Differences in aquaporin levels among cell types of radish and measurement of osmotic water permeability of individual protoplasts

Contaminants of emerging concerns in recycled water: Fate and risks in agroecosystems

Recycled water (RW) has been increasingly recognized as a valuable source of water for alleviating the global water crisis. When RW is used for agricultural irrigation, many contaminants of emerging concern (CECs) are introduced into the agroecosystem. The ubiquity of CECs in field soil, combined with the toxic, carcinogenic, or endocrine-disrupting nature of some CECs, raises significant concerns over their potential risks to the environment and human health. Understanding such risks and delineating the fate processes of CECs in the water-soil-plant continuum contributes to the safe reuse of RW in agriculture. This review summarizes recent findings and provides an overview of CECs in the water-soil-plant continuum, including their occurrence in RW and irrigated soil, fate processes in agricultural soil, offsite transport including runoff and leaching, and plant uptake, metabolism, and accumulation. The potential ecological and human health risks of CECs are also discussed. Studies to date have shown limited accumulation of CECs in irrigated soils and plants, which may be attributed to multiple attenuation processes in the rhizosphere and plant, suggesting minimal health risks from RW-fed food crops. However, our collective understanding of CECs is rather limited and knowledge of their offsite movement and plant accumulation is particularly scarce for field conditions. Given a large number of CECs and their occurrence at trace levels, it is urgent to develop strategies to prioritize CECs so that future research efforts are focused on CECs with elevated risks for offsite contamination or plant accumulation. Irrigating specific crops such as feed crops and fruit trees may be a viable option to further minimize potential plant accumulation under field conditions. To promote the beneficial reuse of RW in agriculture, it is essential to understand the human health and ecological risks imposed by CEC mixtures and metabolites.

Source apportionment and crop bioaccumulation of perfluoroalkyl acids and novel alternatives in an industrial-intensive region with fluorochemical production, China: Health implications for human exposure

Due to their great environmental hazards, the widely used legacy perfluoroalkyl acids (PFAAs) are gradually restricted, and novel alternatives are being developed and applied. For efficient control of emerging environmental risks in agricultural production, we systematically studied the source apportionment in field soils and bioaccumulation characteristics in multiple crops of 12 PFAAs and five novel alternatives in an industrial-intensive region of China, followed by human exposure estimation and health risk assessment. Compared with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), shorter-chained PFAAs and novel alternatives have become the dominant components in local soils and crops, indicating their wide application. A positive matrix factorization (PMF) model coupled with multivariate analysis identified fluoropolymer manufacturing and textile treatment as the principal sources. The bioaccumulation factors (BAFs) of individual PFAAs and alternatives in crops decreased with increasing carbon chain lengths. As a novel alternative of PFOA, hexafluoropropylene oxide dimer acid (GenX) exhibited much higher BAFs; for the alternative of PFOS, 6:2 chlorinated polyfluorinated ether sulfonic acid (6:2 Cl-PFESA) showed lower BAFs. The bioaccumulation capacities of PFAAs and alternatives were also associated with soil organic matter and crop species. Through crop consumption, short-chained PFAAs and novel alternatives might pose emerging human health threats.

Age dependence of the hydraulic resistances of the plasma membrane and the tonoplast (vacuolar membrane) in cells of Chara corallina

Hydraulic resistances (reciprocals of hydraulic conductivities) of the cell (Lp^-1), the cell wall (Lp_w^-1), the membrane (Lp_m^-1), the plasma membrane (Lp_pm^-1), and the tonoplast (Lp_tp^-1) were determined in individual internodal cells of Chara corallina and their dependence on the cell age was studied. The thickness of the cell wall (d) was adopted as an index of the cell age, since the cell wall of spring-grown young cells (sg-cells) was found to be significantly thinner than that of winter-spent old cells (ws-cells). Both Lp_w^-1 and Lp_m^-1 were found to increase with cell age. Since Lp_m^-1 is the sum of Lp_pm^-1 and Lp_tp^-1, their dependence on the wall thickness was studied. It was found that both Lp_pm^-1 and Lp_tp^-1 increase with cell age using d as a proxy and that the former is distinctly higher than the latter. The ratio Lp_pm^-1/Lp_tp^-1 amounts to 30 for 5 μm of d, indicating that the tonoplast is a negligible barrier to osmotic water flow. The ratio decreases with the increase in d and amounts to 5.0 for 11 μm of d, showing that the tonoplast ages faster than the plasma membrane. The physiological meaning of the age dependence of hydraulic resistance of the tonoplast was discussed in terms of the role of the vacuole in the osmoregulation of the cytoplasm.

