Aluminium-induced alteration of ion homeostasis in root tip vacuoles of two maize varieties differing in Al tolerance

Phytoremediation as an Effective Remedy for Removing Trace Elements from Ecosystems

The pollution of soil by trace elements is a global problem. Conventional methods of soil remediation are often inapplicable, so it is necessary to search intensively for innovative and environment-friendly techniques for cleaning up ecosystems, such as phytoremediation. Basic research methods, their strengths and weaknesses, and the effects of microorganisms on metallophytes and plant endophytes resistant to trace elements (TEs) were summarised and described in this manuscript. Prospectively, bio-combined phytoremediation with microorganisms appears to be an ideal, economically viable and environmentally sound solution. The novelty of the work is the description of the potential of "green roofs" to contribute to the capture and accumulation of many metal-bearing and suspended dust and other toxic compounds resulting from anthropopressure. Attention was drawn to the great potential of using phytoremediation on less contaminated soils located along traffic routes and urban parks and green spaces. It also focused on the supportive treatments for phytoremediation using genetic engineering, sorbents, phytohormones, microbiota, microalgae or nanoparticles and highlighted the important role of energy crops in phytoremediation. Perceptions of phytoremediation on different continents are also presented, and new international perspectives are presented. Further development of phytoremediation requires much more funding and increased interdisciplinary research in this direction.

Tolerance mechanisms to aluminum in popcorn inbred lines involving aluminum compartmentalization and ascorbate-glutathione redox pathway

Inbred line 11-133 of popcorn showed the lowest apoplast Al and total Al concentrations and Al-lumogallion complex, associated with a more efficient antioxidant system, mainly due to glutathione metabolism. Popcorn (Zea mays L. var. everta) is largely intended for human consumption. About 40% of the world's arable soils are acidic. In soils acidic, aluminum (Al) ionizes producing the trivalent cation, which is highly toxic to plants. Hence, this work aimed to: (1) evaluate the Al toxicity sites and its effect on the structure of the root tips, (2) quantify Al concentrations in the apoplast and symplast of the roots, and (3) to elucidate the modulation on the activity of antioxidant enzymes and metabolites of the ascorbate-glutathione cycle in two popcorn inbred lines (ILs) 11-133 and 11-60, classified as tolerant and sensitive to this metal, respectively. Aluminum toxicity did not affect the shoot growth; however, there was a yellowing of the oldest leaf blade only in 11-60. The better performance of 11-133 is related to lower apoplastic and total Al concentrations and Al accumulation in the root associated with a lower fluorescence of Al-lumogallion complex at the root tip, indicating the presence of mechanisms of chelation with this metal. Consequently, this IL showed less change in root morphoanatomy and lower reactive oxygen species and malondialdehyde content, which are associated with a more efficient enzymatic and non-enzymatic system, mainly due to the higher content of the glutathione metabolite and the higher activities of superoxide dismutase, monodehydroascorbate reductase, dehydroascorbate reductase, γ-glutamylcysteine synthetase, and glutathione peroxidase enzymes. Thus, these findings illustrated above indicate how internal mechanisms of detoxification respond to Al in popcorn, which can be used as tolerance biomarkers.

ABCG transporter proteins with beneficial activity on plants

Among the several subfamilies of ABC transporters the ABC-G subfamily is very significant. In the model plant Arabidopsis thaliana itself, ABCG subfamily houses highest number of transporters with mostly half-size transporters (called WBC) and fewer full-size transporters (called PDR). During drought stress the stress hormone abscisic acid (ABA) is exported from the root xylem and imported by the leaf stomatal cells by ABCG transporter proteins to reduce the transpiration of water from leaves. Moreover, the ABCG transporters play a chief role in export of prime biotic stress induced hormones like jasmonic acid and salicylic acid among other secondary metabolites. In this way they protect the plant as the first line of defense against pathogenic damages. The ABCG transporters help the plant in becoming kanamycin resistant which help in plant growth. ABCG transporters of Nicotiana plumbaginifolia provide resistance to pathogens like Pseudomonas syringae. Furthermore several ABCG transporters of A. thaliana are efficient in transporting cuticular lipids like cutin to help development of cuticle. Pollen exine wall formation is also aided by one ABCG transporter itself. Some important ABCG transporters like ABCG36 and ABCG40 have been suggested to contribute hugely towards heavy metal resistance and cellular detoxification in Arabidopsis thaliana.

