Expanding roles of plant aquaporins in plasma membranes and cell organelles

Transfer and Degradation of PAHs in the Soil-Plant System: A Review

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic, persistent organic pollutants that threaten ecosystems and human health. Consistent monitoring is essential to minimize the entry of PAHs into plants and reduce food chain contamination. PAHs infiltrate plants through multiple pathways, causing detrimental effects and triggering diverse plant responses, ultimately increasing either toxicity or tolerance. Primary plant detoxification processes include enzymatic transformation, conjugation, and accumulation of contaminants in cell walls/vacuoles. Plants also play a crucial role in stimulating microbial PAHs degradation by producing root exudates, enhancing bioavailability, supplying nutrients, and promoting soil microbial diversity and activity. Thus, synergistic plant-microbe interactions efficiently decrease PAHs uptake by plants and, thereby, their accumulation along the food chain. This review highlights PAHs uptake pathways and their overall fate as contaminants of emerging concern (CEC). Understanding plant uptake mechanisms, responses to contaminants, and interactions with rhizosphere microbiota is vital for addressing PAH pollution in soil and ensuring food safety and quality.

Auxin- and pH-induced guttation in Phycomyces sporangiophores: relation between guttation and diminished elongation growth

Guttation, the formation of exudation water, is widespread among plants and fungi, yet the underlying mechanisms remain largely unknown. We describe the conditions for inducing guttation in sporangiophores of the mucoracean fungus, Phycomyces blakesleeanus. Cultivation on peptone-enriched potato dextrose agar elicits vigorous guttation mainly below the apical growing zone, while sporangiophores raised on a glucose-mineral medium manifest only moderate guttation. Mycelia do not guttate irrespective of the employed media. The topology of guttation droplets allows identifying the non-growing part of the sporangiophore as a guttation zone, which responds to humidity and medium composition in ways that become relevant for turgor homeostasis and thus the sensor physiology of the growing zone. Apparently, the entire sporangiophore, rather than exclusively the growing zone, participates in signal reception and integration to generate a common growth output. Exogenous auxin applied to the growing zones elicits two correlated responses: (i) formation of guttation droplets in the growing and transition zones below the sporangium and (ii) a diminution of the growth rate. In sporangiophore populations, guttation-induction by exogenous control buffer occurs at low frequencies; the bias for guttation increases with increasing auxin concentration. Synthetic auxins and the transport inhibitor NPA suppress guttation completely, but leave growth rates largely unaffected. Mutants C2 carA and C148 carA madC display higher sensitivities for auxin-induced guttation compared to wild type. A working model for guttation includes aquaporins and mechanosensitive ion channels that we identified in Phycomyces by sequence domain searches.

Genome-Wide Analysis of Aquaporins in Japanese Morning Glory (Ipomoea nil)

The aquaporin (AQP) family, also called water channels or major intrinsic proteins, facilitate water transport. AQPs also transport low-molecular-weight solutes, including boric acid, glycerol, urea, and ammonia. Since plants are sessile, water homeostasis is crucial. Therefore, plants have developed diverse AQP variants at higher expression levels than animals. For example, 35 and 33 AQPs have been identified in Arabidopsis and rice, respectively. In the present study, we identified AQPs in morning glory (Ipomoea nil), which has been widely used as a model plant in research on flowering and floral morphology. The importance of AQPs in the opening of morning glory flowers has been reported. In the morning glory genome, 44 AQPs were identified, and their characteristics were analyzed. A phylogenetic analysis revealed five AQP subfamilies in morning glory: plasma membrane-intrinsic proteins (PIPs), tonoplast-intrinsic proteins (TIPs), nodulin 26-like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs), and X-intrinsic proteins (XIPs). Further, transport substrates of morning glory AQPs were estimated based on their homology to the known AQPs in other plant species and their corresponding amino acid motifs that possess permeability pores. It was expected that PIPs are likely to transport water, carbon dioxide, and hydrogen peroxide; TIPs are likely transport water, hydrogen peroxide, ammonia, urea, and boric acid; NIPs are likely transport water, boric acid, ammonia, glycerol, and formamide; and XIPs are likely to transport water, hydrogen peroxide, and glycerol. Overall, these results suggest that AQPs are involved in water and nutrient transport in Japanese morning glory. An in silico gene expression analysis suggested the importance of AQPs in flower opening, water or nutrient uptakes from the soil to roots, and photosynthesis in morning glory. Our findings provide fundamental information that enables further study into the importance of AQPs in morning glory, including their roles in flower opening and other physiological events.

MicroRNA miR1118 contributes to wheat (Triticum aestivum L.) salinity tolerance by regulating the Plasma Membrane Intrinsic Proteins1;5 (PIP1;5) gene

microRNAs (miRNAs) are important regulators of various adaptive stress responses in crops; however, many details about associations among miRNAs, their target genes and physiochemical responses of crops under salinity stress remain poorly understood. We designed this study in a systems biology context and used a collection of computational, experimental and statistical procedures to uncover some regulatory functions of miRNAs in the response of the important crop, wheat, to salinity stress. Accordingly, under salinity conditions, wheat roots' Expressed Sequence Tag (EST) libraries were computationally mined to identify the most reliable differentially expressed miRNA and its related target gene(s). Then, molecular and physiochemical evaluations were carried out in a separate salinity experiment using two contrasting wheat genotypes. Finally, the association between changes in measured characteristics and wheat salinity tolerance was determined. From the results, miR1118 was assigned as a reliable salinity-responsive miRNA in wheat roots. The expression profiles of miR1118 and its predicted target gene, Plasma Membrane Intrinsic Proteins1,5 (PIP1;5), significantly differed between wheat genotypes. Moreover, results revealed that expression profiles of miR1118 and PIP1;5 significantly correlate to Relative Water Content (RWC), root hydraulic conductance (Lp), photosynthetic activities, plasma membrane damages, osmolyte accumulation and ion homeostasis of wheat. Our results suggest a plausible regulatory node through miR1118 adjusting the wheat water status, maintaining ion homeostasis and mitigating membrane damages, mainly through the PIP1;5 gene, under salinity conditions. To our knowledge, this is the first report on the role of miR1118 and PIP1;5 in wheat salinity response.

