Zn-deficient Plants Research Articles

Arbuscular mycorrhizal fungi induce Zn uptake and antioxidant efficiency in broccoli (Brassica oleracea L.) exposed to Zn deficiency

Zinc (Zn) is an essential micronutrient for plant growth and development, and its deficiency in soil can be a significant problem for broccoli (Brassica oleracea L.) production. While the genetic efforts to breed Zn-efficient broccoli are complex, harnessing microbiomes is an emerging way to induce mineral efficiency in crops. In this study, broccoli, a non-host mycorrhizal species, was cultivated with or without zinc deficiency in the presence or absence of arbuscular mycorrhizal fungi (AMF), confirmed by the colonization efficiency in roots. Zn deficiency caused a significant decrease in morphological parameters and photosynthetic attributes being consistent with the decreased Zn levels in root and shoot relative to Zn-sufficient plants. However, the broccoli plants inoculated with AMF showed a substantial improvement in morphological and photosynthetic parameters in Zn-deficient conditions due to the significant increase in tissue Zn levels. In addition, AMF-inoculated plants under Zn deficiency showed a significant decrease in cell death (%), electrolyte leakage, and H2O2 levels compared to Zn-deficient plants, which further suggests the improvements in cellular status due to the AMF colonization. The real-time PCR experiments showed a significant induction in the expression of BoZIP1 and BoNRAMP1 genes in the roots of Zn-deficient broccoli inoculated with AMF, suggesting that AMF may be associated with the induction of these genes responsible for Zn-uptake and transport. We further observed the induction of POD (peroxidase), APX (ascorbate peroxidase) and SOD (superoxide dismutase), and S-metabolites (cysteine and glutathione) predominantly in the roots of Zn-starved broccoli inoculated with AMF which may confer tolerance to Zn-deficiency induced oxidative damage. This is the first report on the role of AMF in mitigating Zn-deficiency in broccoli which may promote the microbiome-aided improvement in plant health suffering from Zn-deficiency in broccoli and other vegetable crops.

Zinc deficiency in cucumber plants can be alleviated by fullerenol

Zinc (Zn) deficiency is one of the most limiting factors in plant growth and productivity worldwide. Among carbon-based nanomaterials, water-soluble fullerene derivates offer unique opportunities to increase crop productivity and protection. In this work, we evaluated the stress mitigation effects of fullerenol in cucumber plants (Cucumis sativus L.) submitted to Zn deficiency conditions. Cucumber plants were grown hydroponically, either with (+Zn) or without (−Zn) a Zn supply, with (+F) or without (−F) a fullerenol supply. Fullerenol was added in a nutrient solution either before the beginning of Zn deficiency (plant priming) or at its start. A nutrient imbalance, leaf yellowing and spotting, a reduced electron transport rate (ETR), enhanced oxidative damage by excessive production of malondialdehyde (MDA) and retarded plant growth were found in Zn-deficient plants. After fullerenol treatment, some symptoms of Zn deficiency were alleviated. Overall, fullerenol decreased leaf MDA and spotting (visual observations). When fullerenol was added in a Zn-free solution, plants exhibited enhanced leaf chlorophyll or shoot dry biomass compared with non-treated plants. Also, an increase in xylem sap or young leaf Zn was observed in fullerenol-treated plants under Zn deficiency. With the exception of the MDA responses and spotting, these effects appear to occur in a dose-dependent manner. In contrast, the Zn-fed plants remained phenotypically unaffected by fullerenol. Thus, fullerenol probably alleviated Zn deficiency due to an improvement in Zn remobilization and the antioxidant capacity of the plants, in turn decreasing lipid peroxidation.

Genome-Wide Investigation and Functional Verification of the ZIP Family Transporters in Wild Emmer Wheat.