Characterization of the Heavy-Metal-Associated Isoprenylated Plant Protein (HIPP) Gene Family from Triticeae Species

Heavy-metal-associated (HMA) isoprenylated plant proteins (HIPPs) only exist in vascular plants. They play important roles in responses to biotic/abiotic stresses, heavy-metal homeostasis, and detoxification. However, research on the distribution, diversification, and function of HIPPs in Triticeae species is limited. In this study, a total of 278 HIPPs were identified from a database from five Triticeae species, and 13 were cloned from Haynaldia villosa. These genes were classified into five groups by phylogenetic analysis. Most HIPPs had one HMA domain, while 51 from Clade I had two, and all HIPPs had good collinear relationships between species or subgenomes. In silico expression profiling revealed that 44 of the 114 wheat HIPPs were dominantly expressed in roots, 43 were upregulated under biotic stresses, and 29 were upregulated upon drought or heat treatment. Subcellular localization analysis of the cloned HIPPs from H. villosa showed that they were expressed on the plasma membrane. HIPP1-V was upregulated in H. villosa after Cd treatment, and transgenic wheat plants overexpressing HIPP1-V showed enhanced Cd tolerance, as shown by the recovery of seed-germination and root-growth inhibition by supplementary Cd. This research provides a genome-wide overview of the Triticeae HIPP genes and proved that HIPP1-V positively regulates Cd tolerance in common wheat.

Sources and doses of aluminum in experiments with rice in nutrient solution

ABSTRACT The aluminum source to produce toxicity in upland rice in nutrient solution experiments is not yet well established, althought the aluminum potassium sulfate has been utilized source to produce aluminum toxicity. However, in recent studies have used aluminum chloride. The aim of this study was to evaluate the capacity of aluminum sources and doses to produce toxicity in upland rice plants grown in nutrient solution. The experiment was arranged in a block randomized design, in a 2 x 5 factorial scheme and four repetitions. The treatments were two aluminum sources (aluminum potassium sulfate - AlK(SO4)2.12H2O and aluminum chloride - AlCl3.6H2O) and five aluminum doses in nutrient solution (0, 370, 740, 1100 and 1480 μmol L-1). The experiment was conducted in a greenhouse in Botucatu city, São Paulo state, Brazil, starting in April 2012, and was carried out for 56 days from transplanting of the seedlings. Using aluminum chloride, the rice plants show lower production of root and total dry weight, area and root volume, medium and thick root length, potassium and sulfur contents and accumulations. Using aluminum potassium sulfate, there are lower aluminum activity and availability, besides the formation of large amount of aluminum compounds non-toxic to the plants (aluminum sulfate) in the nutrient solution. The aluminum doses between 1100 to 1480 µmol L-1, corresponding to aluminum activity of 336.8 to 429.0 µmol L-1 of aluminum chloride as source, are more effective to produce aluminum toxicity in upland rice plants grown in nutrient solution.

Transcriptomic and ionomic analysis provides new insight into the beneficial effect of Al on tea roots' growth and nutrient uptake