Mechanisms Activating Latent Functions of PIP Aquaporin Water Channels via the Interaction between PIP1 and PIP2 Proteins

Plant plasma membrane-type plasma membrane intrinsic protein (PIP) aquaporins are classified into two groups, PIP1s and PIP2s. In this study, we focused on HvPIP1;2, a PIP1 in barley (Hordeum vulgare), to dissect the molecular mechanisms that evoke HvPIP1-mediated water transport. No HvPIP1;2 protein was localized to the plasma membrane when expressed alone in Xenopus laevis oocytes. By contrast, a chimeric HvPIP1;2 protein (HvPIP1;2_24NC), in which the N- and C-terminal regions were replaced with the corresponding regions from HvPIP2;4, was found to localize to the plasma membrane of oocytes. However, HvPIP1;2_24NC showed no water transport activity in swelling assays. These results suggested that the terminal regions of PIP2 proteins direct PIP proteins to the plasma membrane, but the relocalization of PIP1 proteins was not sufficient to PIP1s functionality as a water channel in a membrane. A single amino acid replacement of threonine by methionine in HvPIP2;4 (HvPIP2;4T229M) abolished water transport activity. Co-expression of HvPIP1;2_24NC either with HvPIP2;4_12NC or with HvPIP2;4TM_12NC, in which the N- and C-terminal regions were replaced with the corresponding regions of HvPIP1;2, increased the water transport activity in oocytes. These data provided evidence that the HvPIP1;2 molecule has own water transport activity and an interaction with the middle part of the HvPIP2;4 protein (except for the N- and C-termini) is required for HvPIP1;2 functionality as a water channel. This molecular mechanism could be applied to other PIP1s and PIP2s in addition to the known mechanism that the terminal regions of some PIP2s lead some PIP1s to the plasma membrane.

Versatile Roles of Aquaporins in Plant Growth and Development

Aquaporins (AQPs) are universal membrane integrated water channel proteins that selectively and reversibly facilitate the movement of water, gases, metalloids, and other small neutral solutes across cellular membranes in living organisms. Compared with other organisms, plants have the largest number of AQP members with diverse characteristics, subcellular localizations and substrate permeabilities. AQPs play important roles in plant water relations, cell turgor pressure maintenance, the hydraulic regulation of roots and leaves, and in leaf transpiration, root water uptake, and plant responses to multiple biotic and abiotic stresses. They are also required for plant growth and development. In this review, we comprehensively summarize the expression and roles of diverse AQPs in the growth and development of various vegetative and reproductive organs in plants. The functions of AQPs in the intracellular translocation of hydrogen peroxide are also discussed.

A Survey of Barley PIP Aquaporin Ionic Conductance Reveals Ca2+-Sensitive HvPIP2;8 Na+ and K+ Conductance

Some plasma membrane intrinsic protein (PIP) aquaporins can facilitate ion transport. Here we report that one of the 12 barley PIPs (PIP1 and PIP2) tested, HvPIP2;8, facilitated cation transport when expressed in Xenopus laevis oocytes. HvPIP2;8-associated ion currents were detected with Na+ and K+, but not Cs+, Rb+, or Li+, and was inhibited by Ba2+, Ca2+, and Cd2+ and to a lesser extent Mg2+, which also interacted with Ca2+. Currents were reduced in the presence of K+, Cs+, Rb+, or Li+ relative to Na+ alone. Five HvPIP1 isoforms co-expressed with HvPIP2;8 inhibited the ion conductance relative to HvPIP2;8 alone but HvPIP1;3 and HvPIP1;4 with HvPIP2;8 maintained the ion conductance at a lower level. HvPIP2;8 water permeability was similar to that of a C-terminal phosphorylation mimic mutant HvPIP2;8 S285D, but HvPIP2;8 S285D showed a negative linear correlation between water permeability and ion conductance that was modified by a kinase inhibitor treatment. HvPIP2;8 transcript abundance increased in barley shoot tissues following salt treatments in a salt-tolerant cultivar Haruna-Nijo, but not in salt-sensitive I743. There is potential for HvPIP2;8 to be involved in barley salt-stress responses, and HvPIP2;8 could facilitate both water and Na+/K+ transport activity, depending on the phosphorylation status.

Transpiration difference under high evaporative demand in chickpea (Cicer arietinum L.) may be explained by differences in the water transport pathway in the root cylinder

Terminal drought substantially reduces chickpea yield. Reducing water use at vegetative stage by reducing transpiration under high vapor pressure deficit (VPD), i.e. under dry/hot conditions, contributes to drought adaptation. We hypothesized that this trait could relate to differences in a genotype's dependence on root water transport pathways and hydraulics. Transpiration rate responses in conservative and profligate chickpea genotypes were evaluated under increasing VPD in the presence/absence of apoplastic and cell-to-cell transport inhibitors. Conservative genotypes ICC 4958 and ICC 8058 restricted transpiration under high VPD compared to the profligate genotypes ICC 14799 and ICC 867. Profligate genotypes were more affected by aquaporin inhibition of the cell-to-cell pathway than conservative genotypes, as measured by the root hydraulic conductance and transpiration under high VPD. Aquaporin inhibitor treatment also led to a larger reduction in root hydraulic conductivity in profligate than in conservative genotypes. In contrast, blockage of the apoplastic pathway in roots decreased transpiration more in conservative than in profligate genotypes. Interestingly, conservative genotypes had high early vigour, whereas profligate genotypes had low early vigour. In conclusion, profligate genotypes depend more on the cell-to-cell pathway, which might explain their higher root hydraulic conductivity, whereas water-saving by restricting transpiration led to higher dependence on the apoplastic pathway. This opens the possibility to screen for conservative or profligate chickpea phenotypes using inhibitors, itself opening to the search of the genetic basis of these differences.

Genome Wild Analysis and Molecular Understanding of the Aquaporin Diversity in Olive Trees (Olea Europaea L.)

Cellular aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life, playing important roles in the uptake of water and many solutes across the membranes. In olive trees, AQP diversity, protein features and their biological functions are still largely unknown. This study focuses on the structure and functional and evolution diversity of AQP subfamilies in two olive trees, the wild species Olea europaea var. sylvestris (OeuAQPs) and the domesticated species Olea europaea cv. Picual (OleurAQPs), and describes their involvement in different physiological processes of early plantlet development and in biotic and abiotic stress tolerance in the domesticated species. A scan of genomes from the wild and domesticated olive species revealed the presence of 52 and 79 genes encoding full-length AQP sequences, respectively. Cross-genera phylogenetic analysis with orthologous clustered OleaAQPs into five established subfamilies: PIP, TIP, NIP, SIP, and XIP. Subsequently, gene structures, protein motifs, substrate specificities and cellular localizations of the full length OleaAQPs were predicted. Functional prediction based on the NPA motif, ar/R selectivity filter, Froger's and specificity-determining positions suggested differences in substrate specificities of Olea AQPs. Expression analysis of the OleurAQP genes indicates that some genes are tissue-specific, whereas few others show differential expressions at different developmental stages and in response to various biotic and abiotic stresses. The current study presents the first detailed genome-wide analysis of the AQP gene family in olive trees and it provides valuable information for further functional analysis to infer the role of AQP in the adaptation of olive trees in diverse environmental conditions in order to help the genetic improvement of domesticated olive trees.

