Endocytosis and degradation of BOR1, a boron transporter of Arabidopsis thaliana, regulated by boron availability

Receptor activated Ca(2+) release is inhibited by boric acid in prostate cancer cells

BACKGROUND

The global disparity in cancer incidence remains a major public health problem. We focused on prostate cancer since microscopic disease in men is common, but the incidence of clinical disease varies more than 100 fold worldwide. Ca(2+) signaling is a central regulator of cell proliferation, but has received little attention in cancer prevention. We and others have reported a strong dose-dependent reduction in the incidence of prostate and lung cancer within populations exposed to boron (B) in drinking water and food; and in tumor and cell proliferation in animal and cell culture models.

METHODS/PRINCIPAL FINDINGS

We examined the impact of B on Ca(2+) stores using cancer and non-cancer human prostate cell lines, Ca(2+) indicators Rhod-2 AM and Indo-1 AM and confocal microscopy. In DU-145 cells, inhibition of Ca(2+) release was apparent following treatment with Ringers containing RyR agonists cADPR, 4CmC or caffeine and respective levels of BA (50 microM), (1, 10 microM) or (10, 20, 50,150 microM). Less aggressive LNCaP cancer cells required 20 microM BA and the non-tumor cell line PWR1E required 150 microM BA to significantly inhibit caffeine stimulated Ca(2+) release. BA (10 microM) and the RyR antagonist dantroline (10 microM) were equivalent in their ability to inhibit ER Ca(2+) loss. Flow cytometry and confocal microscopy analysis showed exposure of DU-145 cells to 50 microM BA for 1 hr decreased stored [Ca(2+)] by 32%.

CONCLUSION/SIGNIFICANCE

We show B causes a dose dependent decrease of Ca(2+) release from ryanodine receptor sensitive stores. This occurred at BA concentrations present in blood of geographically disparate populations. Our results suggest higher BA blood levels lower the risk of prostate cancer by reducing intracellular Ca(2+) signals and storage.

Homeostasis of the structurally important micronutrients, B and Si

This review focuses on recent advances in understanding the transport mechanisms of two elements, B and Si in plants. Both are present as noncharged molecules in soil solution as boric acid and silicic acid. Both function in apoplast: pectic polysaccharides crosslinked with borate and polymers of hydrated silica are important for the physical strength of plant cells. In recent years, molecular genetics revealed analogous transport systems of B and Si. Combinations of NIP channels and exporters localized to distal and proximal sides, allow efficient trans-cellular transport of the nutrients. Polar localization, observed in these transport molecules, is likely to be a key to regulate directional transport of nutrients.

Identification of a novel system for boron transport: Atr1 is a main boron exporter in yeast

Boron is a micronutrient in plants and animals, but its specific roles in cellular processes are not known. To understand boron transport and functions, we screened a yeast genomic DNA library for genes that confer resistance to the element in Saccharomyces cerevisiae. Thirty boron-resistant transformants were isolated, and they all contained the ATR1 (YML116w) gene. Atr1 is a multidrug resistance transport protein belonging to the major facilitator superfamily. C-terminal green fluorescent protein-tagged Atr1 localized to the cell membrane and vacuole, and ATR1 gene expression was upregulated by boron and several stress conditions. We found that atr1Delta mutants were highly sensitive to boron treatment, whereas cells overexpressing ATR1 were boron resistant. In addition, atr1Delta cells accumulated boron, whereas ATR1-overexpressing cells had low intracellular levels of the element. Furthermore, atr1Delta cells showed stronger boron-dependent phenotypes than mutants deficient in genes previously reported to be implicated in boron metabolism. ATR1 is widely distributed in bacteria, archaea, and lower eukaryotes. Our data suggest that Atr1 functions as a boron efflux pump and is required for boron tolerance.

Post-Golgi traffic in plants

Secretory and endocytic traffic through the post-Golgi endomembrane system regulates the abundance of plasma-membrane proteins such as receptors, transporters and ion channels, modulating the ability of a cell to communicate with its neighbours and to adapt to a changing environment. The major post-Golgi compartments are numerous and appear to be similar to their counterparts in animals. However, endosomes are rather ill defined morphologically but seem to be involved in specific trafficking pathways. Many plasma-membrane proteins cycle constitutively via endosomal compartments. The trans-Golgi network (TGN) appears to be an early endosome where secretory and endocytic traffic meet. Endocytosed proteins that are to be degraded are targeted to the vacuole via the multivesiculate prevacuolar compartment (PVC) whereas cycling proteins pass through recycling endosomes. The trafficking machinery involves the same classes of proteins as in other eukaryotes. However, there are modifications that match the specifics of post-Golgi traffic in plants. Although plants lack epithelia, some plasma-membrane proteins are located on specific faces of the cell which reflects polarized traffic and influences the physiological performance of the tissue. Plants also differentiate highly polarized tip-growing cells in which post-Golgi traffic is adapted to very high rates of targeted exocytosis, endocytosis and recycling.

Risk assessment of boron in glass wool insulation

BACKGROUND, AIM AND SCOPE

Glass wools are man-made vitreous fibres, which consist principally of sodium, calcium and magnesium silicates, but may contain smaller amounts of other elements, including boron. The boron contents originate from the use of borates in the glass melting process as a glass former and a flux agent. During the production and application of glass wool insulation products, workers may legally be exposed to glass fibre up to the occupational limit value, commonly of 1 fibre/cm3. However, in practice, the fibre exposure will be at least ten times lower. Boron is a non-metallic element widely distributed in nature, where it occurs as boric acid, borates and borosilicates. Humans are mainly exposed to boron via vegetarian food and drinking water, mineral supplements and various consumer products. Boron is an essential element for plant growth, but the essentiality for humans is not proven, although intakes of trace amounts of the element seem to be useful for bone health and proper brain function; higher concentrations of boron, however, may be toxic. In relation to the European Union legislation on dangerous substances, an EU Expert Group has recommended classifying boric acid and borates with risk phrases for reproductive toxicity. The aim of this paper is to assess whether the new EU hazard classification of boron compounds should imply that glass wool products used for building insulation in the future should be labelled, "may impair fertility and cause harm to the unborn child", because of the low boron content.

