Specific plasma membrane aquaporins of the PIP1 subfamily are expressed in sieve elements and guard cells

Stationary sieve element proteins

Vascular plants use the phloem to move sugars and other molecules from source leaves to sink organs such as roots and fruits. Within the phloem, enucleate sieve elements provide the low-resistance pipe system that enable bulk flow of sap. In this review, we provide an overview of the highly specific protein machinery that localize to mature sieve elements without entering the phloem translocation stream. Generally, the proteins either maintain the flow, protect the sieve element against pathogens or transmit system wide signals. A notable exception is found in poppy, where part of the opium biosynthesis is compartmentalized in sieve elements. Biosynthesis of sieve element proteins happens either continuously in companion cell or transiently in immature sieve elements before nuclear disintegration. The latter population is translated during differentiation and stays functional without turnover during the entire lifespan of sieve elements. We discuss how protein longevity imposes some interesting restrictions on plants, especially in arborescent monocots with long living sieve elements.

The contribution of PIP2-type aquaporins to photosynthetic response to increased vapour pressure deficit

The roles of different plasma membrane aquaporins (PIPs) in leaf-level gas exchange of Arabidopsis thaliana were examined using knockout mutants. Since multiple Arabidopsis PIPs are implicated in CO2 transport across cell membranes, we focused on identifying the effects of the knockout mutations on photosynthesis, and whether they are mediated through the control of stomatal conductance of water vapour (gs), mesophyll conductance of CO2 (gm), or both. We grew Arabidopsis plants in low and high humidity environments and found that the contribution of PIPs to gs was larger under low air humidity when the evaporative demand was high, whereas any effect of a lack of PIP function was minimal under higher humidity. The pip2;4 knockout mutant had 44% higher gs than wild-type plants under low humidity, which in turn resulted in an increased net photosynthetic rate (Anet). We also observed a 23% increase in whole-plant transpiration (E) for this knockout mutant. The lack of functional plasma membrane aquaporin AtPIP2;5 did not affect gs or E, but resulted in homeostasis of gm despite changes in humidity, indicating a possible role in regulating CO2 membrane permeability. CO2 transport measurements in yeast expressing AtPIP2;5 confirmed that this aquaporin is indeed permeable to CO2.

Functional Characterization of Date Palm Aquaporin Gene PdPIP1;2 Confers Drought and Salinity Tolerance to Yeast and Arabidopsis

Recent studies on salinity tolerance in date palm revealed the discovery of salt-responsive genes including PdPIP1;2, a highly conserved aquaporin gene in plants, which was functionally characterized in this study to investigate its precise role in drought and salinity tolerance. Immunoblot assay showed a high level of PIP1 protein accumulation only in the leaves of date palm plants when grown under drought, an observation which may imply the involvement of PIP1;2 in CO₂ uptake. Heterologous overexpression of PdPIP1;2 in yeast (Saccharomyces cerevisiae) improved tolerance to salinity and oxidative stress. While, heterologous overexpression of PdPIP1;2 in Arabidopsis had significantly (p < 0.05) increased biomass, chlorophyll content, and root length under drought and salinity. In addition, a significantly (p < 0.05) higher percentage of transgenic plants could be recovered by rewatering after drought stress, indicating the ability of the transgenic plants to maintain water and viability under drought. Transgenic plants under drought and salinity maintained significantly (p < 0.05) higher K⁺/Na⁺ ratios than wild type (WT) plants, an observation which may represent an efficient tolerance mechanism controlled by the transgene. Collectively, this study provided an insight on the mechanism by which PdPIP1;2 conferred tolerance to salt and drought stresses in date palm.

Molecular cloning of a plasma membrane aquaporin in Stipa purpurea, and exploration of its role in drought stress tolerance

Stipa purpurea is widely distributed on the Tibetan Plateau, and has high drought resistance. Plasma membrane intrinsic proteins are a type of aquaporin. They regulate the movement of water and are associated with plant protective reactions to biotic and abiotic stresses. We characterized a plasma membrane intrinsic protein from S. purpurea (SpPIP1) and elucidated its role in molecular aspects of the plant's response to drought stress. The full-length open reading frame of SpPIP1 was 870 bp and encoded 289 amino acids. The transcript level of SpPIP1 was higher in the root of S. purpurea than in the flower, leaf and stem. The level of SpPIP1 transcript increased significantly when treated with drought treatment. Subcellular localization result showed that SpPIP1 was localized in the plasma membrane. Ectopic expression of SpPIP1 in Arabidopsis thaliana resulted in plants with higher tolerance to drought treatment. SpPIP1 of S. purpurea may mediate plant response to arid environments.

Plant and animal aquaporins crosstalk: what can be revealed from distinct perspectives

Aquaporins (AQPs) can be revisited from a distinct and complementary perspective: the outcome from analyzing them from both plant and animal studies. (1) The approach in the study. Diversity found in both kingdoms contrasts with the limited number of crystal structures determined within each group. While the structure of almost half of mammal AQPs was resolved, only a few were resolved in plants. Strikingly, the animal structures resolved are mainly derived from the AQP2-lineage, due to their important roles in water homeostasis regulation in humans. The difference could be attributed to the approach: relevance in animal research is emphasized on pathology and in consequence drug screening that can lead to potential inhibitors, enhancers and/or regulators. By contrast, studies on plants have been mainly focused on the physiological role that AQPs play in growth, development and stress tolerance. (2) The transport capacity. Besides the well-described AQPs with high water transport capacity, large amount of evidence confirms that certain plant AQPs can carry a large list of small solutes. So far, animal AQP list is more restricted. In both kingdoms, there is a great amount of evidence on gas transport, although there is still an unsolved controversy around gas translocation as well as the role of the central pore of the tetramer. (3) More roles than expected. We found it remarkable that the view of AQPs as specific channels has evolved first toward simple transporters to molecules that can experience conformational changes triggered by biochemical and/or mechanical signals, turning them also into signaling components and/or behave as osmosensor molecules.

