A significant positive correlation between endogenous trans-zeatin content and total arsenic in arsenic hyperaccumulator Pteris cretica var. nervosa

Molecular Regulation and Evolution of Cytokinin Signaling in Plant Abiotic Stresses

The sustainable production of crops faces increasing challenges from global climate change and human activities, which leads to increasing instances of many abiotic stressors to plants. Among the abiotic stressors, drought, salinity and excessive levels of toxic metals cause reductions in global agricultural productivity and serious health risks for humans. Cytokinins (CKs) are key phytohormones functioning in both normal development and stress responses in plants. Here, we summarize the molecular mechanisms on the biosynthesis, metabolism, transport and signaling transduction pathways of CKs. CKs act as negative regulators of both root system architecture plasticity and root sodium exclusion in response to salt stress. The functions of CKs in mineral-toxicity tolerance and their detoxification in plants are reviewed. Comparative genomic analyses were performed to trace the origin, evolution and diversification of the critical regulatory networks linking CK signaling and abiotic stress. We found that the production of CKs and their derivatives, pathways of signal transduction and drought-response root growth regulation are evolutionarily conserved in land plants. In addition, the mechanisms of CK-mediated sodium exclusion under salt stress are suggested for further investigations. In summary, we propose that the manipulation of CK levels and their signaling pathways is important for plant abiotic stress and is, therefore, a potential strategy for meeting the increasing demand for global food production under changing climatic conditions.

Cadmium/lead tolerance of six Dianthus species and detoxification mechanism in Dianthus spiculifolius

Toxic heavy metal contaminants seriously affect plant growth and human health. Reducing the accumulation of toxic metals by phytoremediation is an effective way to solve this environmental problem. Dianthus spiculifolius Schur is an ornamental plant with strong cold and drought tolerance. Because of its fast growth, well-developed root system, and large accumulation of biomass, D. spiculifolius has potential applications as a heavy metal hyperaccumulator. Therefore, the aim of this study was evaluate the ability of D. spiculifolius and other Dianthus species to remediate heavy metals, with an ultimate goal to identify available genetic resources for toxic metal removal. The cadmium (Cd) and lead (Pb) tolerance and accumulation of six Dianthus species were analyzed comparatively in physiological and biochemical experiments. Compared with the other Dianthus species, D. spiculifolius showed higher tolerance to, and greater accumulation of, Cd and Pb. Second-generation transcriptome analysis indicated that glutathione transferase activity was increased and the glutathione metabolism pathway was enriched with genes encoding antioxidant enzymes (DsGST, DsGST3, DsGSTU10, DsGGCT2-1, and DsIDH-2) that were up-regulated under Cd/Pb treatment by RT-qPCR in D. spiculifolius. When expressed in yeast, DsGST, DsGST3, DsGSTU10 and DsIDH-2 enhanced Cd or Pb tolerance. These results indicate that D. spiculifolius has potential applications as a new ornamental hyperaccumulator plant, and that antioxidant enzymes might be involved in regulating Cd/Pb accumulation and detoxification. The findings of this study reveal some novel genetic resources that can be used to breed new plant varieties that tolerate and accumulate heavy metals.

Arsenic perception and signaling: The yet unexplored world

Arsenic is one of the most potent carcinogens in the biosphere, jeopardizing the health of millions of people due to its entrance into the human food chain through arsenic-contaminated waters and staple crops, particularly rice. Although the mechanisms of arsenic sensing are widely known in yeast and bacteria, scientific evidence concerning arsenic sensors or components of early arsenic signaling in plants is still in its infancy. However, in recent years, we have gained understanding of the mechanisms involved in arsenic uptake and detoxification in different plant species and started to get insights into arsenic perception and signaling, which allows us to glimpse the possibility to design effective strategies to prevent arsenic accumulation in edible crops or to increase plant arsenic extraction for phytoremediation purposes. In this context, it has been recently described a mechanism according to which arsenite, the reduced form of arsenic, regulates the arsenate/phosphate transporter, consistent with the idea that arsenite functions as a selective signal that coordinates arsenate uptake with detoxification mechanisms. Additionally, several transcriptional and post-translational regulators, miRNAs and phytohormones involved in arsenic signaling and tolerance have been identified. On the other hand, studies concerning the developmental programs triggered to adapt root architecture in order to cope with arsenic toxicity are just starting to be disclosed. In this review, we compile and analyze the latest advances toward understanding how plants perceive arsenic and coordinate its acquisition with detoxification mechanisms and root developmental programs.

