Urea transport by nitrogen-regulated tonoplast intrinsic proteins in Arabidopsis

Tobacco aquaporin NtAQP1 is involved in mesophyll conductance to CO2 in vivo

Leaf mesophyll conductance to CO(2) (g(m)) has been recognized to be finite and variable, rapidly adapting to environmental conditions. The physiological basis for fast changes in g(m) is poorly understood, but current reports suggest the involvement of protein-facilitated CO(2) diffusion across cell membranes. A good candidate for this could be the Nicotiana tabacum L. aquaporin NtAQP1, which was shown to increase membrane permeability to CO(2) in Xenopus oocytes. The objective of the present work was to evaluate its effect on the in vivo mesophyll conductance to CO(2), using plants either deficient in or overexpressing NtAQP1. Antisense plants deficient in NtAQP1 (AS) and NtAQP1 overexpressing tobacco plants (O) were compared with their respective wild-type (WT) genotypes (CAS and CO). Plants grown under optimum conditions showed different photosynthetic rates at saturating light, with a decrease of 13% in AS and an increase of 20% in O, compared with their respective controls. CO(2) response curves of photosynthesis also showed significant differences among genotypes. However, in vitro analysis demonstrated that these differences could not be attributed to alterations in Rubisco activity or ribulose-1,5-bisphosphate content. Analyses of chlorophyll fluorescence and on-line (13)C discrimination indicated that the observed differences in net photosynthesis (A(N)) among genotypes were due to different leaf mesophyll conductances to CO(2), which was estimated to be 30% lower in AS and 20% higher in O compared with their respective WT. These results provide evidence for the in vivo involvement of aquaporin NtAQP1 in mesophyll conductance to CO(2).

The Arabidopsis major intrinsic protein NIP5;1 is essential for efficient boron uptake and plant development under boron limitation

Boron (B) is essential in plants but often present at low concentrations in the environment. To investigate how plants survive under conditions of B limitation, we conducted a transcriptome analysis and identified NIP5;1, a member of the major intrinsic protein family, as a gene upregulated in B-deficient roots of Arabidopsis thaliana. Promoter-beta-glucuronidase fusions indicated that NIP5;1 is strongly upregulated in the root elongation zone and the root hair zone under B limitation, and green fluorescent protein-tagged NIP5;1 proteins localized to the plasma membrane. Expression in Xenopus laevis oocytes demonstrated that NIP5;1 facilitated the transport of boric acid in addition to water. Importantly, two T-DNA insertion lines of NIP5;1 displayed lower boric acid uptake into roots, lower biomass production, and increased sensitivity of root and shoot development to B deficiency. These results identify NIP5;1 as a major plasma membrane boric acid channel crucial for the B uptake required for plant growth and development under B limitation.

Aquaporins in plants

Although very often exposed to a rapid changing environment, plants are in general unable to evade from unfavourable conditions. Therefore, a fine tuned adaptation of physiology including the water balance appears to be of crucial importance. As a consequence a relatively large number of aquaporin genes are present in plant genomes. So far aquaporins in plants were shown to be involved in root water uptake, reproduction or photosynthesis. Accordingly, plant aquaporin classification as simple water pores has changed corresponding to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the respective physiological process could be achieved by regulation mechanisms, which range from post-translational modification, molecular trafficking to heteromerization of aquaporin isoforms. Here the function of the four plant aquaporin family subclasses with regard to substrate specificity, regulation and physiological relevance is described.

Functional aquaporin diversity in plants

Due to the fact that most plants are immobile, a rapid response of physiological processes to changing environmental conditions is essential for their survival. Thus, in comparison to many other organisms, plants might need a more sophisticated tuning of water balance. Among others, this is reflected by the comparable large amount of aquaporin genes in plant genomes. So far, aquaporins were shown to be involved in many physiological processes like root water uptake, reproduction or photosynthesis. Their classification as simple water pores has changed according to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the corresponding physiological process could be achieved by regulation mechanisms. Concerning aquaporins these range from posttranslational modification, molecular trafficking to heteromerization of aquaporin isoforms. The aim of this review is to underline the function of the four plant aquaporin family subclasses with regard to the substrate specificity, regulation and physiological relevance.