Carbon dioxide and water transport through plant aquaporins

Aquaporins are channel proteins that function to increase the permeability of biological membranes. In plants, aquaporins are encoded by multigene families that have undergone substantial diversification in land plants. The plasma membrane intrinsic proteins (PIPs) subfamily of aquaporins is of particular interest given their potential to improve plant water relations and photosynthesis. Flowering plants have between 7 and 28 PIP genes. Their expression varies with tissue and cell type, through development and in response to a variety of factors, contributing to the dynamic and tissue specific control of permeability. There are a growing number of PIPs shown to act as water channels, but those altering membrane permeability to CO₂ are more limited. The structural basis for selective substrate specificities has not yet been resolved, although a few key amino acid positions have been identified. Several regions important for dimerization, gating and trafficking are also known. PIP aquaporins assemble as tetramers and their properties depend on the monomeric composition. PIPs control water flux into and out of veins and stomatal guard cells and also increase membrane permeability to CO₂ in mesophyll and stomatal guard cells. The latter increases the effectiveness of Rubisco and can potentially influence transpiration efficiency.

Are phloem sieve tubes leaky conduits supported by numerous aquaporins?

PREMISE OF THE STUDY

Aquaporin membrane water channels have been previously identified in the phloem of angiosperms, but currently their cellular characterization is lacking, especially in tree species. Pinpointing the cellular location will help generate new hypotheses of how membrane water exchange facilitates sugar transport in plants.

METHODS

We studied histological sections of balsam poplar (Populus balsamifera L.) in leaf, petiole, and stem organs. Immuno-labeling techniques were used to characterize the distribution of PIP1 and PIP2 subfamilies of aquaporins along the phloem pathway. Confocal and super resolution microscopy (3D-SIM) was used to identify the localization of aquaporins at the cellular level.

KEY RESULTS

Sieve tubes of the leaf lamina, petiole, and stem were labeled with antibodies directed at PIP1s and PIP2s. While PIP2s were mostly observed in the plasma membrane, PIP1s showed both an internal membrane and plasma membrane labeling pattern.

CONCLUSIONS

The specificity and consistency of PIP2 labeling in sieve element plasma membranes points to high water exchange rates between sieve tubes and adjacent cells. The PIP1s may relocate between internal membranes and the plasma membrane to facilitate dynamic changes in membrane permeability of sieve elements in response to changing internal or environmental conditions. Aquaporin-mediated changes in membrane permeability of sieve tubes would also allow for some control of radial exchange of water between xylem and phloem.

Expression of the citrus CsTIP2;1 gene improves tobacco plant growth, antioxidant capacity and physiological adaptation under stress conditions

MAIN CONCLUSION

Overexpression of the citrus CsTIP2;1 improves plant growth and tolerance to salt and drought stresses by enhancing cell expansion, H 2 O 2 detoxification and stomatal conductance. Tonoplast intrinsic proteins (TIPs) are a subfamily of aquaporins, belonging to the major intrinsic protein family. In a previous study, we have shown that a citrus TIP isoform, CsTIP2;1, is highly expressed in leaves and also transcriptionally regulated in leaves and roots by salt and drought stresses and infection by 'Candidatus Liberibacter asiaticus', the causal agent of the Huanglongbing disease, suggesting its involvement in the regulation of the flow of water and nutrients required during both normal growth and stress conditions. Here, we show that the overexpression of CsTIP2;1 in transgenic tobacco increases plant growth under optimal and water- and salt-stress conditions and also significantly improves the leaf water and oxidative status, photosynthetic capacity, transpiration rate and water use efficiency of plants subjected to a progressive soil drying. These results correlated with the enhanced mesophyll cell expansion, midrib aquiferous parenchyma abundance, H2O2 detoxification and stomatal conductance observed in the transgenic plants. Taken together, our results indicate that CsTIP2;1 plays an active role in regulating the water and oxidative status required for plant growth and adaptation to stressful environmental conditions and may be potentially useful for engineering stress tolerance in citrus and other crop plants.

Genome-wide expression profiling of aquaporin genes confer responses to abiotic and biotic stresses in Brassica rapa

BACKGROUND

Plants contain a range of aquaporin (AQP) proteins, which act as transporter of water and nutrient molecules through living membranes. AQPs also participate in water uptake through the roots and contribute to water homeostasis in leaves.

RESULTS

In this study, we identified 59 AQP genes in the B. rapa database and Br135K microarray dataset. Phylogenetic analysis revealed four distinct subfamilies of AQP genes: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), NOD26-like intrinsic proteins (NIPs) and small basic intrinsic proteins (SIPs). Microarray analysis showed that the majority of PIP subfamily genes had differential transcript abundance between two B. rapa inbred lines Chiifu and Kenshin that differ in their susceptibility to cold. In addition, all BrPIP genes showed organ-specific expression. Out of 22 genes, 12, 7 and 17 were up-regulated in response to cold, drought and salt stresses, respectively. In addition, 18 BrPIP genes were up-regulated under ABA treatment and 4 BrPIP genes were up-regulated upon F. oxysporum f. sp. conglutinans infection. Moreover, all BrPIP genes showed down-regulation under waterlogging stress, reflecting likely the inactivation of AQPs controlling symplastic water movement.

CONCLUSIONS

This study provides a comprehensive analysis of AQPs in B. rapa and details the expression of 22 members of the BrPIP subfamily. These results provide insight into stress-related biological functions of each PIP gene of the AQP family, which will promote B. rapa breeding programs.

Aquaporins and plant transpiration

Although transpiration and aquaporins have long been identified as two key components influencing plant water status, it is only recently that their relations have been investigated in detail. The present review first examines the various facets of aquaporin function in stomatal guard cells and shows that it involves transport of water but also of other molecules such as carbon dioxide and hydrogen peroxide. At the whole plant level, changes in tissue hydraulics mediated by root and shoot aquaporins can indirectly impact plant transpiration. Recent studies also point to a feedback effect of transpiration on aquaporin function. These mechanisms may contribute to the difference between isohydric and anisohydric stomatal regulation of leaf water status. The contribution of aquaporins to transpiration control goes far beyond the issue of water transport during stomatal movements and involves emerging cellular and long-distance signalling mechanisms which ultimately act on plant growth.

Cold induced changes in the water balance affect immunocytolocalization pattern of one of the aquaporins in the vascular system in the leaves of maize (Zea mays L.)