Plasma Membrane Aquaporin Members PIPs Act in Concert to Regulate Cold Acclimation and Freezing Tolerance Responses in Arabidopsis thaliana

Aquaporins play a major role in plant water uptake at both optimal and environmentally stressed conditions. However, the functional specificity of aquaporins under cold remains obscure. To get a better insight to the role of aquaporins in cold acclimation and freezing tolerance, we took an integrated approach of physiology, transcript profiling and cell biology in Arabidopsis thaliana. Cold acclimation resulted in specific upregulation of PIP1;4 and PIP2;5 aquaporin (plasma membrane intrinsic proteins) expression, and immunoblotting analysis confirmed the increase in amount of PIP2;5 protein and total amount of PIPs during cold acclimation, suggesting that PIP2;5 plays a major role in tackling the cold milieu. Although single mutants of pip1;4 and pip2;5 or their double mutant showed no phenotypic changes in freezing tolerance, they were more sensitive in root elongation and cell survival response under freezing stress conditions compared with the wild type. Consistently, a single mutation in either PIP1;4 or PIP2;5 altered the expression of a number of aquaporins both at the transcriptional and translational levels. Collectively, our results suggest that aquaporin members including PIP1;4 and PIP2;5 function in concert to regulate cold acclimation and freezing tolerance responses.

Dry storage improves the vase quality of cut peony by increasing water uptake efficiency through aquaporins regulation

Proper storage prolongs peony market supply. Here, we determined the changes in fresh weight and expression of four aquaporin genes under dry storage (DS) and wet storage (WS). It has showed that after harvesting, the fresh weight change was accompanied with flower opening. After both short- and long-term of storage, the water uptake efficiency in DS group was greater during the first few vase days, providing a direct material basis of DS improved vase quality. The gene expression results showed that PlPIP1;3 and PlTIP2;1 were mainly expressed in petals, whereas PlNIP1;2-like and PlSIP2;1 were mainly expressed in the green tissues. In addition, the expression of PlTIP2;1 in the petals was consistent with the flower opening process, indicating that it may play a major role in facilitating water uptake. During cold storage, the expression of PlPIP1;3 and PlTIP2;1 was higher or more rapidly induced in the DS group, and thus we deduced that they play important roles in improving the vase quality of DS. Furthermore, the expression of PlNIP1;2-like in the early stage of the DS group was more stable than in WS, which may also be partially responsible for the vase quality improvement. In contrast, PlSIP2;1 may not be involved, since no significant change was observed between the DS and WS group. In short, the expression of PlPIP1;3 and PlTIP2;1 in the DS group during storage may improve water uptake efficiency during the vase period and then improving the vase quality of cut peony.

HKT1;5 Transporter Gene Expression and Association of Amino Acid Substitutions With Salt Tolerance Across Rice Genotypes

Plants need to maintain a low Na+/K+ ratio for their survival and growth when there is high sodium concentration in soil. Under these circumstances, the high affinity K+ transporter (HKT) and its homologs are known to perform a critical role with HKT1;5 as a major player in maintaining Na+ concentration. Preferential expression of HKT1;5 in roots compared to shoots was observed in rice and rice-like genotypes from real time PCR, microarray, and RNAseq experiments and data. Its expression trend was generally higher under increasing salt stress in sensitive IR29, tolerant Pokkali, both glycophytes; as well as the distant wild rice halophyte, Porteresia coarctata, indicative of its importance during salt stress. These results were supported by a low Na+/K+ ratio in Pokkali, but a much lower one in P. coarctata. HKT1;5 has functional variability among salt sensitive and tolerant varieties and multiple sequence alignment of sequences of HKT1;5 from Oryza species and P. coarctata showed 4 major amino acid substitutions (140 P/A/T/I, 184 H/R, D332H, V395L), with similarity amongst the tolerant genotypes and the halophyte but in variance with sensitive ones. The best predicted 3D structure of HKT1;5 was generated using Ktrab potassium transporter as template. Among the four substitutions, conserved presence of aspartate (332) and valine (395) in opposite faces of the membrane along the Na+/K+ channel was observed only for the tolerant and halophytic genotypes. A model based on above, as well as molecular dynamics simulation study showed that valine is unable to generate strong hydrophobic network with its surroundings in comparison to leucine due to reduced side chain length. The resultant alteration in pore rigidity increases the likelihood of Na+ transport from xylem sap to parenchyma and further to soil. The model also proposes that the presence of aspartate at the 332 position possibly leads to frequent polar interactions with the extracellular loop polar residues which may shift the loop away from the opening of the constriction at the pore and therefore permit easy efflux of the Na+. These two substitutions of the HKT1;5 transporter probably help tolerant varieties maintain better Na+/K+ ratio for survival under salt stress.

Genome-wide characterization and expression analysis of aquaporins in salt cress (Eutrema salsugineum)

Aquaporins (AQPs) serve as water channel proteins and belong to major intrinsic proteins (MIPs) family, functioning in rapidly and selectively transporting water and other small solutes across biological membranes. Importantly, AQPs have been shown to play a critical role in abiotic stress response pathways of plants. As a species closely related to Arabidopsis thaliana, Eutrema salsugineum has been proposed as a model for studying salt resistance in plants. Here we surveyed 35 full-length AQP genes in E. salsugineum, which could be grouped into four subfamilies including 12 plasma membrane intrinsic proteins (PIPs), 11 tonoplast intrinsic proteins (TIPs), nine NOD-like intrinsic proteins (NIPs), and three small basic intrinsic proteins (SIPs) by phylogenetic analysis. EsAQPs were comprised of 237-323 amino acids, with a theoretical molecular weight (MW) of 24.31-31.80 kDa and an isoelectric point (pI) value of 4.73-10.49. Functional prediction based on the NPA motif, aromatic/arginine (ar/R) selectivity filter, Froger's position and specificity-determining position suggested quite differences in substrate specificities of EsAQPs. EsAQPs exhibited global expressions in all organs as shown by gene expression profiles and should be play important roles in response to salt, cold and drought stresses. This study provides comprehensive bioinformation on AQPs in E. salsugineum, which would be helpful for gene function analysis for further studies.

Evaluation of water relation parameters in vitis rootstocks with different drought tolerance and their effects on growth of a grafted cultivar

Knowledge of root hydraulic resistance will allow us to better understand water relations arising in the soil-plant-atmosphere continuum. These are the basis for better control of plant behaviour in the current environmental context that is more and more affected by global warming and problems related to increased drought frequency and duration. The objectives of this study were to determine how the growth of a cultivar changes in response to the drought tolerance of the rootstock used in Vitis grown in a semi-arid area and how the root hydraulic resistivity and root hydraulic conductivity change with increased transpiration when adopting a rootstock with a different drought tolerance. These experiments were carried out on intact plants of Gaglioppo grapevines grafted onto rootstocks of 779 P, a drought-tolerant American hybrid, and 420 A, a drought-susceptible American hybrid. Root hydraulic conductivity was significantly higher in the roots of 779 P than in the roots of 420 A. Stomatal conductance, net assimilation of CO₂, leaf water potential, and relative water content were also higher in Gaglioppo grafted onto 779 P than that grafted onto 420 A. Leaf area, leaf dry weight, and specific leaf weight of Gaglioppo were also higher when grafted onto 779 P. Gaglioppo grapevine grafted onto 779 P showed superior growth and physiological performance.

