Tonoplast intrinsic proteins from cauliflower (Brassica oleracea L. var. botrytis): immunological analysis, cDNA cloning and evidence for expression in meristematic tissues

Genome-wide identification and biological relevance of broccoli aquaporins

Broccoli (Brassica oleracea var. italica) is an important crop worldwide, and its regular consumption is associated with health benefits due to the presence of various bioactive compounds. An optimal water balance and homeostasis are needed for plant growth; in this sense, aquaporins play a crucial role. As a result of a genome-wide search, a total of 65 aquaporin genes were identified in broccoli. The aquaporins were classified according to their phylogenetic relationships with other Brassicas species and Arabidopsis thaliana, and evolutionary events of gene duplication were also assessed, highlighting the tendency of NIPs (Nodulin-26-like Intrinsic Proteins) to duplicate. Also, the chromosomal localization, gene duplication, the study of the conserved motifs, and the tertiary structure were determined in broccoli. Functional predictive analyses were also carried out, which, together with the expression analyses in different broccoli plant tissues, allowed the prediction of the biological role of each aquaporin isoform. BoiPIP1-2a and BoiPIP1-2b showed higher expression in all the plant tissues when compared with other aquaporins. BoiTIP1-2b also showed high expression levels and was associated with nitrogen compounds transport such as urea. However, NIPs, through their differential expression and the tandem duplications of the isoforms, were revealed as the putative main actors in the response of broccoli plants to abiotic stress responses. The results of this work pointed to the physiological significance of each aquaporin isoform of broccoli, opening a new field of knowledge and constituting the first step of further in vivo analyses.

Significance of solute specificity, expression, and gating mechanism of tonoplast intrinsic protein during development and stress response in plants

Tonoplast intrinsic proteins (TIPs), belonging to the aquaporin family, are transmembrane channels located mostly at the tonoplast of plant cells. The TIPs are known to transport water and many other small solutes such as ammonia, urea, hydrogen peroxide, and glycerol. In the present review, phylogenetic distribution, structure, transport dynamics, gating mechanism, sub-cellular localization, tissue-specific expression, and co-expression of TIPs are discussed to define their versatile role in plants. Based on the phylogenetic distribution, TIPs are classified into five distinct groups with aromatic-arginine (Ar/R) selectivity filters, typical pore-morphology, and tissue-specific gene expression patterns. The tissue-specific expression of TIPs is conserved among diverse plant species, more particularly for TIP3s, which are expressed exclusively in seeds. Studying TIP3 evolution will help to understand seed development and germination. The solute specificity of TIPs plays an imperative role in physiological processes like stomatal movement and vacuolar sequestration as well as in alleviating environmental stress. TIPs also play an important role in growth and developmental processes like radicle protrusion, anther dehiscence, seed germination, cell elongation, and expansion. The gating mechanism of TIPs regulates the solute flow in response to external signals, which helps to maintain the physiological functions of the cell. The information provided in this review is a base to explore TIP's potential in crop improvement programs.

Plasma membrane vesicles from cauliflower meristematic tissue and their role in water passage

BACKGROUND

Cauliflower (Brassica oleracea L. var. botrytis) inflorescences are composed mainly of meristematic tissue, which has a high cellular proliferation. This considerable cellular density makes the inflorescence an organ with a large proportion of membranes. However, little is known about the specific role of the lipid and protein composition of the plasma membrane present in this organ.

RESULTS

In this work, we analyzed the lipids and proteins present in plasma membrane from two different stages of development of cauliflower inflorescence and compared them with leaf plasma membrane. For this purpose, plasma membrane vesicles were obtained by centrifugation for each sample and the vesicular diameter and osmotic permeability (Pf) were analyzed by dynamic light scattering and the stopped-flow technique, respectively. In addition, fatty acids and sterols were analyzed by gas chromatography and HPLC. The protein composition of the inflorescences and leaves was characterized by HPLC-ESI-QTOF-MS and the data obtained were compared with Brassicaceae proteins present in the UniProt database in relation to the presence of aquaporins determined by western blot analysis. The highest Pf value was found in 90 day inflorescences-derived plasma membrane vesicles (61.4 ± 4.14 μms- 1). For sterols and fatty acids, the concentrations varied according to the organ of origin. The protein profile revealed the presence of aquaporins from the PIP1 and PIP2 subfamilies in both inflorescences and leaves.