MATERIALS AND METHODS

Boron intakes are estimated in a worst-case occupational situation with human exposure to glass wool fibres at the occupational limit of 1 fibre/cm3 by calculation of the mass of the amount of fibres inhaled during an 8-h work day. Fibres are supposed to be cylinders of glass with a length of 30 microm, an average diameter of 1.5-2 microm and containing either 1.5% or 3.5% boron. As a worst-case scenario, the density of the fibres is set to 2,700 kg/m3. The inhalation rate of the individuals at moderate work load was set to 2 m3/h. A worst-case scenario also corresponds to 100% retention and to 100% solubility of the retained fibres in the lungs.

RESULTS

With the normal boron content of 1.5% in glass wool fibres for building insulation, the extra daily occupational boron intake/uptake will be 0.03-0.06 mg B for 5 days a week. For more uncommon glass wool with maximum boron content of 3.5%, the worst-case daily boron intake/uptake will be 0.08-0.16 mg B. The main boron exposure in the general population is from vegetarian food, and the average daily dietary intake with food is estimated to 1.2-1.5 mg B/day. In addition, significant intakes may come with drinking water, especially from mineral water. In some instances, exposure from mineral supplements, cosmetics and other consumer products may be significant. For example, individuals taking mineral supplements, e.g. for bodybuilding, may have an additional intake to that of 1-10 mg/day. During the years, various organisations have recommended safe intake values for boron. Recently, the Scientific Panel on Dietetic Products, Nutrition and Allergies of the European Food Safety Authority (EFSA) has established the 'Tolerable Upper Intake Level' (UL) for the intake of boron (boric acid and borates) at 0.16 mg B/kg body weight per day or about 10 mg B/day for an adult.

DISCUSSION

The calculated, worst-case exposure scenario during an 8-h work day will result in an extra daily boron intake that only corresponds to about 10% of the average daily adult boron intakes through food and drinks of about 1.5 mg. The inter-individual variations in boron intakes from foods, water and supplements will be much greater than an eventual, very worst-case, additional intake of boron from inhalation of glass wool fibres. In addition, the combined intakes are far lower than the 'Tolerable Upper Intake Level' of 10 mg B/day for a person weighing 60 kg, as recommended by the European Food Safety Agency. The potential boron intake from inhalation of glass wool fibres is also much lower than boron intakes by workers in the boron industry, who at the present occupational limit value will be exposed to 50 mg of boron 5 days a week, or 100 times more than the worst case for glass wool fibres. Furthermore, in practice, exposure levels will mostly be ten to 100 times lower than the occupational limit used here as a worst case.

CONCLUSIONS

The estimated boron intake from inhalation of glass wool fibres in occupational settings will be insignificant and without any health risks, even in the case of non-compliance with the occupational limit value. Any proposal requiring hazard labels on commercial glass wool products for building insulation, because of the boron content, is not supported by the present scientific knowledge.

RECOMMENDATIONS AND PERSPECTIVES

The European Commission should ensure that the new EU hazard classification of boron compounds is not applied to commercial glass wool products for building insulation having a low content of boron.

The use of chemical genomics to investigate pathways intersecting auxin-dependent responses and endomembrane trafficking in Arabidopsis thaliana

Plant endomembrane system is essential for viability and necessary for proper development and signal transduction signal processes. Links between the endomembrane system and auxin signaling have been reported by classical genetics screens. However, the relationship between these processes is not well understood. Chemical genomics is a powerful approach to dissect various processes overcoming lethality and redundancy issues. This approach uses small molecules to modify or disrupt the function of specific proteins and biological processes. We present a screen in Arabidopsis thaliana to identify compound affecting auxin-dependent responses and components of the endomembrane system. A gravitropic-response based screen is performed in Arabidopsis seedlings. The identified gravitropic effectors are tested in terms of auxin responsiveness and their effects on endomembrane compartments. These bioactive compounds will be valuable tools for dissecting endomembrane trafficking and auxin signaling processes.

The promoter of the pepper pathogen-induced membrane protein gene CaPIMP1 mediates environmental stress responses in plants

The promoter of the pepper pathogen-induced membrane protein gene CaPIMP1 was analyzed by an Agrobacterium-mediated transient expression assay in tobacco leaves. Several stress-related cis-acting elements (GT-1, W-box and ABRE) are located within the CaPIMP1 promoter. In tobacco leaf tissues transiently transformed with a CaPIMP1 promoter-beta-glucuronidase (GUS) gene fusion, serially 5'-deleted CaPIMP1 promoters were differentially activated by Pseudomonas syringae pv. tabaci, ethylene, methyl jasmonate, abscisic acid, and nitric oxide. The -1,193 bp region of the CaPIMP1 gene promoter sequence exhibited full promoter activity. The -417- and -593 bp promoter regions were sufficient for GUS gene activation by ethylene and methyl jasmonate treatments, respectively. However, CaPIMP1 promoter sequences longer than -793 bp were required for promoter activation by abscisic acid and sodium nitroprusside treatments. CaPIMP1 expression was activated in pepper leaves by treatment with ethylene, methyl jasmonate, abscisic acid, beta-amino-n-butyric acid, NaCl, mechanical wounding, and low temperature, but not with salicylic acid. Overexpression of CaPIMP1 in Arabidopsis conferred hypersensitivity to mannitol, NaCl, and ABA during seed germination but not during seedling development. In contrast, transgenic plants overexpressing CaPIMP1 exhibited enhanced tolerance to oxidative stress induced by methyl viologen during germination and early seedling stages. These results suggest that CaPIMP1 expression may alter responsiveness to environmental stress, as well as to pathogen infection.

