis under aluminum stress in rye

To understand the mechanisms responsible for aluminum (Al) toxicity and tolerance in plants, an expressed sequence tag (EST) approach was used to analyze changes in gene expression in roots of rye (Secale cereale L. cv Blanco) under Al stress. Two cDNA libraries were constructed (Al stressed and unstressed), and a total of 1,194 and 774 ESTs were generated, respectively. The putative proteins encoded by these cDNAs were uncovered by Basic Local Alignment Search Tool searches, and those ESTs showing similarity to proteins of known function were classified according to 13 different functional categories. A total of 671 known function genes were used to analyze the gene expression patterns in rye cv Blanco root tips under Al stress. Many of the previously identified Al-responsive genes showed expression differences between the libraries within 6 h of Al stress. Certain genes were selected, and their expression profiles were studied during a 48-h period using northern analysis. A total of 13 novel genes involved in cell elongation and division (tonoplast aquaporin and ubiquitin-like protein SMT3), oxidative stress (glutathione peroxidase, glucose-6-phosphate-dehydrogenase, and ascorbate peroxidase), iron metabolism (iron deficiency-specific proteins IDS3a, IDS3b, and IDS1; S-adenosyl methionine synthase; and methionine synthase), and other cellular mechanisms (pathogenesis-related protein 1.2, heme oxygenase, and epoxide hydrolase) were demonstrated to be regulated by Al stress. These genes provide new insights about the response of Al-tolerant plants to toxic levels of Al.

Change in root apical protein and peroxidase activity in response to aluminum in tolerant and sensitive maize inbred lines

The effects of a short-term (80 min) exposure to 222 µM aluminum (Al) on the protein content and expression and on peroxidase activity and isoenzymes in the primary root of maize were evaluated. Two inbred lines differing in their level of tolerance to Al were used: Cateto 237 (tolerant) and L36 (sensitive). The apical 20 mm of the primary root was divided into 2-mm-long segments that were analyzed for total protein content and peroxidase activity. These results demonstrate that the total protein content along the root apex was not affected by Al in the tolerant inbred line, but decreased in the sensitive line. In the apical 2 mm of the root of the sensitive line, the expression of low molecular weight proteins (43 kDa or smaller) was decreased. Expression of low molecular proteins increased in the tolerant inbred line, even though total protein content did not increase. This suggests that some of these proteins could play a role in metal tolerance, perhaps as binding peptides. While the peroxidase activity of the tolerant inbred line did not change with exposure to Al, peroxidase activity in the apical 6 mm of the root of the sensitive line decreased. The tolerant inbred line constitutively expressed more anionic peroxidase isoforms. These results demonstrate that maintenance of protein expression may be an important component of the plant's resistance to Al stress, and that resistance to Al stress is associated with the higher expression of anionic peroxidase isoforms.

Response of rice to Al stress and identification of quantitative trait Loci for Al tolerance

Rice (Oryza sativa L.) shows the highest tolerance to Al toxicity among small-grain cereal crops, however, the mechanisms and genetics responsible for its high Al tolerance are not yet well understood. We investigated the response of rice to Al stress using the japonica variety Koshihikari in comparison to the indica variety Kasalath. Koshihikari showed higher tolerance at various Al concentrations than Kasalath. The Al content in root apexes was less in Koshihikari than in Kasalath, suggesting that exclusion mechanisms rather than internal detoxification are acting in Koshihikari. Al-induced secretion of citrate was observed in both Koshihikari and Kasalath, however, it is unlikely to be the mechanism for Al tolerance because there was no significant difference in the amount of citrate secreted between Koshihikari and Kasalath. Quantitative trait loci (QTLs) for Al tolerance were mapped in a population of 183 backcross inbred lines (BILs) derived from a cross of Koshihikari and Kasalath. Three putative QTLs controlling Al tolerance were detected on chromosomes 1, 2 and 6. Kasalath QTL alleles on chromosome 1 and 2 reduced Al tolerance but increased tolerance on chromosome 6. The three QTLs explained about 27% of the phenotypic variation in Al tolerance. The existence of QTLs for Al tolerance was confirmed in substitution lines for corresponding chromosomal segments.

