Giovanni DalCorso

Regulatory networks of cadmium stress in plants

During their life, plants have to cope with a variety of abiotic stresses. Cadmium is highly toxic to plants, water soluble and therefore promptly adsorbed in tissues and its presence greatly influences the entire plant metabolism. In this review, we focus on the signal pathways responsible for the sensing and transduction of the “metal signal” inside the cell, ultimately driving the activation of transcription factors and consequent expression of genes that enable plants to counteract the heavy metal stress.

Proteomic analysis of Arabidopsis halleri shoots in response to the heavy metals cadmium and zinc and rhizosphere microorganisms.

Arabidopsis halleri has the rare ability to colonize heavy metal‐polluted sites and is an emerging model for research on adaptation and metal hyperaccumulation. The aim of this study was to analyze the effect of plant–microbe interaction on the accumulation of cadmium (Cd) and zinc (Zn) in shoots of an ecotype of A. halleri grown in heavy metal‐contaminated soil and to compare the shoot proteome of plants grown solely in the presence of Cd and Zn or in the presence of these two metals and the autochthonous soil rhizosphere‐derived microorganisms. The results of this analysis emphasized the role of plant–microbe interaction in shoot metal accumulation. Differences in protein expression pattern, identified by a proteomic approach involving 2‐DE and MS, indicated a general upregulation of photosynthesis‐related proteins in plants exposed to metals and to metals plus microorganisms, suggesting that metal accumulation in shoots is an energy‐demanding process. The analysis also showed that proteins involved in plant defense mechanisms were downregulated indicating that heavy metals accumulation in leaves supplies a protection system and highlights a cross‐talk between heavy metal signaling and defense signaling.

Interaction between selected bacterial strains and Arabidopsis halleri modulates shoot proteome and cadmium and zinc accumulation

The effects of plant–microbe interactions between the hyperaccumulator Arabidopsis halleri and eight bacterial strains, isolated from the rhizosphere of A. halleri plants grown in a cadmium- and zinc-contaminated site, were analysed for shoot metal accumulation, shoot proteome, and the transcription of genes involved in plant metal homeostasis and hyperaccumulation. Cadmium and zinc concentrations were lower in the shoots of plants cultivated in the presence of these metals plus the selected bacterial strains compared with plants grown solely with these metals or, as previously reported, with plants grown with these metals plus the autochthonous rhizosphere-derived microorganisms. The shoot proteome of plants cultivated in the presence of these selected bacterial strains plus metals, showed an increased abundance of photosynthesis- and abiotic stress-related proteins (e.g. subunits of the photosynthetic complexes, Rubisco, superoxide dismutase, and malate dehydrogenase) counteracted by a decreased amount of plant defence-related proteins (e.g. endochitinases, vegetative storage proteins, and β-glucosidase). The transcription of several homeostasis genes was modulated by the microbial communities and by Cd and Zn content in the shoot. Altogether these results highlight the importance of plant-microbe interactions in plant protein expression and metal accumulation and emphasize the possibility of exploiting microbial consortia for increasing or decreasing shoot metal content.

Recent advances in the analysis of metal hyperaccumulation and hypertolerance in plants using proteomics

Hyperaccumulator/hypertolerant plant species have evolved strategies allowing them to grow in metal-contaminated soils, where they accumulate high concentrations of heavy metals in their shoots without signs of toxicity. The mechanisms that allow enhanced metal uptake, root-to-shoot translocation and detoxification in these species are not fully understood. Complementary approaches such as transcriptomic-based DNA microarrays and proteomics have recently been used to gain insight into the molecular pathways evolved by metal hyperaccumulator/hypertolerant species. Proteomics has the advantage of focusing on the translated portion of the genome and it allows to analyze complex networks of proteins. This review discusses the recent analysis of metal hyperaccumulator/hypertolerant plant species using proteomics. Changes in photosynthetic proteins, sulfur, and glutathione metabolism, transport, biotic and xenobiotic defenses as well as the differential regulation of proteins involved in signaling and secondary metabolism are discussed in relation to metal hyperaccumulation. We also consider the potential contribution of several proteins to the hyperaccumulation phenotype.

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.