Membrane transport of hydrogen peroxide

Hydrogen peroxide (H2O2) belongs to the reactive oxygen species (ROS), known as oxidants that can react with various cellular targets thereby causing cell damage or even cell death. On the other hand, recent work has demonstrated that H2O2 also functions as a signalling molecule controlling different essential processes in plants and mammals. Because of these opposing functions the cellular level of H2O2 is likely to be subjected to tight regulation via processes involved in production, distribution and removal. Substantial progress has been made exploring the formation and scavenging of H2O2, whereas little is known about how this signal molecule is transported from its site of origin to the place of action or detoxification. From work in yeast and bacteria it is clear that the diffusion of H2O2 across membranes is limited. We have now obtained direct evidence that selected aquaporin homologues from plants and mammals have the capacity to channel H2O2 across membranes. The main focus of this review is (i) to summarize the most recent evidence for a signalling role of H2O2 in various pathways in plants and mammals and (ii) to discuss the relevance of specific transport of H2O2.

Regulation of plant aquaporin activity

Accumulating evidence indicates that aquaporins play a key role in plant water relations. Plant aquaporins are part of a large and highly divergent protein family that can be divided into four subfamilies according to amino acid sequence similarity. As in other organisms, plant aquaporins facilitate the transcellular movement of water, but, in some cases, also the flux of small neutral solutes across a cellular membrane. Plant cell membranes are characterized by a large range of osmotic water permeabilities, and recent data indicate that plant aquaporin activity might be regulated by gating mechanisms. The factors affecting the gating behaviour possibly involve phosphorylation, heteromerization, pH, Ca2+, pressure, solute gradients and temperature. Regulation of aquaporin trafficking may also represent a way to modulate membrane water permeability. The aim of this review is to integrate recent molecular and biophysical data on the mechanisms regulating aquaporin activity in plant membranes and to relate them to putative changes in protein structure.

Novel type aquaporin SIPs are mainly localized to the ER membrane and show cell-specific expression inArabidopsis thaliana

We investigated the fourth subgroup of Arabidopsis aquaporin, small and basic intrinsic proteins (SIPs). When they were expressed in yeast, SIP1;1 and SIP1;2, but not SIP2;1, gave water-channel activity. The transient expression of SIPs linked with green fluorescent protein in Arabidopsis cells and the subcellular fractionation of the tissue homogenate showed their ER localization. The SIP proteins were detected in all of the tissues, except for dry seeds. Histochemical analysis of promoter-beta-glucuronidase fusions revealed the cell-specific expression of SIPs. SIP1;1 and SIP1;2 may function as water channels in the ER, while SIP2;1 might act as an ER channel for other small molecules or ions.

Whole gene family expression and drought stress regulation of aquaporins

Since many aquaporins (AQPs) act as water channels, they are thought to play an important role in plant water relations. It is therefore of interest to study the expression patterns of AQP isoforms in order to further elucidate their involvement in plant water transport. We have monitored the expression patterns of all 35 Arabidopsis AQPs in leaves, roots and flowers by cDNA microarrays, specially designed for AQPs, and by quantitative real-time reverse transcriptase PCR (Q-RT-PCR). This showed that many AQPs are pre-dominantly expressed in either root or flower organs, whereas no AQP isoform seem to be leaf specific. Looking at the AQP subfamilies, most plasma membrane intrinsic proteins (PIPs) and some tonoplast intrinsic proteins (TIPs) have a high level of expression, while NOD26-like proteins (NIPs) are present at a much lower level. In addition, we show that PIP transcripts are generally down-regulated upon gradual drought stress in leaves, with the exception of AtPIP1;4 and AtPIP2;5, which are up-regulated. AtPIP2;6 and AtSIP1;1 are constitutively expressed and not significantly affected by the drought stress. The transcriptional down-regulation of PIP genes upon drought stress could also be observed on the protein level.

Identification of 33 rice aquaporin genes and analysis of their expression and function

Plant aquaporins form a large protein family including plasma membrane-type (PIPs) and tonoplast-type aquaporins (TIPs), and facilitate osmotic water transport across membranes as a key physiological function. We identified 33 genes for aquaporins in the genome sequence of rice (Oryza sativa L. cv. Nipponbare). We investigated their expression levels in leaf blades, roots and anthers of rice (cv. Akitakomachi) using semi-quantitative reverse transcription-PCR (RT-PCR). At both early tillering (21 d after germination) and panicle formation (56 d) stages, six genes, including OsPIP2;4 and OsPIP2;5, were expressed predominantly in roots, while 14 genes, including OsPIP2;7 and OsTIP1;2, were found in leaf blades. Eight genes, such as OsPIP1;1 and OsTIP4;1, were evenly expressed in leaf blades, roots and anthers. Analysis by stopped-flow spectrophotometry revealed high water channel activity when OsPIP2;4 or OsPIP2;5 were expressed in yeast but not when OsPIP1;1 or OsPIP1;2 were expressed. Furthermore, the mRNA levels of OsPIP2;4 and OsPIP2;5 showed a clear diurnal fluctuation in roots; they showed a peak 3 h after the onset of light and dropped to a minimum 3 h after the onset of darkness. The mRNA levels of 10 genes including OsPIP2;4 and OsPIP2;5 markedly decreased in roots during chilling treatment and recovered after warming. The changes in mRNA levels during and after the chilling treatment were comparable with that of the bleeding sap volume. These results suggested the relationship between the root water uptake and mRNA levels of several aquaporins with high water channel activity, such as OsPIP2;4 and OsPIP2;5.