Heterologous Expression of Panax ginseng PgTIP1 Confers Enhanced Salt Tolerance of Soybean Cotyledon Hairy Roots, Composite, and Whole Plants

The Panax ginseng TIP gene PgTIP1 was previously demonstrated to have high water channel activity by its heterologous expression in Xenopus laevis oocytes and in yeast; it also plays a significant role in growth of PgTIP1-transgenic Arabidopsis plants under favorable conditions and has enhanced tolerance toward salt and drought treatment. In this work, we first investigated the physiological effects of heterologous PgTIP1 expression in soybean cotyledon hairy roots or composite plants mediated by Agrobacterium rhizogenes toward enhanced salt tolerance. The PgTIP1-transgenic soybean plants mediated by the pollen tube pathway, represented by the lines N and J11, were analyzed at the physiological and molecular levels for enhanced salt tolerance. The results showed that in terms of root-specific heterologous expression, the PgTIP1-transformed soybean cotyledon hairy roots or composite plants displayed superior salt tolerance compared to the empty vector-transformed ones according to the mitigatory effects of hairy root growth reduction, drop in leaf RWC, and rise in REL under salt stress. Additionally, declines in K+ content, increases in Na+ content and Na+/K+ ratios in the hairy roots, stems, or leaves were effectively alleviated by PgTIP1-transformation, particularly the stems and leaves of composite soybean plants. At the whole plant level, PgTIP1-trasgenic soybean lines were found to possess stronger root vigor, reduced root and leaf cell membrane damage, increased SOD, POD, CAT, and APX activities, steadily increased leaf Tr, RWC, and Pn values, and smaller declines in chlorophyll and carotenoid content when exposed to salt stress compared to wild type. Moreover, the distribution patterns of Na+, K+, and Cl- in the roots, stems, and leaves of salt-stressed transgenic plants were readjusted, in that the absorbed Na+ and Cl- were mainly restricted to the roots to reduce their transport to the shoots, and the transport of root-absorbed K+ to the shoots was simultaneously promoted. PgTIP1 transformation into soybean plants enhanced the expression of some stress-related genes (GmPOD, GmAPX1, GmSOS1, and GmCLC1) in the roots and leaves under salt treatment. This indicates that the causes of enhanced salt tolerance of heterologous PgTIP1-transformed soybean are associated with the positive regulation on water relations, ion homeostasis, and ROS scavenging under salt stress both at root-specific and whole plant levels.

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.

Overexpression of MfPIP2-7 from Medicago falcata promotes cold tolerance and growth under NO3 (-) deficiency in transgenic tobacco plants

BACKGROUND

Plasma membrane intrinsic proteins (PIPs), which belong to aquaporins (AQPs) superfamily, are subdivided into two groups, PIP1 and PIP2, based on sequence similarity. Several PIP2s function as water channels, while PIP1s have low or no water channel activity, but have a role in water permeability through interacting with PIP2. A cold responsive PIP2 named as MfPIP2-7 was isolated from Medicago falcata (hereafter falcata), a forage legume with great cold tolerance, and transgenic tobacco plants overexpressing MfPIP2-7 were analyzed in tolerance to multiple stresses including freezing, chilling, and nitrate reduction in this study.

RESULTS

MfPIP2-7 transcript was induced by 4 to 12 h of cold treatment and 2 h of abscisic acid (ABA) treatment. Pretreatment with inhibitor of ABA synthesis blocked the cold induced MfPIP2-7 transcript, indicating that ABA was involved in cold induced transcription of MfPIP2-7 in falcata. Overexpression of MfPIP2-7 resulted in enhanced tolerance to freezing, chilling and NO3 (-) deficiency in transgenic tobacco (Nicotiana tabacum L.) plants as compared with the wild type. Moreover, MfPIP2-7 was demonstrated to facilitate H2O2 diffusion in yeast. Higher transcript levels of several stress responsive genes, such as NtERD10B, NtERD10C, NtDREB1, and 2, and nitrate reductase (NR) encoding genes (NtNIA1, and NtNIA2) were observed in transgenic plants as compared with the wild type with dependence upon H2O2. In addition, NR activity was increased in transgenic plants, which led to alterations in free amino acid components and concentrations.

CONCLUSIONS

The results suggest that MfPIP2-7 plays an important role in plant tolerance to freezing, chilling, and NO3 (-) deficiency by promoted H2O2 diffusion that in turn up-regulates expression of NIAs and multiple stress responsive genes.

Localized and non-localized effects of arbuscular mycorrhizal symbiosis on accumulation of osmolytes and aquaporins and on antioxidant systems in maize plants subjected to total or partial root drying

The arbuscular mycorrhizal (AM) symbiosis alters host plant physiology under drought stress, but no information is available on whether or not the AM affects respond to drought locally or systemically. A split-root system was used to obtain AM plants with total or only half root system colonized as well as to induce physiological drought affecting the whole plant or non-physiological drought affecting only the half root system. We analysed the local and/or systemic nature of the AM effects on accumulation of osmoregulatory compounds and aquaporins and on antioxidant systems. Maize plants accumulated proline both, locally in roots affected by drought and systemically when the drought affected the whole root system, being the last effect ampler in AM plants. PIPs (plasma membrane intrinsic proteins) aquaporins were also differently regulated by drought in AM and non-AM root compartments. When the drought affected only the AM root compartment, the rise of lipid peroxidation was restricted to such compartment. On the contrary, when the drought affected the non-AM root fraction, the rise of lipid peroxidation was similar in both root compartments. Thus, the benefits of the AM symbiosis not only rely in a lower oxidative stress in the host plant, but it also restricts locally such oxidative stress.

A ginseng PgTIP1 gene whose protein biological activity related to Ser(128) residue confers faster growth and enhanced salt stress tolerance in Arabidopsis

Water movement across cellular membranes is mostly regulated by aquaporins. A tonoplast intrinsic protein PgTIP1 from Panax ginseng has been found to play an important role in plant growth and development, and also in the response of plants to abiotic stress. However, the regulation of its function and activity remains unknown. To answer this question, mutated forms of PgTIP1 were made by replacing Ser(128) with Ala (named S128A) or Asp (named S128D), and also by replacing Thr(54) with Ala (named T54A) or Asp (named T54D). Then, wild type or mutated PgTIP1 was expressed in yeast and water transport was monitored in protoplasts. The substitution of Ser(128) abolished the water channel activity of PgTIP1, while the substitution of Thr(54) did not inhibit its activity. Moreover, the overexpression of PgTIP1 but not S128A or S128D in Arabidopsis significantly increased plant growth as determined by biomass production, it also had a beneficial effect on salt stress tolerance. Importantly, the overexpression of PgTIP1 led to the altered expression of stress-related genes, which made the plants more tolerant to salt stress. Our results demonstrated that PgTIP1 conferred faster growth and enhanced tolerance to salt in Arabidopsis, and that its biological activity related to Ser(128) residue.