The zinc/iron-regulated transporter-like protein (ZIP) family has a crucial role in Zn homeostasis of plants. Although the ZIP genes have been systematically studied in many plant species, the significance of this family in wild emmer wheat (Triticum turgidum ssp. dicoccoides) is not yet well understood. In this study, a genome-wide investigation of ZIPs genes based on the wild emmer reference genome was conducted, and 33 TdZIP genes were identified. Protein structure analysis revealed that TdZIP proteins had 1 to 13 transmembrane (TM) domains and most of them were predicted to be located on the plasma membrane. These TdZIPs can be classified into three clades in a phylogenetic tree. They were annotated as being involved in inorganic ion transport and metabolism. Cis-acting analysis showed that several elements were involved in hormone, stresses, grain-filling, and plant development. Expression pattern analysis indicated that TdZIP genes were highly expressed in different tissues. TdZIP genes showed different expression patterns in response to Zn deficiency and that 11 genes were significantly induced in either roots or both roots and shoots of Zn-deficient plants. Yeast complementation analysis showed that TdZIP1A-3, TdZIP6B-1, TdZIP6B-2, TdZIP7A-3, and TdZIP7B-2 have the capacity to transport Zn. Overexpression of TdZIP6B-1 in rice showed increased Zn concentration in roots compared with wild-type plants. The expression levels of TdZIP6B-1 in transgenic rice were upregulated in normal Zn concentration compared to that of no Zn. This work provides a comprehensive understanding of the ZIP gene family in wild emmer wheat and paves the way for future functional analysis and genetic improvement of Zn deficiency tolerance in wheat.

Peningkatan Kadar Zn Beras Pecah-Kulit pada Sistem Penggenangan Berselang Melalui Aplikasi Pupuk Kandang Diperkaya Zn Heptahidrat

Zn levels in rice are essential for maintaining consumer health. However, Zn deficient plants can reduce levels of Zn in grains and the ability of plants to express their genetic potentials. This study examines the use of cow manure enriched with Zn heptahydrate on increasing Zn levels of broken skin rice. The experiment was arranged in a randomized single factor group design with seven levels of manure dose treatment, namely: 0; 2.5; 5, 7.5, 10, 12.5, 15 tons ha-1, with 3 replications. Spacing was 30 cm x 30 cm with a size of 3 m x 4 m. The parameters observed were plant growth, the weight of 1,000-grain, percentage of empty grain, production, Zn and protein contents of the brown rice. The collected data were analyzed using analysis of variance with a confidence interval of 5%, followed by the Duncan test of 5%. The results showed that the application of cow manure with a dose of 10 tons ha-1 enriched by 5 kg ha-1 Zn heptahydrate could increase the number of productive tillers, weight of 1,000-grains, production, Zn and protein contents of the brown rice as well as suppress the grainy rice grain of Mekongga variety of lowland rice on the intermittent flooding.
 
 Keywords: brownrice, cow manure, irrigation system, Zn heptahydrate

Interactions between zinc and Phomopsis longicolla infection in roots of Glycine max.

Phomopsis. longicolla is a hemibiotrophic fungus causing significant soybean yield loss worldwide. To reveal the role of zinc in plant-pathogen interactions, soybean seedlings were grown hydroponically with a range of Zn concentrations, 0.06 µM (deficient, Zn0), 0.4 µM (optimal growth), 1.5 µM, 4 µM, 12 µM, and toxic 38 μM, and were subsequently inoculated with P. longicolla via the roots. In vivo analysis of metal distribution in tissues by micro-X-ray fluorescence showed local Zn mobilization in the root maturation zone in all treatments. Decreased root and pod biomass, and photosynthetic performance in infected plants treated with 0.4 µM Zn were accompanied with accumulation of Zn, jasmonoyl-L-isoleucine (JA-Ile), jasmonic acid, and cell wall-bound syringic acid (cwSyA) in roots. Zn concentration in roots of infected plants treated with 1.5 µM Zn was seven-fold higher than in the 0.4 µM Zn treatment, which together with accumulation of JA-Ile, cwSyA, cell wall-bound vanilic acid and leaf jasmonates contributed to maintaining photosynthesis and pod biomass. Host-pathogen nutrient competition and phenolics accumulation limited the infection in Zn-deficient plants. The low infection rate in Zn 4 µM-treated roots correlated with salicylic and 4-hydroxybenzoic acid, and cell wall-bound p-coumaric acid accumulation. Zn toxicity promoted pathogen invasion and depleted cell wall-bound phenolics. The results show that manipulation of Zn availability improves soybean resistance to P. longicolla by stimulating phenolics biosynthesis and stress-inducible phytohormones.