Heavy Metal Toxicity in Plants

Plants are sessile organisms that must cope with the surrounding soil composition in order to survive and reproduce. Soils often contain excessive levels of essential and non-essential elements, which may be toxic at high concentrations depending on the plant species and the soil characteristics. Many metals share common toxicity mechanisms, and plants deal with these metals using similar scavenging pathways. The impact of metal toxicity is made more complex by competition, since high levels of one metal may imbalance the uptake and transport of others, therefore contributing to the toxicity symptoms. Here, the toxicity symptoms and mechanisms of the most common essential and non-essential heavy metals will be considered.

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.

The Role of the Atypical Kinases ABC1K7 and ABC1K8 in Abscisic Acid Responses

The ABC1K family of atypical kinases (activity of bc1 complex kinase) is represented in bacteria, archaea, and eukaryotes. In plants they regulate diverse physiological processes in the chloroplasts and mitochondria, but their precise functions are poorly defined. ABC1K7 and ABC1K8 are probably involved in oxidative stress responses, isoprenyl lipid synthesis and distribution of iron within chloroplasts. Because reactive oxygen species take part in abscisic acid (ABA)-mediated processes, we investigated the functions of ABC1K7 and ABC1K8 during germination, stomatal movement, and leaf senescence. Both genes were upregulated by ABA treatment and some ABA-responsive physiological processes were affected in abc1k7 and abc1k8 mutants. Germination was more severely affected by ABA, osmotic stress and salt stress in the single and double mutants; the stomatal aperture was smaller in the mutants under standard growth conditions and was not further reduced by exogenous ABA application; ABA-induced senescence symptoms were more severe in the leaves of the single and double mutants compared to wild type leaves. Taken together, our results suggest that ABC1K7 and ABC1K8 might be involved in the cross-talk between ABA and ROS signaling.