Unique mechanism of plant endocytic/vacuolar transport pathways

The post-Golgi traffic network in plant cells is highly complex, which is correlated with the large number of genes related to this function. RABs and SNAREs are key regulators of tethering and fusion of transport vesicles to target membranes, and the numbers of these regulators have also expanded in plant lineages. In addition to this increase in the net number of genes, plants also seem to have evolved new gene families tailored to fulfill plant-unique functions. In this article, we summarize recent progress in studies on plant-unique RABs and SNAREs functioning in post-Golgi trafficking, with a special focus on the endocytic pathway.

Highly boron deficiency-tolerant plants generated by enhanced expression of NIP5;1, a boric acid channel

Boron (B) is an essential element for plants, and B deficiency is a worldwide agricultural problem. In B-deficient areas, B is often supplied as fertilizer, but excess B can be toxic to both plants and animals. Generation of B deficiency-tolerant plants could reduce B fertilizer use. Improved fertility under B-limiting conditions in Arabidopsis thaliana by overexpression of BOR1, a B transporter, has been reported, but the root growth was not improved by the BOR1 overexpression. In this study, we report that enhanced expression of NIP5;1, a boric acid channel for efficient B uptake, resulted in improved root elongation under B-limiting conditions in A. thaliana. An NIP5;1 activation tag line, which has a T-DNA insertion with enhancer sequences near the NIP5;1 gene, showed improved root elongation under B limitation. We generated a construct which mimics the tag line: the cauliflower mosaic virus 35S RNA promoter was inserted at 1,357 bp upstream of the NIP5;1 transcription initiation site. Introduction of this construct into the nip5;1-1 mutant and the BOR1 overexpresser resulted in enhanced expression of NIP5;1 and improved root elongation under low B supply. Furthermore, one of the transgenic lines exhibited improved fertility and short-term B uptake. Our results demonstrate successful improvement of B deficiency tolerance and the potential of enhancing expression of a mineral nutrient channel gene to improve growth under nutrient-limiting conditions.

Polar targeting and endocytic recycling in auxin-dependent plant development

Plant development is characterized by a profound phenotypic plasticity that often involves redefining of the developmental fate and polarity of cells within differentiated tissues. The plant hormone auxin and its directional intercellular transport play a major role in these processes because they provide positional information and link cell polarity with tissue patterning. This plant-specific mechanism of transport-dependent auxin gradients depends on subcellular dynamics of auxin transport components, in particular on endocytic recycling and polar targeting. Recent insights into these cellular processes in plants have revealed important parallels to yeast and animal systems, including clathrin-dependent endocytosis, retromer function, and transcytosis, but have also emphasized unique features of plant cells such as diversity of polar targeting pathways; integration of environmental signals into subcellular trafficking; and the link between endocytosis, cell polarity, and cell fate specification. We review these advances and focus on the translation of the subcellular dynamics to the regulation of whole-plant development.

The functions of Rab GTPases in plant membrane traffic

Rab GTPases are important determinants of membrane identity and membrane targeting. Higher plants have evolved a unique set of Rab GTPases that presumably reflects the specific demands of plant cell trafficking. In recent years, significant progress has been made in identifying Rab GTPases involved in endosome organisation, cytokinesis and in post-Golgi traffic to the plasma membrane and vacuoles. These include members of the Rab-F1, Rab-F2, Rab-A1, Rab-A2 and Rab-A4 subclasses. Some important regulators or effectors have also been identified for Rab-F, Rab-A1 and Rab-A4 proteins. However, uncertainties remain about the trafficking pathways that connect the compartments in the trans-Golgi/prevacuolar/endosomal system and there is still little or no insight into the functions of several major subclasses within the Rab GTPase family.

SNAREs--molecular governors in signalling and development

SNARE (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) proteins drive membrane fusion and contribute to membrane and protein targeting and delivery in all eukaryotic cells. SNAREs are essential to the mechanics of cell growth and development, and they facilitate a number of homeostatic and evoked responses in plants, from hormone signalling to pathogen defence. Additionally, there is now unambiguous evidence that SNAREs play roles in anchoring other membrane proteins and in facilitating ion channel gating through direct, physical interaction with channel proteins. What is the physiological significance of these additional features of plant SNAREs? We explore possible interpretations and suggest functions as scaffolds for effective signal transmission between proteins and, by analogy with a mechanical device invented by James Watt, as molecular governors to coordinate solute transport with cell expansion and growth.

Boron in plants: deficiency and toxicity

Boron (B) is an essential nutrient for normal growth of higher plants, and B availability in soil and irrigation water is an important determinant of agricultural production. To date, a primordial function of B is undoubtedly its structural role in the cell wall; however, there is increasing evidence for a possible role of B in other processes such as the maintenance of plasma membrane function and several metabolic pathways. In recent years, the knowledge of the molecular basis of B deficiency and toxicity responses in plants has advanced greatly. The aim of this review is to provide an update on recent findings related to these topics, which can contribute to a better understanding of the role of B in plants.

NIP6;1 is a boric acid channel for preferential transport of boron to growing shoot tissues in Arabidopsis

Boron (B) in soil is taken up by roots through NIP5;1, a boric acid channel, and is loaded into the xylem by BOR1, a borate exporter. Here, the function of Arabidopsis thaliana NIP6;1, the most similar gene to NIP5;1, was studied. NIP6;1 facilitates the rapid permeation of boric acid across the membrane but is completely impermeable to water. NIP6;1 transcript accumulation is elevated in response to B deprivation in shoots but not in roots. NIP6;1 promoter-beta-glucuronidase is predominantly expressed in nodal regions of shoots, especially the phloem region of vascular tissues. Three independently identified T-DNA insertion lines for the NIP6;1 gene exhibited reduced expansion of young rosette leaves only under low-B conditions. B concentrations are reduced in young rosette leaves but not in the old leaves of these mutants. Taken together, these data strongly suggest that NIP6;1 is a boric acid channel required for proper distribution of boric acid, particularly among young developing shoot tissues. We propose that NIP6;1 is involved in xylem-phloem transfer of boric acid at the nodal regions and that the water-tight property of NIP6;1 is important for this function. It is proposed that during evolution, NIP5;1 and NIP6;1 were diversified in terms of both the specificity of their expression in plant tissues and their water permeation properties, while maintaining their ability to be induced under low B and their boric acid transport activities.