Transcriptome profiling of roots indicated that genes involved in cell wall modification, cytoskeleton, H⁺ exchange and K⁺ influx played important roles in tea root growth under Al addition. Tea (Camellia sinensis) is considered as an Al accumulator species. It can accumulate a high concentration of Al in mature leaves without any symptom of toxicity, even improve roots' growth and nutrient uptake. However, the molecular mechanisms underlying this tolerance remain unclear. Here, we investigated the accumulation of elements and transcriptional profiles in tea roots treated with various Al doses. The results showed that the growth of tea plants was improved by a low dose of Al (0.2, 0.4, 0.6, 1 mM); however, this beneficial effect disappeared when higher concentrations of Al were supplied (2, 4, 10 mM). Ionomic analysis suggested that accumulation of P and K increased under a low Al supply (< 1 mM), while Ca and Mg contents were negatively correlated with external Al doses. The RNA seq obtained 523,391 unigenes, among which 20,448 were annotated in all databases. In total, 1876 unigenes were expressed significantly different in any Al treatment. A large number of DEGs involved in cell growth and division, such as those linked to cell wall-modifying enzymes, actin cytoskeleton, cyclin and H⁺-ATPase were identified, suggesting that these pathways were involved in root growth under different Al supply. Furthermore, expression of transporters significantly changed in roots supplied with Al. Among them, HAK5, which is involved in K uptake by plants, had a significant positive correlation with the K content.

Alterations of root architecture and cell wall modifications in Tilia cordata Miller (Linden) growing on mining sludge

Trees are considered good candidates for phytoremediation of soils contaminated with trace elements (TE), e.g. mine tailings. Using two year-old Tilia cordata plants, we demonstrated the nature and the scale of root architecture, especially root apices, as an indicator of mining sludge toxicity and plant capability to cope with these stress conditions. The novelty of our research is the analysis of the root response to substrate with extremely high concentrations of numerous toxic TE, and the 3D illustration of the disorders in root apex architecture using a clarity technique for confocal microscopy. The analysis demonstrates (1) a marked reduction in the size of the root apex zones (2) the occurrence of vascular tissues abnormally close to the root apex (3) collapse of the internal tissues in many root apices. Simultaneously, at the cellular level we observed some signs of a defensive response - such as a common increase of cell wall (CW) thickness and the formation of local CW thickenings - that enlarge the CW capacity for TE sequestration. However, we also detected harmful effects. Among others, a massive deposition of TE in the middle lamella which caused major damage - probably one of the reasons why the inner tissues of the root apex often collapsed - and the formation of incomplete CWs resulting in the occurrence of extremely large cells. Moreover, many cells of the root apex exhibited degenerated protoplasts. All these alterations indicate the harsh conditions for lime growth and survival and simultaneously, the manifestation of a defensive response. The obtained results allowed us to conclude that analysis of the nature and scale of structural alterations in roots can be useful indicators of plant ability to cope with stress conditions, e.g. in prospect of using the examined plants for reclamation of soils contaminated with TE.

Psidium guajava bioactive product chemical analysis and heavy metal toxicity reduction

The present study had as its objective to verify the Psidium guajava var. Pomifera L. chelating, antioxidant and cytoprotective effects against mercury and aluminum. The ethanolic extract, tannic and flavonoid fractions were subjected to LC-MS analysis. The Ferric Reducing Antioxidant Power (FRAP) and ferric ion reduction demonstrated a present antioxidant activity. The fungicidal and bactericidal activity of these metals were established. After determining the sub-allelopathic doses, germination tests using Lactuca sativa were performed. Quercetin and its derivatives were the main compounds identified in the extract and the fractions. Mercury chloride significantly reduced the bactericidal effect of the flavonoid fraction (p < 0.001). None of the fractions were cytoprotective against mercury or aluminum in the fungal model assays. Using a sub-allelopathic concentration (64 μg/mL), the ethanolic extract, flavonoid and tannic fractions were found to be cytoprotective against aluminum for radicles, however only the tannic fraction was cytoprotective for caulicles. These data suggest that natural P. guajava products are promising cytoprotective compound sources. This activity may be related to the antioxidant effect of secondary metabolites, mainly flavonoids. Our results point to a potential for environmental intervention product and technique development aimed at mitigating contamination by toxic metals such as mercury and aluminum.

Al(3+) -promoted fluoride accumulation in tea plants (Camellia sinensis) was inhibited by an anion channel inhibitor DIDS

BACKGROUND

Generally, tea plants are grown in acid soil which is rich in aluminum (Al) and fluoride (F). A recent publication showed that pretreatment with Al(3+) promoted F accumulation in tea plants by increasing endogenous Ca(2+) and calmodulin (CaM). A high level of F in tea leaves not only impairs tea quality but also might pose a health risk for people drinking tea regularly. Therefore it is important to try to find some clues which might be beneficial in controlling F accumulation in tea plants grown in acid soil (Al(3+) ).