CONCLUSION

This study shows that the composition of the sterols, the degree of unsaturation of the fatty acids, and the proteins present in the membranes analyzed give them high functionality for water passage. This represents an important addition to the limited information available in this field.

Developmental pattern of aquaporin expression in barley (Hordeum vulgare L.) leaves

Aquaporins are multifunctional membrane channels which belong to the family of major intrinsic proteins (MIPs) and are best known for their ability to facilitate the movement of water. In the present study, earlier results from microarray experiments were followed up. These experiments had suggested that, in barley (Hordeum vulgare L.), aquaporin family members are expressed in distinct patterns during leaf development. Real-time PCR and in situ hybridization were used to analyse the level and tissue-distribution of expression of candidate aquaporins, focusing on plasma membrane and tonoplast intrinsic proteins (PIPs, TIPs). Water channel function of seven aquaporins, whose transcripts were the most abundant and the most variable, was tested through expression in yeast and, in part, through expression in oocytes. All PIP1 and PIP2 subfamily members changed in expression during leaf development, with expression being much higher or lower in growing compared with mature tissue. The same applied to those TIPs which were expressed at detectable levels. Specific roles during leaf development are proposed for particular aquaporins.

Plant aquaporins: membrane channels with multiple integrated functions

Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide). Recent progress was made in understanding the molecular bases of aquaporin transport selectivity and gating. The present review examines how a wide range of selectivity profiles and regulation properties allows aquaporins to be integrated in numerous functions, throughout plant development, and during adaptations to variable living conditions. Although they play a central role in water relations of roots, leaves, seeds, and flowers, aquaporins have also been linked to plant mineral nutrition and carbon and nitrogen fixation.

Newly formed vacuoles in root meristems of barley and pea seedlings have characteristics of both protein storage and lytic vacuoles

Plant cells are considered to possess functionally different types of vacuoles in the same cell. One of the papers cited in support of this concept reported that protein storage and lytic vacuoles in root tips of barley (Hordeum vulgare) and pea (Pisum sativum) seedlings were initially separate compartments that later fused to form a central vacuole during cell elongation. We have reinvestigated the situation in these two roots using immunogold electron microscopy as well as immunofluorescence microscopy of histological sections. Using antisera generated against the whole protein of alpha-tonoplast intrinsic protein (TIP) as well as specific C-terminal TIP peptide antisera against alpha-, gamma-, and delta-TIP, together with antisera against the storage proteins barley lectin and pea legumin and vicilin, we were unable to obtain evidence for separate vacuole populations. Instead, our observations point to the formation of a single type of vacuole in cells differentiating both proximally and distally from the root meristem. This is a hybrid-type vacuole containing storage proteins and having both alpha- and gamma-TIPs, but not delta-TIP, in its tonoplast. As cells differentiate toward the zone of elongation, their vacuoles are characterized by increasing amounts of gamma-TIP and decreasing amounts of alpha-TIP.

Novel tonoplast transporters identified using a proteomic approach with vacuoles isolated from cauliflower buds