Foliar-Applied Silicon in Sorghum (Sorghum bicolor L.) Alleviate Zinc Deficiency

Foliar-applied silicon may reduce the damage caused by zinc (Zn) deficiency in plants. While adding silicon (Si) and Zn to the solution may increase foliar fertilization efficiency, Si effects on sorghum plants are not known yet. Therefore, this study investigates how four different Zn chelate concentrations, with and without Si, applied via leaf, affect sorghum nutrition and development. The experimental design consisted of a 4 × 2 factorial scheme with four Zn-EDTA (0, 0.60, 1.20, and 1.80 g L−1) and two Si (control and 0.50 g L−1) concentrations applied via leaf, and five replicates. The evaluated parameters were Zn and Si accumulation, Zn use efficiency, cell electrolyte leakage, as well as chlorophyll, carotenoids, protein, and dry matter content. In Zn deficient plants, foliar-applied Si increased Zn accumulation, photosynthetic pigments, shoot and root dry matter while decreasing cell leakage. Zn chelates with added foliar-applied Si increased Zn accumulation, photosynthetic pigments, protein, and root dry matter. In conclusion, foliar spraying of Si alleviates Zn deficiency in sorghum plants and, therefore, adding Si to the Zn solution is agronomically feasible.

Interaction Between Zn Deficiency, Toxicity and Turnip Yellow Mosaic Virus Infection in Noccaea ochroleucum.

Zinc is essential for the functioning of numerous proteins in plants. To investigate how Zn homeostasis interacts with virus infection, Zn-tolerant Noccaea ochroleucum plants exposed to deficient (Zn’0’), optimal (Zn10), and excess Zn (Zn100) concentrations, as well as Cd amendment, were infected with Turnip yellow mosaic virus (TYMV). Imaging analysis of fluorescence kinetics from the μs (OJIP) to the minutes (Kautsky effect, quenching analysis) time domain revealed strong patchiness of systemic virus-induced photosystem II (PSII) inhibition. That was more pronounced in Zn-deficient plants, while Zn excess acted synergistically with TYMV, in both cases resulting in reduced PSII reaction centers. Infected Cd-treated plants, already severely stressed, showed inhibited non-photochemical quenching and PSII activity. Quantitative in situ hybridization at the cellular level showed increased gene expression of ZNT5 and downregulation of HMA4 in infected Zn-deficient leaves. In Zn10 and Zn100 infected leaves, vacuolar sequestration of Zn increased by activation of HMA3 (mesophyll) and MTP1 (epidermis). This correlated with Zn accumulation in the mesophyll and formation of biomineralization dots in the cell wall (Zn100) visible by micro X-ray fluorescence tomography. The study reveals the importance of adequate Zn supply and distribution in the maintenance of photosynthesis under TYMV infection, achieved by tissue-targeted activation of metal transporter gene expression.

Morpho-physiological and transcriptome profiling reveal novel zinc deficiency-responsive genes in rice.

Intensive farming has depleted the soil zinc (Zn) availability resulting in decreased crop productivity. Here, we attempt to understand the Zn deficiency response in rice through temporal transcriptome analysis. For this, rice seedlings were raised under Zn-deficient conditions up to 4weeks followed by Zn re-supply for 3days. Zn-deficient plants developed characteristic deficiency symptoms such as leaf bronzing, decrease in biomass, total chlorophyll, PSII efficiency, decreased carbonic anhydrase activity and increased ROS production. Interestingly, severe alterations in root system architecture were also observed. Comprehensive transcriptome analyses of rice seedlings were carried out after 2 (DEF2W) and 4weeks (DEF4W) of Zn deficiency with respect to transcriptome profiles of corresponding Zn sufficient conditions (SUF2W, SUF4W). Additionally, to detect the potential Zn-responsive genes, transcriptome profile of Zn-recovered seedlings was compared with DEF4W. All differentially expressed Zn-responsive genes were categorized into early and late Zn deficiency response, and a set of 77 genes, induced and repressed on Zn deficiency and re-supply, respectively, was identified. These genes could be used as low Zn-responsive marker genes. Further, genes involved in membrane transport, phytosiderophore activity and organic acid biosynthesis showed high differential expression. Additionally, the present study unravelled several genes putatively associated with alterations in root system architecture under Zn deficiency and provides novel insights into the interpretation of morpho-physiological, biochemical and molecular regulation of zinc deficiency responses in rice.