Chilling stress is known to affect the water balance in plants, which often manifests itself in the decrease of the water potential in different organs. Relationships between chilling, assimilate transport and water balance are far from being understood. Although aquaporins play a key role in regulating water balance in plants, especially under stress conditions, the role of individual aquaporins in stress response remains unclear. In this report we show the specific localization within plasma membranes of one of the aquaporins (PIP2;3) in the leaves of two maize inbred lines differing in their chilling-sensitivity. This form of aquaporin has been also observed in thick-walled sieve elements - an additional type of sieve tubes of unclear function found only in monocotyledons. Moderate chilling (about 15°C) caused significant reduction of labelling in these cells accompanied by a steep decrease in the water potential in leaves of chilling-sensitive maize line. Our results suggest that both PIP2;3 and thick-walled sieve tubes may be an unknown element of the mechanism of the response of maize to cold stress.

Transcriptome analysis of the Chinese bread wheat cultivar Yunong 201 and its ethyl methanesulfonate mutant line

Roche 454 next-generation sequencing was applied to obtain extensive information about the transcriptomes of the bread wheat cultivar Yunong 201 and its EMS mutant line Yunong 3114. Totals of 1.43 million and 1.44 million raw reads were generated, 14,432, 17,845 and 27,867 isotigs were constructed using the reads in Yunong 201, Yunong 3114 and their combination, respectively. Moreover, 29,042, 34,722, and 48,486 unigenes were generated in Yunong 201, Yunong 3114, and combined cultivars, respectively. A total of 50,382 and 59,891 unigenes from the Yunong 201 and Yunong 3114 were mapped on different chromosomes. Of all unigenes, 1363 DEGs were identified in Yunong 201 and Yunong 3114. qRT-PCR analysis confirmed the expression profiles of 40 candidate unigenes possibly related to abiotic stresses. The expression patterns of four annotated DEGs were also verified in the two wheat cultivars under abiotic stresses. This study provided useful information for further analysis of wheat functional genomics.

PIP1 aquaporins: Intrinsic water channels or PIP2 aquaporin modulators?

The highly conserved plant aquaporins, known as Plasma membrane Intrinsic Proteins (PIPs), are the main gateways for cell membrane water exchange. Years of research have described in detail the properties of the PIP2 subfamily. However, characterizing the PIP1 subfamily has been difficult due to the failure to localize to the plasma membrane. In addition, the discovery of the PIP1-PIP2 interaction suggested that PIP1 aquaporins could be regulated by a complex posttranslational mechanism that involves trafficking, heteromerization and fine-tuning of channel activity. This review not only considers the evidence and findings but also discusses the complexity of PIP aquaporins. To establish a new benchmark in PIP regulation, we propose to consider PIP1-PIP2 pairs as functional units for the purpose of future research into their physiological roles.

RNAi-mediated downregulation of poplar plasma membrane intrinsic proteins (PIPs) changes plasma membrane proteome composition and affects leaf physiology

Plasma membrane intrinsic proteins (PIPs) are one subfamily of aquaporins that mediate the transmembrane transport of water. To reveal their function in poplar, we generated transgenic poplar plants in which the translation of PIP genes was downregulated by RNA interference investigated these plants with a comprehensive leaf plasma membrane proteome and physiome analysis. First, inhibition of PIP synthesis strongly altered the leaf plasma membrane protein composition. Strikingly, several signaling components and transporters involved in the regulation of stomatal movement were differentially regulated in transgenic poplars. Furthermore, hormonal crosstalk related to abscisic acid, auxin and brassinosteroids was altered, in addition to cell wall biosynthesis/cutinization, the organization of cellular structures and membrane trafficking. A physiological analysis confirmed the proteomic results. The leaves had wider opened stomata and higher net CO2 assimilation and transpiration rates as well as greater mesophyll conductance for CO2 (gm) and leaf hydraulic conductance (Kleaf). Based on these results, we conclude that PIP proteins not only play essential roles in whole leaf water and CO2 flux but have important roles in the regulation of stomatal movement.

Diffusion or bulk flow: how plasmodesmata facilitate pre-phloem transport of assimilates

Assimilates synthesized in the mesophyll of mature leaves move along the pre-phloem transport pathway to the bundle sheath of the minor veins from which they are loaded into the phloem. The present review discusses the most probable driving force(s) for the pre-phloem pathway, diffusion down the concentration gradient or bulk flow along a pressure gradient. The driving force seems to depend on the mode of phloem loading. In a majority of plant species phloem loading is a thermodynamically active process, involving the activity of membrane transporters in the sieve-element companion cell complex. Since assimilate movement includes an apoplasmic step, this mode is called apoplasmic loading. Well established is also the polymer-trap loading mode, where the phloem-transport sugars are raffinose-family oligomers in herbaceous plants. Also this mode depends on the investment of energy, here for sugar oligomerization, and leads to a high sugar accumulation in the phloem, even though the phloem is not symplasmically isolated, but well coupled by plasmodesmata (PD). Hence the mode polymer-trap mode is also designated active symplasmic loading. For woody angiosperms and gymnosperms an alternate loading mode is currently matter of discussion, called passive symplasmic loading. Based on the limited material available, this review compares the different loading modes and suggests that diffusion is the driving force in apoplasmic loaders, while bulk flow plays an increasing role in plants having a continuous symplasmic pathway from mesophyll to sieve elements. Crucial for the driving force is the question where water enters the pre-phloem pathway. Surprisingly, the role of PD in water movement has not been addressed so far appropriately. Modeling of assimilate and water fluxes indicates that in symplasmic loaders a considerable part of water flux happens through the PD between bundle sheath and phloem.

The plasma membrane transport systems and adaptation to salinity

Salt stress represents one of the environmental challenges that drastically affect plant growth and yield. Evidence suggests that glycophytes and halophytes have a salt tolerance mechanisms working at the cellular level, and the plasma membrane (PM) is believed to be one facet of the cellular mechanisms. The responses of the PM transport proteins to salinity in contrasting species/cultivars were discussed. The review provides a comprehensive overview of the recent advances describing the crucial roles that the PM transport systems have in plant adaptation to salt. Several lines of evidence were presented to demonstrate the correlation between the PM transport proteins and adaptation of plants to high salinity. How alterations in these transport systems of the PM allow plants to cope with the salt stress was also addressed. Although inconsistencies exist in some of the information related to the responses of the PM transport proteins to salinity in different species/cultivars, their key roles in adaptation of plants to high salinity is obvious and evident, and cannot be precluded. Despite the promising results, detailed investigations at the cellular/molecular level are needed in some issues of the PM transport systems in response to salinity to further evaluate their implication in salt tolerance.

