The metal ion transporter IRT1 is necessary for iron homeostasis and efficient photosynthesis in Arabidopsis thaliana

The mutants irt1-1 and irt1-2 of Arabidopsis thaliana were identified among a collection of T-DNA-tagged lines on the basis of a decrease in the effective quantum yield of photosystem II. The mutations responsible interfere with expression of IRT1, a nuclear gene that encodes the metal ion transporter IRT1. In irt1 mutants, photosensitivity and chlorophyll fluorescence parameters, as well as abundance and composition of the photosynthetic apparatus, are significantly altered. Additional effects of the mutation under greenhouse conditions, including chlorosis and a drastic reduction in growth rate and fertility, are compatible with a deficiency in iron transport. Propagation of irt1 plants on media supplemented with additional quantities of iron salts restores almost all aspects of wild-type behaviour. The irt2-1 mutant, which carries an En insertion in the highly homologous IRT2 gene of Arabidopsis thaliana, was identified by reverse genetics and shows no symptoms of iron deficiency. This, together with the finding that irt1-1 can be complemented by 35S::IRT1 but not by 35S::IRT2, demonstrates that, although the products of the two genes are closely related, only AtIRT1 is required for iron homeostasis under physiological conditions.     

Members of the Arabidopsis dynamin-like gene family,ADL1, are essential for plant cytokinesis and polarized cell growth

Polarized membrane trafficking during plant cytokinesis and cell expansion are critical for plant morphogenesis, yet very little is known about the molecular mechanisms that guide this process. Dynamin and dynamin-related proteins are large GTP binding proteins that are involved in membrane trafficking. Here, we show that two functionally redundant members of the Arabidopsis dynamin-related protein family, ADL1A and ADL1E, are essential for polar cell expansion and cell plate biogenesis. adl1A-2 adl1E-1 double mutants show defects in cell plate assembly, cell wall formation, and plasma membrane recycling. Using a functional green fluorescent protein fusion protein, we show that the distribution of ADL1A is dynamic and that the protein is localized asymmetrically to the plasma membrane of newly formed and mature root cells. We propose that ADL1-mediated membrane recycling is essential for plasma membrane formation and maintenance in plants.     

Assessment of methylammonium as an analog for ammonium in plant uptake from soil

We tested radioactive methylammonium (¹⁴CH₃NH _inf3 ^sup+ ) as a tracer for ammonium (NH₄ ⁺) in root uptake measurements from soil. Tomato (Lycopersicon esculentum Moll. cv T5) in 3 L pots filled with loamy sand soil received 40, 200, or 600 μmol ¹⁴CH₃NH₃ ⁺ or ¹⁵NH₄ ⁺. During a 4 h period, the plants absorbed ¹⁴CH₃NH₃ ⁺ at slower rates than ¹⁵NH₄ ⁺. Estimates of NH₄ ⁺ absorption based on ¹⁵NH₄ ⁺ absorption were 0.9–7.9 μmol NH₄ ⁺ g⁻¹ plant dry weight h⁻¹, whereas those based on ¹⁴CH₃NH₃ ⁺ absorption were 0.2–1.0 μmol NH₄ ⁺ g⁻¹ plant dry weight h⁻¹. After 4 h, approximately one-half of the applied ¹⁵N was not recovered in the plants or soil KCl extracts; apparently, this ¹⁵N was either immobilized or nitrified and denitrified by soil biota. By contrast, almost all the ¹⁴CH₃NH₃ ⁺ remained in the soil solution after 4 h, but after a 10 d incubation, approximately 20% had been released as ¹⁴CO₂. These differences in plant absorption rates and movement through soil pools indicate that CH₃NH₃ ⁺ cannot be used reliably as an NH₄ ⁺ analog in soil.     

