A comparative analysis of nickel (Ni) accumulation by the hyperaccumulator Noccaea japonica (H. Boissieu) F.K. Mey originating from ultramafic (serpentine) soil and plants from 16 populations of the hyperaccumulator Noccaea caerulescens F.K. Mey originating from ultramafic (serpentine), calamine and non-metalliferous soils, was performed. The plants were grown for 2 weeks in half-strength Hoagland’s solution without Ni, followed by a 6-week exposure to NiSO4 at a non-toxic concentration (1 μM). The Ni concentration in the roots and shoots was determined by atomic absorption spectrophotometry. In N. japonica, the Ni concentration in the shoots was significantly lower than in the roots, and lower than that in the shoots of N. caerulescens from the ultramafic populations. The ability of plants from different populations of N. caerulescens to accumulate Ni in roots (per unit dry weight) decreased in the following order: Puente Basadre ≈ Le Coulet > St-Baudille ≈ Cira ≈ Prémanon > Viviez ≈ Monte Prinzera > Les Avinières > Moravskoslezké > Le Bleymard ≈ Krušné Hory ≈ Wilwerwiltz ≈ La Calamine ≈ St-Félix-de-Palliéres ≈ Kuopio > Prayon. The value of the translocation factor in N. japonica did not significantly differ from that in the ultramafic population Puente Basadre of N. caerulescens, whereas it varied within wide limits among the N. caerulescens populations. The highest Ni translocation factor was obtained for the population Monte Prinzera from the ultramafic group and the populations Krušné Hory and Kuopio from the non-metallicolous group, whereas the lowest values were obtained for the calamine populations La Calamine and Prayon. In N. caerulescens, the Ni concentration in the roots was uncorrelated with the Ni concentration in the shoots, but significantly positively correlated with Ni tolerance. The high Ni tolerance in ultramafic populations is apparently explained by a high capacity to sequester Ni in the roots themselves, and not directly related to the root-to-shoot translocation capacity.

In this work, the effect of exogenous histidine supply on zinc (Zn) and nickel (Ni) translocation in shoot-excised root systems of the non-accumulating species Arabidopsis thaliana (L.) Heynh. and Lepidium ruderale L. was studied. Intact 7-week-old plants of A. thaliana and L. ruderale were pretreated for 4 h (13:00 till 17:00) with MES/KOH buffered 1 mM L-histidine, L-alanine or demineralized water. After the pretreatment, the leaf rosettes were cut off with a razor blade and the root systems were transferred to a fresh nutrient solution amended with 25 µM Ni(NO3)2 or Zn(NO3)2. Root pressure exudates were collected overnight (till 11:00 AM). The Ni and Zn concentrations in the roots and root pressure exudates were determined by atomic absorption spectrophotometry. The amount of Ni or Zn loaded into the xylem exudate (‘total amount of Ni or Zn exudated’) was calculated as the product of the metal concentration and the volume of root pressure exudate, expressed on a root dry weight basis. The ‘total Zn or Ni amount’ present in the root system and the root pressure exudate together (‘total uptake’), at the end of the experiment, was calculated as the sum of the total amount of Zn or Ni present in the root pressure exudates and the remaining amount in the root system after sap collection, and expressed on a root dry weight basis. Zn or Ni xylem loading was then recalculated as a percentage of the total Zn or Ni uptake. Pretreatment with L-histidine caused a significant increase in the xylem loading and the total amount of exudated Ni and Zn in L. ruderale and Zn in A. thaliana. No increase in Ni xylem loading, Ni concentration in the root pressure exudate, or total amount of Ni exudated was observed in A. thaliana after pretreatment with L-histidine. No decrease in the volume of root pressure exudates was observed in histidine-pretreated plants, indicating that the increase in metal concentration in the root pressure exudates is certainly not attributable to decreased root pressure exudation in any of the treatments. In contrast to L-histidine, pretreatment with L-alanine did not increase the metal concentrations in the root pressure exudates or the percentage of metal loaded into the xylem. The total uptake of Ni and Zn in A. thaliana was significantly higher than in L. ruderale, which is consistent with the higher concentrations of both metals in the roots of A. thaliana. Exogenous L-histidine and L-alanine did not affect the uptake of metals and their concentration in the roots, except for a slight increase in the uptake of Zn and its concentration in the roots of alanine-pretreated A. thaliana. It is concluded that Ni and Zn translocation in L. ruderale and Zn translocation in A. thaliana may be limited by the concentration of free histidine in their roots.