Expression and characterization of plasma membrane aquaporins in stomatal complexes of Zea mays

Stomata, the microscopic pores on the surface of the aerial parts of plants, are bordered by two specialized cells, known as guard cells, which control the stomatal aperture according to endogenous and environmental signals. Like most movements occurring in plants, the opening and closing of stomata are based on hydraulic forces. During opening, the activation of plasma membrane and tonoplast transporters results in solute accumulation in the guard cells. To re-establish the perturbed osmotic equilibrium, water follows the solutes into the cells, leading to their swelling. Numerous studies have contributed to the understanding of the mechanism and regulation of stomatal movements. However, despite the importance of transmembrane water flow during this process, only a few studies have provided evidence for the involvement of water channels, called aquaporins. Here, we microdissected Zea mays stomatal complexes and showed that members of the aquaporin plasma membrane intrinsic protein (PIP) subfamily are expressed in these complexes and that their mRNA expression generally follows a diurnal pattern. The substrate specificity of two of the expressed ZmPIPs, ZmPIP1;5 and ZmPIP1;6, was investigated by heterologous expression in Xenopus oocytes and yeast cells. Our data show that both isoforms facilitate transmembrane water diffusion in the presence of the ZmPIP2;1 isoform. In addition, both display CO2 permeability comparable to that of the CO2 diffusion facilitator NtAQP1. These data indicate that ZmPIPs may have various physiological roles in stomatal complexes.

Exploring Picea glauca aquaporins in the context of needle water uptake and xylem refilling

Conifer needles have been reported to absorb water under certain conditions. Radial water movement across needle tissues is likely influenced by aquaporin (AQP) water channels. Foliar water uptake and AQP localization in Picea glauca needles were studied using physiological and microscopic methods. AQP expression was measured using quantitative real-time PCR. Members of the AQP gene family in spruce were identified using homology search tools. Needles of drought-stressed plants absorbed water when exposed to high relative humidity (RH). AQPs were present in the endodermis-like bundle sheath, in phloem cells and in the transfusion parenchyma of needles. Up-regulation of AQPs in high RH coincided with embolism repair in stem xylem. The present study also provides the most comprehensive functional and phylogenetic analysis of spruce AQPs to date. Thirty putative complete AQP sequences were found. Our findings are consistent with the hypothesis that AQPs facilitate radial water movement from the needle epidermis towards the vascular tissue. Foliar water uptake may occur in late winter when needles are covered by melting snow and may provide a water source for embolism repair before the beginning of the growing season.

Heteromerization of PIP aquaporins affects their intrinsic permeability

The plant aquaporin plasma membrane intrinsic proteins (PIP) subfamily represents one of the main gateways for water exchange at the plasma membrane (PM). A fraction of this subfamily, known as PIP1, does not reach the PM unless they are coexpressed with a PIP2 aquaporin. Although ubiquitous and abundantly expressed, the role and properties of PIP1 aquaporins have therefore remained masked. Here, we unravel how FaPIP1;1, a fruit-specific PIP1 aquaporin from Fragaria x ananassa, contributes to the modulation of membrane water permeability (Pf) and pH aquaporin regulation. Our approach was to combine an experimental and mathematical model design to test its activity without affecting its trafficking dynamics. We demonstrate that FaPIP1;1 has a high water channel activity when coexpressed as well as how PIP1-PIP2 affects gating sensitivity in terms of cytosolic acidification. PIP1-PIP2 random heterotetramerization not only allows FaPIP1;1 to arrive at the PM but also produces an enhancement of FaPIP2;1 activity. In this context, we propose that FaPIP1;1 is a key participant in the regulation of water movement across the membranes of cells expressing both aquaporins.

Water uptake along the length of grapevine fine roots: developmental anatomy, tissue-specific aquaporin expression, and pathways of water transport

To better understand water uptake patterns in root systems of woody perennial crops, we detailed the developmental anatomy and hydraulic physiology along the length of grapevine (Vitis berlandieri × Vitis rupestris) fine roots from the tip to secondary growth zones. Our characterization included the localization of suberized structures and aquaporin gene expression and the determination of hydraulic conductivity (Lpr) and aquaporin protein activity (via chemical inhibition) in different root zones under both osmotic and hydrostatic pressure gradients. Tissue-specific messenger RNA levels of the plasma membrane aquaporin isogenes (VvPIPs) were quantified using laser-capture microdissection and quantitative polymerase chain reaction. Our results highlight dramatic changes in structure and function along the length of grapevine fine roots. Although the root tip lacked suberization altogether, a suberized exodermis and endodermis developed in the maturation zone, which gave way to the secondary growth zone containing a multilayer suberized periderm. Longitudinally, VvPIP isogenes exhibited strong peaks of expression in the root tip that decreased precipitously along the root length in a pattern similar to Arabidopsis (Arabidopsis thaliana) roots. In the radial orientation, expression was always greatest in interior tissues (i.e. stele, endodermis, and/or vascular tissues) for all root zones. High Lpr and aquaporin protein activity were associated with peak VvPIP expression levels in the root tip. This suggests that aquaporins play a limited role in controlling water uptake in secondary growth zones, which contradicts existing theoretical predictions. Despite having significantly lower Lpr, woody roots can constitute the vast majority of the root system surface area in mature vines and thus provide for significant water uptake potential.

Regulation of leaf hydraulics: from molecular to whole plant levels

The water status of plant leaves is dependent on both stomatal regulation and water supply from the vasculature to inner tissues. The present review addresses the multiple physiological and mechanistic facets of the latter process. Inner leaf tissues contribute to at least a third of the whole resistance to water flow within the plant. Physiological studies indicated that leaf hydraulic conductance (K leaf) is highly dependent on the anatomy, development and age of the leaf and can vary rapidly in response to physiological or environmental factors such as leaf hydration, light, temperature, or nutrient supply. Differences in venation pattern provide a basis for variations in K leaf during development and between species. On a short time (hour) scale, the hydraulic resistance of the vessels can be influenced by transpiration-induced cavitations, wall collapses, and changes in xylem sap composition. The extravascular compartment includes all living tissues (xylem parenchyma, bundle sheath, and mesophyll) that transport water from xylem vessels to substomatal chambers. Pharmacological inhibition and reverse genetics studies have shown that this compartment involves water channel proteins called aquaporins (AQPs) that facilitate water transport across cell membranes. In many plant species, AQPs are present in all leaf tissues with a preferential expression in the vascular bundles. The various mechanisms that allow adjustment of K leaf to specific environmental conditions include transcriptional regulation of AQPs and changes in their abundance, trafficking, and intrinsic activity. Finally, the hydraulics of inner leaf tissues can have a strong impact on the dynamic responses of leaf water potential and stomata, and as a consequence on plant carbon economy and leaf expansion growth. The manipulation of these functions could help optimize the entire plant performance and its adaptation to extreme conditions over short and long time scales.