Role and regulation of urea transporters

In the past few years, significant knowledge has been gained about the physiological role and regulation of urea transporters, which have been now cloned in many species. The two major mammalian urea transporters, UT-A and UT-B, have been best studied in the kidney, where they mediate the facilitated diffusion of urea across tubular, interstitial, and vascular compartments, necessary to maintain an osmolar gradient along the renal corticomedullary axis. The genes encoding these transporters, Slc14A2 for UT-A and Slc14A1 for UT-B, have been characterized in rodents and humans, allowing identification of transcriptional mechanisms involved in the regulation of UT-A expression. The crucial role that urea transporters play in renal physiology is underscored by the phenotypic characteristics of UT-A and UT-B knockout mice, in which lack of specific urea transporters impairs the ability to concentrate urine. Expression of the UT-A and UT-B transporters has also been identified in extra-renal sites, where their physiological significance is only beginning to be elucidated. More information on the mechanisms modulating urea transporter expression is becoming available, and the possible involvement of aberrant regulation of these transporters in pathological conditions, or as a result of certain pharmacological treatments, has emerged from recent studies.

The yeast osmosensitive mutantfps1Δ transformed by the cauliflower BobTIP1;1 aquaporin withstand a hypo-osmotic shock

Osmoregulation plays an important role in cellular responses to osmotic stress in plants and in yeast. Aquaporins contribute to osmotic adjustment by facilitating transport of water or solutes across membranes. The tonoplastic water channel BobTIP1;1 (original name BobTIP26-1) genes are upregulated during dessication stress in cauliflower meristematic tissue. To investigate the physiological importance of BobTIP1;1, we expressed it in a Saccharomyces cerevisiae osmosensitive mutant fps1Delta. We showed that the defect in the yeast glycerol plasma membrane transporter is complemented by a plant cDNA encoding the aquaporin BobTIP1;1 which is localized in the vacuolar membrane of the complemented yeast cells. To our knowledge, this is the first example of a plant aquaporin for which localization in the vacuolar membrane of yeast cells is related to an osmoresistant phenotype under hypo-osmotic shock.

Tonoplast intrinsic proteins AtTIP2;1 and AtTIP2;3 facilitate NH3 transport into the vacuole

While membrane transporters mediating ammonium uptake across the plasma membrane have been well described at the molecular level, little is known about compartmentation and cellular export of ammonium. (The term ammonium is used to denote both NH3 and NH4+ and chemical symbols are used when specificity is required.) We therefore developed a yeast (Saccharomyces cerevisiae) complementation approach and isolated two Arabidopsis (Arabidopsis thaliana) genes that conferred tolerance to the toxic ammonium analog methylammonium in yeast. Both genes, AtTIP2;1 and AtTIP2;3, encode aquaporins of the tonoplast intrinsic protein subfamily and transported methylammonium or ammonium in yeast preferentially at high medium pH. AtTIP2;1 expression in Xenopus oocytes increased 14C-methylammonium accumulation with increasing pH. AtTIP2;1- and AtTIP2;3-mediated methylammonium detoxification in yeast depended on a functional vacuole, which was in agreement with the subcellular localization of green fluorescent protein-fusion proteins on the tonoplast in planta. Transcript levels of both AtTIPs were influenced by nitrogen supply but did not follow those of the nitrogen-derepressed ammonium transporter gene AtAMT1;1. Transgenic Arabidopsis plants overexpressing AtTIP2;1 did not show altered ammonium accumulation in roots after ammonium supply, although AtTIP2;1 mRNA levels in wild-type plants were up-regulated under these conditions. This study shows that AtTIP2;1 and AtTIP2;3 can mediate the extracytosolic transport of methyl-NH2 and NH3 across the tonoplast membrane and may thus participate in vacuolar ammonium compartmentation.