Membrane nanodomains in plants: capturing form, function, and movement

The plasma membrane is the interface between the cell and the external environment. Plasma membrane lipids provide scaffolds for proteins and protein complexes that are involved in cell to cell communication, signal transduction, immune responses, and transport of small molecules. In animals, fungi, and plants, a substantial subset of these plasma membrane proteins function within ordered sterol- and sphingolipid-rich nanodomains. High-resolution microscopy, lipid dyes, pharmacological inhibitors of lipid biosynthesis, and lipid biosynthetic mutants have been employed to examine the relationship between the lipid environment and protein activity in plants. They have also been used to identify proteins associated with nanodomains and the pathways by which nanodomain-associated proteins are trafficked to their plasma membrane destinations. These studies suggest that plant membrane nanodomains function in a context-specific manner, analogous to similar structures in animals and fungi. In addition to the highly conserved flotillin and remorin markers, some members of the B and G subclasses of ATP binding cassette transporters have emerged as functional markers for plant nanodomains. Further, the glycophosphatidylinositol-anchored fasciclin-like arabinogalactan proteins, that are often associated with detergent-resistant membranes, appear also to have a functional role in membrane nanodomains.

Environmental fate and exposure; neonicotinoids and fipronil

Systemic insecticides are applied to plants using a wide variety of methods, ranging from foliar sprays to seed treatments and soil drenches. Neonicotinoids and fipronil are among the most widely used pesticides in the world. Their popularity is largely due to their high toxicity to invertebrates, the ease and flexibility with which they can be applied, their long persistence, and their systemic nature, which ensures that they spread to all parts of the target crop. However, these properties also increase the probability of environmental contamination and exposure of nontarget organisms. Environmental contamination occurs via a number of routes including dust generated during drilling of dressed seeds, contamination and accumulation in arable soils and soil water, runoff into waterways, and uptake of pesticides by nontarget plants via their roots or dust deposition on leaves. Persistence in soils, waterways, and nontarget plants is variable but can be prolonged; for example, the half-lives of neonicotinoids in soils can exceed 1,000 days, so they can accumulate when used repeatedly. Similarly, they can persist in woody plants for periods exceeding 1 year. Breakdown results in toxic metabolites, though concentrations of these in the environment are rarely measured. Overall, there is strong evidence that soils, waterways, and plants in agricultural environments and neighboring areas are contaminated with variable levels of neonicotinoids or fipronil mixtures and their metabolites (soil, parts per billion (ppb)-parts per million (ppm) range; water, parts per trillion (ppt)-ppb range; and plants, ppb-ppm range). This provides multiple routes for chronic (and acute in some cases) exposure of nontarget animals. For example, pollinators are exposed through direct contact with dust during drilling; consumption of pollen, nectar, or guttation drops from seed-treated crops, water, and consumption of contaminated pollen and nectar from wild flowers and trees growing near-treated crops. Studies of food stores in honeybee colonies from across the globe demonstrate that colonies are routinely and chronically exposed to neonicotinoids, fipronil, and their metabolites (generally in the 1-100 ppb range), mixed with other pesticides some of which are known to act synergistically with neonicotinoids. Other nontarget organisms, particularly those inhabiting soils, aquatic habitats, or herbivorous insects feeding on noncrop plants in farmland, will also inevitably receive exposure, although data are generally lacking for these groups. We summarize the current state of knowledge regarding the environmental fate of these compounds by outlining what is known about the chemical properties of these compounds, and placing these properties in the context of modern agricultural practices.

Osmotic stress decreases PIP aquaporin transcripts in barley roots but H2O2 is not involved in this process

Previous reports indicate that salt stress reduces the root hydraulic conductance and the expression of plasmamembrane-type aquaporins (PIPs). As a molecular mechanism for this phenomenon, the present study found evidence that the osmotic component, but probably not an ion-specific component, decreases PIP transcripts. Eight of ten PIP transcripts were reduced to less than half by 360 mM mannitol treatment for 12 h in comparison with control samples. A large decrease of HvPIP2;1 protein was also recorded. This reduction of both transcripts and proteins of HvPIP2s should be physiologically effective for preventing or reducing dehydration at an initial phase of severe salt/osmotic stress. Root cell sap osmolality increased from 278 to 372 mOsm 24 h after 360 mM mannitol treatment. These steps can secure survival and growth recovery with water reabsorption in barley. Our data also suggest that H2O2 seems not to be the main cause of osmotic stress-induced transcriptional down-regulation within the concentrations (20-500 μM) and time periods (24 h) examined, although H2O2 was previously proposed to be involved in the mechanisms of salinity/osmotic tolerance.

New insights into the regulation of aquaporins by the arbuscular mycorrhizal symbiosis in maize plants under drought stress and possible implications for plant performance

The relationship between modulation by arbuscular mycorrhizae (AM) of aquaporin expression in the host plant and changes in root hydraulic conductance, plant water status, and performance under stressful conditions is not well known. This investigation aimed to elucidate how the AM symbiosis modulates the expression of the whole set of aquaporin genes in maize plants under different growing and drought stress conditions, as well as to characterize some of these aquaporins in order to shed further light on the molecules that may be involved in the mycorrhizal responses to drought. The AM symbiosis regulated a wide number of aquaporins in the host plant, comprising members of the different aquaporin subfamilies. The regulation of these genes depends on the watering conditions and the severity of the drought stress imposed. Some of these aquaporins can transport water and also other molecules which are of physiological importance for plant performance. AM plants grew and developed better than non-AM plants under the different conditions assayed. Thus, for the first time, this study relates the well-known better performance of AM plants under drought stress to not only the water movement in their tissues but also the mobilization of N compounds, glycerol, signaling molecules, or metalloids with a role in abiotic stress tolerance. Future studies should elucidate the specific function of each aquaporin isoform regulated by the AM symbiosis in order to shed further light on how the symbiosis alters the plant fitness under stressful conditions.

Accumulation of TIP2;2 Aquaporin during Dark Adaptation Is Partially PhyA Dependent in Roots of Arabidopsis Seedlings

Light regulates the expression and function of aquaporins, which are involved in water and solute transport. In Arabidopsis thaliana, mRNA levels of one of the aquaporin genes, TIP2;2, increase during dark adaptation and decrease under far-red light illumination, but the effects of light at the protein level and on the mechanism of light regulation remain unknown. Numerous studies have described the light regulation of aquaporin genes, but none have identified the regulatory mechanisms behind this regulation via specific photoreceptor signaling. In this paper, we focus on the role of phytochrome A (phyA) signaling in the regulation of the TIP2;2 protein. We generated Arabidopsis transgenic plants expressing a TIP2;2-GFP fusion protein driven by its own promoter, and showed several differences in TIP2;2 behavior between wild type and the phyA mutant. Fluorescence of TIP2;2-GFP protein in the endodermis of roots in the wild-type seedlings increased during dark adaptation, but not in the phyA mutant. The amount of the TIP2;2-GFP protein in wild-type seedlings decreased rapidly under far-red light illumination, and a delay in reduction of TIP2;2-GFP was observed in the phyA mutant. Our results imply that phyA, cooperating with other photoreceptors, modulates the level of TIP2;2 in Arabidopsis roots.