Young meristematic plant cells contain a large number of small vacuoles, while the largest part of the vacuome in mature cells is composed by a large central vacuole, occupying 80% to 90% of the cell volume. Thus far, only a limited number of vacuolar membrane proteins have been identified and characterized. The proteomic approach is a powerful tool to identify new vacuolar membrane proteins. To analyze vacuoles from growing tissues we isolated vacuoles from cauliflower (Brassica oleracea) buds, which are constituted by a large amount of small cells but also contain cells in expansion as well as fully expanded cells. Here we show that using purified cauliflower vacuoles and different extraction procedures such as saline, NaOH, acetone, and chloroform/methanol and analyzing the data against the Arabidopsis (Arabidopsis thaliana) database 102 cauliflower integral proteins and 214 peripheral proteins could be identified. The vacuolar pyrophosphatase was the most prominent protein. From the 102 identified proteins 45 proteins were already described. Nine of these, corresponding to 46% of peptides detected, are known vacuolar proteins. We identified 57 proteins (55.9%) containing at least one membrane spanning domain with unknown subcellular localization. A comparison of the newly identified proteins with expression profiles from in silico data revealed that most of them are highly expressed in young, developing tissues. To verify whether the newly identified proteins were indeed localized in the vacuole we constructed and expressed green fluorescence protein fusion proteins for five putative vacuolar membrane proteins exhibiting three to 11 transmembrane domains. Four of them, a putative organic cation transporter, a nodulin N21 family protein, a membrane protein of unknown function, and a senescence related membrane protein were localized in the vacuolar membrane, while a white-brown ATP-binding cassette transporter homolog was shown to reside in the plasma membrane. These results demonstrate that proteomic analysis of highly purified vacuoles from specific tissues allows the identification of new vacuolar proteins and provides an additional view of tonoplastic proteins.

Major intrinsic proteins (MIPs) in plants: a complex gene family with major impacts on plant phenotype

The ubiquitous cell membrane proteins called aquaporins are now firmly established as channel proteins that control the specific transport of water molecules across cell membranes in all living organisms. The aquaporins are thus likely to be of fundamental significance to all facets of plant growth and development affected by plant-water relations. A majority of plant aquaporins have been found to share essential structural features with the human aquaporin and exhibit water-transporting ability in various functional assays, and some have been shown experimentally to be of critical importance to plant survival. Furthermore, substantial evidence is now available from a number of plant species that shows differential gene expression of aquaporins in response to abiotic stresses such as salinity, drought, or cold and clearly establishes the aquaporins as major players in the response of plants to conditions that affect water availability. This review summarizes the function and regulation of these genes to develop a greater understanding of the response of plants to water insufficiency, and particularly, to identify tolerant genotypes of major crop species including wheat and rice and plants that are important in agroforestry.

Expression and functional analysis of the rice plasma-membrane intrinsic protein gene family

Plasma membrane intrinsic proteins (PIPs) are a subfamily of aquaporins that enable fast and controlled translocation of water across the membrane. In this study, we systematically identified and cloned ten PIP genes from rice. Based on the similarity of the amino acid sequences they encoded, these rice PIP genes were classified into two groups and designated as OsPIP1-1 to OsPIP1-3 and OsPIP2-1 to OsPIP2-7 following the nomenclature of PIP genes in maize. Quantitative RT-PCR analysis identified three root-specific and one leaf-specific OsPIP genes. Furthermore, the expression profile of each OsPIP gene in response to salt, drought and ABA treatment was examined in detail. Analysis on transgenic plants over-expressing of either OsPIP1 (OsPIP1-1) or OsPIP2 (OsPIP2-2) in wild-type Arabidopsis, showed enhanced tolerance to salt (100 mM of NaCl) and drought (200 mM of mannitol), but not to salt treatment of higher concentration (150 mM of NaCl). Taken together, these data suggest a distinct role of each OsPIP gene in response to different stresses, and should add a new layer to the understanding of the physiological function of rice PIP genes.

New insights into the tonoplast architecture of plant vacuoles and vacuolar dynamics during osmotic stress

BACKGROUND

The vegetative plant vacuole occupies >90% of the volume in mature plant cells. Vacuoles play fundamental roles in adjusting cellular homeostasis and allowing cell growth. The composition of the vacuole and the regulation of its volume depend on the coordinated activities of the transporters and channels localized in the membrane (named tonoplast) surrounding the vacuole. While the tonoplast protein complexes are well studied, the tonoplast itself is less well described. To extend our knowledge of how the vacuole folds inside the plant cell, we present three-dimensional reconstructions of vacuoles from tobacco suspension cells expressing the tonoplast aquaporin fusion gene BobTIP26-1::gfp.