Identification of differentially expressed transcripts from leaves of the boron tolerant plant Gypsophila perfoliata L

Very recently some of the species of Gypsophila genus collected from the boron rich soils in Turkey were shown to be remarkably tolerant to high levels of boron. A limited amount of boron is necessary for the normal development of plants; however, a high level of boron in soil is generally toxic. Nevertheless, the adaptability of plant species allows them to withstand the presence of extreme amounts of metal ion by various strategies. This study is conducted on highly boron tolerant Gypsophila perfoliata L. collected from a location in the boron mining area. The plant samples were transferred into plant nutritional medium in the presence high; approximately 500 (35 mg/kg), 1,000, and 30 microM (considered normal) boron concentrations. We compared the transcriptome of the plant sample treated with the excess levels of boron to that of the samples grown under normal concentration using differential display PCR (DDRT-PCR) method. Thirty bands showing differential expression levels (presence or absence of bands or varying intensities) in either of approximately 500 or 30 microM B concentrations at varying time points were excised, cloned, and sequenced. Among which, 18 of them were confirmed via quantitative reverse transcription real time PCR (qRT-PCR). We are reporting the first preliminary molecular level study of boron tolerance on this organism by attempting to identify putative genes related in the tolerance mechanism. The gene fragments are consistent with the literature data obtained from a proteomics study and a metabolomics study performed in barley under varying boron concentrations.

Boron transport mechanisms: collaboration of channels and transporters

Boron (B) is an essential element for plants, but is also toxic when present in excess. B deficiency and toxicity are both major agricultural problems worldwide, and elucidating the molecular mechanisms of B transport should allow us to develop technology to alleviate B deficiency and toxicity problems. Recent milestones include the identification of a boric acid channel, NIP5;1, and a boric acid/borate exporter, BOR1, from Arabidopsis thaliana. Both proteins were shown to be required for plant growth under B limitation. In addition, BOR1 homologs are required for B homeostasis in mammalian cells and B-toxicity tolerance in yeast and plants. Here, we discuss how transgenic approaches show promise for generating crops that are tolerant of B deficiency and toxicity.

Comparison of the dynamics and functional redundancy of the Arabidopsis dynamin-related isoforms DRP1A and DRP1C during plant development

Members of the Arabidopsis (Arabidopsis thaliana) DYNAMIN-RELATED PROTEIN1 (DRP1) family are required for cytokinesis and cell expansion. Two isoforms, DRP1A and DRP1C, are required for plasma membrane maintenance during stigmatic papillae expansion and pollen development, respectively. It is unknown whether the DRP1s function interchangeably or if they have distinct roles during cell division and expansion. DRP1C was previously shown to form dynamic foci in the cell cortex, which colocalize with part of the clathrin endocytic machinery in plants. DRP1A localizes to the plasma membrane, but its cortical organization and dynamics have not been determined. Using dual color labeling with live cell imaging techniques, we showed that DRP1A also forms discreet dynamic foci in the epidermal cell cortex. Although the foci overlap with those formed by DRP1C and clathrin light chain, there are clear differences in behavior and response to pharmacological inhibitors between DRP1A and DRP1C foci. Possible functional or regulatory differences between DRP1A and DRP1C were supported by the failure of DRP1C to functionally compensate for the absence of DRP1A. Our studies indicated that the DRP1 isoforms function or are regulated differently during cell expansion.

Endosomal functions in plants

Plant endosomes are highly dynamic organelles that are involved in the constitutive recycling of plasma membrane cargo and the trafficking of polarized plasma membrane proteins such as auxin carriers. In addition, recent studies have shown that surface receptors such as the plant defense-related FLS2 receptor and the brassinosteroid receptor BRI1 appear to signal from endosomes upon ligand binding and internalization. In yeast and mammals, endosomes are also known to recycle vacuolar cargo receptors back to the trans Golgi network and sort membrane proteins for degradation in the vacuole/lysosome. Some of these sorting mechanisms are mediated by the retromer and endosomal sorting complex required for transport (ESCRT) complexes. Plants contain orthologs of all major retromer and ESCRT complex subunits, but they have also evolved variations in endosomal functions connected to plant-specific features such as the diversity of vacuolar transport pathways. This review focuses on recent studies in plants dealing with the regulation of endosomal recycling functions, architecture and formation of multivesicular bodies, ligand-mediated endocytosis and receptor signaling from endosomes as well as novel endosomal markers and the function of endosomes in the transport and processing of soluble vacuolar proteins.

Endosidin1 defines a compartment involved in endocytosis of the brassinosteroid receptor BRI1 and the auxin transporters PIN2 and AUX1

Although it is known that proteins are delivered to and recycled from the plasma membrane (PM) via endosomes, the nature of the compartments and pathways responsible for cargo and vesicle sorting and cellular signaling is poorly understood. To define and dissect specific recycling pathways, chemical effectors of proteins involved in vesicle trafficking, especially through endosomes, would be invaluable. Thus, we identified chemicals affecting essential steps in PM/endosome trafficking, using the intensely localized PM transport at the tips of germinating pollen tubes. The basic mechanisms of this localized growth are likely similar to those of non-tip growing cells in seedlings. The compound endosidin 1 (ES1) interfered selectively with endocytosis in seedlings, providing a unique tool to dissect recycling pathways. ES1 treatment induced the rapid agglomeration of the auxin translocators PIN2 and AUX1 and the brassinosteroid receptor BRI1 into distinct endomembrane compartments termed "endosidin bodies"; however, the markers PIN1, PIN7, and other PM proteins were unaffected. Endosidin bodies were defined by the syntaxin SYP61 and the V-ATPase subunit VHA-a1, two trans-Golgi network (TGN)/endosomal proteins. Interestingly, brassinosteroid (BR)-induced gene expression was inhibited by ES1 and treated seedlings displayed a brassinolide (BL)-insensitive phenotype similar to a bri1 loss-of-function mutant. No effect was detected in auxin signaling. Thus, PIN2, AUX1, and BRI1 use interactive pathways involving an early SYP61/VHA-a1 endosomal compartment.

