A complex containing PGRL1 and PGR5 is involved in the switch between linear and cyclic electron flow in Arabidopsis.

During photosynthesis, two photoreaction centers located in the thylakoid membranes of the chloroplast, photosystems I and II (PSI and PSII), use light energy to mobilize electrons to generate ATP and NADPH. Different modes of electron flow exist, of which the linear electron flow is driven by PSI and PSII, generating ATP and NADPH, whereas the cyclic electron flow (CEF) only generates ATP and is driven by the PSI alone. Different environmental and metabolic conditions require the adjustment of ATP/NADPH ratios and a switch of electron distribution between the two photosystems. With the exception of PGR5, other components facilitating CEF are unknown. Here, we report the identification of PGRL1, a transmembrane protein present in thylakoids of Arabidopsis thaliana. Plants lacking PGRL1 show perturbation of CEF, similar to PGR5-deficient plants. We find that PGRL1 and PGR5 interact physically and associate with PSI. We therefore propose that the PGRL1-PGR5 complex facilitates CEF in eukaryotes.

Enhancement of Zn tolerance and accumulation in plants mediated by the expression of Saccharomyces cerevisiae vacuolar transporter ZRC1.

MAIN CONCLUSION: Transgenic Arabidopsis thaliana and Populus alba plants overexpressing the zinc transporter ScZRC1 in shoots exhibit Zn tolerance. Increased Zn concentrations were observed in shoots of P. alba, a species suitable for phytoremediation. Genetic engineering of plants for phytoremediation is worth to consider if genes leading to heavy metal accumulation and tolerance are expressed in high biomass producing plants. The Saccharomyces cerevisiae ZRC1 gene encodes a zinc transporter which is primarily involved in the uptake of Zn into the vacuole. The ZRC1 gene was expressed in the model species A. thaliana and P. alba (cv. Villafranca). Both species were transformed with constructs carrying ScZRC1 under the control of either the CaMV35S promoter for constitutive expression or the active promoter region of the tobacco Rubisco small subunit (pRbcS) to limit the expression to the above-ground tissues. In hydroponic cultures, A. thaliana and poplar ScZRC1-expressing plants accumulated more Zn in vegetative tissues and were more tolerant than untransformed plants. No differences were found between plants carrying the CaMV35::ScZRC1 or pRbcS::ScZRC1 constructs. The higher Zn accumulation in transgenic plants was accompanied by an increased superoxide dismutase (SOD) activity, indicating the activation of defense mechanisms to prevent cellular damage. In the presence of cadmium in addition to Zn, plants did not show symptoms of metal toxicity, neither in hydroponic cultures nor in soil. Zn accumulation increased in shoots, while no differences were observed for Cd accumulation, in comparison to control plants. These data suggest that ectopic expression of ScZRC1 can increase the potential of poplar for the remediation of Zn-polluted soils, although further tests are required to assay its application in remediating multimetal polluted soils.

Comparative analysis identifies micro-RNA associated with nutrient homeostasis, development and stress response in Arabidopsis thaliana upon high Zn and metal hyperaccumulator Arabidopsis halleri.

miRNAs have been found to be key players in mineral homeostasis, both in the control of nutrient balance and in the response to toxic trace elements. However, the effect of Zn excess on miRNAs has not been elucidated; moreover, no data are present regarding miRNAs in hyperaccumulator species, where metal homeostasis is tightly regulated. Therefore, expression levels of mature miRNAs were measured by RNA-Seq in Zn-sensitive Arabidopsis thaliana grown in control conditions and upon high Zn, in soil and in Zn-hyperaccumulator Arabidopsis halleri grown in control conditions. Differential expression of notable miRNAs and their targets was confirmed by real-time RT-PCR. The comparison in A. thaliana revealed a small subset modulated upon Zn treatment that is associated with stress response and nutrient homeostasis. On the other hand, a more consistent group of miRNAs was differentially expressed in A. halleri compared with A. thaliana, reflecting inherent differences in nutritional requirements and response to stresses and plant growth and development. Overall, these results confirm the involvement of miRNAs in Zn homeostasis and support the hypothesis of distinct regulatory pathways in hyperaccumulator species.