Loss of TIP1;1 aquaporin in Arabidopsis leads to cell and plant death

Arabidopsis TIP1;1 (gammaTIP) is a member of the tonoplast family of aquaporins (AQP). Using RNA interference (RNAi) we reduced TIP1;1 to different extent in various lines. When most severely affected, miniature plants died, a phenotype partially complemented by the TIP1;1 homolog McMIP-F. Less severely affected lines produced small plants, early senescence, and showed lesion formation. The relative water content in TIP1;1 RNAi plants was not significantly affected. Global expression profiling suggested a disturbance in carbon metabolism in RNAi lines with upregulated transcripts for functions in carbon acquisition and respiration, vesicle transport, signaling and transcription, and radical oxygen stress. Metabolite profiles showed low glucose, fructose, inositol, and threonic, succinic, fumaric, and malic acids, but sucrose levels were similar to WT. Increased amounts were found for raffinose and several unknown compounds. TIP1;1 RNAi plants also contained high starch and apoplastic carbohydrate increased. A GFP-TIP1;1 fusion protein indicated tonoplast location in spongy mesophyll cells, and high signal intensity in palisade mesophyll associated with vesicles near plastids. Signals in vascular tissues were strongest not only in vesicle-like structures but also outlined large vacuoles. Compromised routing of carbohydrate and lack of sucrose provision for cell-autonomous functions seems to characterize this RNAi phenotype. We suggest a function for TIP1;1 in vesicle-based metabolite routing through or between pre-vacuolar compartments and the central vacuole. Phenotype and expression characteristics support a view of TIP1;1 functioning as a marker for vesicles that are targeted to the central vacuole.

Aquaporin homologues in plants and mammals transport ammonia

Using functional complementation and a yeast mutant deficient in ammonium (NH4+) transport (Deltamep1-3), three wheat (Triticum aestivum) TIP2 aquaporin homologues were isolated that restored the ability of the mutant to grow when 2 mM NH4+ was supplied as the sole nitrogen source. When expressed in Xenopus oocytes, TaTIP2;1 increased the uptake of NH4+ analogues methylammonium and formamide. Furthermore, expression of TaTIP2;1 increased acidification of the oocyte-bathing medium containing NH4+ in accordance with NH3 diffusion through the aquaporin. Homology modeling of TaTIP2;1 in combination with site directed mutagenesis suggested a new subgroup of NH3-transporting aquaporins here called aquaammoniaporins. Mammalian AQP8 sharing the aquaammoniaporin signature also complemented NH4+ transport deficiency in yeast.

The potassium-dependent transcriptome of Arabidopsis reveals a prominent role of jasmonic acid in nutrient signaling

Full genome microarrays were used to assess transcriptional responses of Arabidopsis seedlings to changing external supply of the essential macronutrient potassium (K(+)). Rank product statistics and iterative group analysis were employed to identify differentially regulated genes and statistically significant coregulated sets of functionally related genes. The most prominent response was found for genes linked to the phytohormone jasmonic acid (JA). Transcript levels for the JA biosynthetic enzymes lipoxygenase, allene oxide synthase, and allene oxide cyclase were strongly increased during K(+) starvation and quickly decreased after K(+) resupply. A large number of well-known JA responsive genes showed the same expression profile, including genes involved in storage of amino acids (VSP), glucosinolate production (CYP79), polyamine biosynthesis (ADC2), and defense (PDF1.2). Our findings highlight a novel role of JA in nutrient signaling and stress management through a variety of physiological processes such as nutrient storage, recycling, and reallocation. Other highly significant K(+)-responsive genes discovered in our study encoded cell wall proteins (e.g. extensins and arabinogalactans) and ion transporters (e.g. the high-affinity K(+) transporter HAK5 and the nitrate transporter NRT2.1) as well as proteins with a putative role in Ca(2+) signaling (e.g. calmodulins). On the basis of our results, we propose candidate genes involved in K(+) perception and signaling as well as a network of molecular processes underlying plant adaptation to K(+) deficiency.

Renal urea transporters

PURPOSE OF REVIEW

Urea is transported across the kidney inner medullary collecting duct by urea-transporter proteins. Two urea-transporter genes have been cloned from humans and rodents: the UT-A (Slc14A2) gene encodes five protein and eight cDNA isoforms; the UT-B (Slc14A1) gene encodes a single isoform. In the past year, significant progress has been made in understanding the regulation of urea-transporter protein abundance in kidney, studies of genetically engineered mice that lack a urea transporter, identification of urea transporters outside of the kidney, cloning of urea transporters in nonmammalian species, and active urea transport in microorganisms.