Boric acid inhibits stored Ca2+ release in DU-145 prostate cancer cells

Boron (B) is a developmental and reproductive toxin. It is also essential for some organisms. Plants use uptake and efflux transport proteins to maintain homeostasis, and in humans, boron has been reported to reduce prostate cancer. Ca2+ signaling is one of the primary mechanisms used by cells to respond to their environment. In this paper, we report that boric acid (BA) inhibits NAD+ and NADP+ as well as mechanically induced release of stored Ca2+ in growing DU-145 prostate cancer cells. Cell proliferation was inhibited by 30% at 100 microM, 60% at 250 microM, and 97% at 1,000 microM BA. NAD+-induced Ca2+ transients were partly inhibited at 250 microM BA and completely at 1,000 microM BA, whereas both NADP+ and mechanically induced transients were inhibited by 1,000 microM BA. Expression of CD38 protein increased in proportion to BA exposure (0-1,000 microM). In vitro mass spectrometry analysis showed that BA formed adducts with the CD38 products and Ca2+ channel agonists cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Vesicles positive for the Ca2+ fluorophore fluo-3 acetoxymethyl ester accumulated in cells exposed to 250 and 1,000 microM BA. The BA analog, methylboronic acid (MBA; 250 and 1,000 microM), did not inhibit cell proliferation or NAD+, NADP+, or mechanically stimulated Ca2+ store release. Nor did MBA increase CD38 expression or cause the formation of intracellular vesicles. Thus, mammalian cells can distinguish between BA and its synthetic analog MBA and exhibit graded concentration-dependent responses. Based on these observations, we hypothesize that toxicity of BA stems from the ability of high concentrations to impair Ca2+ signaling.

Iron-induced turnover of the Arabidopsis IRON-REGULATED TRANSPORTER1 metal transporter requires lysine residues

Iron is an essential micronutrient but is toxic if accumulated at high levels. Thus, iron uptake and distribution in plants are controlled by precise regulatory mechanisms. IRON-REGULATED TRANSPORTER1 (IRT1) is the major high affinity iron transporter responsible for iron uptake from the soil in Arabidopsis (Arabidopsis thaliana). Previously, we showed that IRT1 is subject to posttranscriptional regulation; when expressed from the constitutive cauliflower mosaic virus 35S promoter, IRT1 protein accumulates only in iron-deficient roots. IRT1 contains an intracellular loop that may be critical for posttranslational regulation by metals. Of particular interest are a histidine (His) motif (HGHGHGH) that might bind metals and two lysine residues that could serve as attachment sites for ubiquitin. We constructed a set of mutant IRT1 alleles: IRT1H154Q, IRT1H156Q, IRT1H158Q, IRT1H160Q, IRT14HQ (quadruple His mutant), IRT1K146R, IRT1K171R, and a double mutant (IRT1K146R,K171R). Mutation of the His or lysine residues did not eliminate the ability of IRT1 to transport iron or zinc. Expression of each of the IRT1 variants and an IRT1intact construct in plants from the 35S promoter revealed that either K146 or K171 is required for iron-induced protein turnover, and 35S-IRT1K146R,K171R plants contain higher levels of iron as compared to 35S-IRT1 and wild type. Furthermore, accumulation of metals in 35S-IRT1K146R,K171R plants was not associated with an increase in ferric chelate reductase activity; this result indicates that, at least under conditions when iron is abundant, reduction of ferric iron may not be the rate-limiting step in iron uptake by strategy I plants such as Arabidopsis.

Multivariate analysis of tronchuda cabbage (Brassica oleracea L. var. costata DC) phenolics: influence of fertilizers

A field experiment was carried out to investigate the effect of fertilization level on the phenolic composition of tronchuda cabbage ( Brassica oleracea L. var. costata DC) external and internal leaves. Eight different plots were constituted: a control without fertilization, one with organic matter, and six experiments with conventional fertilizers (nitrogen, boron, and sulfur, two levels each). The phenolic compounds were analyzed by reversed-phase HPLC-DAD. External and internal leaves revealed distinct qualitative composition. In the internal leaves were found 15 phenolics (5 kaempferol and 10 cinnamic acid derivatives), whereas the external leaves presented 3- p-coumaroylquinic acid and 13 kaempferol derivatives. Principal component analysis (PCA) was applied to assess the relationships between phenolic compounds, agronomical practices, and harvesting time. Samples obtained with conventional practices were quite effectively separated from organic samples, for both types of leaves. In general, samples developed without any fertilization presented the highest phenolics amounts: external and internal leaves contained 1.4- and 4.6-fold more phenolic compounds than the ones that received conventional fertilizer, respectively, and the internal leaves presented 2.4 times more phenolics than the ones grown with organic amendment. Additionally, samples from organic production exhibited higher total phenolics content than those from conventional practices, collected at the same time. Samples harvested first were revealed to be distinct from the ones collected later. The results show that it is possible to grow tronchuda cabbage without excess fertilizers, with highest amounts of phenolics and reduced environment contamination.

Biologically active compounds of semi-metals

Semi-metals (boron, silicon, arsenic and selenium) form organo-metal compounds, some of which are found in nature and affect the physiology of living organisms. They include, e.g., the boron-containing antibiotics aplasmomycin, borophycin, boromycin, and tartrolon or the silicon compounds present in "silicate" bacteria, relatives of the genus Bacillus, which release silicon from aluminosilicates through the secretion of organic acids. Arsenic is incorporated into arsenosugars and arsenobetaines by marine algae and invertebrates, and fungi and bacteria can produce volatile methylated arsenic compounds. Some prokaryotes can use arsenate as a terminal electron acceptor while others can utilize arsenite as an electron donor to generate energy. Selenium is incorporated into selenocysteine that is found in some proteins. Biomethylation of selenide produces methylselenide and dimethylselenide. Selenium analogues of amino acids, antitumor, antibacterial, antifungal, antiviral, anti-infective drugs are often used as analogues of important pharmacological sulfur compounds. Other metalloids, i.e. the rare and toxic tellurium and the radioactive short-lived astatine, have no biological significance.