Morphological characteristics and accumulation of arsenic in Argyrochosma formosa (Liebm.) Windham developed in a highly contaminated site with arsenic in Matehuala, SLP, México

We studied the ability of Argyrochosma formosa growing in an arsenic heavily contaminated site to accumulate this metalloid; morphological characteristics and translocation of arsenic were evaluated in the organs. Population census of wild specimens of A. formosa was done, and 14 samples of ferns and rhizosphere soil were collected randomly. We recorded morphological characteristics with scanning electronic microscopy (SEM); concentrations of As in organs of fern plants (root, rhizome, and fronds) were evaluated with inductively coupled plasma-optic emission spectrometry (ICP-OES). Two hundred ninety-four individuals at different stages of development were identified, indicating the establishment of fern on the site. Morphological characteristics of A. formosa in fern plant organs did not show structural effects, compared with herbarium plants. Arsenic distribution in fern plant tissues was 192.2-763.6 mg/kg, 188-1017 mg/kg, and 113-2008 mg/kg, in roots, rhizomes, and fronds, respectively. The calculated bioaccumulation factor in fronds ranged from 2 to 7 and the translocation factor from 0.6 to 2.1. Our data suggest that A. formosa is an arsenic-tolerant species and propose it for phytoremediation on contaminated sites with As concentrations similar to that of the studied location. Further studies should be performed to evaluate the mechanisms of accumulation of As in plant tissues.

Arsenic accumulation and speciation in two cultivars of Pteris cretica L. and characterization of arsenate reductase PcACR2 and arsenite transporter PcACR3 genes in the hyperaccumulating cv. Albo-lineata

Pollution and poisoning with carcinogenic arsenic (As) is of major concern globally. Interestingly, there are ferns that can naturally tolerate remarkably high As concentrations in soils while hyperaccumulating this metalloid in their fronds. Besides Pteris vittata in which As-related traits and molecular determinants have been studied in detail, the As hyperaccumulation status has been attributed also to Pteris cretica. We thus inspected two P. cretica cultivars, Parkerii and Albo-lineata, for As hyperaccumulation traits. The cultivars were grown in soils supplemented with 20, 100, and 250 mg kg-1 of inorganic arsenate (iAsV). Unlike Parkerii, Albo-lineata was confirmed to be As tolerant and hyperaccumulating, with up to 1.3 and 6.4 g As kg-1 dry weight in roots and fronds, respectively, from soils amended with 250 mg iAsV kg-1. As speciation analyses rejected that organoarsenical species and binding with phytochelatins and other proteinaceous ligands would play any significant role in the biology of As in either cultivar. While in Parkerii, the dominating As species, particularly in roots, occurred as iAsV, in Albo-lineata the majority of the root and frond As was apparently converted to iAsIII. Parkerii markedly accumulated iAsIII in its fronds when grown on As spiked soils. Considering the roles iAsV reductase ACR2 and iAsIII transporter ACR3 may have in the handling of iAs, we isolated Albo-lineata PcACR2 and PcACR3 genes closely related to P. vittata PvACR2 and PvACR3. The gene expression analysis in Albo-lineata fronds revealed that the transcription of PcACR2 and PcACR3 was clearly As responsive (up to 6.5- and 45-times increase in transcript levels compared to control soil conditions, respectively). The tolerance and uptake assays in yeasts showed that PcACRs can complement corresponding As-sensitive mutations, indicating that PcACR2 and PcACR3 encode functional proteins that can perform, respectively, iAsV reduction and membrane iAsIII transport tasks in As-hyperaccumulating Albo-lineata.

Auxins and Cytokinins Regulate Phytohormone Homeostasis and Thiol-Mediated Detoxification in the Green Alga Acutodesmus obliquus Exposed to Lead Stress

Phytohormones, such as auxins and cytokinins, take part in the integration of growth control and stress response, but their role in algal adaptation to heavy metal remains to be elucidated. The current research indicated that lead (Pb), one of the most toxic metals in nature, causes severe depletion of endogenous cytokinins, auxins, and gibberellin and an increase in abscisic acid content in the green alga Acutodesmus obliquus. Exogenous auxins and cytokinins alleviate Pb toxicity through the regulation of the endogenous phytohormones’ levels. Exogenously applied auxins provoked the coordinated activation metal tolerance mechanisms leading to the increase in phytochelatin synthase activity and accumulation of phytochelatins and their precursors, which are essential for Pb sequestration. On the other hand, phytochelatin synthesis decreased in algal cells treated with cytokinins. Significant changes in the levels of low molecular weight metabolites, mainly involved in metal chelation and glutathione synthesis pathway under the influence of phytohormones in algal cells growing in the presence of Pb stress, were observed. This is the first report showing that auxins and cytokinins are important regulatory factors in algal adaptation strategies to heavy metal stress based on thiol-mediated compounds and the maintenance of phytohormone homeostasis.