Genome-wide identification and expression analysis of aquaporins in tomato

The family of aquaporins, also called water channels or major intrinsic proteins, is characterized by six transmembrane domains that together facilitate the transport of water and a variety of low molecular weight solutes. They are found in all domains of life, but show their highest diversity in plants. Numerous studies identified aquaporins as important targets for improving plant performance under drought stress. The phylogeny of aquaporins is well established based on model species like Arabidopsis thaliana, which can be used as a template to investigate aquaporins in other species. In this study we comprehensively identified aquaporin encoding genes in tomato (Solanum lycopersicum), which is an important vegetable crop and also serves as a model for fleshy fruit development. We found 47 aquaporin genes in the tomato genome and analyzed their structural features. Based on a phylogenetic analysis of the deduced amino acid sequences the aquaporin genes were assigned to five subfamilies (PIPs, TIPs, NIPs, SIPs and XIPs) and their substrate specificity was assessed on the basis of key amino acid residues. As ESTs were available for 32 genes, expression of these genes was analyzed in 13 different tissues and developmental stages of tomato. We detected tissue-specific and development-specific expression of tomato aquaporin genes, which is a first step towards revealing the contribution of aquaporins to water and solute transport in leaves and during fruit development.

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.

Plant homeostasis of foliar manganese sinks: specific variation in hyperaccumulators

Plant manganese (Mn) hyperaccumulation provides unusual insight into homeostasis of this essential micronutrient, in particular its excessive storage in shoot tissues. The compartmentation of hyperaccumulated foliar Mn appears exceptional among metal hyperaccumulators, since it occurs via specific microdistribution patterns. Here, three associated Mn hyperaccumulators, Virotia neurophylla, Maytenus fournieri, and Garcinia amplexicaulis exhibiting distinctly different Mn detoxification strategies were examined. Non-invasive sample preparation in conjunction with cryo scanning electron microscopy (SEM) was used to obtain in vivo quantitative microprobe X-ray and anatomical data from fully hydrated cells. Highly vacuolated large palisade mesophyll cells in V. neurophylla leaves were found to contain around 650 mM Mn. The large non-photosynthetic hypodermal cells of M. fournieri leaves, also with high vacuolar content, and the main site for Mn disposal, had an estimated mean vacuolar Mn concentration of around 600 mM. Previous qualitative X-ray mapping had shown Mn to be almost evenly sequestered across the entire leaf cross section of G. amplexicaulis. However, quantitative data obtained here showed a marked variation in localised concentrations that ranged between ~15 and >800 mM. Notable among these were mean values of >600 mM in spongy mesophyll cells, and ~800 mM within cells of a narrow sub epidermal layer preceding the palisade mesophyll. This study demonstrated the extraordinary Mn carrying capacities of different types of leaf cell vacuoles.

Correlation of aquaporins and transmembrane solute transporters revealed by genome-wide analysis in developing maize leaf

Aquaporins are multifunctional membrane channels that facilitate the transmembrane transport of water and solutes. When transmembrane mineral nutrient transporters exhibit the same expression patterns as aquaporins under diverse temporal and physiological conditions, there is a greater probability that they interact. In this study, genome-wide temporal profiling of transcripts analysis and coexpression network-based approaches are used to examine the significant specificity correlation of aquaporins and transmembrane solute transporters in developing maize leaf. The results indicate that specific maize aquaporins are related to specific transmembrane solute transporters. The analysis demonstrates a systems-level correlation between aquaporins, nutrient transporters, and the homeostasis of mineral nutrients in developing maize leaf. Our results provide a resource for further studies into the physiological function of these aquaporins.

Cold stress-induced acclimation in rice is mediated by root-specific aquaporins

Cold acclimation process plays a vital role in the survival of chilling- and freezing-tolerant plants subjected to cold temperature stress. However, it remains elusive whether a cold acclimation process enhances root water uptake (a component of chilling tolerance) in chilling-sensitive crops such as rice. By analyzing the root hydraulic conductivity under cold stress for a prolonged time, we found that cold stress induced a gradual increase in root osmotic hydraulic conductivity [Lp(r(os))]. Compared with the control treatment (roots and shoots at 25°C), low root temperature (LRT) treatment (roots at 10°C; shoots at 25°C) dramatically reduced Lp(r(os)) within 1 h. However, Lp(r(os)) gradually increased during prolonged LRT treatment and it reached 10-fold higher values at day 5. Moreover, a coordinated up-regulation of root aquaporin gene expression, particularly OsPIP2;5, was observed during LRT treatment. Further, comparison of aquaporin gene expression under root-only chilling (LRT) and whole-plant chilling conditions, and in the roots of intact plants vs. shootless plants, suggests that a shoot to root signal is necessary for inducing the expression of aquaporin genes in the root. Collectively, these results demonstrate that a cold acclimation process for root water uptake functions in rice and is possibly regulated through aquaporins.

Regulation by arbuscular mycorrhizae of the integrated physiological response to salinity in plants: new challenges in physiological and molecular studies

Excessive salt accumulation in soils is a major ecological and agronomical problem, in particular in arid and semi-arid areas. Excessive soil salinity affects the establishment, development, and growth of plants, resulting in important losses in productivity. Plants have evolved biochemical and molecular mechanisms that may act in a concerted manner and constitute the integrated physiological response to soil salinity. These include the synthesis and accumulation of compatible solutes to avoid cell dehydration and maintain root water uptake, the regulation of ion homeostasis to control ion uptake by roots, compartmentation and transport into shoots, the fine regulation of water uptake and distribution to plant tissues by the action of aquaporins, the reduction of oxidative damage through improved antioxidant capacity and the maintenance of photosynthesis at values adequate for plant growth. Arbuscular mycorrhizal (AM) symbiosis can help the host plants to cope with the detrimental effects of high soil salinity. There is evidence that AM symbiosis affects and regulates several of the above mentioned mechanisms, but the molecular bases of such effects are almost completely unknown. This review summarizes current knowledge about the effects of AM symbiosis on these physiological mechanisms, emphasizing new perspectives and challenges in physiological and molecular studies on salt-stress alleviation by AM symbiosis.

Surveying the plant's world by magnetic resonance imaging

Understanding the way in which plants develop, grow and interact with their environment requires tools capable of a high degree of both spatial and temporal resolution. Magnetic resonance imaging (MRI), a technique which is able to visualize internal structures and metabolites, has the great virtue that it is non-invasive and therefore has the potential to monitor physiological processes occurring in vivo. The major aim of this review is to attract plant biologists to MRI by explaining its advantages and wide range of possible applications for solving outstanding issues in plant science. We discuss the challenges and opportunities of MRI in the study of plant physiology and development, plant-environment interactions, biodiversity, gene functions and metabolism. Overall, it is our view that the potential benefit of harnessing MRI for plant research purposes is hard to overrate.