A new biochemical marker for aluminium tolerance in plants

Al was shown to elicit the induction of several pathogenesis-related genes, suggesting that a common signalling pathway may be involved in the early response to Al and pathogens. However, we found no evidence of oxidative burst involving either H2O2 or O2- during the first hours of Al exposure distinguishing the early response to Al from a common response to pathogen infection. We identified a strong superoxide dismutase insensitive nitro blue tetrazolium (NBT) reduction activity in the root tips of control plants. This activity was rapidly inhibited by Al exposure in the meristematic/distal transition zones of roots in all species examined. In wheat (Triticum aestivum), the inhibition of NBT reduction occurred in less than 1 min in vivo suggesting that Al either directly blocks an enzyme responsible for NBT reduction, or affects a signal pathway involved in the regulation of this activity. The sensitivity of NBT reduction to KCN and NaN3 suggests that an enzymatic, rather than a chemical reaction is involved. In tolerant plants, the inhibition of NBT reduction caused by Al was reversed within 24 h of exposure. The level of recovery was a function of the degree of Al tolerance. We show that NBT reduction is a simple biochemical marker allowing the rapid identification of tolerant individuals within a segregating population.

A gene encoding multidrug resistance (MDR)-like protein is induced by aluminum and inhibitors of calcium flux in wheat

A cDNA clone exclusively induced by aluminum (Al) was isolated from root apices of wheat (Triticum aestivum L.) by the differential display method. The predicted amino acid sequence exhibited homology to the multidrug resistance (MDR) proteins that is known as a member of the ATP-binding cassette (ABC) protein superfamily. Thus this gene was named TaMDR1 (Triticum aestivum MDR). TaMDR1 was induced as a function of Al concentration in the range from 5 to 50 microM, which is in the range of Al content in natural acid soil environment. The concentration required for the induction was lower in the Al-sensitive cultivar than in the Al-tolerant cultivar, indicating that the accumulation of TaMDR1 mRNA was associated with the events caused by Al toxicity rather than Al tolerance. TaMDR1 was significantly induced by the exposure to lanthanum, gadolinium and ruthenium red, which are known as inhibitors of calcium channels. Furthermore, decreasing of calcium ion in growth medium caused stimulation of the gene expression. These results suggested that the induction of TaMDR1 is caused by the breaking of calcium homeostasis which occurred at early stage of Al toxicity.

Physical and genetic mapping of barley (Hordeum vulgare) germin-like cDNAs

Germin with oxalate oxidase and superoxide dismutase activity is a homohexamer of six manganese-containing interlocked beta-jellyroll monomers with extreme resistance to heat and proteolytic degradation [Woo, E.-J., Dunwell, J. M., Goodenough, P. W., Marvier, A. C. & Pickersill, R. W. (2000) Nat. Struct. Biol. 7, 1036-1038]. This structure is conserved in germin-like proteins (GLPs) with other enzymatic functions and characteristic for proteins deposited in plant cell walls in response to pathogen attack and abiotic stress. Comparative nucleotide and amino acid sequence analyses of 49,610 barley expressed sequence tags identified 124 germin and germin-like cDNAs, which distributed into five subfamilies designated HvGER-I to HvGER-V. Representative cDNAs for these subfamilies hybridized to 67 bacterial artificial chromosome (BAC) clones from a library containing 6.3 genomic equivalents. Twenty-six BAC clones hybridized to the subfamily IV probe and identified a gene-rich region including clone 418E1 of 96 kb encoding eight GLPs (i.e., 1 gene per 12 kb). This BAC clone lacked highly repeated sequences and mapped to the subtelomeric region of the long arm of chromosome 4(4H). Among the six genes of the contig expressed in leaves, one specifies a protein known to be associated with papilla formation in the epidermis upon powdery mildew infection. Three structural genes for oxalate oxidase are present in subfamily I and eight GLPs of various functions in the other subfamilies. These genes map at loci in chromosomes 1(7H), 2 (2H), 3(3H), 4(4H), and 7(5H). Some are present on a single BAC clone. The results are discussed in relation to cereal genome organization.

Prospecting sugarcane genes involved in aluminum tolerance

Aluminum is one of the major factors that affect plant development in acid soils, causing a substantial reduction in yield in many crops. In South America, about 66% of the land surface is made up of acid soils where high aluminum saturation is one of the main limiting factors for agriculture. The biochemical and molecular basis of aluminum tolerance in plants is far from being completely understood despite a growing number of studies, and in the specific case of sugarcane there are virtually no reports on the effects of gene regulation on aluminum stress. The objective of the work presented in this paper was to prospect the sugarcane expressed sequence tag (SUCEST) data bank for sugarcane genes related to several biochemical pathways known to be involved in the responses to aluminum toxicity in other plant species and yeast. Sugarcane genes similar to most of these genes were found, including those coding for enzymes that alleviate oxidative stress or combat infection by pathogens and those which code for proteins responsible for the release of organic acids and signal transducers. The role of these genes in aluminum tolerance mechanisms is reviewed. Due to the high level of genomic conservation in related grasses such as maize, barley, sorghum and sugarcane, these genes may be valuable tools which will help us to better understand and to manipulate aluminum tolerance in these species.