RECENT FINDINGS

UT-A1 protein abundance is increased by 12 days of vasopressin, but not by 5 days. Analysis of the UT-A1 promoter suggests that vasopressin increases UT-A1 indirectly following a direct effect to increase the transcription of other genes, such as the Na(+)-K(+)-2Cl- cotransporter NKCC2/BSC1 and the aquaporin (AQP) 2 water channel, that begin to increase inner medullary osmolality. UT-A1 protein abundance is also increased by adrenalectomy, and is decreased by glucocorticoids or mineralocorticoids. However, each hormone works through its own receptor. Knockout mice that lack UT-A1 and UT-A3, or lack UT-B, have a urine-concentrating defect and a decrease in inner medullary interstitial urea content.

SUMMARY

Urea transporters play a critical role in the urine-concentrating mechanism. Their abundance is regulated by vasopressin, glucocorticoids, and mineralocorticoids. These regulatory mechanisms may be important in disease states such as diabetes because changes in urea-transporter abundance in diabetic rats require glucocorticoids and vasopressin.

The potassium-dependent transcriptome of Arabidopsis reveals a prominent role of jasmonic acid in nutrient signaling

Full genome microarrays were used to assess transcriptional responses of Arabidopsis seedlings to changing external supply of the essential macronutrient potassium (K(+)). Rank product statistics and iterative group analysis were employed to identify differentially regulated genes and statistically significant coregulated sets of functionally related genes. The most prominent response was found for genes linked to the phytohormone jasmonic acid (JA). Transcript levels for the JA biosynthetic enzymes lipoxygenase, allene oxide synthase, and allene oxide cyclase were strongly increased during K(+) starvation and quickly decreased after K(+) resupply. A large number of well-known JA responsive genes showed the same expression profile, including genes involved in storage of amino acids (VSP), glucosinolate production (CYP79), polyamine biosynthesis (ADC2), and defense (PDF1.2). Our findings highlight a novel role of JA in nutrient signaling and stress management through a variety of physiological processes such as nutrient storage, recycling, and reallocation. Other highly significant K(+)-responsive genes discovered in our study encoded cell wall proteins (e.g. extensins and arabinogalactans) and ion transporters (e.g. the high-affinity K(+) transporter HAK5 and the nitrate transporter NRT2.1) as well as proteins with a putative role in Ca(2+) signaling (e.g. calmodulins). On the basis of our results, we propose candidate genes involved in K(+) perception and signaling as well as a network of molecular processes underlying plant adaptation to K(+) deficiency.

Novel regulation of aquaporins during osmotic stress

Aquaporin protein regulation and redistribution in response to osmotic stress was investigated. Ice plant (Mesembryanthemum crystallinum) McTIP1;2 (McMIPF) mediated water flux when expressed in Xenopus leavis oocytes. Mannitol-induced water imbalance resulted in increased protein amounts in tonoplast fractions and a shift in protein distribution to other membrane fractions, suggesting aquaporin relocalization. Indirect immunofluorescence labeling also supports a change in membrane distribution for McTIP1;2 and the appearance of a unique compartment where McTIP1;2 is expressed. Mannitol-induced redistribution of McTIP1;2 was arrested by pretreatment with brefeldin A, wortmannin, and cytochalasin D, inhibitors of vesicle trafficking-related processes. Evidence suggests a role for glycosylation and involvement of a cAMP-dependent signaling pathway in McTIP1;2 redistribution. McTIP1;2 redistribution to endosomal compartments may be part of a homeostatic process to restore and maintain cellular osmolarity under osmotic-stress conditions.

Regulatory levels for the transport of ammonium in plant roots

Ammonium is an attractive nitrogen form for root uptake due to its permanent availability and the reduced state of the nitrogen. On the other hand, ammonium fluxes are difficult to control because ammonium represents an equilibrium between NH4+ and NH3, which are two N forms with different membrane permeabilities. There is increasing evidence that AMT-type ammonium transporters represent the major entry pathways for root uptake of NH4+. Since excess uptake of ammonium might cause toxicity and since ammonium is also released from catabolic processes within the cell, ammonium uptake across the root plasma membrane has to be tightly regulated. To take over a function in cellular ammonium homeostasis, various AMT transporters are synthesized that differ in their biochemical properties, their localization, and in their regulation at the transcriptional level. At the same time, AMT-driven transport is subject to control by the nitrogen status of a local root portion as well as of the whole plant. In this review, the focus is on the different levels at which AMT-dependent ammonium uptake is regulated and the gaps in current knowledge are highlighted.