The Péclet effect on leaf water enrichment correlates with leaf hydraulic conductance and mesophyll conductance for CO(2)

Leaf water gets isotopically enriched through transpiration, and diffusion of enriched water through the leaf depends on transpiration flow and the effective path length (L). The aim of this work was to relate L with physiological variables likely to respond to similar processes. We studied the response to drought and vein severing of leaf lamina hydraulic conductance (K(lamina) ), mesophyll conductance for CO(2) (g(m) ) and leaf water isotope enrichment in Vitis vinifera L cv. Grenache. We hypothesized that restrictions in water pathways would reduce K(lamina) and increase L. As a secondary hypothesis, we proposed that, given the common pathways for water and CO(2) involved, a similar response should be found in g(m) . Our results showed that L was strongly related to mesophyll variables, such as K(lamina) or g(m) across experimental drought and vein-cutting treatments, showing stronger relationships than with variables included as input parameters for the models, such as transpiration. Our findings were further supported by a literature survey showing a close link between L and leaf hydraulic conductance (K(leaf) = 31.5 × L(-0.43) , r(2) = 0.60, n = 24). The strong correlation found between L, K(lamina) and g(m) supports the idea that water and CO(2) share an important part of their diffusion pathways through the mesophyll.

H(+)/phenanthrene symporter and aquaglyceroporin are implicated in phenanthrene uptake by wheat (Triticum aestivum L.) roots

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic pollutants that are toxic to human and nonhuman organisms. Dietary intake of PAHs is a dominant route of exposure for the general population because food crops are a major source of dietary PAHs. The mechanism for crop root uptake of PAHs remains unclear. Here we reveal that wheat root uptake of PAHs involves active and passive processes. The passive uptake is mercury and glycerol dependent. Mercury and glycerol inhibit uptake, indicating that aquaglyceroporins sensitive to mercury contribute to passive uptake. Active uptake is mediated by a phenanthrene/H symporter. The electrical response of wheat roots triggered by phenanthrene consists of two sequential phases: depolarization followed by repolarization. The depolarization is phenanthrene concentration dependent, with saturation kinetics that have an apparent of K(m) 10.8 μmol L(-1). As uptake proceeds, external solution pH increase is noticed. Lower pH favors the uptake. Vanadate and 2,4-dinitrophenol suppress the electrical response to phenanthrene and phenanthrene uptake, suggesting that plasma membrane H(+)-ATPase is involved in the establishment of an electrochemical proton gradient acting as a driving force for active uptake. Therefore, it is suggested that aquaglyceroporin and phenanthrene/H symporter are implicated in phenanthrene uptake. Our results provide insight into PAH uptake mechanism in wheat roots that is relevant to strategies for reducing PAH accumulation in wheat for food safety, improving phytoremediation of PAH-contaminated soils or water by agronomic practices and genetic modification to target remedial plants for higher PAH uptake capacity.

Carbonic anhydrase and the molecular evolution of C4 photosynthesis

C(4) photosynthesis, a biochemical CO(2)-concentrating mechanism (CCM), evolved more than 60 times within the angiosperms from C(3) ancestors. The genus Flaveria, which contains species demonstrating C(3), C(3)-C(4), C(4)-like or C(4) photosynthesis, is a model for examining the molecular evolution of the C(4) pathway. Work with carbonic anhydrase (CA), and C(3) and C(4) Flaveria congeners has added significantly to the understanding of this process. The C(4) form of CA3, a β-CA, which catalyses the first reaction in the C(4) pathway by hydrating atmospheric CO(2) to bicarbonate in the cytosol of mesophyll cells (mcs), evolved from a chloroplastic C(3) ancestor. The molecular modifications to the ancestral CA3 gene included the loss of the sequence encoding the chloroplast transit peptide, and mutations in regulatory regions that resulted in high levels of expression in the C(4) mesophyll. Analyses of the CA3 proteins and regulatory elements from Flaveria photosynthetic intermediates indicated C(4) biochemistry very likely evolved in a specific, stepwise manner in this genus. The details of the mechanisms involved in the molecular evolution of other C(4) plant β-CAs are unknown; however, comparative genetics indicate gene duplication and neofunctionalization played significant roles as they did in Flaveria.

Regulation of root water uptake under abiotic stress conditions

A common effect of several abiotic stresses is to cause tissue dehydration. Such dehydration is caused by the imbalance between root water uptake and leaf transpiration. Under some specific stress conditions, regulation of root water uptake is more crucial to overcome stress injury than regulation of leaf transpiration. This review first describes present knowledge about how water is taken up by roots and then discusses how specific stress situations such as drought, salinity, low temperature, and flooding modify root water uptake. The rate of root water uptake of a given plant is the result of its root hydraulic characteristics, which are ultimately regulated by aquaporin activity and, to some extent, by suberin deposition. Present knowledge about the effects of different stresses on these features is also summarized. Finally, current findings regarding how molecular signals such as the plant hormones abscisic acid, ethylene, and salicylic acid, and how reactive oxygen species may modulate the final response of root water uptake under stress conditions are discussed.

Single-molecule analysis of PIP2;1 dynamics and partitioning reveals multiple modes of Arabidopsis plasma membrane aquaporin regulation

PIP2;1 is an integral membrane protein that facilitates water transport across plasma membranes. To address the dynamics of Arabidopsis thaliana PIP2;1 at the single-molecule level as well as their role in PIP2;1 regulation, we tracked green fluorescent protein-PIP2;1 molecules by variable-angle evanescent wave microscopy and fluorescence correlation spectroscopy (FCS). Single-particle tracking analysis revealed that PIP2;1 presented four diffusion modes with large dispersion of diffusion coefficients, suggesting that partitioning and dynamics of PIP2;1 are heterogeneous and, more importantly, that PIP2;1 can move into or out of membrane microdomains. In response to salt stress, the diffusion coefficients and percentage of restricted diffusion increased, implying that PIP2;1 internalization was enhanced. This was further supported by the decrease in PIP2;1 density on plasma membranes by FCS. We additionally demonstrated that PIP2;1 internalization involves a combination of two pathways: a tyrphostin A23-sensitive clathrin-dependent pathway and a methyl-β-cyclodextrin-sensitive, membrane raft-associated pathway. The latter was efficiently stimulated under NaCl conditions. Taken together, our findings demonstrate that PIP2;1 molecules are heterogeneously distributed on the plasma membrane and that clathrin and membrane raft pathways cooperate to mediate the subcellular trafficking of PIP2;1, suggesting that the dynamic partitioning and recycling pathways might be involved in the multiple modes of regulating water permeability.