RESULTS

It was found that diisothiocyanostilbene-2,2-disulfonic acid (DIDS) significantly reduced Al(3+) -promoted F accumulation in tea plants. Additionally, Al(3+) plus DIDS treatment stimulated significantly higher Ca(2+) efflux and decreased the CaM level in tea roots compared with Al(3+) treatment. Besides, significantly higher depolarization of membrane potential was shown in tea roots treated with Al(3+) plus DIDS than in those treated with Al(3+) , as well as higher net total H(+) efflux and plasma membrane H(+) -ATPase activity.

CONCLUSION

Al(3+) -promoted F accumulation in tea plants was inhibited by an anion channel inhibitor DIDS. Ca(2+) /CaM and membrane potential depolarization may be the components involved in this process. © 2016 Society of Chemical Industry.

Molecular Scanning and Morpho-Physiological Dissection of Component Mechanism in Lens Species in Response to Aluminium Stress

Aluminium (Al) stress was imposed on 285 lentil genotypes at seedling stage under hydroponics to study its effects on morpho-physiological traits where resistant cultigens and wilds showed minimum reduction in root and shoot length and maximum root re-growth (RRG) after staining. Molecular assortment based on 46 simple sequence repeat (SSR) markers clustered the genotypes into 11 groups, where wilds were separated from the cultigens. Genetic diversity and polymorphism information content (PIC) varied between 0.148-0.775 and 0.140-0.739, respectively. Breeding lines which were found to be most resistant (L-7903, L-4602); sensitive cultivars (BM-4, L-4147) and wilds ILWL-185 (resistant), ILWL-436 (sensitive) were grouped into different clusters. These genotypes were also separated on the basis of population structure and Jaccard's similarity index and analysed to study Al resistance mechanism through determination of different attributes like localization of Al and callose, lipid peroxidation, secretion of organic acids and production of antioxidant enzymes. In contrast to sensitive genotypes, in resistant ones most of the Al was localized in the epidermal cells, where its movement to apoplastic region was restricted due to release of citrate and malate. Under acidic field conditions, resistant genotypes produced maximum seed yield/plant as compared to sensitive genotypes at two different locations i.e. Imphal, Manipur, India and Basar, Arunanchal Pradesh, India during 2012-13, 2013-14 and 2014-15. These findings suggest that Al stress adaptation in lentil is through exclusion mechanism and hybridization between the contrasting genotypes from distinct clusters can help in development of resistant varieties.

Heavy Metal Tolerance in Plants: Role of Transcriptomics, Proteomics, Metabolomics, and Ionomics

Heavy metal contamination of soil and water causing toxicity/stress has become one important constraint to crop productivity and quality. This situation has further worsened by the increasing population growth and inherent food demand. It has been reported in several studies that counterbalancing toxicity due to heavy metal requires complex mechanisms at molecular, biochemical, physiological, cellular, tissue, and whole plant level, which might manifest in terms of improved crop productivity. Recent advances in various disciplines of biological sciences such as metabolomics, transcriptomics, proteomics, etc., have assisted in the characterization of metabolites, transcription factors, and stress-inducible proteins involved in heavy metal tolerance, which in turn can be utilized for generating heavy metal-tolerant crops. This review summarizes various tolerance strategies of plants under heavy metal toxicity covering the role of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), various stress-inducible proteins (proteomics) as well as the role of plant hormones. We also provide a glance of some strategies adopted by metal-accumulating plants, also known as "metallophytes."

Ca(2+) and CaM are involved in Al(3+) pretreatment-promoted fluoride accumulation in tea plants (Camellia sinesis L.)