In this work, a comparative analysis of zinc (Zn) accumulation by the excluder Microthlaspi perfoliatum (L.) F.K. Mey, the hyperaccumulator Noccaea japonicum (H. Boissieu) F.K. Mey from ultramafic (serpentine) soil and plants from 19 populations of the hyperaccumulator Noccaea caerulescens F.K. Mey originating from ultramafic, calamine and non-metalliferous soils was carried out. The seedlings were grown for 2 weeks on half-strength Hoagland’s solution at 2 μM ZnSO4, followed by incubation for 6 weeks at 5 μM ZnSO4 (these Zn concentrations were non-toxic for all the populations of all species). The Zn concentration in the roots and shoots was determined by atomic absorption spectrophotometry. In M. perfoliatum, the Zn concentration in the roots was significantly higher than in the shoots, whereas in the hyperaccumulators, N. japonicum and N. caerulescens, the Zn concentrations in both organs were in most cases similar or higher in the shoots. Within N. caerulescens the greatest differences in Zn accumulation in roots and shoots were found among the calamine populations whereas the smallest differences were found among the ultramafic populations. Zn accumulation in roots per unit dry weight decreased in the following order: Les Avinières ≈ St-Baudille ≈ Viviez ≈ Le Coulet ≈ Le Puy de Wolf > Cira > Puente Basadre > Moravskoslezské ≈ Monte Prinzera > St‑Fé-lix-de-Pallières ≈ Prémanon > La Calamine > Le Bleymard > Krušné Hory ≈ Kuopio > Prayon > Wilwerwiltz ≈ Jean Arsac ≈ Plombières. The value of the translocation factor (TF) was the lowest in the excluder M. perfoliatum. In N. japonicum, TF did not significantly differ from the ultramafic populations of N. caerulescens. Among populations of N. caerulescens, the mean TF values varied to a large extent. The highest TF value (5.83) was obtained for Prayon and the lowest value (0.37) for Les Avinières, both belonging to the calamine ecotype. No correlation was found between the Zn concentration in the roots and the Zn concentration in the shoots in N. caerulescens. A significant negative correlation was found between the Zn accumulation in the roots and plant Zn tolerance estimated by the root growth test, which indicates that root Zn tolerance in N. caerulescens might depend, to some extent, on the capacity to restrict the accumulation of Zn in the root, mainly through restricting its uptake into the root. The difference in root Zn tolerance between the calamine and non-metallicolous ecotypes seems to be largely explained by an enhanced Zn sequestration capacity in the calamine ecotype, compared to the non-metallicolous one.

In this work, the effect of exogenous histidine supply on zinc (Zn) and nickel (Ni) translocation to the shoots in intact plants of the hyperaccumulator Noccaea caerulescens F.K. Mey was studied. Three series of experiments were carried out. (1) Intact N. caerulescens plants (St-Félix-de-Pallières population) were pretreated for 4 h (12:00 till 16:00) with a MES/KOH-buffered 1 mM L-histidine solution or demineralized water, then exposed overnight (20 h) to 5, 25 or 250 µM Ni or Zn and harvested. (2) Intact N. caerulescens plants of the same population were pretreated with 1 mM L-histidine solution or demineralized water overnight (20 h) and then exposed to 250 µM Ni or Zn for 8 h during the day (10:00 till 18:00) and harvested. (3) Intact N. caerulescens plants (the calamine populations St-Félix-de-Pallières (SF) and La Calamine (LC), and the ultramafic population Monte Prinzera (MP)) were exposed for 8 h (10:00 till 18:00) to 250 µM Ni or Zn and then to 1 mM L-histidine solution or demineralized water overnight (20 h) and harvested. The Ni and Zn concentrations in the roots and shoots were determined by atomic absorption spectrophotometry. The translocation factor (TF), expressed as the shoot to root metal concentration ratio, the total plant Ni or Zn content, and the percentage of the total Ni or Zn content present in the shoot (% translocated) were calculated. A 4 h pretreatment with L-histidine during the afternoon (before metal exposure overnight) significantly decreased the Ni and Zn concentrations in the root and increased the concentration of Ni, but not of Zn, in the shoot, significantly increased both TF and the % translocated for both metals, albeit much more strongly for Ni, and also slightly, but significantly, increased the total plant content of Ni, but not of Zn. Overnight pretreatment with L-histidine (followed by metal exposure during the day) of the same population (SF) had basically similar effects on Ni translocation, but significantly decreased the plant total Ni content, and was without significant effects on Zn translocation, but considerably decreased the root Zn concentration. The different populations under study (SF, MP, LC) showed significant differences in their Ni and Zn uptake and translocation capacities, but in general showed qualitatively similar responses to post-treatment with L‑histidine that strongly increased the TF and the % translocated for both metals in SF and MP, whereas in LC the effect was prominent only for Ni. Significant population × histidine treatment effect interactions were obtained for the root Zn concentration, and the TF and % translocated for Ni, which were largely explained by a relatively low responsiveness to the L-histidine treatment in LC, compared to SF and/or MP. It is concluded that the high endogenous L-histidine concentrations in N. caerulescens are probably functional in the hyperaccumulation of both Ni and Zn. The overall stronger effect of exogenous L-histidine supply on the translocation of Ni, compared to Zn, seems to result, at least in part, from the high Zn burdens at the start of the treatments, particularly in the shoots, which largely mask the apparent effects of exogenous L-histidine supply on the shoot Zn concentration and, to a lower degree, the % Zn translocated.