Extending the knowledge about PFAS bioaccumulation factors for agricultural plants - A review

A main source of perfluoroalkyl and polyfluoroalkyl substances (PFASs) residues in agricultural plants is their uptake from contaminated soil. Bioaccumulation factors (BAFs) can be an important tool to derive recommendations for cultivation or handling of crops prior consumption. This review compiles >4500 soil-to-plant BAFs for 45 PFASs from 24 studies involving 27 genera of agricultural crops. Grasses (Poaceae) provided most BAFs with the highest number of values for perfluorooctanoic acid and perfluorooctane sulfonic acid. Influencing factors on PFAS transfer like compound-specific properties (hydrophobicity, chain length, functional group, etc.), plant species, compartments, and other boundary conditions are critically discussed. Throughout the literature, BAFs were higher for vegetative plant compartments than for reproductive and storage organs. Decreasing BAFs per additional perfluorinated carbon were clearly apparent for aboveground parts (up to 1.16 in grains) but not always for roots (partly down to zero). Combining all BAFs per single perfluoroalkyl carboxylic acid (C4-C14) and sulfonic acid (C4-C10), median log BAFs decreased by -0.25(±0.029) and -0.24(±0.013) per fluorinated carbon, respectively. For the first time, the plant uptake of ultra-short-chain (≤ C3) perfluoroalkyl acids (PFAAs) was reviewed and showed a ubiquitous occurrence of trifluoroacetic acid in plants independent from the presence of other PFAAs. Based on identified knowledge gaps, it is suggested to focus on the uptake of precursors to PFAAs, PFAAs ≤C3, and additional emerging PFASs such as GenX or fluorinated ethers in future research. Studies regarding the uptake of PFASs by sugar cane, which accounts for about one fifth of the global crop production, are completely lacking and are also recommended. Furthermore, aqueous soil leachates should be tested as an alternative to the solvent extraction of soils as a base for BAF calculations.

Multiple crop bioaccumulation and human exposure of perfluoroalkyl substances around a mega fluorochemical industrial park, China: Implication for planting optimization and food safety

Perfluoroalkyl substances (PFASs) have become a recognized concern due to their mobility, persistence, ubiquity and health hazards in the environment. In this study, ten types of vegetables and three types of grain crops were collected in two open-air fields with different distances (0.3 km, 10 km) from a mega fluorochemical industrial park (FIP), China. Bioaccumulation characteristics of PFASs in light of crop types and organs were explored, followed by analyzing human exposure and risks to local residents with different age groups and dietary habits. Elevated levels of ∑PFASs were found nearby the FIP ranging from 79.9 ng/g to 200 ng/g in soils and from 58.8 ng/g to 8085 ng/g in crops. Perfluorooctanoic acid (PFOA) was the predominant PFAS component in soil; while shorter-chain perfluoroalkyl carboxylic acids (PFCAs), especially perfluorobutanoic acid (PFBA), were the major PFAS contaminants in multiple crops, resulting from their bioaccumulation preference. Depending on the crop types, the bioaccumulation factors (BAFs) of ∑PFASs for edible parts varied from 0.36 to 48.0, and the highest values were found in shoot vegetables compared with those in fruit vegetables, flower vegetables, root vegetables and grain crops. For typical grains, the BAFs of ∑PFASs decreased in the order of soybean (Glycine max (Linn.) Merr.), wheat (Triticum aestivum L.) and corn (Zea mays L.), possibly related to their protein and lipid content. Among specific organs in the whole plants, leaves exhibited the highest BAFs of ∑PFASs compared with corresponding roots, stems, husks or grains. With increasing carbon chain lengths of individual PFCAs (C4-C8), the logarithm of their BAFs for edible parts of various crops showed a linear decrease (0.1-1.16 log decrease per CF₂ unit), and the largest decrease was observed in grains. Human exposure to PFOA via the consumption of contaminated crops represents a health risk for local residents, especially for low-age consumers or urban consumers with higher vegetable diet. Implications for planting optimization and food safety were provided aiming to reduce health hazards of PFASs.