Light plays an essential role in intracellular distribution of auxin efflux carrier PIN2 in Arabidopsis thaliana

Background

Light plays a key role in multiple plant developmental processes. It has been shown that root development is modulated by shoot-localized light signaling and requires shoot-derived transport of the plant hormone, auxin. However, the mechanism by which light regulates root development is not largely understood. In plants, the endogenous auxin, indole-3-acetic acid, is directionally transported by plasma-membrane (PM)-localized auxin influx and efflux carriers in transporting cells. Remarkably, the auxin efflux carrier PIN proteins exhibit asymmetric PM localization, determining the polarity of auxin transport. Similar to PM-resident receptors and transporters in animal and yeast cells, PIN proteins undergo constitutive cycling between the PM and endosomal compartments. Auxin plays multiple roles in PIN protein intracellular trafficking, inhibiting PIN2 endocytosis at some concentrations and promoting PIN2 degradation at others. However, how PIN proteins are turned over in plant cells is yet to be addressed.

Methodology and Principle Findings

Using laser confocal scanning microscopy, and physiological and molecular genetic approaches, here, we show that in dark-grown seedlings, the PM localization of auxin efflux carrier PIN2 was largely reduced, and, in addition, PIN2 signal was detected in vacuolar compartments. This is in contrast to light-grown seedlings where PIN2 was predominantly PM-localized. In light-grown plants after shift to dark or to continuous red or far-red light, PIN2 also accumulated in vacuolar compartments. We show that PIN2 vacuolar targeting was derived from the PM via endocytic trafficking and inhibited by HY5-dependent light signaling. In addition, the ubiquitin 26S proteasome is involved in the process, since its inhibition by mutations in COP9 and a proteasome inhibitor MG132 impaired the process.

Conclusions and Significance

Collectively, our data indicate that light plays an essential role in PIN2 intracellular trafficking, promoting PM-localization in the presence of light and, on the other hand, vacuolar targeting for protein degradation in the absence of light. Based on these results, we postulate that light regulation of root development is mediated at least in part by changes in the intracellular distribution of auxin efflux carriers, PIN proteins, in response to the light environment.

Evidence for a sorting endosome in Arabidopsis root cells

In eukaryotic cells, the endocytic and secretory pathways are key players in several physiological processes. These pathways are largely inter-connected in animal and yeast cells through organelles named sorting endosomes. Sorting endosomes are multi-vesicular compartments that redirect proteins towards various destinations, such as the lysosomes or vacuoles for degradation, the trans-Golgi network for retrograde transport and the plasma membrane for recycling. In contrast, cross-talk between the endocytic and secretory pathways has not been clearly established in plants, especially in terms of cargo protein trafficking. Here we show by co-localization analyses that endosomes labelled with the AtSORTING NEXIN1 (AtSNX1) protein overlap with the pre-vacuolar compartment in Arabidopsis root cells. In addition, alteration of the routing functions of AtSNX1 endosomes by drug treatments leads to mis-routing of endocytic and secretory cargo proteins. Based on these results, we propose that the AtSNX1 endosomal compartment represents a sorting endosome in root cells, and that this specialized organelle is conserved throughout eukaryotes.

Physiological and genetic basis of plant tolerance to excess boron

Boron (B) deficit as well as excess may significantly limit the organic production in plants. In extreme cases they may kill the affected plants. Boron excess occurs primarily in arid and semiarid regions, in saline soils or in consequence to human action. Excessive boron concentrations retard plant growth and cause physiological and morphological changes (chlorosis and necrosis) first of all in leaf tips and then in marginal or intercostal parts of the lamina. Physiological mechanisms of plant tolerance to boron excess have not been studied in sufficient detail. The predominant opinion holds that they are based on restricted uptake and accumulation of boron in the root and aboveground plant parts. Significant differences in boron excess tolerance have been observed not only between different crops but even between different genotypes of the same crop. This has enabled the breeding of crop genotypes and crops adapted to growing on soils rich in available boron and intensified the research on the inheritance of plant tolerance to high B concentration. Sources of tolerance to high B concentration have been found in many crops (wheat, mustard, pea, lentil, eucalypt). Using different molecular techniques based on PCR (RAPD, SRAP), plant parents and progenies have been analyzed in an attempt to map as precisely as possible the position of B-tolerant genes. Small grains have been studied in greatest detail for inheritance of B tolerance. B tolerance in wheat is controlled by at least four additive genes, Bo1, Bo2, Bo3 and Bo4. Consequently, there exists a broad range of tolerance levels. Studies of Arabidopsis have broadened our understanding of regulation mechanisms of B transport from roots to above ground parts, allowing more direct genetic manipulations.

Boron uptake by the root cortex symplast of tomato and pea plants: evidence for low-boron-induced active transport

The objective of this research was to test the hypothesis of the existence of an active boron (B) uptake into the cortical cells induced by low B supply. The uptake of B was characterised in two tomato (Lycopersicon esculentum Mill.) genotypes: B-efficient FER and B-inefficient mutant T3238. In addition, pea (Pisum sativum L.) was used as an anatomically appropriate model for obtaining intact root cortex. Time course uptake studies in tomato indicate that the B-inefficient mutant was defective by the absence of an active low-B-induced uptake system in the cortex. Pea roots showed up to 10-fold higher accumulation of B into the cortex symplast at low (0.5 µm) external B supply in comparison to adequate B (10 µm) supply. Also, low-B-induced uptake of B was strongly inhibited by 2,4-dinitrophenol, indicating a metabolic energy-derived active component of B uptake at low external supply. Uptake of B by the cortical cells of tomato and pea plants appears to be a combination of both passive and active components, with a passive component prevailing at higher external B. An active component of B uptake suppressed by either adequate or high B supply might indicate a downregulation of plasma membrane-associated B transporter(s) in root cortical cells.