A rice phenolic efflux transporter is essential for solubilizing precipitated apoplasmic iron in the plant stele

Iron deficiency is one of the major agricultural problems, as 30% of the arable land of the world is too alkaline for optimal crop production, rendering plants short of available iron despite its abundance. To take up apoplasmic precipitated iron, plants secrete phenolics such as protocatechuic acid (PCA) and caffeic acid. The molecular pathways and genes of iron uptake strategies are already characterized, whereas the molecular mechanisms of phenolics synthesis and secretion have not been clarified, and no phenolics efflux transporters have been identified in plants yet. Here we describe the identification of a phenolics efflux transporter in rice. We identified a cadmium-accumulating rice mutant in which the amount of PCA and caffeic acid in the xylem sap was dramatically reduced and hence named it phenolics efflux zero 1 (pez1). PEZ1 localized to the plasma membrane and transported PCA when expressed in Xenopus laevis oocytes. PEZ1 localized mainly in the stele of roots. In the roots of pez1, precipitated apoplasmic iron increased. The growth of PEZ1 overexpression lines was severely restricted, and these lines accumulated more iron as a result of the high solubilization of precipitated apoplasmic iron in the stele. We show that PEZ1 is responsible for an increase of PCA concentration in the xylem sap and is essential for the utilization of apoplasmic precipitated iron in the stele.     

COS1: an Arabidopsis coronatine insensitive1 suppressor essential for regulation of jasmonate-mediated plant defense and senescence

The Arabidopsis thaliana CORONATINE INSENSITIVE1 (COI1) gene encodes an F-box protein to assemble SCF(COI1) complexes essential for response to jasmonates (JAs), which are a family of plant signaling molecules required for many essential functions, including plant defense and reproduction. To better understand the molecular basis of JA action, we screened for suppressors of coi1 and isolated a coi1 suppressor1 (cos1) mutant. The cos1 mutation restores the coi1-related phenotypes, including defects in JA sensitivity, senescence, and plant defense responses. The COS1 gene was cloned through a map-based approach and found to encode lumazine synthase, a key component in the riboflavin pathway that is essential for diverse yet critical cellular processes. We demonstrated a novel function for the riboflavin pathway that acts downstream of COI1 in the JA signaling pathway and is required for suppression of the COI1-mediated root growth, senescence, and plant defense.     

Iron supply prevents Cd uptake in Arabidopsis by inhibiting IRT1 expression and favoring competition between Fe and Cd uptake

Background

Anthropogenic nitrogen (N) addition has dramatically increased and significantly affected global nitrogen cycling. The natural abundance of stable N isotope ratios (δ¹⁵N) has been used as an indicator of the N status of an ecosystem. However, how plant and soil δ¹⁵N signatures would respond to N addition is still unclear.

Methods and aims

Herein, we synthesized the data of 951 observations from 48 individual studies associated with responses of plant and soil δ¹⁵N values to N addition and conducted a meta-analysis to explore a general pattern of N addition effects on δ¹⁵N values of plant and soil.

Results

Our results showed that δ¹⁵N values of plant, soil total N, and soil NO₃ ^? were significantly increased by N addition, while δ¹⁵N value of soil N₂O was significantly decreased and δ¹⁵N value of soil NH₄ ⁺ was not significantly changed. The δ¹⁵N value of soil total N of different ecosystems showed similar responses to N addition, whereas δ¹⁵N values of different plant types showed different responses. Increasing treatment duration significantly increased the effects of inorganic N addition on δ¹⁵N values of shrubs and soil NH₄ ⁺ but did not affect the responses of δ¹⁵N values of soil total N and NO₃ ^?. With increasing inorganic N addition rate, only δ¹⁵N value of plant was significantly increased, but no significant relationship was found between the effect of N addition on other components and N addition rate because of the input of isotopically depleted sources.

Conclusions

Our study revealed a comprehensive picture of the effects of N addition on δ¹⁵N signatures in terrestrial ecosystems and could help us understand how plant and soil δ¹⁵N signatures change with N addition and how these signatures can be used as an indicator of ecosystem N status under increasing N deposition or fertilization.

     

Analysis of sequence, map position, and gene expression reveals conserved essential genes for iron uptake in Arabidopsis and tomato