Phylogeny, function, and evolution of the cupins, a structurally conserved, functionally diverse superfamily of proteins

The cupin superfamily is a group of functionally diverse proteins that are found in all three kingdoms of life, Archaea, Eubacteria, and Eukaryota. These proteins have a characteristic signature domain comprising two histidine- containing motifs separated by an intermotif region of variable length. This domain consists of six beta strands within a conserved beta barrel structure. Most cupins, such as microbial phosphomannose isomerases (PMIs), AraC- type transcriptional regulators, and cereal oxalate oxidases (OXOs), contain only a single domain, whereas others, such as seed storage proteins and oxalate decarboxylases (OXDCs), are bi-cupins with two pairs of motifs. Although some cupins have known functions and have been characterized at the biochemical level, the majority are known only from gene cloning or sequencing projects. In this study, phylogenetic analyses were conducted on the conserved domain to investigate the evolution and structure/function relationships of cupins, with an emphasis on single- domain plant germin-like proteins (GLPs). An unrooted phylogeny of cupins from a wide spectrum of evolutionary lineages identified three main clusters, microbial PMIs, OXDCs, and plant GLPs. The sister group to the plant GLPs in the global analysis was then used to root a phylogeny of all available plant GLPs. The resulting phylogeny contained three main clades, classifying the GLPs into distinct subfamilies. It is suggested that these subfamilies correlate with functional categories, one of which contains the bifunctional barley germin that has both OXO and superoxide dismutase (SOD) activity. It is proposed that GLPs function primarily as SODs, enzymes that protect plants from the effects of oxidative stress. Closer inspection of the DNA sequence encoding the intermotif region in plant GLPs showed global conservation of thymine in the second codon position, a character associated with hydrophobic residues. Since many of these proteins are multimeric and enzymatically inactive in their monomeric state, this conservation of hydrophobicity is thought to be associated with the need to maintain the various monomer- monomer interactions. The type of structure-based predictive analysis presented in this paper is an important approach for understanding gene function and evolution in an era when genomes from a wide range of organisms are being sequenced at a rapid rate.

Induction of vacuolar ATPase and mitochondrial ATP synthase by aluminum in an aluminum-resistant cultivar of wheat

Two 51-kD aluminum (Al)-induced proteins (RMP51, root membrane proteins of 51 kD) were recently discovered in an aluminum-resistant cultivar of wheat (Triticum aestivum) cv PT741 (Basu et al., 1994a). These proteins segregate with the aluminum resistance phenotype in a segregating population arising from a cross between Al-resistant cv PT741 and Al-sensitive cv Katepwa (Taylor et al., 1997). The proteins have been purified by continuous elution electrophoresis and analyzed by peptide microsequencing. Sequence analysis of the purified peptides revealed that they are homologous to the B subunit of the vacuolar H+-ATPase (V-ATPase) and the alpha- and beta-subunits of the mitochondrial ATP synthase (F1F0-ATPase). To confirm that these ATPases are induced by Al, ATPase activity and transcript levels were analyzed under Al stress. Both V-ATPase and F1F0-ATPase activities were induced by Al and responded in a dose-dependent manner to 0 to 150 microM Al. In contrast, plasma membrane H+-ATPase (P-ATPase) activity decreased to 0.5x control levels, even when plants were exposed to 25 microM Al. Northern analysis showed that the transcript encoding the B subunit of V-ATPase increased by 2.2x in a dose-dependent manner, whereas levels of the transcript encoding the alpha-subunit of F1F0-ATPase remained constant. The effect of Al on ATPase activity in other cultivars was also examined. The Al-resistant cultivar, cv PT741, was the only cultivar to show induction of V- and F1F0-ATPases. These results suggest that the V-ATPase in cv PT741 is responding specifically to Al stress with the ATP required for its activity supplied by ATP synthase to maintain energy balance within the cell.

Characterization of oxalate oxidase and cell death in Al-sensitive and tolerant wheat roots

Several genes including oxalate oxidase (Oxo) are up-regulated in Triticum aestivum L. root tips exposed to Al. To better understand the function of Oxo during Al exposure, the protein level and enzyme activity were measured. The data indicate that both Oxo protein and activity are increased proportionally to the level of root growth inhibition (RGI). A high level of Oxo expression may result in excess H(2)O(2) production which could become toxic and induce cell death. However, the timing of H(2)O(2) production (observed after 24 h) indicates that it cannot be the primary cause of cell death first observed after 8 h. Moreover, at Al concentrations resulting in 50% RGI, we did not observe any cell death in the sensitive cultivar while a punctated pattern of death involving small groups of cells was found in the tolerant cultivar. This pattern was maintained for several days in the tolerant cultivar, suggesting the involvement of a cell death mechanism aimed at replacing epidermal cells intoxicated with Al while root growth is maintained. The accelerated epidermal cell turnover may represent a new detoxification mechanism helping to protect deeper cell layers of the meristematic and elongation zone essential for root growth.

