Zinc in plants

Zinc (Zn) is an essential component of thousands of proteins in plants, although it is toxic in excess. In this review, the dominant fluxes of Zn in the soil-root-shoot continuum are described, including Zn inputs to soils, the plant availability of soluble Zn(2+) at the root surface, and plant uptake and accumulation of Zn. Knowledge of these fluxes can inform agronomic and genetic strategies to address the widespread problem of Zn-limited crop growth. Substantial within-species genetic variation in Zn composition is being used to alleviate human dietary Zn deficiencies through biofortification. Intriguingly, a meta-analysis of data from an extensive literature survey indicates that a small proportion of the genetic variation in shoot Zn concentration can be attributed to evolutionary processes whose effects manifest above the family level. Remarkable insights into the evolutionary potential of plants to respond to elevated soil Zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous Zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri.

Tandem quadruplication of HMA4 in the zinc (Zn) and cadmium (Cd) hyperaccumulator Noccaea caerulescens

Zinc (Zn) and cadmium (Cd) hyperaccumulation may have evolved twice in the Brassicaceae, in Arabidopsis halleri and in the Noccaea genus. Tandem gene duplication and deregulated expression of the Zn transporter, HMA4, has previously been linked to Zn/Cd hyperaccumulation in A. halleri. Here, we tested the hypothesis that tandem duplication and deregulation of HMA4 expression also occurs in Noccaea. A Noccaea caerulescens genomic library was generated, containing 36,864 fosmid pCC1FOS (TM) clones with insert sizes similar to 20-40 kbp, and screened with a PCR-generated HMA4 genomic probe. Gene copy number within the genome was estimated through DNA fingerprinting and pooled fosmid pyrosequencing. Gene copy numbers within individual clones was determined by PCR analyses with novel locus specific primers. Entire fosmids were then sequenced individually and reads equivalent to 20-fold coverage were assembled to generate complete whole contigs. Four tandem HMA4 repeats were identified in a contiguous sequence of 101,480 bp based on sequence overlap identities. These were flanked by regions syntenous with up and downstream regions of AtHMA4 in Arabidopsis thaliana. Promoter-reporter beta-glucuronidase (GUS) fusion analysis of a NcHMA4 in A. thaliana revealed deregulated expression in roots and shoots, analogous to AhHMA4 promoters, but distinct from AtHMA4 expression which localised to the root vascular tissue. This remarkable consistency in tandem duplication and deregulated expression of metal transport genes between N. caerulescens and A. halleri, which last shared a common ancestor > 40 mya, provides intriguing evidence that parallel evolutionary pathways may underlie Zn/Cd hyperaccumulation in Brassicaceae.

Bio-fortification of potato tubers using foliar zinc-fertiliser

Worldwide, many people are zinc (Zn)-deficient. Dietary Zn intake can be increased by producing crops with higher concentrations of Zn in their edible portions. This can be achieved by applying Zn-fertilisers to varieties with an increased ability to acquire Zn and to accumulate Zn in their edible portions. Potato (Solanum tuberosum L.) is an important food crop and is, therefore, a target for bio-fortification with Zn. Field trials incorporating a core collection of 23 potato genotypes, performed over 4 years (2006 – 2009), indicated significant genotypic effects on tuber Zn concentration and suggested that tuber Zn concentration was influenced by environmental effects, but also found that genotype environment (G E) interactions were not significant. Tuber Zn concentrations averaged 10.8 mg kg–1 dry matter (DM), and the ratio between the lowest and the highest varietal tuber Zn-concentration averaged 1.76. Tuber Zn concentrations could be increased by foliar Zn-fertilisation. Tuber yields of ‘Maris Piper’ were unaffected by foliar applications of < 1.08 g Zn plant–1. The relationship between tuber Zn concentration and foliar Zn application followed a saturation curve, reaching a maximum at approx. 30 mg Zn kg–1 DM at a foliar Zn application rate of 1.08 g plant–1. Despite a 40-fold increase in shoot Zn concentration compared to the unfertilised controls following foliar Zn fertilisation with 2.16 g Zn plant–1, only a doubling in tuber Zn concentration was observed. This suggests that the biofortification of tubers with Zn was restricted by the limited mobility of Zn in the phloem. A significant positive linear relationship between tuber Zn concentration and tuber N concentration supported the hypothesis of co-transport of Zn and N-compounds in the phloem.

Response to zinc deficiency of two rice lines with contrasting tolerance is determined by root growth maintenance and organic acid exudation rates, and not by zinc-transporter activity

Zinc (Zn)-deficient soils constrain rice (Oryza sativa) production and cause Zn malnutrition. The identification of Zn-deficiency-tolerant rice lines indicates that breeding might overcome these constraints. Here, we seek to identify processes underlying Zn-deficiency tolerance in rice at the physiological and transcriptional levels. A Zn-deficiency-tolerant line RIL46 acquires Zn more efficiently and produces more biomass than its nontolerant maternal line (IR74) at low Zn(ext) under field conditions. We tested if this was the result of increased expression of Zn(2+) transporters; increased root exudation of deoxymugineic acid (DMA) or low-molecular-weight organic acids (LMWOAs); and/or increased root production. Experiments were performed in field and controlled environment conditions. There was little genotypic variation in transcript abundance of Zn-responsive root Zn(2+)-transporters between the RIL46 and IR74. However, root exudation of DMA and LMWOA was greater in RIL46, coinciding with increased root expression of putative ligand-efflux genes. Adventitious root production was maintained in RIL46 at low Zn(ext), correlating with altered expression of root-specific auxin-responsive genes. Zinc-deficiency tolerance in RIL46 is most likely the result of maintenance of root growth, increased efflux of Zn ligands, and increased uptake of Zn-ligand complexes at low Zn(ext); these traits are potential breeding targets.