Tea plant (Camellia sinensis (L.) O. kuntze) is known to be a fluoride (F) and aluminum (Al(3+)) hyper-accumulator. Previous study showed that pre-treatment of Al(3+) caused a significant increase of F accumulation in tea plants. However, less is known about the intricate network of Al(3+) promoted F accumulation in tea plants. In this study, the involvement of endogenous Ca(2+) and CaM in Al(3+) pretreatment-promoted F accumulation in tea plants was investigated. Our results showed that Al(3+) induced the inverse change of intracellular Ca(2+) fluorescence intensity and stimulated Ca(2+) trans-membrane transport in the mature zone of tea root. Also, a link between internal Ca(2+) and CaM was found in tea roots under the presence of Al(3+). In order to investigate whether Ca(2+) and CaM were related to F accumulation promoted by Al(3+) pretreatment, Ca(2+) chelator EGTA and CaM antagonists CPZ and TFP were used. EGTA, CPZ, and TFP pretreatment inhibited Al(3+)-induced increase of Ca(2+) fluorescence intensity and CaM content in tea roots, and also significantly reduced Al(3+)-promoted F accumulation in tea plants. Taken together, our results suggested that the endogenous Ca(2+) and CaM are involved in Al(3+) pretreatment-promoted F accumulation in tea roots.

Mitochondrial alterations during Al-induced PCD in peanut root tips

Previous study found there was a negative relationship between Al-induced PCD and Al-resistance in peanut. The present research was undertaken to verify whether mitochondria play a significant role in PCD induced by Al in peanut. The roots of Al-tolerant plants were found to exhibit more intensive root growth, while accumulating less Al³⁺ than Al-sensitive plants under Al treatment. The different enhancement of ROS production was observed in the mitochondria isolated from two peanut cultivars. The concentration of mitochondrial MDA in root tips increased after Al treatment, which was higher in Zhonghua 2 than in 99-1507. With the increase of Al concentration, mitochondrial Ca²⁺ concentration decreased, and Ca²⁺ concentration of Zhonghua 2 decreased faster than that of 99-1507. The opening of mitochondrial permeability transition pore was more extensively in mitochondria isolated from Zhonghua 2 than from 99-1507. The collapse of inner mitochondrial membrane potential (ΔΨm) was also observed with a release of Cytochrome c (Cyt c) from mitochondria, it was more obvious in Zhonghua 2 than in 99-1507 with Al concentration increasing. The results showed that mitochondrial membrane structure and function were damaged seriously in Al-induced PCD, the increase of mitochondrial antioxidant system activity decreased cellular damages under Al stress. To sum up, compared with Al-sensitive peanut cultivar, Al-tolerant peanut cultivar has less Al³⁺ absorption, mitochondrial ROS and membrane lipid peroxidation level, higher control of MPT opening, ΔΨm maintaining, Cty c release from mitochondria and mitochondrial respiratory functions so that it is not easy to produce PCD under Al stress.

Genome-wide association analysis of aluminum tolerance in cultivated and Tibetan wild barley

Tibetan wild barley (Hordeum vulgare L. ssp. spontaneum), originated and grown in harsh enviroNment in Tibet, is well-known for its rich germpalsm with high tolerance to abiotic stresses. However, the genetic variation and genes involved in Al tolerance are not totally known for the wild barley. In this study, a genome-wide association analysis (GWAS) was performed by using four root parameters related with Al tolerance and 469 DArT markers on 7 chromosomes within or across 110 Tibetan wild accessions and 56 cultivated cultivars. Population structure and cluster analysis revealed that a wide genetic diversity was present in Tibetan wild barley. Linkage disequilibrium (LD) decayed more rapidly in Tibetan wild barley (9.30 cM) than cultivated barley (11.52 cM), indicating that GWAS may provide higher resolution in the Tibetan group. Two novel Tibetan group-specific loci, bpb-9458 and bpb-8524 were identified, which were associated with relative longest root growth (RLRG), located at 2H and 7H on barely genome, and could explain 12.9% and 9.7% of the phenotypic variation, respectively. Moreover, a common locus bpb-6949, localized 0.8 cM away from a candidate gene HvMATE, was detected in both wild and cultivated barleys, and showed significant association with total root growth (TRG). The present study highlights that Tibetan wild barley could provide elite germplasm novel genes for barley Al-tolerant improvement.