Arabidopsis (Arabidopsis thaliana) and tomato (Lycopersicon esculentum) show similar physiological responses to iron deficiency, suggesting that homologous genes are involved. Essential gene functions are generally considered to be carried out by orthologs that have remained conserved in sequence and map position in evolutionarily related species. This assumption has not yet been proven for plant genomes that underwent large genome rearrangements. We addressed this question in an attempt to deduce functional gene pairs for iron reduction, iron transport, and iron regulation between Arabidopsis and tomato. Iron uptake processes are essential for plant growth. We investigated iron uptake gene pairs from tomato and Arabidopsis, namely sequence, conserved gene content of the regions containing iron uptake homologs based on conserved orthologous set marker analysis, gene expression patterns, and, in two cases, genetic data. Compared to tomato, the Arabidopsis genome revealed more and larger gene families coding for the iron uptake functions. The number of possible homologous pairs was reduced if functional expression data were taken into account in addition to sequence and map position. We predict novel homologous as well as partially redundant functions of ferric reductase-like and iron-regulated transporter-like genes in Arabidopsis and tomato. Arabidopsis nicotianamine synthase genes encode a partially redundant family. In this study, Arabidopsis gene redundancy generally reflected the presumed genome duplication structure. In some cases, statistical analysis of conserved gene regions between tomato and Arabidopsis suggested a common evolutionary origin. Although involvement of conserved genes in iron uptake was found, these essential genes seem to be of paralogous rather than orthologous origin in tomato and Arabidopsis.     

The possibility of predicting solute uptake and plant growth response from independently measured soil and plant characteristics

Summary A model of the way the rate of growth of a plant may be affected by the level of supply of a nutrient is presented. Growth rate is linked to the nutrient level of the photosynthetic tissues, which is assumed to control changes in the net assimilation rate, the leaf area per unit shoot weight, the shoot: root ratio, the root surface area, and the distribution of nutrient between root and shoot. The uptake of nutrient depends on the concentration of nutrient at the root surface, the root surface area and its absorbing power. All these relationships may be determined in stirred solution culture. A method of applying this information to soil grown plants is suggested.Soil Science Laboratory, Department of Agricultural Science, University of Oxford     

The possibility of predicting solute uptake and plant growth response from independently measured soil and plant characteristics

Summary The growth and nitrogen uptake response of rape plants to nitrate concentration at the root surface were studied in solution culture in a controlled environment cabinet over a period of 24 days. NO₃ ^– was supplied at the rates of 10^–5 M, 5×10^–5 M, 10^–4 M, 10^–3 M and 10^–2 M in solution and was maintained near these levels.With increasing mean N concentration in the tissues, the relative growth rate and leaf area ratio increased and unit leaf rate decreased slightly. Values of all three growth parameters decreased with plant age.The shoot: root dry weight ratios and their N content ratios increased with increasing mean per cent N in the plant. The length or surface area per unit dry weight of roots was correlated negatively with per cent N and positively with age.The maximum mean inflow of nitrate to rape roots decreased sharply with age. The concentration at which half maximal mean inflow was attained was 3.44×10^–5 M NO₃ ^–.     

ANT1, an aromatic and neutral amino acid transporter in Arabidopsis

A new amino acid transporter was identified from the Arabidopsis expressed sequence tag cDNAs by expressing the cDNA in a yeast amino acid transport mutant. Transport analysis of the expressed protein in yeast showed that it transports aromatic and neutral amino acids, as well as arginine. This transporter (ANT1, aromatic and neutral transporter) also transports indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid. The cDNA is 1.6 kb in length with an open reading frame that codes for a protein with 432 amino acids and a calculated molecular mass of 50 kD. Hydropathy analysis showed ANT1 is an integral membrane protein with 11 putative membrane-spanning domains. Southern analysis and a BLAST search of the Arabidopsis genome database suggests that ANT1 is part of a small gene family containing at least five members. Phylogenetic comparisons with other known amino acid transporters in plants suggests that ANT1 represents a new class of amino acid transporter. RNA gel-blot analysis showed that this transporter is expressed in all organs with highest abundance in flowers and cauline leaves.     

A plant DJ-1 homolog is essential for Arabidopsis thaliana chloroplast development

Protein superfamilies can exhibit considerable diversification of function among their members in various organisms. The DJ-1 superfamily is composed of proteins that are principally involved in stress response and are widely distributed in all kingdoms of life. The model flowering plant Arabidopsis thaliana contains three close homologs of animal DJ-1, all of which are tandem duplications of the DJ-1 domain. Consequently, the plant DJ-1 homologs are likely pseudo-dimeric proteins composed of a single polypeptide chain. We report that one A. thaliana DJ-1 homolog (AtDJ1C) is the first DJ-1 homolog in any organism that is required for viability. Homozygous disruption of the AtDJ1C gene results in non-viable, albino seedlings that can be complemented by expression of wild-type or epitope-tagged AtDJ1C. The plastids from these dj1c plants lack thylakoid membranes and granal stacks, indicating that AtDJ1C is required for proper chloroplast development. AtDJ1C is expressed early in leaf development when chloroplasts mature, but is downregulated in older tissue, consistent with a proposed role in plastid development. In addition to its plant-specific function, AtDJ1C is an atypical member of the DJ-1 superfamily that lacks a conserved cysteine residue that is required for the functions of most other superfamily members. The essential role for AtDJ1C in chloroplast maturation expands the known functional diversity of the DJ-1 superfamily and provides the first evidence of a role for specialized DJ-1-like proteins in eukaryotic development.     