Silicon addition to soybean (Glycine max L.) plants alleviate zinc deficiency

It is well established the beneficial role of silicon (Si) in alleviating abiotic stress. However, it remains poorly understood the mechanisms of the Si-mediated protection against metal deficiency, especially the zinc (Zn) one. Recently, it has been proposed that Si may act by an interaction with this biometal in the root apoplast contributing to its movement through the plant, as in the case of Fe deficiency. In the present work, the effect of initial or continuous Si doses in soybean Zn deficient plants has been studied. For that purpose, plants grown in hydroponic culture were treated with different Si doses (0.0, 0.5 and 1.0 mM) under Zn limiting conditions. SPAD index in leaves, several growth parameters, mineral content in the whole plant and the formation of Zn pools in roots were determined. An initial addition of 0.5 mM of Si to the nutrient solution led to an enhancement of plants growth, Zn and Si content in leaves, and a higher storage of Zn in the root apoplast. The results suggest that this treatment enhanced Zn accumulation on roots and its movement to shoots when needed, mitigating Zn deficiency symptoms.

Increased expression of six ZIP family genes by zinc (Zn) deficiency is associated with enhanced uptake and root-to-shoot translocation of Zn in barley (Hordeum vulgare).

Low zinc (Zn) in soils reduces yield and grain Zn content. Regulation of ZRT/IRT-like protein (ZIP) family genes is a major mechanism in plant adaptation to low and fluctuating Zn in soil. Although several Zn deficiency-inducible ZIP genes are identified in cereals, there has been no systematic study on the association of Zn deficiency-induced uptake and root-to-shoot translocation with expression of ZIP family genes. We measured Zn deficiency-induced uptake and root-to-shoot translocation of Zn in barley (Hordeum vulgare) plants by resupplying 0.5 μM Zn, and quantified the transcripts of thirteen HvZIP genes. Subcellular localization and tissue-specific expression were also determined for Zn deficiency-inducible HvZIP genes. Zn deficiency enhanced the capacity of uptake and root-to-shoot translocation of Zn, and sustained the enhanced capacity for 6 d after Zn resupply. Six HvZIP genes were highly induced in roots of Zn-deficient plants, and their proteins were localized in the plasma membrane. Tissue-specific expression in roots supports their roles in uptake and root-to-shoot translocation of Zn under low Zn conditions. Our results provide a comprehensive view on the physiological roles of ZIP genes in plant adaptation to low and fluctuating Zn in soil, and pave the way for development of new strategies to improve Zn-deficiency tolerance and biofortification in cereals.

Identification of Zinc Deficiency-Responsive MicroRNAs in Brassica juncea Roots by Small RNA Sequencing

The importance of zinc (Zn) as a micronutrient essential for plant growth and development is becoming increasingly apparent. Much of the world's soil is Zn-deficient, and soil-based Zn deficiency is often accompanied by Zn deficiency in human populations. MicroRNAs (miRNAs) play important roles in the regulation of plant gene expression at the level of translation. Many miRNAs involved in the modulation of heavy metal toxicity responses in plants have been identified; however, the role of miRNAs in the plant Zn deficiency response is almost completely unknown. Using high-throughput Solexa sequencing, we identified several miRNAs that respond to Zn deficiency in Brassica juncea roots. At least 21 conserved candidate miRNA families, and 101 individual members within those families, were identified in both the control and the Zn-deficient B. juncea roots. Among this, 15 miRNAs from 9 miRNA families were differentially expressed in the control and Zn-deficient plants. Of the 15 differentially expressed miRNAs, 13 were up-regulated in the Zn-deficient B. juncea roots, and only two, miR399b and miR845a, were down-regulated. Bioinformatics analysis indicated that these miRNAs were involved in modulating phytohormone response, plant growth and development, and abiotic stress responses in B. juncea roots. These data help to lay the foundation for further understanding of miRNA function in the regulation of the plant Zn deficiency response and its impact on plant growth and development.