Zinc and Cadmium Mapping in the Apical Shoot and Hypocotyl Tissues of Radish by High-Resolution Secondary Ion Mass Spectrometry (NanoSIMS) after Short-Term Exposure to Metal Contamination

Zinc (as an essential phytonutrient) and cadmium (as a toxic but readily bioavailable nonessential metal for plants) share similar routes for crossing plant biomembranes, although with a substantially different potential for translocation into above-ground tissues. The in situ distribution of these metals in plant cells and tissues (particularly intensively-dividing and fast-growing areas) is poorly understood. In this study, 17-day-old radish (Raphanus sativus L.) plants grown in nutrient solution were subjected to short-term (24 h) equimolar contamination (2.2 µM of each ⁷⁰Zn and Cd) to investigate their accumulation and distribution in the shoot apex (leaf primordia) and edible fleshy hypocotyl tissues. After 24-h exposure, radish hypocotyl had similar concentration (in µg/g dry weight) of ⁷⁰Zn (12.1 ± 1.1) and total Cd (12.9 ± 0.8), with relatively limited translocation of both metals to shoots (concentrations lower by 2.5-fold for ⁷⁰Zn and 4.8-fold for Cd) as determined by inductively-coupled plasma mass spectrometry (ICP-MS). The in situ Zn/Cd distribution maps created by high-resolution secondary ion mass spectrometry (NanoSIMS, Cameca, Gennevilliers, France) imaging corresponded well with the ICP-MS data, confirming a similar pattern and uniform distribution of ⁷⁰Zn and Cd across the examined areas. Both applied techniques can be powerful tools for quantification (ICP-MS) and localisation and visualisation (NanoSIMS) of some ultra-trace isotopes in the intensively-dividing cells and fast-growing tissues of non-metalophytes even after short-term metal exposure. The results emphasise the importance of the quality of (agro)ecosystem resources (growing media, metal-contaminated soils/waters) in the public health risk, given that, even under low contamination and short-term exposure, some of the most toxic metallic ions (e.g., Cd) can relatively rapidly enter the human food chain.

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.

Genome-Wide Characterization and Expression Analysis of Major Intrinsic Proteins during Abiotic and Biotic Stresses in Sweet Orange (Citrus sinensis L. Osb.)

The family of aquaporins (AQPs), or major intrinsic proteins (MIPs), includes integral membrane proteins that function as transmembrane channels for water and other small molecules of physiological significance. MIPs are classified into five subfamilies in higher plants, including plasma membrane (PIPs), tonoplast (TIPs), NOD26-like (NIPs), small basic (SIPs) and unclassified X (XIPs) intrinsic proteins. This study reports a genome-wide survey of MIP encoding genes in sweet orange (Citrus sinensis L. Osb.), the most widely cultivated Citrus spp. A total of 34 different genes encoding C. sinensis MIPs (CsMIPs) were identified and assigned into five subfamilies (CsPIPs, CsTIPs, CsNIPs, CsSIPs and CsXIPs) based on sequence analysis and also on their phylogenetic relationships with clearly classified MIPs of Arabidopsis thaliana. Analysis of key amino acid residues allowed the assessment of the substrate specificity of each CsMIP. Gene structure analysis revealed that the CsMIPs possess an exon-intron organization that is highly conserved within each subfamily. CsMIP loci were precisely mapped on every sweet orange chromosome, indicating a wide distribution of the gene family in the sweet orange genome. Investigation of their expression patterns in different tissues and upon drought and salt stress treatments, as well as with ‘Candidatus Liberibacter asiaticus’ infection, revealed a tissue-specific and coordinated regulation of the different CsMIP isoforms, consistent with the organization of the stress-responsive cis-acting regulatory elements observed in their promoter regions. A special role in regulating the flow of water and nutrients is proposed for CsTIPs and CsXIPs during drought stress, and for most CsMIPs during salt stress and the development of HLB disease. These results provide a valuable reference for further exploration of the CsMIPs functions and applications to the genetic improvement of both abiotic and biotic stress tolerance in citrus.