Comprehensive screening of Arabidopsis mutants suggests the lysine histidine transporter 1 to be involved in plant uptake of amino acids

Plant nitrogen (N) uptake is a key process in the global N cycle and is usually considered a "bottleneck" for biomass production in land ecosystems. Earlier, mineral N was considered the only form available to plants. Recent studies have questioned this dogma and shown that plants may access organic N sources such as amino acids. The actual mechanism enabling plants to access amino acid N is still unknown. However, a recent study suggested the Lysine Histidine Transporter 1 (LHT1) to be involved in root amino acid uptake. In this study, we isolated mutants defective in root amino acid uptake by screening Arabidopsis (Arabidopsis thaliana) seeds from ethyl methanesulfonate-treated plants and seeds from amino acid transporter T-DNA knockout mutants for resistance against the toxic D-enantiomer of alanine (Ala). Both ethyl methanesulfonate and T-DNA knockout plants identified as D-Ala resistant were found to be mutated in the LHT1 gene. LHT1 mutants displayed impaired capacity for uptake of a range of amino acids from solutions, displayed impaired growth when N was supplied in organic forms, and acquired substantially lower amounts of amino acids than wild-type plants from solid growth media. LHT1 mutants grown on mineral N did not display a phenotype until at the stage of flowering, when premature senescence of old leaf pairs occurred, suggesting that LHT1 may fulfill an important function at this developmental stage. Based on the broad and unbiased screening of mutants resistant to D-Ala, we suggest that LHT1 is an important mediator of root uptake of amino acids. This provides a molecular background for plant acquisition of organic N from the soil.     

Role of the iron transporter OsNRAMP1 in cadmium uptake and accumulation in rice

The heavy metal cadmium (Cd) is toxic to humans, and its accumulation in rice grains is a major agricultural problem. Rice has seven putative metal transporter NRAMP genes, but microarray analysis showed that only OsNRAMP1 is highly up-regulated by iron (Fe) deficiency. OsNRAMP1 localized to the plasma membrane and transported Cd as well as Fe. OsNRAMP1 expression was observed mainly in roots and was higher in the roots of a high-Cd-accumulating cultivar (Habataki) than in those of a low-Cd-accumulating cultivar (Sasanishiki). The amino acid sequence of OsNRAMP1 in the Sasanishiki and Habataki cultivars was found to be 100% identical. These results suggest that OsNRAMP1 participates in cellular Cd uptake and that the differences observed in Cd accumulation among cultivars are because of differences in OsNRAMP1 expression levels in roots.     

Iron requirement and iron uptake from various iron compounds by different plant species

The Fe requirements of four monocotyledonous plant species (Avena sativa L., Triticum aestivum L., Oryza sativa L., Zea mays L.) and of three dicotyledonous species (Lycopersicum esculentum Mill., Cucumis sativus L., Glycine maxima (L.) Merr.) in hydroponic cultures were ascertained. Fe was given as NaFe-EDDHA chelate (Fe ethylenediamine di (O-hydroxyphenylacetate). I found that the monocotyledonous species required a substantially higher Fe concentration in the nutrient solution in order to attain optimum growth than did the dicotyledonous species. Analyses showed that the process of iron uptake was less efficient with the monocotyledonous species. When the results obtained by using chelated Fe were compared with those using ionic Fe, it was shown that the inefficient species were equally inefficient in utilizing Fe³⁺ ions. However, the differences between the efficient and the inefficient species disappeared when Fe²⁺ was used. This confirms the work of others who postulated that Fe³⁺ is reduced before uptake of chelated iron by the root. In addition, it was shown that reduction also takes place when Fe is used in ionic form. The efficiency of Fe uptake seems to depend on the efficiency of the root system of the particular plant species in reducing Fe³⁺. The removal of Fe from the chelate complex after reduction to Fe²⁺ seems to present no difficulties to the various plant species.