Local and average structure in zinc cyanide: towards an understanding of the atomistic origin of negative thermal expansion

Neutron diffraction at 11.4 and 295 K and solid-state 67Zn NMR are used to determine both the local and average structures in the disordered, negative thermal expansion (NTE) material, Zn(CN)2. Solid-state NMR not only confirms that there is head-to-tail disorder of the C≡N groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4-n(NC)n tetrahedral species, which do not follow a simple binomial distribution. The Zn(CN)4 and Zn(NC)4 species occur with much lower probabilities than are predicted by binomial theory, supporting the conclusion that they are of higher energy than the other local arrangements. The lowest energy arrangement is Zn(CN)2(NC)2. The use of total neutron diffraction at 11.4 K, with analysis of both the Bragg diffraction and the derived total correlation function, yields the first experimental determination of the individual Zn−N and Zn−C bond lengths as 1.969(2) and 2.030(2) Å, respectively. The very small difference in bond lengths, of ~0.06 Å, means that it is impossible to obtain these bond lengths using Bragg diffraction in isolation. Total neutron diffraction also provides information on both the average and local atomic displacements responsible for NTE in Zn(CN)2. The principal motions giving rise to NTE are shown to be those in which the carbon and nitrogen atoms within individual Zn−C≡N−Zn linkages are displaced to the same side of the Zn···Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the Zn···Zn axis, suggested previously in X-ray studies as being responsible for NTE behavior, in fact make negligible contribution at temperatures up to 295 K.

Zinc chelatase in human lymphocytes: detection of the enzymatic defect in erythropoietic protoporphyria

We describe a fluorometric assay for heme synthetase, the enzyme that is genetically deficient in erythropoietic protoporphyria. The method, which can readily detect activity in 1 microliter of packed human lymphocytes, is based on the formation of zinc protoheme from protoporphyrin IX. That zinc chelatase and ferrochelatase activities reside in the same enzyme was shown by the competitive action of ferrous ions and the inhibitory effects of N-methyl protoporphyrin (a specific inhibitor of heme synthetase) on zinc chelatase. The Km for zinc was 11 micrograms and that for protoporphyrin IX was 6 microM. The Ki fro ferrous ions was 14 microM. Zinc chelatase was reduced to 15.3% of the mean control activity in lymphocytes obtained from patients with protoporphyria, thus confirming the defect of heme biosynthesis in this disorder. The assay should prove to be useful for determining heme synthetase in tissues with low specific activity and to investigate further the enzymatic defect in protoporphyria.

Zinc mediated azide-alkyne ligation to 1,5- and 1,4,5-substituted 1,2,3-triazoles

A mild method for regioselective formation of 1,5-substituted 1,2,3-triazoles is described. The zinc-mediated reaction works at room temperature and is successful across a wide range of azido/alkynyl substrates. Additionally, the triazole 4-position can be further functionalized through the intermediate aryl-zinc to accommodate a diverse three-component coupling strategy.

Contrasting behaviour of cadmium and zinc in a soil–plant–arthropod system

This study investigates the transfer of Cd and Zn from a soil amended with sewage sludge at rates up to 100 t ha(-1) through a multi-trophic system consisting of barley, the aphid Sitobion avenae and the larvae of the lacewing Chrysoperla carnae. Results show marked differences in the transfer of the two metals. Cadmium was freely accumulated in barley roots, but accumulation in the shoot was restricted to a concentration of around 0.22 mg kg(-1) (dry weight). This limited the transfer of Cd to higher trophic levels and resulted in no significant accumulation of Cd in S. avenae or in C. carnae. Zinc transfer in the system was largely unrestricted, resulting in significant accumulation in roots and shoots, in S. avenae and in C. carnae. Cadmium biomagnification occurred in lacewing pupae, with concentrations up to 3.6 times greater than in aphids. S. avenae biomagnified Zn by a factor of ca. 2.5 at low sludge amendment rates, but biomagnification decreased to a factor of 1.4 at the highest amendment rate. Biomagnification of Zn did not occur in C. carnae, but concentrations were up to 3.5 time higher than in soil. Results are discussed in light of the mechanisms regulating transfer of the two metals in the system.

Transfer of cadmium and zinc from sewage sludge amended soil through a plant–aphid system to newly emerged adult ladybirds (Coccinella septempunctata)

An agricultural soil was amended with sewage sludge at rates equivalent to 0, 10 and 30 t (dry solids) ha−1 and the subsequent transfer of zinc and cadmium through a soil–plant–arthropod system was investigated. Zinc concentration in soil, wheat and aphids increased significantly with sludge amendment rate. Zinc was biomagnified during transfer along the pathway, resulting in concentrations in the aphids four times greater than in the soil. Cadmium concentration in the soil was also significantly elevated by the addition of sludge, but there was no significant difference in cadmium concentration in the shoots of wheat plants. Cadmium concentration in aphids followed the pattern found in plants, but again, differences between treatments were not significant. Aphids collected from the plants were subsequently fed to fourth instar Coccinella septempunctata. Consumption of these aphids did not result in significant differences between treatments in the body burden of newly emerged adult C. septempunctata for either metal. Sequestration of cadmium in the pupal exuviae had a greater effect on the body burden of newly emerged adult ladybirds than for zinc. Results are discussed in relation to possible risks posed by the transfer of trace metals via the soil–plant–arthropod system to predatory arthropods.