Genetic analysis of the effect of zinc deficiency on Arabidopsis growth and mineral concentrations

Zinc deficiency is a common micronutrient deficiency in plants growing in many different regions of the world and is associated with disturbances in uptake and accumulation of mineral nutrients. Despite many published data on physiological factors affecting ion accumulation in Zn deficient plants, there is very little information about the genetic factors underlying this. We aim to identify genetic loci involved in mineral accumulation and plant performance under Zn deficiency. Genetic loci were identified using the genetically segregating Ler × Cvi recombinant inbred line (RIL) population grown under Zn deficient conditions. Lines were analysed for the concentrations of Zn, Fe, Mn, K, Ca, Mg, P, Cu, S and Al in shoot dry matter. The same was done for the same lines grown under Zn sufficient conditions. We found considerable heritable variation for most mineral concentrations. In general, there was a positive correlation between mineral concentrations. For Zn only condition-dependent QTLs were identified, while for most other mineral concentrations both condition-dependent and -independent QTLs were identified. Several QTLs co-localize, including co-localization to loci controlling shoot biomass and to mineral concentration loci found previously in this and other RIL populations. There are different genetic loci controlling Zn accumulation under deficient and sufficient Zn supply. Only for few minerals, their accumulation is controlled by Zn-supply-specific loci.

Photochemistry and gas exchange in cold conditions in Zn-deficient red cabbage (Brassica oleracea L. var. capitata f. rubra) plants

Abstract The responses of red cabbage (Brassica oleracea L. var. capitata f. rubra) plants to a low Zn supply and cold conditions (10°/7°C day/night temperature) were investigated in a hydroponic growing medium. A low Zn supply caused a significant reduction of shoot and root dry weight - up to 55% and 45% for the control and 62% and 52% for cold-treated plants, respectively. The total soluble carbohydrates and starch declined in Zn-deficient plants. Exposure to low temperatures, however, led to a decline in starch but an increase in soluble sugars. In Zn-sufficient plants, low temperatures increased the excitation capture efficiency of open photosystem II (PS II) reaction centres (RCs) (F'v /F'm ), the quantum yield of PS II (ΦPSII), the electron transport rate (ETR) and the proportion of active chlorophyll associated with the RCs of PS II (Fv /F0 ). Low temperatures did not affect net CO2 uptake in Zn-sufficient plants, though a reduction of stomatal conductance occurred. The results demonstrated that although cold-treated plants were slightly more susceptible to Zn deficiency, cold treatment caused greater shoot biomass (up to 32%) in plants supplied with adequate Zn. The adaptation of red cabbage plants to cold conditions is attributable to improved photochemical events in the leaves, a maintenance of the net CO2 assimilation rate, lower water loss and the accumulation of anthocyanins as antioxidants.

Adequate zinc nutrition alleviates the adverse effects of heat stress in bread wheat

Zinc (Zn) deficiency and heat stress affect the productivity of cereal crops in many semi arid regions of the world. Zinc nutrition affects membrane integrity and activity of superoxide dismutase (SOD) enzymes which are also important to a plant’s ability to cope with high temperatures. Two experiments were conducted to examine the interaction between Zn nutrition and heat stress at two growth stages. Wheat was grown at deficient and adequate levels of Zn at 22/16°C before being exposed to a short period of high temperature (40/20°C). One experiment examined the response to a 3 day heat stress treatment at 10 days after anthesis and the second experiment examined the response to 6 days of heat stress in 30-day old seedlings. Varieties differing in thermotolerance and sensitivity to Zn deficiency were compared. Zinc deficiency and heat stress reduced kernel growth rates, kernel weight and grain yield and the greatest reductions were measured in Zn-deficient, heat stressed plants. The greater reduction in kernel growth rates from heat stress in Zn deficient plants occurred in both thermotolerant and sensitive varieties and was negatively associated with grain Zn concentration. Genetic differences in tolerance to Zn deficiency did not have consistent effects on the response to heat stress. Chlorophyll fluorescence and chlorophyll content were reduced and chloroplast ultrastructure was disrupted by heat stress and the effect was exacerbated by low supplies of Zn. While the responses to heat stress and low Zn were largely additive effects, the experiments demonstrated that low levels of Zn nutrition can exacerbate the adverse effects of short periods of heat stress on kernel growth and chloroplast function.