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

The ability to accumulate nickel (Ni) was compared in hyperaccumulator Noccaea сaerulescens F.K. Mey and excluder Thlaspi arvense L. after a short-term (1, 2, or 3 days) and long-term (8 weeks) exposure. T. arvense and four accessions of N. сaerulescens (La Calamine (LC), Saint Félix de Palliéres (SF), Monte Prinzera (MP), and Lellingen (LE)) were grown on a half-strength Hoagland`s solution in the presence of 25 µM Ni(NO3)2 (N. сaerulescens and T. arvense) and 250 µM Ni(NO3)2 (N. сaerulescens; T. arvense for only 1–3 days). Metal content in the roots and shoots was determined by atomic absorption spectroscopy. The Ni content per unit mass in the roots and shoots of N. сaerulescens in most cases did not differ significantly after the short-term incubation. At 25 µM Ni in the nutrient solution, its content in the roots of LC plants after 2–3 days of incubation was lower than in T. arvense, whereas Ni content in the shoots of these plants was similar. In the plants of other accessions of N. сaerulescens, Ni content in the roots and shoots in most cases was higher than in T. arvense. At 250 µM Ni, the differences in metal content in the roots were insignificant, and its content in the shoots in all the accessions of the hyperaccumulator was much higher than in the excluder. The Ni translocation factor was higher in N. сaerulescens than in T. arvense and exceeded unity only in the plants of MP accession. After the long-term exposure, the Ni translocation factor was higher than 1 in plants of all accessions of N. сaerulescens and decreased in the following order: MP ≈ LC > LE ≥ SF; in T. arvense, it did not exceed 0.3. Upon both long-term and short-term exposure, the ability to accumulate Ni by N. сaerulescens plants of different accessions generally increased in the following order: LC < SF < LE < MP. However, minor changes were observed depending on the duration of exposure and Ni concentration in the medium. Thus, considerable differences in the ability to accumulate Ni among the plants of different accessions of hyperaccumulator N. сaerulescens became apparent as early as during the first days of exposure to Ni and hardly depended on the duration of incubation or metal concentration in the medium. The obtained data confirm the assumption about a constitutive or genetically predetermined ability of plants of different N. сaerulescens accessions to accumulate Ni.

The work dealt with the influence of free L-histidine on nickel (Ni) translocation into the shoots of the hyperaccumulator plants Alyssum murale, A. fallacinum, A. corsicum, A. tenium, A. lesbiacum, A. bertolonii, A. pintodasilvae, and A. obovatum and of the closely related non-hyperaccumulator Aurinia saxatilis (formerly Alyssum saxatile ). The Ni concentration in the xylem sap was determined by graphite furnace or flame atomic absorption spectrophotometry. If plants were not treated with L-histidine or L-alanine, the highest Ni concentration was found in the xylem sap of A. murale and A. corsicum . When the plants were pretreated with L‑histidine, the Ni loading into the xylem vessels increased in only two hyperaccumulator species, A. pintodasilvae and A. obovatum , and in the non-hyperaccumulator A. saxatilis . The plant pretreatment with L-alanine did not increase the Ni level in the xylem sap. This indicates that the stimulation of Ni xylem loading is histidine-specific and not characteristic of any amino acid. Therefore, the role of histidine in the selective nickel accumulation in the shoots may considerably differ even in closely related plant species of one genus. This may presumably be accounted for by both different contents of endogenous histidine in the roots and specific patterns of the metal transport and distribution in different species.