Cell biology of aluminum toxicity and tolerance in higher plants

Aluminum is the major element in the soil and exists as a stable complex with oxygen and silicate in neutral and weakly acidic soil. When the soil pH is lower than 4.5-5.0, Al is solubilized in the soil water and absorbed by plant roots. Absorbed Al inhibits root elongation severely, and the elongation of roots exposed to Al3+ as low as mumol level is inhibited within an hour(s). Thus much research has been conducted to understand the mechanism of Al toxicity and tolerance. Al is located specifically at the root apex. Al-sensitive plants absorb more Al than do Al-tolerant plants, and thus the exclusion mechanism of Al is the major idea for Al tolerance. The understanding of Al stress in plants is important for stable food production in future. Al is a complicated ion in its chemical form and biological function. In this chapter, mechanisms of Al toxicity and tolerance proposed during the past few decades as well as future topics are described from physiological and molecular points of view.

Expression of aluminum-induced genes in transgenic arabidopsis plants can ameliorate aluminum stress and/or oxidative stress

To examine the biological role of Al-stress-induced genes, nine genes derived from Arabidopsis, tobacco (Nicotiana tabacum L.), wheat (Triticum aestivum L.), and yeast (Saccharomyces cerevisiae) were expressed in Arabidopsis ecotype Landsberg. Lines containing eight of these genes were phenotypically normal and were tested in root elongation assays for their sensitivity to Al, Cd, Cu, Na, Zn, and to oxidative stresses. An Arabidopsis blue-copper-binding protein gene (AtBCB), a tobacco glutathione S-transferase gene (parB), a tobacco peroxidase gene (NtPox), and a tobacco GDP-dissociation inhibitor gene (NtGDI1) conferred a degree of resistance to Al. Two of these genes, AtBCB and parB, and a peroxidase gene from Arabidopsis (AtPox) also showed increased resistance to oxidative stress induced by diamide, while parB conferred resistance to Cu and Na. Al content of Al-treated root tips was reduced in the four Al-resistant plant lines compared with wild-type Ler-0, as judged by morin staining. All four Al-resistant lines also showed reduced staining of roots with 2',7'-dichloro fluorescein diacetate (H(2)DCFDA), an indicator of oxidative stress. We conclude that Al-induced genes can serve to protect against Al toxicity, and also provide genetic evidence for a link between Al stress and oxidative stress in plants.

Microbial relatives of the seed storage proteins of higher plants: conservation of structure and diversification of function during evolution of the cupin superfamily

This review summarizes the recent discovery of the cupin superfamily (from the Latin term "cupa," a small barrel) of functionally diverse proteins that initially were limited to several higher plant proteins such as seed storage proteins, germin (an oxalate oxidase), germin-like proteins, and auxin-binding protein. Knowledge of the three-dimensional structure of two vicilins, seed proteins with a characteristic beta-barrel core, led to the identification of a small number of conserved residues and thence to the discovery of several microbial proteins which share these key amino acids. In particular, there is a highly conserved pattern of two histidine-containing motifs with a varied intermotif spacing. This cupin signature is found as a central component of many microbial proteins including certain types of phosphomannose isomerase, polyketide synthase, epimerase, and dioxygenase. In addition, the signature has been identified within the N-terminal effector domain in a subgroup of bacterial AraC transcription factors. As well as these single-domain cupins, this survey has identified other classes of two-domain bicupins including bacterial gentisate 1, 2-dioxygenases and 1-hydroxy-2-naphthoate dioxygenases, fungal oxalate decarboxylases, and legume sucrose-binding proteins. Cupin evolution is discussed from the perspective of the structure-function relationships, using data from the genomes of several prokaryotes, especially Bacillus subtilis. Many of these functions involve aspects of sugar metabolism and cell wall synthesis and are concerned with responses to abiotic stress such as heat, desiccation, or starvation. Particular emphasis is also given to the oxalate-degrading enzymes from microbes, their biological significance, and their value in a range of medical and other applications.

Barley oxalate oxidase is a hexameric protein related to seed storage proteins: evidence from X-ray crystallography

The oxalate oxidase enzyme expressed in barley roots is a thermostable, protease-resistant enzyme that generates H2O2. It has great medical importance because of its use to assay plasma and urinary oxalate, and it has also been used to generate transgenic, pathogen-resistant crops. This protein has now been purified and three types of crystals grown. X-ray analysis shows that the symmetry present in these crystals is consistent with a hexameric arrangement of subunits, probably a trimer of dimers. This structure may be similar to that found in the related seed storage proteins.