Overexpression of ZNT1 and NRAMP4 from the Ni Hyperaccumulator Noccaea caerulescens Population Monte Prinzera in Arabidopsis thaliana Perturbs Fe, Mn, and Ni Accumulation.

Metalliferous soils are characterized by a high content of metal compounds that can hamper plant growth. The pseudometallophyte Noccaea caerulescens is able to grow on metalliferous substrates by implementing both tolerance and accumulation of usually toxic metal ions. Expression of particular transmembrane transporter proteins (e.g., members of the ZIP and NRAMP families) leads to metal tolerance and accumulation, and its comparison between hyperaccumulator N. caerulescens with non-accumulator relatives Arabidopsis thaliana and Thlaspi arvense has deepened our knowledge on mechanisms adopted by plants to survive in metalliferous soils. In this work, two transporters, ZNT1 and NRAMP4, expressed in a serpentinic population of N. caerulescens identified on the Monte Prinzera (Italy) are considered, and their expression has been induced in yeast and in A. thaliana. In the latter, single transgenic lines were crossed to test the effect of the combined over-expression of the two transporters. An enhanced iron and manganese translocation towards the shoot was induced by overexpression of NcZNT1. The combined overexpression of NcZNT1 and NcNRAMP4 did perturb the metal accumulation in plants.

Evolution of the metal hyperaccumulation and hypertolerance traits.

To succeed in life, living organisms have to adapt to the environmental issues to which they are subjected. Some plants, defined as hyperaccumulators, have adapted to metalliferous environments, acquiring the ability to tolerate and accommodate high amounts of toxic metal into their shoot, without showing symptoms of toxicity. The determinants for these traits and their mode of action have long been the subject of research, whose attention lately moved to the evolution of the hypertolerance and hyperaccumulation traits. Genetic evidence indicates that the evolution of both traits includes significant evolutionary events that result in species-wide tolerant and accumulating backgrounds. Different edaphic environments are responsible for subsequent refinement, by local adaptive processes, leading to specific strategies and various degrees of hypertolerance and hyperaccumulation, which characterize metallicolous from non-metallicolous ecotypes belonging to the same genetic unit. In this review, we overview the most updated concepts regarding the evolution of hyperaccumulation and hypertolerance, highlighting also the ecological context concerning the plant populations displaying this fascinating phenomenon.

The Tomato Metallocarboxypeptidase Inhibitor I, which Interacts with a Heavy Metal-Associated Isoprenylated Protein, Is Implicated in Plant Response to Cadmium.

Metallocarboxypeptidases are metal-dependent enzymes, whose biological activity is regulated by inhibitors directed on the metal-containing active site. Some metallocarboxypeptidase inhibitors are induced under stress conditions and have a role in defense against pests. This paper is aimed at investigating the response of the tomato metallocarboxypeptidase inhibitor (TCMP)-1 to Cd and other abiotic stresses. To this aim, the tomato TCMP-1 was ectopically expressed in the model species Arabidopsis thaliana, and a yeast two-hybrid analysis was performed to identify interacting proteins. We demonstrate that TCMP-1 is responsive to Cd, NaCl, and abscisic acid (ABA) and interacts with the tomato heavy metal-associated isoprenylated plant protein (HIPP)26. A. thaliana plants overexpressing TCMP-1 accumulate lower amount of Cd in shoots, display an increased expression of AtHIPP26 in comparison with wild-type plants, and are characterized by a modulation in the expression of antioxidant enzymes. Overall, these results suggest a possible role for the TCMP-1/HIPP26 complex in Cd response and compartmentalization.

The Arabidopsis thaliana transcription factor MYB59 regulates calcium signalling during plant growth and stress response.