Growth, photosynthesis and antioxidant defense systém in Zn-deficient red cabbage plants

The effect of Zn deficiency was studied in red cabbage (Brassica oleracea L. var. capitata f. rubra) plants grown in nutrient solution under controlled environmental conditions. Zinc starvation affected the number (61%), surface area (72%) and biomass (62%) of leaves more than root biomass (42%). Although chlorophyll fluorescence parameters revealed occurrence of photoinhibition following declined stomatal conductance and reduction of CO2 available at carboxylation sites, photosynthesis apparatus was not damaged seriously under Zn deficiency conditions. Chlorophyll a, chlorophyll a/b ratio, soluble carbohydrates and starch declined but anthocyanins and free phenolics were accumulated under Zn deficiency conditions. Activity of ascorbate peroxidase, catalase and peroxidase enhanced under Zn deficiency conditions, whereas activity of superoxide dismutase declined in leaves but not in roots of Zn-deficient plants. Maintenance of superoxide dismutase activity and malondialdehyde content in roots demonstrated that roots were more protected against reactive oxygen species imbalance under Zn deficiency conditions compared with leaves that was correlated well with the lower sensitivity of roots to low Zn supply.

Impairment in reproductive development is a major factor limiting yield of black gram under zinc deficiency

Black gram [Vigna mungo (L.) Hepper] cv. IPU 94 plants grown in sand culture with deficient zinc (0.1 µM Zn) nutrition and those deprived of normal (1 µM) Zn supply at the initiation of flowering, showed decrease in dry matter production and especially seed yield. These plants showed a decrease in the size of anthers and stigmatic heads, pollen producing capacity of the anthers and stigmatic exudations. Zn deficiency caused structural alterations in exine and retarded germination of pollen grains and tube growth. The pollen extracts and stigmatic exudates of the Zn-deficient plants showed increase in activity of acid phosphatase isoforms and inhibition of esterase isoforms. Zn deficiency led to decrease in number of pods, seeds per pod and seed mass, altered seed coat topography and reduced seeds germinability. Low seed yield under Zn deficiency is attributed to a role of Zn in pollen function, as also in pollen-pistil interaction conducive to fertilization and development of seeds.

Deoxymugineic acid increases Zn translocation in Zn-deficient rice plants

Deoxymugineic acid (DMA) is a member of the mugineic acid family phytosiderophores (MAs), which are natural metal chelators produced by graminaceous plants. Rice secretes DMA in response to Fe deficiency to take up Fe in the form of Fe(III)–MAs complex. In contrast with barley, the roots of which secrete MAs in response to Zn deficiency, the amount of DMA secreted by rice roots was slightly decreased under conditions of low Zn supply. There was a concomitant increase in endogenous DMA in rice shoots, suggesting that DMA plays a role in the translocation of Zn within Zn-deficient rice plants. The expression of OsNAS1 and OsNAS2 was not increased in Zn-deficient roots but that of OsNAS3 was increased in Zn-deficient roots and shoots. The expression of OsNAAT1 was also increased in Zn-deficient roots and dramatically increased in shoots; correspondingly, HPLC analysis was unable to detect nicotianamine in Zn-deficient shoots. The expression of OsDMAS1 was increased in Zn-deficient shoots. Analyses using the positron-emitting tracer imaging system (PETIS) showed that Zn-deficient rice roots absorbed less 62Zn-DMA than 62Zn2+. Importantly, supply of 62Zn-DMA rather than 62Zn2+ increased the translocation of 62Zn into the leaves of Zn-deficient plants. This was especially evident in the discrimination center (DC). These results suggest that DMA in Zn-deficient rice plants has an important role in the distribution of Zn within the plant rather than in the absorption of Zn from the soil.Electronic supplementary materialThe online version of this article (doi:10.1007/s11103-008-9292-x) contains supplementary material, which is available to authorized users.

Selecting Rice Genotypes Tolerant to Zinc Deficiency and Sodicity: Differences in Concentration of Major Cations and Sodium/Potassium Ratio in Leaves