The photosynthetic response of tobacco plants overexpressing ice plant aquaporin McMIPB to a soil water deficit and high vapor pressure deficit

We investigated the photosynthetic capacity and plant growth of tobacco plants overexpressing ice plant (Mesembryanthemum crystallinum L.) aquaporin McMIPB under (1) a well-watered growth condition, (2) a well-watered and temporal higher vapor pressure deficit (VPD) condition, and (3) a soil water deficit growth condition to investigate the effect of McMIPB on photosynthetic responses under moderate soil and atmospheric humidity and water deficit conditions. Transgenic plants showed a significantly higher photosynthesis rate (by 48 %), higher mesophyll conductance (by 52 %), and enhanced growth under the well-watered growth condition than those of control plants. Decreases in the photosynthesis rate and stomatal conductance from ambient to higher VPD were slightly higher in transgenic plants than those in control plants. When plants were grown under the soil water deficit condition, decreases in the photosynthesis rate and stomatal conductance were less significant in transgenic plants than those in control plants. McMIPB is likely to work as a CO2 transporter, as well as control the regulation of stomata to water deficits.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

The relationship between root hydraulics and scion vigour across Vitis rootstocks: what role do root aquaporins play?

Vitis vinifera scions are commonly grafted onto rootstocks of other grape species to influence scion vigour and provide resistance to soil-borne pests and abiotic stress; however, the mechanisms by which rootstocks affect scion physiology remain unknown. This study characterized the hydraulic physiology of Vitis rootstocks that vary in vigour classification by investigating aquaporin (VvPIP) gene expression, fine-root hydraulic conductivity (Lp(r)), % aquaporin contribution to Lp(r), scion transpiration, and the size of root systems. Expression of several VvPIP genes was consistently greater in higher-vigour rootstocks under favourable growing conditions in a variety of media and in root tips compared to mature fine roots. Similar to VvPIP expression patterns, fine-root Lp(r) and % aquaporin contribution to Lp(r) determined under both osmotic (Lp(r)(Osm)) and hydrostatic (Lp(r)(Hyd)) pressure gradients were consistently greater in high-vigour rootstocks. Interestingly, the % aquaporin contribution was nearly identical for Lp(r)(Osm) and Lp(r)(Hyd) even though a hydrostatic gradient would induce a predominant flow across the apoplastic pathway. In common scion greenhouse experiments, leaf area-specific transpiration (E) and total leaf area increased with rootstock vigour and were positively correlated with fine-root Lp(r). These results suggest that increased canopy water demands for scion grafted onto high-vigour rootstocks are matched by adjustments in root-system hydraulic conductivity through the combination of fine-root Lp(r) and increased root surface area.

Plasma membrane localization of the type I H(+)-PPase AVP1 in sieve element-companion cell complexes from Arabidopsis thaliana

Previous literature has shown the presence of a plasma membrane (PM) localized type I H(+)-PPase in sieve elements of Ricinus communis. Unfortunately, the physiological relevance of these findings remains obscure due to the lack of genetic and molecular reagents to study R. communis. The availability of H(+)-PPase gain and loss-of-function mutants in Arabidopsis thaliana makes this plant an attractive genetic model to address the question, but data on the PM localization of this H(+)-PPase in A. thaliana are limited to two proteomic approaches. Here we present the first report on the localization of the type I H(+)-PPase AVP1 in sieve element-companion cell complexes (SE-CCc) from A. thaliana. Double epifluorescence and immunogold labeling experiments are consistent with the co-localization of AVP1 and PIP1 (a bona fide PM maker) in PM of SE-CCc from A. thaliana.

Structure and stability of the spinach aquaporin SoPIP2;1 in detergent micelles and lipid membranes

Background

SoPIP2;1 constitutes one of the major integral proteins in spinach leaf plasma membranes and belongs to the aquaporin family. SoPIP2;1 is a highly permeable and selective water channel that has been successfully overexpressed and purified with high yields. In order to optimize reconstitution of the purified protein into biomimetic systems, we have here for the first time characterized the structural stability of SoPIP2;1.

Methodology/Principal Finding

We have characterized the protein structural stability after purification and after reconstitution into detergent micelles and proteoliposomes using circular dichroism and fluorescence spectroscopy techniques. The structure of SoPIP2;1 was analyzed either with the protein solubilized with octyl-β-D-glucopyranoside (OG) or reconstituted into lipid membranes formed by E. coli lipids, diphytanoylphosphatidylcholine (DPhPC), or reconstituted into lipid membranes formed from mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE), 1-palmitoyl-2oleoyl-phosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-phosphatidylserine (POPS), and ergosterol. Generally, SoPIP2;1 secondary structure was found to be predominantly α-helical in accordance with crystallographic data. The protein has a high thermal structural stability in detergent solutions, with an irreversible thermal unfolding occurring at a melting temperature of 58°C. Incorporation of the protein into lipid membranes increases the structural stability as evidenced by an increased melting temperature of up to 70°C.

Conclusion/Significance

The results of this study provide insights into SoPIP2;1 stability in various host membranes and suggest suitable choices of detergent and lipid composition for reconstitution of SoPIP2;1 into biomimetic membranes for biotechnological applications.

Aquaporins: a family of highly regulated multifunctional channels

Aquaporins (AQPs) were discovered as channels facilitatingwater movement across cellular membranes. Whereas much of the research has focused on characterizing AQPs with respect to cell water homeostasis, recent discoveries in terms of the transport selectivity of AQP homologs has shed new light on their physiological roles. In fact, whereas some AQPs behave as "strict" water channels, others can conduct a wide range ofnonpolar solutes, such as urea or glycerol and even more unconventional permeants, such as the nonpolar gases carbon dioxide and nitric oxide, the polar gas ammonia, the reactive oxygen species hydrogen peroxide and the metalloids antimonite, arsenite, boron and silicon. This suggests that AQPs are also key players in various physiological processes not related to water homeostasis. The function, regulation and biological importance of AQPs in the different kingdoms is reviewed in this chapter, with special emphasis on animal and plant AQPs.