KEY MESSAGE: Transcription factor MYB59 is involved in plant growth and stress responses by acting as negative regulator of Ca signalling and homeostasis. The Arabidopsis thaliana transcription factor MYB59 is induced by cadmium (Cd) and plays a key role in the regulation of cell cycle progression and root elongation, but its mechanism of action is poorly understood. We investigated the expression of MYB59 and differences between wild-type plants, the myb59 mutant and MYB59-overexpressing lines (obtained by transformation in the mutant genotype) during plant growth and in response to various forms of stress. We also compared the transcriptomes of wild-type and myb59 mutant plants to determine putative MYB59 targets. The myb59 mutant has longer roots, smaller leaves and smaller cells than wild-type plants and responds differently to stress in germination assay. Transcriptomic analysis revealed the upregulation in the myb59 mutant of multiple genes involved in calcium (Ca) homeostasis and signalling, including those encoding calmodulin-like proteins and Ca transporters. Notably, MYB59 was strongly induced by Ca deficiency, and the myb59 mutant was characterized by higher levels of cytosolic Ca in root cells and showed a modest alteration of Ca transient frequency in guard cells, associated with the absence of Ca-induced stomatal closure. These results indicate that MYB59 negatively regulates Ca homeostasis and signalling during Ca deficiency, thus controlling plant growth and stress responses.

Heavy Metal Pollutions: State of the Art and Innovation in Phytoremediation.

Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils.

The potential of genetic engineering of plants for the remediation of soils contaminated with heavy metals.

The genetic engineering of plants to facilitate the reclamation of soils and waters contaminated with inorganic pollutants is a relatively new and evolving field, benefiting from the heterologous expression of genes that increase the capacity of plants to mobilize, stabilize and/or accumulate metals. The efficiency of phytoremediation relies on the mechanisms underlying metal accumulation and tolerance, such as metal uptake, translocation and detoxification. The transfer of genes involved in any of these processes into fast-growing, high-biomass crops may improve their reclamation potential. The successful phytoextraction of metals/metalloids and their accumulation in aerial organs have been achieved by expressing metal ligands or transporters, enzymes involved in sulfur metabolism, enzymes that alter the chemical form or redox state of metals/metalloids and even the components of primary metabolism. This review article considers the potential of genetic engineering as a strategy to improve the phytoremediation capacity of plants in the context of heavy metals and metalloids, using recent case studies to demonstrate the practical application of this approach in the field.

The MTP1 promoters from Arabidopsis halleri reveal cis-regulating elements for the evolution of metal tolerance.

In the hyperaccumulator Arabidopsis halleri, the zinc (Zn) vacuolar transporter MTP1 is a key component of hypertolerance. Because protein sequences and functions are highly conserved between A. halleri and Arabidopsis thaliana, Zn tolerance in A. halleri may reflect the constitutively higher MTP1 expression compared with A. thaliana, based on copy number expansion and different cis regulation. Three MTP1 promoters were characterized in A. halleri ecotype I16. The comparison with the A. thaliana MTP1 promoter revealed different expression profiles correlated with specific cis-acting regulatory elements. The MTP1 5' untranslated region, highly conserved among A. thaliana, Arabidopsis lyrata and A. halleri, contains a dimer of MYB-binding motifs in the A. halleri promoters absent in the A. thaliana and A. lyrata sequences. Site-directed mutagenesis of these motifs revealed their role for expression in trichomes. A. thaliana mtp1 transgenic lines expressing AtMTP1 controlled by the native A. halleri promoter were more Zn-tolerant than lines carrying mutations on MYB-binding motifs. Differences in Zn tolerance were associated with different distribution of Zn among plant organs and in trichomes. The different cis-acting elements in the MTP1 promoters of A. halleri, particularly the MYB-binding sites, are probably involved in the evolution of Zn tolerance.

Loss of the Atypical Kinases ABC1K7 and ABC1K8 Changes the Lipid Composition of the Chloroplast Membrane.