Rab-A2 and Rab-A3 GTPases define a trans-golgi endosomal membrane domain in Arabidopsis that contributes substantially to the cell plate

The Ypt3/Rab11/Rab25 subfamily of Rab GTPases has expanded greatly in Arabidopsis thaliana, comprising 26 members in six provisional subclasses, Rab-A1 to Rab-A6. We show that the Rab-A2 and Rab-A3 subclasses define a novel post-Golgi membrane domain in Arabidopsis root tips. The Rab-A2/A3 compartment was distinct from but often close to Golgi stacks and prevacuolar compartments and partly overlapped the VHA-a1 trans-Golgi compartment. It was also sensitive to brefeldin A and accumulated FM4-64 before prevacuolar compartments did. Mutations in RAB-A2a that were predicted to stabilize the GDP- or GTP-bound state shifted the location of the protein to the Golgi or plasma membrane, respectively. In mitosis, KNOLLE accumulated principally in the Rab-A2/A3 compartment. During cytokinesis, Rab-A2 and Rab-A3 proteins localized precisely to the growing margins of the cell plate, but VHA-a1, GNOM, and prevacuolar markers were excluded. Inducible expression of dominant-inhibitory mutants of RAB-A2a resulted in enlarged, polynucleate, meristematic cells with cell wall stubs. The Rab-A2/A3 compartment, therefore, is a trans-Golgi compartment that communicates with the plasma membrane and early endosomal system and contributes substantially to the cell plate. Despite the unique features of plant cytokinesis, membrane traffic to the division plane exhibits surprising molecular similarity across eukaryotic kingdoms in its reliance on Ypt3/Rab11/Rab-A GTPases.

The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis

Caesium (Cs(+)) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs(+) is chemically similar to potassium (K(+)), and much is known about K(+) transport mechanisms, it is not clear through which K(+) transport mechanisms Cs(+) is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K(+) and Cs(+) uptake was characterized. It is demonstrated that AtHAK5 is localized to the plasma membrane under conditions of K(+) deprivation, when it is expressed. Growth analysis showed that AtHAK5 plays a role during severe K(+) deprivation. Under K(+)-deficient conditions in the presence of Cs(+), Arabidopsis seedlings lacking AtHAK5 had increased inhibition of root growth and lower Cs(+) accumulation, and significantly higher leaf chlorophyll concentrations than wild type. These data indicate that, in addition to transporting K(+) in planta, AtHAK5 also transports Cs(+). Further experiments showed that AtHAK5 mediated Cs(+) uptake into yeast cells and that, although the K(+) deficiency-induced expression of AtHAK5 was inhibited by low concentrations of NH(4)(+) in planta, Cs(+) uptake by yeast was stimulated by low concentrations of NH(4)(+). Interestingly, the growth of the Arabidopsis atakt1-1 mutant was more sensitive to Cs(+) than the wild type. This may be explained, in part, by increased expression of AtHAK5 in the atakt1-1 mutant. It is concluded that AtHAK5 is a root plasma membrane uptake mechanism for K(+) and Cs(+) under conditions of low K(+) availability.

Identification of boron transporter genes likely to be responsible for tolerance to boron toxicity in wheat and barley

Tolerance to boron (B) toxicity in cereals is known to be associated with reduced tissue accumulation of B. Genes from roots of B-tolerant cultivars of wheat and barley with high similarities to previously reported B efflux transporters from Arabidopsis and rice were cloned. Expression of these genes was strongly correlated with the ability of tolerant genotypes to lower the concentration of B in roots. The gene from barley located to chromosome 4. Backcross lines containing a B tolerance locus on chromosome 4 showed tolerance in proportion to the level of expression of the transporter gene, whereas those lacking the locus were sensitive to B and had very low levels of gene expression. The results are consistent with a widespread mechanism of tolerance to high B based on efflux of B from root cells.

The retromer protein VPS29 links cell polarity and organ initiation in plants

A key feature of plants (as opposed to animals) is their ability to establish new organs not only during embryogenesis, but also throughout their development. A master regulator of organ initiation in plants is the phytohormone auxin. Auxin acts locally as a morphogen and is directionally transported from cell to cell by polarized auxin efflux carriers, termed PIN-FORMED (PIN) proteins. Here we report that the Arabidopsis ortholog of the yeast and mammalian vacuolar protein sorting 29 (VPS29), a member of the retromer complex, mediates the formation of new axes of development. Furthermore, we show that VPS29 is required for endosome homeostasis, PIN protein cycling, and dynamic PIN1 repolarization during development. We propose a model that links VPS29 function, PIN1 polarity, and organ initiation in plants.

Nitrate efflux at the root plasma membrane: identification of an Arabidopsis excretion transporter

Root NO(3)(-) efflux to the outer medium is a component of NO(3)(-) net uptake and can even overcome influx upon various stresses. Its role and molecular basis are unknown. Following a functional biochemical approach, NAXT1 (for NITRATE EXCRETION TRANSPORTER1) was identified by mass spectrometry in the plasma membrane (PM) of Arabidopsis thaliana suspension cells, a localization confirmed using a NAXT1-Green Fluorescent Protein fusion protein. NAXT1 belongs to a subclass of seven NAXT members from the large NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER family and is mainly expressed in the cortex of mature roots. The passive NO(3)(-) transport activity (K(m) = 5 mM) in isolated root PM, electrically coupled to the ATP-dependant H(+)-pumping activity, is inhibited by anti-NAXT antibodies. In standard culture conditions, NO(3)(-) contents were altered in plants expressing NAXT-interfering RNAs but not in naxt1 mutant plants. Upon acid load, unidirectional root NO(3)(-) efflux markedly increased in wild-type plants, leading to a prolonged NO(3)(-) excretion regime concomitant with a decrease in root NO(3)(-) content. In vivo and in vitro mutant phenotypes revealed that this response is mediated by NAXT1, whose expression is upregulated at the posttranscriptional level. Strong medium acidification generated a similar response. In vitro, the passive efflux of NO(3)(-) (but not of Cl(-)) was strongly impaired in naxt1 mutant PM. This identification of NO(3)(-) efflux transporters at the PM of plant cells opens the way to molecular studies of the physiological role of NO(3)(-) efflux in stressed or unstressed plants.