RESULTS

3-D reconstruction of the cell vacuole made possible an accurate analysis of large spanning folds of the vacuolar membrane under both normal and stressed conditions, and suggested interactions between surrounding plastids. Dynamic, high resolution 3-D pictures of the vacuole in tobacco suspension cells monitored under different growth conditions provide additional details about vacuolar architecture. The GFP-decorated vacuole is a single continuous compartment transected by tubular-like transvacuolar strands and large membrane surfaces. Cell culture under osmotic stress led to a complex vacuolar network with an increased tonoplast surface area. In-depth 3-D realistic inspections showed that the unity of the vacuole is maintained during acclimation to osmotic stress. Vacuolar unity exhibited during stress adaptation, coupled with the intimate associations of vacuoles with other organelles, suggests a physiological role for the vacuole in metabolism, and communication between the vacuole and organelles, respectively, in plant cells. Desiccation stress ensuing from PEG treatment generates "double" membrane structures closely linked to the tonoplast within the vacuole. These membrane structures may serve as membrane reservoirs for membrane reversion when cells are reintroduced to normal growth conditions.

CONCLUSION

3-D processing of a GFP-labeled tonoplast provides compelling visual constructions of the plant cell vacuole and elaborates on the nature of tonoplast folding and architecture. Furthermore, these methods allow real-time determination of membrane rearrangements during stresses.

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.

Molecular physiology of aquaporins in plants

In plants, membrane channels of the major intrinsic protein (MIP) super-family exhibit a high diversity with, for instance, 35 homologues in the model species Arabidopsis thaliana. As has been found in other organisms, plant MIPs function as membrane channels permeable to water (aquaporins) and in some cases to small nonelectrolytes. The aim of the present article is to integrate into plant physiology what has been recently learned about the molecular and functional properties of aquaporins in plants. Exhaustive compilation of data in the literature shows that the numerous aquaporin isoforms of plants have specific expression patterns throughout plant development and in response to environmental stimuli. The diversity of aquaporin homologues in plants can also be explained in part by their presence in multiple subcellular compartments. In recent years, there have been numerous reports that describe the activity of water channels in purified membrane vesicles, in isolated organelles or protoplasts, and in intact plant cells or even tissues. Altogether, these data suggest that the transport of water and solutes across plant membranes concerns many facets of plant physiology. Because of the high degree of compartmentation of plant cells, aquaporins may play a critical role in cell osmoregulation. Water uptake in roots represents a typical process in which to investigate the role of aquaporins in transcellular water transport, and the mechanisms and regulations involved are discussed.

The tobacco plasma membrane aquaporin NtAQP1

This paper gives a summary of a project to characterize a tobacco aquaporin. The cDNA and gene, including the 5' upstream region, for the tobacco aquaporin NtAQP1 has been isolated and the encoded protein characterized. The significance of promoter regions for an abscisic acid- and gibberellic acid-induced gene expression could be restricted to a region between -1450 and -1112 upstream of the transcription start point by transient transformation of a bicistronic vector into tobacco protoplasts. NtAQP1 expression in tobacco plants was found to be elevated in flowers, stems and roots. In roots, the protein was detected close to xylem vessels in pitch-like structures. Studies with a NtAQP1-GFP fusion indicated a plasma membrane location. For a functional analysis, the cDNA was expressed in Xenopus oocytes. NtAQP1 was found to be a heavy metal-insensitive aquaporin with additional permeability for glycerol. Mutation of a threonine at position 233 to a cysteine transformed NtAQP1 into a heavy metal-sensitive aquaporin.