The activity of bc1 complex kinase (ABC1K) family is a large group of atypical protein kinases found in prokaryotes and eukaryotes. In bacteria and mitochondria, ABC1K kinases are necessary for the synthesis of coenzyme Q and are therefore involved in the respiratory pathway. In chloroplasts, they are involved in prenylquinone synthesis and stress responses, but their functional role remains unclear. Plants can respond to biotic and abiotic stress by modifying membrane fluidity in order to create a suitable environment for the activity of integral membrane proteins. Therefore, this work was focused on the analysis of the effect of ABC1K7 and ABC1K8 on the production of polar lipids and their accumulation in Arabidopsis thaliana leaves. A comparison of abc1k7 and abc1k8 single mutants and the abc1k7/abc1k8 double mutant with wild-type plants and transgenic lines overexpressing ABC1K7 and ABC1K8 was performed using untargeted lipidomic analysis based on liquid chromatography coupled to mass spectrometry. Multivariate data analysis identified sets of chloroplast lipids representing the different genotypes. The abc1k7 and abc1k8 single mutants produced lower levels of the highly unsaturated lipid digalactosyldiacylglycerol (DGDG) than wild-type plants and also different forms of monogalactosyldiacylglycerol (MGDG) and kaempferol. The abc1k8 mutant also produced higher levels of oxylipin-conjugated DGDG and sinapates. The double mutant produced even higher levels of oxylipin-conjugated MGDG and DGDG. These results show that ABC1K7 and ABC1K8 influence chloroplast lipid synthesis or accumulation and modulate chloroplast membrane composition in response to stress.

AtSIA1 AND AtOSA1: two Abc1 proteins involved in oxidative stress responses and iron distribution within chloroplasts.

The Abc1 protein kinases are a large family of functionally diverse proteins with multiple roles in the regulation of respiration and oxidative stress tolerance. A functional characterization was carried out for AtSIA1, an Arabidopsis thaliana Abc1-like protein, focusing on its potential redundancy with its homolog AtOSA1. Both proteins are located within chloroplasts, even if a different subplastidial localization seems probable. The comparison of atsia1 and atosa1 mutants, atsia1/atosa1 double mutant and wild-type plants revealed a reduction in plastidial iron-containing proteins of the Cytb6 f complex in the mutants. Iron uptake from soil is not hampered in mutant lines, suggesting that AtSIA1 and AtOSA1 affect iron distribution within the chloroplast. Mutants accumulated more ferritin and superoxide, and showed reduced tolerance to reactive oxygen species (ROS), potentially indicating a basal role in oxidative stress. The mutants produced higher concentrations of plastochromanol and plastoquinones than wild-type plants, but only atsia1 plants developed larger plastoglobules and contained higher concentrations of α- and γ-tocopherol and VTE1. Taken together, these data suggest that AtSIA1 and AtOSA1 probably act in signaling pathways that influence responses to ROS production and oxidative stress.

Genetic approaches to exploit landraces for improvement of Triticum turgidum ssp. durum in the age of climate change.

Addressing the challenges of climate change and durum wheat production is becoming an important driver for food and nutrition security in the Mediterranean area, where are located the major producing countries (Italy, Spain, France, Greece, Morocco, Algeria, Tunisia, Turkey, and Syria). One of the emergent strategies, to cope with durum wheat adaptation, is the exploration and exploitation of the existing genetic variability in landrace populations. In this context, this review aims to highlight the important role of durum wheat landraces as a useful genetic resource to improve the sustainability of Mediterranean agroecosystems, with a focus on adaptation to environmental stresses. We described the most recent molecular techniques and statistical approaches suitable for the identification of beneficial genes/alleles related to the most important traits in landraces and the development of molecular markers for marker-assisted selection. Finally, we outline the state of the art about landraces genetic diversity and signature of selection, already identified from these accessions, for adaptability to the environment.

Metal Interactions in the Ni Hyperaccumulating Population of Noccaea caerulescens Monte Prinzera.