Sorting out the sorting functions of endosomes in Arabidopsis

In animals, sorting of membrane proteins following their internalization from the plasma membrane (PM) by endocytosis occurs through a series of different endosomal compartments. In plants, how and where these sorting events take place is still poorly understood and our current view of the endocytic pathway still largely relies on analogies made from the animal system. However, extensive differences seem to exist between animal and plant endosomal functions, as exemplified by the role of the trans-Golgi network (TGN) as an early endosomal compartment in plants or the functional diversification of conserved sorting complexes. By using the Arabidopsis root tip as a reference model, we and other have begun to shed light on the complexity of the plant endocytic pathways. Notably, we have recently characterized the functions of an endosomal compartment, the SNX1-endosomes, also referred to as the prevacuolar compartment (PVC) or multivesicular bodies (MVB), in the sorting of different cargo proteins, including two related auxin-efflux carriers, PIN1 and PIN2. We have shown that routing decisions take place at this endosomal level, such as the sorting of PIN2 toward the lytic vacuole for degradation or PIN1 toward the PM for recycling.

Tracking down the elusive early endosome

Despite significant progress in understanding protein trafficking and compartmentation in plants, the identification and protein compartmentalization for organelles that belong to both the secretory and endocytic pathways have been difficult because protein trafficking has generally been studied separately in these two pathways. However, recent data indicate that the trans-Golgi network serves as an early endosome merging the secretory and endocytic pathways in plant cells. Here, we discuss the proteins identified as markers for post-Golgi compartments in these two pathways and propose that the trans-Golgi network is a pivotal organelle with multiple sorting domains for post-Golgi protein trafficking in plant cells.

VPS9a, the common activator for two distinct types of Rab5 GTPases, is essential for the development of Arabidopsis thaliana

Rab5, a subfamily of Rab GTPases, regulates a variety of endosomal functions as a molecular switch. Arabidopsis thaliana has two different types of Rab5-member GTPases: conventional type, ARA7 and RHA1, and a plant-specific type, ARA6. We found that only one guanine nucleotide exchange factor (GEF), named VPS9a, can activate all Rab5 members to GTP-bound forms in vitro in spite of their diverged structures. In the vps9a-1 mutant, whose GEF activity is completely lost, embryogenesis was arrested at the torpedo stage. Green fluorescent protein (GFP)-ARA7 and ARA6-GFP were diffused in cytosol like GDP-fixed mutants of Rab5 in vps9a-1, indicating that both types of GTPase are regulated by VPS9a. In the leaky vps9a-2 mutant, elongation of the primary root was severely affected. Overexpression of the GTP-fixed form of ARA7 suppressed the vps9a-2 mutation, but overexpression of ARA6 had no apparent effects. These results indicate that the two types of plant Rab5 members are functionally differentiated, even though they are regulated by the same activator, VPS9a.

AtDUR3 represents the major transporter for high-affinity urea transport across the plasma membrane of nitrogen-deficient Arabidopsis roots

Despite the fact that urea is a ubiquitous nitrogen source in soils and the most widespread form of nitrogen fertilizer used in agricultural plant production, membrane transporters that might contribute to the uptake of urea in plant roots have so far been characterized only in heterologous systems. Two T-DNA insertion lines, atdur3-1 and atdur3-3, that showed impaired growth on urea as a sole nitrogen source were used to investigate a role of the H+/urea co-transporter AtDUR3 in nitrogen nutrition in Arabidopsis. In transgenic lines expressing AtDUR3-promoter:GFP constructs, promoter activity was upregulated under nitrogen deficiency and localized to the rhizodermis, including root hairs, as well as to the cortex in more basal root zones. Protein gel blot analysis of two-phase partitioned root membrane fractions and whole-mount immunolocalization in root hairs revealed the plasma membrane to be enriched in AtDUR3 protein. Expression of the AtDUR3 gene in nitrogen-deficient roots was repressed by ammonium and nitrate but induced after supply of urea. Higher accumulation of urea in roots of wild-type plants relative to atdur3-1 and atdur3-3 confirmed that urea was the substrate transported by AtDUR3. Influx of 15N-labeled urea in atdur3-1 and atdur3-3 showed a linear concentration dependency up to 200 microM external urea, whereas influx in wild-type roots followed saturation kinetics with an apparent Km of 4 microM. The results indicate that AtDUR3 is the major transporter for high-affinity urea uptake in Arabidopsis roots and suggest that the high substrate affinity of AtDUR3 reflects an adaptation to the low urea levels usually found in unfertilized soils.

MODULATOR OF PIN genes control steady-state levels of Arabidopsis PIN proteins

Polar transport of the phytohormone auxin controls numerous growth responses in plants. Molecular characterization of auxin transport in Arabidopsis thaliana has provided important insights into the mechanisms underlying the regulation of auxin distribution. In particular, the control of subcellular localization and expression of PIN-type auxin efflux components appears to be fundamental for orchestrated distribution of the growth regulator throughout the entire plant body. Here we describe the identification of two Arabidopsis loci, MOP2 and MOP3 (for MODULATOR OF PIN), that are involved in control of the steady-state levels of PIN protein. Mutations in both loci result in defects in auxin distribution and polar auxin transport, and cause phenotypes consistent with a reduction of PIN protein levels. Genetic interaction between PIN2 and both MOP loci is suggestive of functional cross-talk, which is further substantiated by findings demonstrating that ectopic PIN up-regulation is compensated in the mop background. Thus, in addition to pathways that control PIN localization and transcription, MOP2 and MOP3 appear to be involved in fine-tuning of auxin distribution via post-transcriptional regulation of PIN expression.