Editorial: Environmental phytoremediation: plants and microorganisms at work

Human industry, farming, and waste disposal practices have resulted in the large-scale contamination of soil and water with organic compounds and heavy metals, with detrimental effects on ecosystems and human health. Conventional soil remediation methods are expensive and often involve the storage of soil in designated areas, postponing rather than solving the problem. In the last decade, the pressing need to find alternative methods has highlighted the scientific and economic benefits of plants and their associated microorganisms, which can be used for the reclamation of polluted soil and water (Meagher, 2000). This is an elegant and low-cost approach for the decontamination of polluted sites and has been greeted with a high degree of public acceptance, therefore prompting research into the use of phytoremediation technology to address the large areas of land and water currently affected (reviewed by Kramer, 2005; Vangronsveld et al., 2009; Lee, 2013). This Frontiers in Plant Science research topic provides a snapshot of current research into the application of environmental phytoremediation strategies. Many scientists are currently investigating the phenomenon of metal hyperaccumulation in different species, aiming to determine the mechanisms associated with the accumulation and detoxification of heavy metals and ultimately to use these plants and their rhizosphere-derived microorganisms for the decontamination of polluted sites. A greenhouse experiment using Pteris vittata with or without bacterial strains selected from autochthonous rhizosphere-derived microorganisms [chosen for their resistance to high concentrations of arsenic (As) and their ability to reduce arsenate to arsenite] showed that the efficiency of phytoextraction increased when P. vittata plants were inoculated with the selected microbial communities (Lampis et al., 2015). A detailed comparative analysis of the endophytic bacteria and fungi from the selenium (Se) hyperaccumulator species Stanleya pinnata (Brassicaceae) and Astragalus bisulcatus (Fabaceae), and the related non-accumulators Physaria bellis (Brassicaceae) and Medicago sativa (Fabaceae), revealed that isolates from Se hyperaccumulator species were more resistant to selenate and selenite, could reduce selenite to elemental Se, could reduce nitrite and produce siderophores, and several strains also showed the ability to promote plant growth (Jong et al., 2015). Microorganisms with high Se tolerance and the ability to produce elemental Se would be useful for wastewater treatment and/or the production of Se nanoparticles (Staicu et al., 2015). The use of omics analysis and advanced microscopy to study the interaction between metal hyperaccumulators and the bacterial rhizobiome is considered in a review article by Visioli et al. (2015). This emphasizes emerging techniques for the analysis of microbial communities in polluted soils that help to determine the impact of pollution on those communities (Berg et al., 2012). It also highlights the advantages of in situ analysis to monitor the colonization of plants and the survival of microbial inoculums under real conditions, particularly the use of environmental scanning electron microscopy, a powerful approach for the in situ analysis of biological specimens without sample preparation (Stabentheiner et al., 2010; Visioli et al., 2014). The phytoremediation potential of plants inoculated with bacteria isolated from the rhizosphere and endosphere of other plants grown in soil contaminated with heavy metals is discussed in two articles (Khan et al., 2015; Ma et al., 2015). The arboreal species Prosopis juliflora, native to South America, was previously considered as a bioindicator species for polluted sites (Senthilkumar et al., 2005) and was shown to tolerate high concentrations of heavy metals and therefore may be useful in soil reclamation (Varun et al., 2011). Several bacterial strains with resistance to chromium (Cr), isolated from the rhizosphere and endosphere of P. juliflora plants grown on soil contaminated with tannery effluent, also showed tolerance toward other heavy metals such as Cd, Cu, Pb, and Zn. The inoculation of ryegrass (Lolium multiflorum L.) with three of these isolates promoted plant growth and the removal of toxic metals from polluted soil, demonstrating that the interaction between plants and bacterial strains identified in contaminated areas could improve plant growth and the efficiency of phytoremediation (Khan et al., 2015). Likewise, Brassica juncea and Ricinus communis plants inoculated with rhizospheric and endophytic bacteria isolated from a polluted serpentine environment accumulated more biomass and heavy metals than non-inoculated control plants (Ma et al., 2015). These effects were clearly attributed to the production of bacterial metabolites that promoted plant growth and metal mobilization. However, the low metal translocation factor obtained upon inoculation indicated that metal-resistant serpentine bacteria are suitable for the phytostabilization of contaminated soil (Ma et al., 2015). The beneficial interaction between plants and rhizobia for the remediation of contaminated soil is discussed by Teng et al. (2015). Certain symbiotic relationships between legumes and nitrogen-fixing bacteria are resistant to heavy metals, promoting the dissipation of organic pollutants and enhancing their removal (Fan et al., 2008; Glick, 2010; Li et al., 2013). Rhizobia not only fix nitrogen but also promote plant growth, thus increasing plant biomass, soil fertility, the bioavailability, uptake and translocation of pollutants, the degradation of organic contaminants and the phytostabilization of metals. All these features make rhizobia valuable phytoremediation tools. Endophytic rhizobia degrade organic contaminants that have accumulated in nodules, thus reducing phytovolatilization and facilitating phytoremediation in polluted environments (Teng et al., 2015). Two further articles discuss the use of plants and their associated microorganisms for the reclamation of land polluted with organic contaminants (Germaine et al., 2015; Sauvetre and Schroder, 2015). In the first project (Sauvetre and Schroder, 2015), Phragmites australis plants were exposed to carbamazepine, a widely-used drug that is present in the environment as a persistent and recalcitrant contaminant (Ternes et al., 2007; Huerta-Fontela et al., 2011). After 9 days, the plants reduced the initial drug concentration by 90%, and characterization of the endophytic bacteria revealed that all isolates possessed at least one plant growth-promoting trait. Several had the ability to remove carbamazepine from soil, whereas others produced siderophores and were able to solubilize phosphate, suggesting they would be beneficial in phytoremediation programs. The second article addresses the effectiveness of a large-scale combined phytoremediation/biopiling system, termed ecopiling, for the removal of hydrocarbons from soil affected by industrial contamination (Germaine et al., 2015). Bacterial communities capable of total petroleum hydrocarbon (TPH) degradation were used to inoculate soil contaminated with chemical fertilizers. Perennial rye grass and white clover were then sown to complete the ecopile. During a 2-year trial, there was a consistent reduction in the TPH level suggesting that this multifactorial approach involving biostimulation, bio-augmentation and phytoremediation is suitable for the remediation soils contaminated with industrial hydrocarbons. It is notable that all the articles submitted in this research topic focused on the use of naturally-occurring hyperaccumulator species rather than transgenic plants and/or microorganisms, although genetically-engineered plants and microbes can also be used for the efficient treatment of polluted soil and water (Van Aben, 2009; Singh et al., 2011). This highlights the diverse and promising approaches that are being developed by the environmental phytoremediation research community.