Hyperaccumulation is a fascinating trait displayed by a few plant species able to accumulate large amounts of metal ions in above-ground tissues without symptoms of toxicity. Noccaea caerulescens is a recognized model system to study metal hyperaccumulation and hypertolerance. A N. caerulescens population naturally growing on a serpentine soil in the Italian Apennine Mountains, Monte Prinzera, was chosen for the study here reported. Plants were grown hydroponically and treated with different metals, in excess or limiting concentrations. Accumulated metals were quantified in shoots and roots by means of ICP-MS. By real-time PCR analysis, the expression of metal transporters and Fe deficiency-regulated genes was compared in the shoots and roots of treated plants. N. caerulescens Monte Prinzera confirmed its ability to hypertolerate and hyperaccumulate Ni but not Zn. Moreover, excess Ni does not induce Fe deficiency as in Ni-sensitive species and instead competes with Fe translocation rather than its uptake.

Epigenetic Control of Plant Response to Heavy Metals.

Plants are sessile organisms that must adapt to environmental conditions, such as soil characteristics, by adjusting their development during their entire life cycle. In case of low-distance seed dispersal, the new generations are challenged with the same abiotic stress encountered by the parents. Epigenetic modification is an effective option that allows plants to face an environmental constraint and to share the same adaptative strategy with their progeny through transgenerational inheritance. This is the topic of the presented review that reports the scientific progress, up to date, gained in unravelling the epigenetic response of plants to soil contamination by heavy metals and metalloids, collectively known as potentially toxic elements. The effect of the microbial community inhabiting the rhizosphere is also considered, as the evidence of a transgenerational transfer of the epigenetic status that contributes to the activation in plants of response mechanisms to soil pollution.

DNA methylation is enhanced during Cd hyperaccumulation in Noccaea caerulescens ecotype Ganges.

In this study, we assess the DNA damage occurring in response to cadmium (Cd) in the Cd hyperaccumulator Noccaea caerulescens Ganges (GA) vs the non-accumulator and close-relative species Arabidopsis thaliana. At this purpose, the alkaline comet assay was utilized to evaluate the Cd-induced variations in nucleoids and the methy-sens comet assay, and semiquantitative real-time (qRT)-PCR were also performed to associate nucleus variations to possible DNA modifications. Cadmium induced high DNA damages in nuclei of A. thaliana while only a small increase in DNA migration was observed in N. caerulescens GA. In addition, in N. caerulescens GA, CpG DNA methylation increase upon Cd when compared to control condition, along with an increase in the expression of MET1 gene, coding for the DNA-methyltransferase. N. caerulescens GA does not show any oxidative stress under Cd treatment, while A. thaliana Cd-treated plants showed an upregulation of transcripts of the respiratory burst oxidase, accumulation of reactive oxygen species, and enhanced superoxide dismutase activity. These data suggest that epigenetic modifications occur in the N. caerulescens GA exposed to Cd to preserve genome integrity, contributing to Cd tolerance.

Pseudomonas putida response to cadmium: changes in membrane and cytosolic proteomes.

Pseudomonas putida is a saprophytic bacterium with remarkable environmental adaptability and the capacity to tolerate high concentrations of heavy metals. The strain P. putida-Cd001 was isolated from soil contaminated with Cd, Zn and Pb. Membrane-associated and cytosolic proteomes were analyzed to identify proteins whose expression was modulated in response to 250 µM CdSO4. We identified 44 protein spots in the membrane and 21 in the cytosolic fraction differentially expressed in Cd-treated samples compared to untreated controls. Outer membrane porins from the OprD and OprI families were less abundant in bacteria exposed to Cd, whereas those from the OprF and OprL, OprH and OprB families were more abundant, reflecting the increased need to acquire energy sources, the need to maintain membrane integrity and the process of adaptation. Components of the efflux system, such as the CzcB subunit of the CBA system, were also induced by Cd. Analysis of the cytosolic proteome revealed that proteins involved in protein synthesis, degradation and folding were induced along with enzymes that combat oxidative stress, showing that the entire bacterial proteome is modulated by heavy metal exposure. This analysis provides new insights into the adaptation mechanisms used by P. putida-Cd001 to survive in Cd-polluted environments.