Comparative analysis of growth and composition of Atropa belladonna L. plants was performed after separate and combined additions of NaCl and NiCl 2 to the nutrient medium. Plants were grown in water culture on modified Johnson solution for 8 weeks until the formation of the fifth leaf pair. Thereafter, NiCl 2 was introduced at final concentrations of 100 and 150 μM into the medium either separately or in combination with 100 mM NaCl. After completing the 7-day treatment with Ni ions, the plants' weight and the content of water and photosynthetic pigments were determined. The content of Ni, free polyamines (putrescine, spermidine, spermine), and atropine was determined in plant roots and leaves, whereas the content of Fe, proline, and malondialdehyde (MDA) was examined in leaves only. The distribution of Ni in various tissues was inspected using the dimethylglyoxime method. The presence of NiCl 2 in growth media diminished the increments in fresh weight of shoots and roots; lowered the content of water, pigments, and iron in leaves; and initiated chlorosis. The leaves of Ni-treated plants accumulated larger amounts of atropine, putrescine, proline, and MDA with respect to the control levels of these compounds. In contrast to the action of Ni alone, the combined application of NaCl and NiCl 2 was followed by the increased content of water and pigments in leaves. The presence of NaCl in the medium restricted the entry of Ni into roots and diminished the levels of MDA and proline in leaves. After growing the plants in the presence of 100 and 150 μM NiCl 2 , nickel was located in the root outer cortex and the rhizoderm. In plants treated with 150 μM NiCl 2 , nickel was also observed in tissues of the central cylinder, mostly in the pericycle, phloem, and xylem. In plants grown in the presence of 150 μM NiCl 2 and 100 mM NaCl, the decreased accumulation of nickel was noted in the tissues of the central cylinder in the root hair zone. Thus, the combined action of Ni and moderate salinity reduced nickel accumulation in roots and aboveground organs of A. belladonna plants. The reduced Ni content in plants mitigated the toxic effect of Ni present in the medium. This was manifested in stabilization of leaf water status, an increase in the content of photosynthetic pigments, and alleviation of oxidative stress, which was assessed from the content of low-molecular organic compounds exhibiting stress-protective and antioxidant action (proline, MDA, free polyamines, and atropine).

Enrichment and pure cultures of hyperthermophilic archaea capable of anaerobic growth on one-carbon compounds (CO and/or formate) were obtained from deep-sea sites of hydrothermal activity at the Mid-Atlantic Ridge, Lau Basin, and Guaymas Basin. All isolates belonged to the T. barophilus‒T. paralvinellae group within the genus Thermococcus. In all cases available for analysis, the genomes of Thermococcus strains capable of growth by hydrogenogenic utilization of CO and/or formate contained clusters of genes encoding energy-converting hydrogenase and either CO dehydrogenase or formate dehydrogenase and formate transporter. Apart from the previously known processes of hydrogenogenic oxidation of CO and formate, the oxidation of these substrates coupled to sulfur reduction was observed, processes previously unknown among archaea. The capacities for hydrogenogenic or sulfidogenic oxidation of CO and formate occurred in the studied strains in all possible combinations, which could only in part be explained by peculiarities of organization of genetic determinants revealed in the genomes. Investigation of CO and formate consumption kinetics revealed that T. barophilus strain Ch5 was able to grow at concentrations close to the environmental ones. Thus, it was shown that hyperthermophilic archaea from deep-sea hydrothermal vents are able to utilize one-carbon substrates of abiotic origin both in the presence of an electron acceptor (sulfur) and in its absence. These processes were probably of importance under the conditions of the early Earth biosphere.

