The presence of arsenic in soil and water is a constant threat to plant growth in many regions of the world. Phytohormones act in the integration of growth control and stress response, but their role in plant responses to arsenic remains to be elucidated. Here, we show that arsenate [As(V)], the most prevalent arsenic chemical species in nature, causes severe depletion of endogenous cytokinins (CKs) in the model plant Arabidopsis (Arabidopsis thaliana). We found that CK signaling mutants and transgenic plants with reduced endogenous CK levels showed an As(V)-tolerant phenotype. Our data indicate that in CK-depleted plants exposed to As(V), transcript levels of As(V)/phosphate-transporters were similar or even higher than in wild-type plants. In contrast, CK depletion provoked the coordinated activation of As(V) tolerance mechanisms, leading to the accumulation of thiol compounds such as phytochelatins and glutathione, which are essential for arsenic sequestration. Transgenic CK-deficient Arabidopsis and tobacco lines show a marked increase in arsenic accumulation. Our findings indicate that CK is an important regulatory factor in plant adaptation to arsenic stress.Since the inception of life, arsenic in the biosphere has been a constant challenge to the survival of life forms. Although all organisms on earth have developed strategies to cope with this extremely toxic metalloid (Rosen, 2002;Tripathi et al., 2007), arsenic currently poses a major worldwide environmental problem (Naujokas et al., 2013).Arsenate [As(V)] is the most abundant chemical form of arsenic. Due to its structural similarity to phosphate (Pi), it is easily incorporated into plants and other organisms through Pi transporters. Once taken up by the cell, As(V) is rapidly reduced to arsenite [As(III)] by As (V) reductases. As(III) is either extruded from the cytoplasm or is sequestered by phytochelatins (PCs) and other related thiol-containing compounds and is compartmentalized into vacuoles (Tripathi et al., 2007).In plants, when As(V) is perceived, Pi transporters are rapidly downregulated, and thiol compound accumulation increases concomitantly to cope with the metalloid. These two responses are key As(V) tolerance strategies for natural plant populations (Meharg and Macnair 1992;Bleeker et al., 2006). Accumulation of PC and other related thiol-containing compounds is widely used by plants for heavy metal detoxification. PCs are synthesized from glutathione (GSH) by the enzyme PHYTOCHELATIN SYNTHASE1 (PCS1). As(V) induces PCS1, together with two other genes involved in GSH biosynthesis: g-GLUTAMYLCYSTEINE SYNTHETASE and GLUTATHIONE SYNTHETASE (GSH2; Sung et al., 2009). In addition, As(III) activates GSH and PC accumulation; As(V) reductase activity is thus essential for arsenic tolerance (Bleeker et al., 2006). Arabidopsis thaliana ARSENATE REDUCTASE QTL1 (AtARQ1, At2g21045; also termed HAC1; Chao et al., 2014), an As(V) reductaseencoding gene, is also transcriptionally regulated by As(V) (Sánchez-Bermejo et al., 2014). Arsenic tolerance thus ...
Arsenic, a class-1 carcinogenic, is a ubiquitous metalloid found in the atmosphere, soils, natural waters, and organisms. The World Health Organization (WHO) estimates that hundred million people worldwide might be chronically exposed to arsenic in drinking water at concentrations above the safety standard. Conventionally applied techniques to remove arsenic species show low removal efficiency, high operational costs, and high-energy requirements. The biological methods, especially phytoremediation, could be cost-effective for protecting human health and the environment from toxic metal contamination. Plants, as sessile organisms, have developed an extraordinary capacity to tolerate arsenic through three main strategies: uptake repression, sequestration into the vacuole, or extrusion. Therefore, arsenic perception and tolerance require a coordinated response that involves arsenic transporters, extrusion pumps, vacuole transporters, and the activation of the phytochelatin biosynthetic pathway. For phytoremediation to become a feasible strategy for arsenic removal from contaminated sites, it is essential to completely understand the molecular mechanisms of arsenic uptake, extrusion, and sequestration, as well as how this response is coordinated. The new genome-wide technologies provide a unique opportunity to understand the molecular mechanisms underlying arsenic perception and accumulation in plants that will open up new possibilities for phytoremediation of arsenic-contaminated waters and soils.
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