Influence of evaporative demand on aquaporin expression and root hydraulics of hybrid poplar

When light levels and evaporative demand increase, dynamic physiological changes in roots may be required to restore the water balance at the whole plant level. We hypothesized that a dynamic increase in root hydraulic conductance (L(P)) and aquaporin (AQP) expression could moderate the transpiration-induced drop in water potential (Ψ), allowing continued gas exchange in hybrid poplar (Populus trichocarpa × deltoides) saplings. Fifty-six AQPs have been identified in poplar, but little information about their expression patterns in roots is available, especially from a whole-plant water relations perspective. We measured AQP expression and L(P) in plants subjected to different levels of light and evaporative demand. Shaded plants had only one-tenth the root area of plants growing at higher light levels. Shade-grown saplings experiencing a sudden increase in light exhibited a threefold higher L(P) than plants remaining in shade. This dynamic increase in L(P) corresponded with increased transcript abundance of 15 AQPs out of a total of 33 genes simultaneously assessed by quantitative RT-PCR. The tissue-level localization of transcripts of four AQPs was studied with in situ hybridization. Comprehensive expression profiling in conjunction with physiological and morphological measurements is a valuable reference for future studies on AQP function in poplar.

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.

Aquaporin-facilitated water uptake in barley (Hordeum vulgare L.) roots

It is not known to what degree aquaporin-facilitated water uptake differs between root developmental regions and types of root. The aim of this study was to measure aquaporin-dependent water flow in the main types of root and root developmental regions of 14- to 17-d-old barley plants and to identify candidate aquaporins which mediate this flow. Water flow at root level was related to flow at cell and plant level. Plants were grown hydroponically. Hydraulic conductivity of cells and roots was determined with a pressure probe and through exudation, respectively, and whole-plant water flow (transpiration) determined gravimetrically in response to the commonly used aquaporin inhibitor HgCl(2). Expression of aquaporins was analysed by real-time PCR and in situ hybridization. Hydraulic conductivity of cortical cells in seminal roots was largest in lateral roots; it was smallest in the fully mature zone and intermediate in the not fully mature 'transition' zone along the main root axis. Adventitious roots displayed an even higher (3- to 4-fold) cortical cell hydraulic conductivity in the transition zone. This coincided with 3- to 4-fold higher expression of three aquaporins (HvPIP2;2, HvPIP2;5, HvTIP1:1). These were expressed (also) in cortical tissue. The largest inhibition of water flow (83-95%) in response to HgCl(2) was observed in cortical cells. Water flow through roots and plants was reduced less (40-74%). It is concluded that aquaporins contribute substantially to root water uptake in 14- to 17-d-old barley plants. Most water uptake occurs through lateral roots. HvPIP2;5, HvPIP2;2, and HvTIP1;1 are prime candidates to mediate water flow in cortical tissue.

Mechanisms of water transport mediated by PIP aquaporins and their regulation via phosphorylation events under salinity stress in barley roots

Water homeostasis is crucial to the growth and survival of plants under water-related stress. Plasma membrane intrinsic proteins (PIPs) have been shown to be primary channels mediating water uptake in plant cells. Here we report the water transport activity and mechanisms for the regulation of barley (Hordeum vulgare) PIP aquaporins. HvPIP2 but not HvPIP1 channels were found to show robust water transport activity when expressed alone in Xenopus laevis oocytes. However, the co-expression of HvPIP1 with HvPIP2 in oocytes resulted in significant increases in activity compared with the expression of HvPIP2 alone, suggesting the participation of HvPIP1 in water transport together with HvPIP2 presumably through heteromerization. Severe salinity stress (200 mM NaCl) significantly reduced root hydraulic conductivity (Lp(r)) and the accumulation of six of 10 HvPIP mRNAs. However, under relatively mild stress (100 mM NaCl), only a moderate reduction in Lp(r) with no significant difference in HvPIP mRNA levels was observed. Sorbitol-mediated osmotic stress equivalent to 100 and 200 mM NaCl induced nearly identical Lp(r) reductions in barley roots. Furthermore, the water transport activity in intact barley roots was suggested to require phosphorylation that is sensitive to a kinase inhibitor, staurosporine. HvPIP2s also showed water efflux activity in Xenopus oocytes, suggesting a potential ability to mediate water loss from cells under hypertonic conditions. Water transport via HvPIP aquaporins and the significance of reductions of Lp(r) in barley plants during salinity stress are discussed.

Vacuolar proton pumps and aquaporins involved in rapid internode elongation of deepwater rice

Rapid growth of the submerged shoots of deepwater rice is essential for survival during the rainy season. We investigated changes in the expression of vacuolar H(+)-ATPase (V-ATPase), H(+)-pyrophosphatase (V-PPase), and aquaporins under submerged conditions. The amounts of vacuolar proton pumps, which support the active transport of ions into the vacuoles, were maintained on a membrane protein basis in the developing vacuoles. Among the six isogenes of V-PPase, OsVHP1;3 was markedly enhanced by submersion. The gene expression of efficient water channels, OsTIP1;1, OsTIP2;2, OsPIP1;1, OsPIP2;1, and OsPIP2;2, was markedly enhanced by submersion. The increase in aquaporin expression might support quick elongation of internodes. The mRNA levels of OsNIP2;2 and OsNIP3;1, which transport silicic and boric acids respectively, clearly decreased. The present study indicates that internodes of deepwater rice upregulate vacuolar proton pumps and water channel aquaporins and downregulate aquaporins that allow permeation of the substrates that suppress internode growth.

Water uptake by seminal and adventitious roots in relation to whole-plant water flow in barley (Hordeum vulgare L.)

Prior to an assessment of the role of aquaporins in root water uptake, the main path of water movement in different types of root and driving forces during day and night need to be known. In the present study on hydroponically grown barley (Hordeum vulgare L.) the two main root types of 14- to 17-d-old plants were analysed for hydraulic conductivity in dependence of the main driving force (hydrostatic, osmotic). Seminal roots contributed 92% and adventitious roots 8% to plant water uptake. The lower contribution of adventitious compared with seminal roots was associated with a smaller surface area and number of roots per plant and a lower axial hydraulic conductance, and occurred despite a less-developed endodermis. The radial hydraulic conductivity of the two types of root was similar and depended little on the prevailing driving force, suggesting that water uptake occurred along a pathway that involved crossing of membrane(s). Exudation experiments showed that osmotic forces were sufficient to support night-time transpiration, yet transpiration experiments and cuticle permeance data questioned the significance of osmotic forces. During the day, 90% of water uptake was driven by a tension of about -0.15 MPa.

A conserved primary salt tolerance mechanism mediated by HKT transporters: a mechanism for sodium exclusion and maintenance of high K(+)/Na(+) ratio in leaves during salinity stress

Increasing soil salinity is a serious threat to agricultural productions worldwide in the 21st century. Several essential Na(+) transporters such as AtNHX1 and AtSOS1 function in Na(+) tolerance under salinity stress in plants. Recently, evidence for a new primary salt tolerance mechanism has been reported, which is mediated by a class of HKT transporters both in dicots such as Arabidopsis and monocot crops such as rice and wheat. Here we present a review on vital physiological functions of HKT transporters including AtHKT1;1 and OsHKT1;5 in preventing shoot Na(+) over-accumulation by mediating Na(+) exclusion from xylem vessels in the presence of a large amount of Na(+) thereby protecting leaves from salinity stress. Findings of the HKT2 transporter sub-family are also updated in this review. Subjects regarding function and regulation of HKT transporters, which need to be elucidated in future research, are discussed.