Metal Detoxification in Land Plants: From Bryophytes to Vascular Plants. STATE of the Art and Opportunities.

Potentially toxic elements are a widespread concern due to their increasing diffusion into the environment. To counteract this problem, the relationship between plants and metal(loid)s has been investigated in the last 30 years. In this field, research has mainly dealt with angiosperms, whereas plant clades that are lower in the evolutive scale have been somewhat overlooked. However, recent studies have revealed the potential of bryophytes, pteridophytes and gymnosperms in environmental sciences, either as suitable indicators of habitat health and elemental pollution or as efficient tools for the reclamation of degraded soils and waters. In this review, we summarize recent research on the interaction between plants and potentially toxic elements, considering all land plant clades. The focus is on plant applicability in the identification and restoration of polluted environments, as well as on the characterization of molecular mechanisms with a potential outlet in the engineering of element tolerance and accumulation.

MYB59 transcription factor behaves differently in metallicolous and non-metallicolous populations of Arabidopsis halleri.

In Arabidopsis thaliana (L.) Heynh., MYB59 transcription factor participates in regulating Ca homeostasis and signal transduction and is induced by Cd excess. To investigate its role in the facultative metallophyte Arabidopsis halleri ssp. halleri (L.) O'Kane and Al-Shehbaz, MYB59 expression was investigated under Cd treatment or Ca depletion in three populations belonging to distinct phylogeographic units (metallicolous PL22 and I16 and non-metallicolous I29), and compared with the expression in A. thaliana. In control conditions, MYB59 transcription in A. thaliana and the non-metallicolous population I29 follow a comparable trend with higher expression in roots than shoots, whereas in metallicolous populations I16 and PL22 its expression is similar in roots and shoots, suggesting a convergent evolution associated with adaptation to metalliferous environments. After 6 h of Ca depletion, MYB59 transcript levels were very high in I16 and PL22 populations, indicating that the adaptation to metalliferous environments requires tightly regulated Ca homeostasis and signalling. Cd treatment caused variability in MYB59 expression. In I29, MYB59 expression, as in A. thaliana, is likely associated to stress response, whereas its modulation in the two metallicolous populations reflects the different strategies for Cd tolerance and accumulation. In conclusion, MYB59 regulation in A. halleri is part of the network linking mineral nutrition and Cd tolerance/accumulation.

Heavy metals modulate DNA compaction and methylation at CpG sites in the metal hyperaccumulator Arabidopsis halleri.

Excess heavy metals affect plant physiology by inducing stress symptoms, however several species have evolved the ability to hyperaccumulate metals in above-ground tissues without phytotoxic effects. In this study we assume that at subcellular level, different strategies were adopted by hyperaccumulator versus the non-accumulator plant species to face the excess of heavy metals. At this purpose the comet assay was used to investigate the nucleoid structure modifications occurring in response to Zn and Cd treatments in the I16 and PL22 populations of the hyperaccumulator Arabidopsis halleri versus the nonaccumulator species Arabidopsis thaliana. Methy-sens comet assay and RT-qPCR were also performed to associate metal induced variations in nucleoids with possible epigenetic modifications. The comet assay showed that Zn induced a mild but non significant reduction in the tail moment in A. thaliana and in both I16 and PL22. Cd treatment induced an increase in DNA migration in nuclei of A. thaliana, whereas no differences in DNA migration was observed for I16, and a significant increase in nucleoid condensation was found in PL22 Cd treated samples. This last population showed higher CpG DNA methylation upon Cd treatment than in control conditions, and an up-regulation of genes involved in symmetric methylation and histone deacetylation. Our data support the hypothesis of a possible role of epigenetic modifications in the hyperaccumulation trait to cope with the high Cd shoot concentrations. In addition, the differences observed between PL22 and I16 could reinforce previous suggestions of divergent strategies for metals detoxification developing in the two metallicolous populations.