ABSTRACT Zinc (Zn) deficiency is a major nutritional problem for rice under sodic conditions. Seedlings (35-d old) of 30 rice genotypes were transplanted in pots at pH2 9.8 [diethylene triamine penta acetic acid (DTPA) Zn 1.8 ppm] to identify genotypes tolerant to both sodicity and Zn deficiency. Ten genotypes (group A) showed potential to tolerate both the stresses. Sixteen genotypes (group B) were sensitive to Zn deficiency. However, some of the seedlings of group B genotypes were normal (without Zn deficiency symptoms). Four genotypes (group C) were sensitive to sodicity. Leaves and their leaf sheaths were analyzed at 33 d after transplanting for Ca, Mg, K, and Na. Group A genotypes (CSR-88IR15, CSR-89IR14, IR4630-22-2-5-1-2, and Trichi) had significantly less Na concentrations in their leaves and the leaf sheaths compared to group B genotypes (CSR10, CSR23, CSR-88IR1, 89H1-931098, and IR47538-3B-9-3B-1). The concentration of Na was invariably higher in the leaf sheath than its leaf in both the groups, but reverse was true for Ca, Mg, and K. Zinc deficient plants had relatively higher concentrations of Ca and Mg in their leaves and the leaf sheaths than group A. Concentration of K was somewhat better in group A than group B genotypes. Higher Na/K ratio in group B genotypes compared to group A may be attributed to increased concentrations of Na rather than decreases in K concentrations. Further studies are needed to understand the processes associated with differential uptake of Na and K by Zn deficient plants of group B genotypes resulting in higher Na/K ratio compared to group A genotypes.

Mobilization of Cu and Zn by root exudates of dicotyledonous plants in resin-buffered solutions and in soil

It has been frequently suggested that root exudates play a role in trace metal mobilization and uptake by plants, but there is little in vivo evidence. We studied root exudation of dicotyledonous plants in relation to mobilization and uptake of Cu and Zn in nutrient solutions and in a calcareous soil at varying Cu and Zn supply. Spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) were grown on resin-buffered nutrient solutions at varying free ion activities of Cu (pCu 13.0–10.4) and Zn (pZn 10.1–6.6). The Cu and Zn concentrations in the nutrient solution increased with time, except in plant-free controls, indicating that the plant roots released organic ligands that mobilized Cu and Zn from the resin. At same pCu, soluble Cu increased more at low Zn supply, as long as Zn deficiency effects on growth were small. Zinc deficiency was observed in most treatment solutions with pZn ≥ 9.3, but not in nutrient solutions of a smaller volume/plant ratio in which higher Zn concentrations were observed at same pZn. Root exudates of Zn-deficient plants showed higher specific UV absorbance (SUVA, an indicator of aromaticity and metal affinity) than those of non-deficient plants. Measurement of the metal diffusion flux with the DGT technique showed that the Cu and Zn complexes in the nutrient solutions were highly labile. Diffusive transport (through the unstirred layer surrounding the roots) of the free ion only could not explain the observed plant uptake of Cu and of Zn at low Zn2+ activity. The Cu and Zn uptake by the plants was well explained if it was assumed that the complexes with root exudates contributed 0.4% (Cu) or 20% (Zn) relative to the free ion. In the soil experiment, metal concentrations and organic C concentrations were larger in the solution of planted soils than in unplanted controls. The SUVA of the soil solution after plant growth was higher for unamended soils, on which the plants were Zn-deficient, than for Zn-amended soils. In conclusion, root exudates of dicotyledonous plants are able to mobilize Cu and Zn, and plants appear to respond to Zn deficiency by exuding root exudates with higher metal affinity.

Overexpression of the OsZIP4 zinc transporter confers disarrangement of zinc distribution in rice plants

Zinc (Zn), an essential nutrient in cells, plays a vital role in controlling cellular processes such as growth, development, and differentiation. Although the mechanisms of Zn translocation in rice plants (Oryza sativa) are not fully understood, it has recently received increased interest. OsZIP4 is a Zn transporter that localizes to apical cells. Transgenic rice plants overexpressing the OsZIP4 gene under the control of the cauliflower mosaic virus (CaMV) 35S promoter were produced. The Zn concentration in roots of 35S-OsZIP4 transgenic plants was 10 times higher than in those of vector controls, but it was five times lower in shoots. The Zn concentration in seeds of 35S-OsZIP4 plants was four times lower compared with vector controls. Northern blot analysis and quantitative real-time reverse transcription-PCR revealed transcripts of OsZIP4 expression driven by the CaMV 35S promoter in roots and shoots of 35S-OsZIP4 plants, but levels of endogenous OsZIP4 transcripts were low in roots and high in shoots compared with vector controls. Microarray analysis revealed that the genes expressed in shoots of 35S-OsZIP4 plants coincided with those induced in shoots of Zn-deficient plants. These results indicate that constitutive expression of OsZIP4 changes the Zn distribution within rice plants, and that OsZIP4 is a critical Zn transporter that must be strictly regulated.