Cadmium (Cd) accumulation and tolerance were analyzed in hyperaccumulator Noccaea caerulescens F.K. Mey and excluder Thlaspi arvense L.. Five accessions of N. caerulescens (La Calamine (LC, Belgium), Saint Félix de Palliéres (SF, France), Col du Mas de l’Aire (CMA, France), Ganges (GA, France) from metalliferous soils and Lellingen (LE, Luxembourg) from nonmetalliferous soils) were grown in halfstrength Hoagland solution for 8 weeks in the presence of 1, 5, 25, and 50 μM Cd(NO3)2 and T. arvense in the presence of 0.1, 0.2, 1.0, and 5.0 μM Cd(NO3)2. The toxic effect of Cd was assessed by changes in root and shoot dry weight. The content of Cd in roots and shoots was determined by atomic absorption spectrophotometry and expressed in mg/kg dry weight of plant material and calculated per plant root system or shoot. The tolerance of N. caerulescens to Cd was higher than that of T. arvense and increased in various accessions of N. caerulescens in the row GA < CMA < LE < SF ≈ LC. The ability to accumulate Cd in roots of N. caerulescens accessions increased in the row LC < LE ≈ GA < CMA ≈ SF, while that in shoots were in the row LC < LE ≈ GA < SF < CMA. Reduction in accumulation of root biomass of hyperaccumulator N. caerulescens started at lower Cd content in them compared with that in shoots, while an opposite pattern was observed for excluder T. arvense. Thus, accessions of hyperaccumulator N. caerulescens, having a higher tolerance to Cd compared with excluder T. arvense, differed significantly from each other not only in their capacity to accumulate heavy metals but also in tolerance to them. LC accession from calamine soils accumulated less Cd and, possibly, this was the reason why it was more tolerant than the other accessions. SF accession, also growing on calamine soils, was characterized both by high Cd tolerance and accumulation, which is probably due to more efficient mechanisms of Cd detoxification. The results obtained suggest that there are differences in the mechanisms and causes of tolerance to Cd in various accessions of hyperaccumulator N. caerulescens.

Representatives of Brassicaceae species—the hyperaccumulator Noccaea caerulescens F.K. Mey and the metal excluder Thlaspi arvense L.—were compared in terms of their ability to accumulate nickel (Ni) and zinc (Zn) and their tolerance to these metals. Four ecotypes of N. caerulescens were used: the ecotypes La Calamine (LC, Belgium) and Saint Felix de Palliéres (SF, France) grow naturally on calamine soils rich in Zn, Cd, and Pb; the ecotype Monte Prinzera (MP, Italy) originates from serpentine soils rich in Ni, Co, and Cr; and the ecotype Lellingen (LE, Luxembourg) inhabits non-metalliferous soils. The plants of N. caerulescens were grown for 8 weeks in a half-strength Hoagland solution supplemented with 25, 100, 200, 300, and 400 μM Ni(NO3)2 (ecotypes LC, SF, MP, LE) or 100, 200, 400, 800, and 1000 μM Zn(NO3)2 (ecotypes LC, SF, LE); the plants of T. arvense were grown in the presence of 10, 20, 25, and 30 μM Ni(NO3)2 or 40, 50, 60, 70, 80 μM Zn(NO3)2. The toxic effect of Ni and Zn was assessed from changes in dry matter of roots and shoots of treated plants compared to untreated. The content of metals in roots and shoots was determined by means of atomic absorption spectrophotometry. The Ni-accumulating capacity of N. caerulescens ecotypes increased in the order: LC < SF < LE < MP, and the Zn-accumulating capacity increased in the row: LC < SF < LE. In the hyperaccumulating plant N. caerulescens, the increments of biomass started to decrease at a lower metal content in roots than in shoots, whereas the opposite pattern was observed in the metal excluder T. arvense. Since T. arvense plants accumulated Ni and Zn in roots, whereas N. caerulescens accumulated these metals in shoots, one may assume that the greater sensitivity of root growth compared with shoots in N. caerulescens was determined by more effective mechanisms of metal detoxification in shoots. Conversely, the higher sensitivity of shoot growth compared to root growth in T. arvense was determined by more effective mechanisms of metal detoxification in roots. Being more tolerant to Ni and Zn than T. arvense plants, the N. caerulescens ecotypes differed substantially in terms of metal-accumulating capacity and their tolerance to heavy metals. The ecotype originating from non-metalliferous soils (LE) accumulated larger amounts of Zn, but was less tolerant compared with ecotypes growing naturally on calamine soils (SF and LC), whereas the ecotype occurring on serpentine soils (MP) exhibited a markedly greater tolerance to Ni, compared with other ecotypes examined, as well as the largest accumulation of this metal. The results indicate the existence of different mechanisms responsible for plant tolerance to Ni and Zn; the study of these mechanisms is a promising direction for future research.