Münch, morphology, microfluidics - our structural problem with the phloem

The sieve tubes of the phloem are enigmatic structures. Their role as channels for the distribution of assimilates was established in the 19th century, but their sensitivity to disturbations has hampered the elucidation of their transport mechanisms and its regulation ever since. Ernst Münch's classical monograph of 1930 is generally regarded as the first coherent theory of phloem transport, but the 'Münchian' pressure flow mechanism had been discussed already before the turn of the century. Münch's impact rather rested on his simple physical models of the phloem that visualized pressure flow in an intuitive way, and we argue that the downscaling of such models to realistic, low-Reynolds-number sizes will boost our understanding of phloem transport in this century just as Münch's models did in the previous one. However, biologically meaningful physical models that could be used to test predictions of the many existing mathematical models would have to be designed in analogy with natural phloem structures. Unfortunately, the study of phloem anatomy seems in decline, and we still lack basic quantitative data required for evaluating the plausibility of our theoretical deductions. In this review, we provide a subjective overview of unresolved problems in angiosperm phloem structure research within a functional context.

Transcriptional regulation of aquaporins in accessions of Arabidopsis in response to drought stress

Aquaporins facilitate water transport over cellular membranes, and are therefore believed to play an important role in water homeostasis. In higher plants aquaporin-like proteins, also called major intrinsic proteins (MIPs), are divided into five subfamilies. We have previously shown that MIP transcription in Arabidopsis thaliana is generally downregulated in leaves upon drought stress, apart from two members of the plasma membrane intrinsic protein (PIP) subfamily, AtPIP1;4 and AtPIP2;5, which are upregulated. In order to assess whether this regulation is general or accession-specific we monitored the gene expression of all PIPs in five Arabidopsis accessions. The overall drought regulation of PIPs was well conserved for all five accessions tested, suggesting a general and fundamental physiological role of this drought response. In addition, significant differences among accessions were identified for transcripts of three PIP genes. Principal component analysis showed that most of the PIP transcriptional variation during drought stress could be explained by one variable linked to leaf water content. Promoter-GUS constructs of AtPIP1;4, AtPIP2;5 and also AtPIP2;6, which is unresponsive to drought stress, had distinct expression patterns concentrated in the base of the leaf petioles and parts of the flowers. The presence of drought stress response elements within the 1.6-kb promoter regions of AtPIP1;4 and AtPIP2;5 was demonstrated by comparing transcription of the promoter reporter construct and the endogenous gene upon drought stress. Analysis by ATTED-II and other web-based bioinformatical tools showed that several of the MIPs downregulated upon drought are strongly co-expressed, whereas AtPIP1;4, AtPIP2;5 and AtPIP2;6 are not co-expressed.

Electron crystallography and aquaporins

Electron crystallography of two-dimensional (2D) crystals can provide information on the structure of membrane proteins at near-atomic resolution. Originally developed and used to determine the structure of bacteriorhodopsin (bR), electron crystallography has recently been applied to elucidate the structure of aquaporins (AQPs), a family of membrane proteins that form pores mostly for water but also other solutes. While electron crystallography has made major contributions to our understanding of the structure and function of AQPs, structural studies on AQPs, in turn, have fostered a number of technical developments in electron crystallography. In this contribution, we summarize the insights electron crystallography has provided into the biology of AQPs, and describe technical advancements in electron crystallography that were driven by structural studies on AQP 2D crystals. In addition, we discuss some of the lessons that were learned from electron crystallographic work on AQPs.

Abscisic acid regulates root hydraulic conductance via aquaporin expression modulation in Nicotiana tabacum

Abscisic acid (ABA) modifies the hydraulic properties of roots by increasing root water flux. The effects of ABA on aquaporin content and root hydraulic conductance are controversial. We addressed these effects via a combination of experiments. Tobacco (Nicotiana tabacum) plants were grown hydroponically, and ABA (1 microM) was exogenously applied to the roots. Then, the water transport properties of tobacco roots and expression of PIP-type aquaporins were examined. ABA increased the sap flow rate (Jv) and also the osmotic root hydraulic conductance (Lp(r-o)) of excised tobacco roots after 24h. The expression of three aquaporin PIP-type genes and PIP1s proteins abundance in tobacco roots were analyzed by real-time PCR and protein gel blot analysis, respectively. Interestingly, the accumulation of NtAQP1, NtPIP1;1 and NtPIP2;1 transcripts and NtPIP1;1 and NtAQP1 proteins abundance was significantly increased. Although the antibody used recognize NtPIP1;1 and NtAQP1, most probably it also recognizes other PIP1 proteins present in tobacco. Thus, the increase in the expression of the three PIP-type genes and other PIP1s proteins abundance caused by ABA were correlated with an increase in Lp(r-o) and Jv. ABA therefore facilitated the cell-to-cell component of water transport across the root cylinder. The subcellular localization of NtPIP1;1- and NtPIP2;1-GFP was investigated by protoplast transformation with chimeric gene, showing NtPIP2;1 localization in plasma membrane and NtPIP1;1 retention in the endoplasmic reticulum (ER). However, ABA did not change subcellular localization of NtPIP1;1 from ER to plasma membrane.

Role of aquaporins in leaf physiology

Playing a key role in plant growth and development, leaves need to be continuously supplied with water and carbon dioxide to fulfil their photosynthetic function. On its way through the leaf from the xylem to the stomata, water can either move through cell walls or pass from cell to cell to cross the different tissues. Although both pathways are probably used to some degree, evidence is accumulating that living cells contribute substantially to the overall leaf hydraulic conductance (K(leaf)). Transcellular water flow is facilitated and regulated by water channels in the membranes, named aquaporins (AQPs). This review addresses how AQP expression and activity effectively regulate the leaf water balance in normal conditions and modify the cell membrane water permeability in response to different environmental factors, such as irradiance, temperature, and water supply. The role of AQPs in leaf growth and movement, and in CO(2) transport is also discussed.