Response of cytokinins and nitrogen metabolism in the fronds of Pteris sp. under arsenic stress

Given the close relationship between cytokinins (CKs), photosynthesis and nitrogen metabolism, this study assessed the effect of arsenic (As) contamination on these metabolic components in the As-hyperaccumulators Pteris cretica L. var. Albo-lineata (Pc-A) and var. Parkerii (Pc-P) as well as the As-non-hyperaccumulator Pteris straminea Mett. ex Baker (Ps). The ferns were cultivated in a pot experiment for 23 weeks in soil spiked with As at the levels 20 and 100 mg·kg-1. For the purpose of this study, the CKs were placed into five functionally different groups according to their structure and physiological roles: bioactive forms (bCKs; CK free bases); inactive or weakly active forms (dCKs; CK N-glucosides); transport forms (tCKs; CK ribosides); storage forms (sCKs; O-glucosides); and primary products of CK biosynthesis (ppbCKs; CK nucleotides). An important finding was higher CKs total content, accumulation of sCKs and reduction of dCKs in As-hyperaccumulators in contrast to non-hyperaccumulator ferns. A significant depletion of C resources was confirmed in ferns, especially Ps, which was determined by measuring the photosynthetic rate and chlorophyll fluorescence. A fluorescence decrease signified a reduction in the C/N ratio, inducing an increase of bioactive CKs forms in Pc-P and Ps. The impact of As on N utilization was significant in As-hyperaccumulators. The glutamic acid/glutamine ratio, an indicator of primary N assimilation, diminished in all ferns with increased As level in the soil. In conclusion, the results indicate a large phenotypic diversity of Pteris species to As and suggest that the CKs composition and the glutamic acid/glutamine ratio can be used as a tool to diagnose As stress in plants.

Effect of arsenate on endogenous levels of cytokinins with different existing forms in two Pteris species

Our previous results showed that content of trans-zeatin (tZ) increases in leaves of heavy metal hyperaccumulators but decreases in non-hyperaccumulators growing in multiple heavy metal polluted soils. However, the relationship between arsenic (As) accumulation and endogenous forms of cytokinins (CTKs) in As hyperaccumulators remains unknown. Here a hydroponic experiment was conducted to compare the CTK forms in the As hyperaccumulator Pteris cretica var. nervosa and non-hyperaccumulator Pteris ensiformis under arsenate stress (0, 2, 5, and 10 mg L^-1). A simple and cost-effective procedure for the determination of CTK forms in plants was established, and a stepwise regression analysis was used to study the relationship among total As contents and different forms of endogenous CTKs in fronds of two plants. The results showed that the optimized chromatographic parameters were Zobax SB-C18 column (5 μm × 4.6 mm × 250 mm), UV detection detector at 269 nm, a flow rate of 0.6 mL min^-1, constant temperature of 45 °C and gradient elution with methanol-acetonitrile-1% acetic acid. Contents of chlorophylls in the fronds of P. ensiformis were significantly decreased with addition of As compared to P. cretica var. nervosa. Furthermore, the total As content in fronds of P. cretica var. nervosa was positively correlated to the contents of N6-(2-isopentenyl) adenine-7-β-D-glucoside (iP7G) and N6-(2-isopentenyl) adenosine (iPR). However, the total As content in fronds of P. ensiformis was negatively correlated to its trans-zeatin riboside (ZR) content. Therefore, iP7G and iPR could positively improve As accumulation by P. cretica var. nervosa.

Two facets of world arsenic problem solution: crop poisoning restriction and enforcement of phytoremediation

This review provides insights into As toxicity in plants with focus on photosynthesis and sugar metabolism as important arsenic targets and simultaneously defence tools against accompanying oxidative stress. Heavy metal contamination is a great problem all over the world. Arsenic, a metalloid occurring naturally in the Earth's crust, also massively spreads out in the environment by human activities. Its accumulation in crops poses a severe health risk to humans and animals. Besides the restriction of human-caused contamination, there are two basic ways how to cope with the problem: first, to limit arsenic accumulation in harvestable parts of the crops; second, to make use of some arsenic hyperaccumulating plants for phytoremediation of contaminated soils and waters. Progress in the use of both strategies depends strongly on the level of our knowledge on the physiological and morphological processes resulting from arsenic exposure. Arsenic uptake is mediated preferentially by P and Si transporters and its accumulation substantially impairs plant metabolism at numerous levels including damages through oxidative stress. Rice is a predominantly studied crop where substantial progress has been made in understanding of the mechanisms of arsenic uptake, distribution, and detoxification, though many questions still remain. Full exploitation of plant potential for soil and water phytoremediations also requires deep understanding of the plant response to this toxic metalloid. The aim of this review is to summarize data regarding the effect of arsenic on plant physiology with a focus on mechanisms providing increased arsenic tolerance and/or hyperaccumulation. The emphasis is placed on the topic unjustifiably neglected in the previous reviews - i.e., carbohydrate metabolism, tightly connected to photosynthesis, and beside others involved in plant ability to cope with arsenic-induced oxidative and nitrosative stresses.