Expression of water channel proteins in Mesembryanthemum crystallinum

We have characterized transcripts for nine major intrinsic proteins (MIPs), some of which function as water channels (aquaporins), from the ice plant Mesembryanthemum crystallinum. To determine the cellular distribution and expression of these MIPs, oligopeptide-based antibodies were generated against MIP-A, MIP-B, MIP-C, or MIP-F, which, according to sequence and functional characteristics, are located in the plasma membrane (PM) and tonoplast, respectively. MIPs were most abundant in cells involved in bulk water flow and solute flux. The tonoplast MIP-F was found in all cells, while signature cell types identified different PM-MIPs: MIP-A predominantly in phloem-associated cells, MIP-B in xylem parenchyma, and MIP-C in the epidermis and endodermis of immature roots. Membrane protein analysis confirmed MIP-F as tonoplast located. MIP-A and MIP-B were found in tonoplast fractions and also in fractions distinct from either the tonoplast or PM. MIP-C was most abundant but not exclusive to PM fractions, where it is expected based on its sequence signature. We suggest that within the cell, MIPs are mobile, which is similar to aquaporins cycling through animal endosomes. MIP cycling and the differential regulation of these proteins observed under conditions of salt stress may be fundamental for the control of tissue water flux.

Recent advances on proteins of plant terminal membranes

Since the beginning of the 1990s, our knowledge of the protein equipment of plant membranes progresses at an accelerating pace, owing to the irruption of molecular biology tools and genetics strategies in plant biology. Map-based cloning strategies and exploration of EST databases rapidly enrich the catalog of cDNA or gene sequences expected to code for membrane proteins. The accumulation of 'putative' membrane proteins reinforces the need for structural, functional and physiological information. Indeed, ambiguities often exist concerning the association to a membrane, the membrane identity and the topology of the protein inserted in the membrane. The combination of directed mutagenesis and heterologous expression of plant genes in various systems and plant reverse genetics has opened the possibility to study molecular and physiological functions. This review will emphasize how these tools have been essential for the exciting recent discoveries on plant terminal membrane proteins. These discoveries concern a variety of transport systems for ions, organic solutes including auxin, water channels, a large collection of systems suspected to act as receptors of chemical signals, proteins thought to control vesicle trafficking and enzymatic systems.

High expression of the tonoplast aquaporin ZmTIP1 in epidermal and conducting tissues of maize

Aquaporins are integral membrane proteins of the tonoplast and the plasma membrane that facilitate the passage of water through these membranes. Because of their potentially important role in regulating water flow in plants, studies documenting aquaporin gene expression in specialized tissues involved in water and solute transport are important. We used in situ hybridization to examine the expression pattern of the tonoplast aquaporin ZmTIP1 in different organs of maize (Zea mays L.). This tonoplast water channel is highly expressed in the root epidermis, the root endodermis, the small parenchyma cells surrounding mature xylem vessels in the root and the stem, phloem companion cells and a ring of cells around the phloem strand in the stem and the leaf sheath, and the basal endosperm transfer cells in developing kernels. We postulate that the high level of expression of ZmTIP1 in these tissues facilitates rapid flow of water through the tonoplast to permit osmotic equilibration between the cytosol and the vacuolar content, and to permit rapid transcellular water flow through living cells when required.

Characterization of a maize tonoplast aquaporin expressed in zones of cell division and elongation

We studied aquaporins in maize (Zea mays), an important crop in which numerous studies on plant water relations have been carried out. A maize cDNA, ZmTIP1, was isolated by reverse transcription-coupled PCR using conserved motifs from plant aquaporins. The derived amino acid sequence of ZmTIP1 shows 76% sequence identity with the tonoplast aquaporin gamma-TIP (tonoplast intrinsic protein) from Arabidopsis. Expression of ZmTIP1 in Xenopus laevis oocytes showed that it increased the osmotic water permeability of oocytes 5-fold; this water transport was inhibited by mercuric chloride. A cross-reacting antiserum made against bean alpha-TIP was used for immunocytochemical localization of ZmTIP1. These results indicate that this and/or other aquaporins is abundantly present in the small vacuoles of meristematic cells. Northern analysis demonstrated that ZmTIP1 is expressed in all plant organs. In situ hybridization showed a high ZmTIP1 expression in meristems and zones of cell enlargement: tips of primary and lateral roots, leaf primordia, and male and female inflorescence meristems. The high ZmTIP1 expression in meristems and expanding cells suggests that ZmTIP1 is needed (a) for vacuole biogenesis and (b) to support the rapid influx of water into vacuoles during cell expansion.