The expression pattern of plasma membrane aquaporins in maize leaf highlights their role in hydraulic regulation

Leaves are key organs for evaporation and photosynthesis and play a crucial role in plant growth and development. In order to function properly, they need to maintain a balanced water content. Water movement through a leaf occurs by a combination of different pathways: water can follow an apoplastic route through the cell wall or a cell-to-cell route via the symplastic and transcellular paths. As aquaporins (AQPs) play an important role in regulating transcellular water flow and CO(2) conductance, studies on AQP mRNA and protein expression in leaves are essential to better understand their role in these physiological processes. Here, we quantified and localized the expression of Zea mays plasma membrane aquaporins (ZmPIPs, plasma membrane intrinsic proteins) in the leaf using quantitative RT-PCR and immunodetection. All ZmPIP genes except ZmPIP2;7 were expressed in leaves. Expression was found to be dependent on the developmental stage of the leaf tissue, with, in general, an increase in expression at the end of the elongation zone and a decrease in mature leaf tissue. These data correlated with the cell water permeability, as determined using a protoplast swelling assay. The diurnal expression of ZmPIPs was also investigated and expression was found to be higher during the first hours of the light period than at night. Immunocytochemical localization of four ZmPIP isoforms indicated that they are involved in leaf radial water movement, in particular in vascular bundles and the mesophyll.

Deactivation of aquaporins decreases internal conductance to CO2 diffusion in tobacco leaves grown under long-term drought

We compared the diffusion conductance to CO₂ from the intercellular air space to the chloroplasts (internal conductance (g _i)) between tobacco leaves acclimated to long-term drought (drought-acclimated (DA)) and those grown under sufficient irrigation (well-watered (WW)), and analysed the changes in g _i in relation to the leaf anatomical characteristics and a possible CO₂ transporter, aquaporin. The g _i, which was estimated by combined analyses of CO₂ gas exchange with chlorophyll fluorescence, in the DA plants was approximately half of that in the WW plants. The mesophyll and chloroplast surface areas exposing the intercellular air space, which potentially affect g _i, were not significantly different between the WW and DA plants. The amounts of plasma membrane aquaporins (PIP), immunochemically determined using radish PIP antibodies, were unrelated to g _i. After treatment with HgCl₂, an aquaporin inhibitor, the water permeability of the leaf tissues (measured as the weight loss of fully-turgid leaf disks without the abaxial epidermis in 1 m sorbitol) in WW plants decreased with an increase in HgCl₂ concentration. The g _i in the WW plants decreased to similar levels to the DA plants when the detached leaflets were fed with 0.5 mm HgCl₂. In contrast, both water permeability and g _i were insensitive to HgCl₂ treatments in DA plants. These results suggest that deactivation of aquaporins is responsible for the significant reduction in g _i observed in plants growing under long-term drought.

Identification of grapevine aquaporins and expression analysis in developing berries

Aquaporins are membrane water channels that play critical roles in controlling the water content of cells and tissues. In this work, nine full-length cDNAs encoding putative aquaporins were isolated from grape berry cDNA libraries. A phylogenetic analysis conducted with 28 aquaporin genes identified in the grapevine genome and previously characterized aquaporins from Arabidopsis indicates that three cDNAs encode putative tonoplast aquaporins (TIPs) whereas six cDNAs belong to the plasma membrane aquaporin subfamily (PIPs). Specific probes designed on the 3' untranslated regions of each cDNA were used for the preparation of cDNA macroarray filters and in situ hybridization experiments. Macroarray data indicate that expression levels of most TIP and PIP genes depend on grape berry developmental stages and point out to a global decrease of aquaporin gene expression during berry ripening. In young berries, high expression of aquaporin genes was preferentially observed in dividing and elongating cells and in cells involved in water and solutes transport. Taken together, the data provided in this paper indicate that aquaporins are implicated in various physiological aspects of grape berry development.

Aquaporins and plant water balance

The impact of aquaporin function on plant water balance is discussed. The significance of these proteins for root water uptake, water conductance in the xylem, including embolism refilling and the role of plant aquaporins in leaf physiology, is described. Emphasis is placed on certain aspects of water stress reactions and the correlation of aquaporins to abscisic acid as well as on the relation of water and CO2 permeability in leaves.

A plasma membrane-anchored fluorescent protein fusion illuminates sieve element plasma membranes in Arabidopsis and tobacco

Rapid acquisition of quantitative anatomical data from the sieve tubes of angiosperm phloem has been confounded by their small size, their distance from organ surfaces, and the time-consuming nature of traditional methods, such as transmission electron microscopy. To improve access to these cells, for which good anatomical data are critical, a monomeric yellow fluorescent protein (mCitrine) was N-terminally fused to a small (approximately 6 kD) membrane protein (AtRCI2A) and stably expressed in Arabidopsis thaliana (Columbia-0 ecotype) and Nicotiana tabacum ('Samsun') under the control of a companion cell-specific promoter (AtSUC2p). The construct, called by its abbreviation SUmCR, yielded stable sieve element (SE) plasma membrane fluorescence labeling, even after plastic (methacrylate) embedding. In conjunction with wide-field fluorescence measurements of sieve pore number and position using aniline blue-stained callose, mCitrine-labeled material was used to calculate rough estimates of sieve tube-specific conductivity for both species. The SUmCR construct also revealed a hitherto unknown expression domain of the AtSUC2 Suc-H(+) symporter in the epidermis of the cell division zone of developing root tips. The success of this construct in targeting plasma membrane-anchored fluorescent proteins to SEs could be attributable to the small size of AtRCI2A or to the presence of other signals innate to AtRCI2A that permit the protein to be trafficked to SEs. The construct provides a hitherto unique entrée into companion cell-to-SE protein targeting, as well as a new tool for studying whole-plant phloem anatomy and architecture.

Tissue and cell-specific localization of rice aquaporins and their water transport activities

Water transport in plants is greatly dependent on the expression and activity of water transport channels, called aquaporins. Here, we have clarified the tissue- and cell-specific localization of aquaporins in rice plants by immunoblotting and immunocytochemistry using seven isoform-specific aquaporin antibodies. We also examined water transport activities of typical aquaporin family members using a yeast expression system in combination with a stopped-flow spectrophotometry assay. OsPIP1 members, OsPIP2;1, OsTIP1;1 and OsTIP2;2 were expressed in both leaf blades and roots, while OsPIP2;3, OsPIP2;5 and OsTIP2;1 were expressed only in roots. In roots, large amounts of aquaporins accumulated in the region adjacent to the root tip (around 1.5-4 mm from the root tip). In this region, cell-specific localization of the various aquaporin members was observed. OsPIP1 members and OsTIP2;2 accumulated predominantly in the endodermis and the central cylinder, respectively. OsTIP1;1 showed specific localization in the rhizodermis and exodermis. OsPIP2;1, OsPIP2;3 and OsPIP2;5 accumulated in all root cells, but they showed higher levels of accumulation in endodermis than other cells. In the region at 35 mm from the root tip, where aerenchyma develops, aquaporins accumulated at low levels. In leaf blades, OsPIP1 members and OsPIP2;1 were localized mainly in mesophyll cells. OsPIP2;1, OsPIP2;3, OsPIP2;5 and OsTIP2;2 expressed in yeast showed high water transport activities. These results suggest that rice aquaporins with various water transport activities may play distinct roles in facilitating water flux and maintaining the water potential in different tissues and cells.

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.

The signal transduction pathways controlling in planta tuberization in potato: an emerging synthesis

Tuberization is one of the multiple outputs of a single-input phytochrome B sensory system, involving several regulatory genes. Phytochrome B- and GA-mediated photoperiodic perception occurs in the leaf, and then the RNA acts as a systemic signal in the long-distance signaling pathway to initiate tuberization in the subapical region of an underground stolon. There is good evidence that flowering and tuberizing signals might be similar. Is there a cross-talk with an oxidative burst-mediated redox signaling pathway during tuberization? Is the lipoxygenase cascade involved in the formation of the perimedullary tissue in a growing tuber? Do aquaporins regulate cell division, expansion and elongation during stolon growth and tuber induction in potato? Is the adaptive diversity for tuberization under varying photoperiods a micro-evolutionary indicator of differential transduction of cell-to-cell signal molecules under spatial and temporal expression of regulatory genes encoding transcriptional activators? Taking these views into consideration, the review presents an interim synthesis of a signaling network regulating in planta tuberization in potato.

Differential accumulation of mRNAs in drought-tolerant and susceptible common bean cultivars in response to water deficit

The physiological response to drought was measured in two common bean varieties with contrastive susceptibility to drought stress. A subtractive cDNA library was constructed from the two cultivars, Phaseolus vulgaris'Pinto Villa' (tolerant) and 'Carioca' (susceptible). 18 cDNAs displayed protein-coding genes associated with drought, cold and oxidative stress, signal transduction, plant defense, chloroplast function and unknown function. A cDNA coding for an aquaporin (AQP) was selected for further analyses. The open reading frames (ORFs) of AQPs from 'Pinto Villa' and 'Carioca' were compared and despite their similarity, accumulated differentially in the plant organs, as demonstrated by Northern blot and in situ hybridization. A phylogenetic analysis of the deduced amino acid sequence with other AQPs suggested a tonoplast-located protein. Under drought conditions, the levels of AQP mRNA from the susceptible cultivar decreased to undetectable levels; by contrast, 'Pinto Villa' mRNA was present and restricted the phloem tissue. This would allow 'Pinto Villa' to maintain vascular tissue functions under drought stress.

Cloning of salt stress responsive cDNA from wheat and resistant analysis of differential fragment SR07 in transgenic tobacco

Analysis of the gene expression differentiation in leaves of wheat (Triticum aestivum L.) cultivar Baofeng 7228, under salt stress, was carried out by Differential-Display Reverse Transcription-polymerase Chain Reaction (DDRT-PCR.) Twenty-seven differential cDNA fragments were obtained. The expression of the SR07 fragment was induced noticeably by salt treatment, and the nucleotide sequence homology of 87% between the SR07 fragment and PIPs (water channel proteins) was observed. Further research showed that a 561 bp open read frame was present in the SR07 fragment. Plant expression vector of pCAMBIA-SR07 was constructed and three transformants of tobacco (Nicotiana tobacum) mediated by Agrobacterium tumefaciens plasmid were obtained. Resistance to salt, PEG, and mannitol stresses of the three transformants were examined. No significant difference (P > 0.05) was observed between the control and the transformants in resistance to salt stress, but there was significant difference (P < 0.05) between the control and the transformants in resistance to PEG and mannitol stresses. Therefore, the expression of the SR07 fragment may play an important role in the water regulation of the plant.

Modulating the expression of aquaporin genes in planta: A key to understand their physiological functions?

Aquaporins (AQPs) are believed to act as "cellular plumbers", allowing plants to rapidly alter their membrane water permeability in response to environmental cues. This study of AQP regulation at both the RNA and protein levels has revealed a large number of possible mechanisms. Currently, modulation of AQP expression in planta is considered the strategy of choice for elucidating the role of AQPs in plant physiology. This review highlights the fact that this strategy is complicated by many factors, such as the incomplete characterization of transport selectivity of the targeted AQP, the fact that AQPs might act as multifunctional channels with multiple physiological roles, and the number of post-translational regulation mechanisms. The classification of AQPs as constitutive or stress-responsive isoforms is also proposed.

A phloem-enriched cDNA library from Ricinus: insights into phloem function

The aim of this study was to identify genes that are expressed in the phloem. Increased knowledge of phloem regulation will contribute to our understanding of its many roles, from transport of solutes to information about interactions with pathogens. A cDNA library constructed from phloem-enriched sap exuding from cut Ricinus communis (L.) hypocotyls was sequenced. To assess contamination from other tissues, two libraries were constructed: one using the first 15 min of exudation and the other from sap collected after 120 min of exudation had elapsed. Of 1012 clones sequenced, 158 unique transcripts were identified. The presence of marker molecules such as profilin, the low occurrence of chloroplast-related mRNAs, and the sieve element localization of constituent mRNA using in situ hybridization were consistent with a phloem origin of the sap. Functional analysis of the cDNAs revealed classifications including ribosomal function, interaction with the environment, transport, DNA/RNA binding, and protein turnover. An analysis of the closest Arabidopsis thaliana (L.) homologue for each clone indicated that genes involved in cell localization, protein synthesis, tissue localization, organ localization, organ differentiation, and cell fate were represented at twice the level occurring in the whole Arabidopsis genome. The transcripts found in this phloem-enriched library are discussed in the context of phloem function and the relationship between the companion cell and sieve element.

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.