Measuring the osmotic water permeability coefficient (Pf) of spherical cells: isolated plant protoplasts as an example

Studying AQP regulation mechanisms is crucial for the understanding of water relations at both the cellular and the whole plant levels. Presented here is a simple and very efficient method for the determination of the osmotic water permeability coefficient (P(f)) in plant protoplasts, applicable in principle also to other spherical cells such as frog oocytes. The first step of the assay is the isolation of protoplasts from the plant tissue of interest by enzymatic digestion into a chamber with an appropriate isotonic solution. The second step consists of an osmotic challenge assay: protoplasts immobilized on the bottom of the chamber are submitted to a constant perfusion starting with an isotonic solution and followed by a hypotonic solution. The cell swelling is video recorded. In the third step, the images are processed offline to yield volume changes, and the time course of the volume changes is correlated with the time course of the change in osmolarity of the chamber perfusion medium, using a curve fitting procedure written in Matlab (the 'PfFit'), to yield P(f).

Perfluoroalkyl acid distribution in various plant compartments of edible crops grown in biosolids-amended soils

Crop uptake of perfluoroalkyl acids (PFAAs) from biosolids-amended soil has been identified as a potential pathway for PFAA entry into the terrestrial food chain. This study compared the uptake of PFAAs in greenhouse-grown radish (Raphanus sativus), celery (Apium graveolens var. dulce), tomato (Lycopersicon lycopersicum), and sugar snap pea (Pisum sativum var. macrocarpon) from an industrially impacted biosolids-amended soil, a municipal biosolids-amended soil, and a control soil. Individual concentrations of PFAAs, on a dry weight basis, in mature, edible portions of crops grown in soil amended with PFAA industrially impacted biosolids were highest for perfluorooctanoate (PFOA; 67 ng/g) in radish root, perfluorobutanoate (PFBA; 232 ng/g) in celery shoot, and PFBA (150 ng/g) in pea fruit. Comparatively, PFAA concentrations in edible compartments of crops grown in the municipal biosolids-amended soil and in the control soil were less than 25 ng/g. Bioaccumulation factors (BAFs) were calculated for the root, shoot, and fruit compartments (as applicable) of all crops grown in the industrially impacted soil. BAFs were highest for PFBA in the shoots of all crops, as well as in the fruit compartment of pea. Root-soil concentration factors (RCFs) for tomato and pea were independent of PFAA chain length, while radish and celery RCFs showed a slight decrease with increasing chain length. Shoot-soil concentration factors (SCFs) for all crops showed a decrease with increasing chain length (0.11 to 0.36 log decrease per CF2 group). The biggest decrease (0.54-0.58 log decrease per CF2 group) was seen in fruit-soil concentration factors (FCFs). Crop anatomy and PFAA properties were utilized to explain data trends. In general, fruit crops were found to accumulate fewer long-chain PFAAs than shoot or root crops presumably due to an increasing number of biological barriers as the contaminant is transported throughout the plant (roots to shoots to fruits). These data were incorporated into a preliminary conceptual framework for PFAA accumulation in edible crops. In addition, these data suggest that edible crops grown in soils conventionally amended for nutrients with biosolids (that are not impacted by PFAA industries) are unlikely a significant source of long-chain PFAA exposure to humans.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.

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.

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.

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.

Overexpression of PIP2;5 aquaporin alleviates effects of low root temperature on cell hydraulic conductivity and growth in Arabidopsis

The effects of low root temperature on growth and root cell water transport were compared between wild-type Arabidopsis (Arabidopsis thaliana) and plants overexpressing plasma membrane intrinsic protein 1;4 (PIP1;4) and PIP2;5. Descending root temperature from 25°C to 10°C quickly reduced cell hydraulic conductivity (L(p)) in wild-type plants but did not affect L(p) in plants overexpressing PIP1;4 and PIP2;5. Similarly, when the roots of wild-type plants were exposed to 10°C for 1 d, L(p) was lower compared with 25°C. However, there was no effect of low root temperature on L(p) in PIP1;4- and PIP2;5-overexpressing plants after 1 d of treatment. When the roots were exposed to 10°C for 5 d, L(p) was reduced in wild-type plants and in plants overexpressing PIP1;4, whereas there was still no effect in PIP2;5-overexpressing plants. These results suggest that the gating mechanism in PIP1;4 may be more sensitive to prolonged low temperature compared with PIP2;5. The reduction of L(p) at 10°C in roots of wild-type plants was partly restored to the preexposure level by 5 mm Ca(NO(3))(2) and protein phosphatase inhibitors (75 nm okadaic acid or 1 μm Na(3)VO(4)), suggesting that aquaporin phosphorylation/dephosphorylation processes were involved in this response. The temperature sensitivity of cell water transport in roots was reflected by a reduction in shoot and root growth rates in the wild-type and PIP1;4-overexpressing plants exposed to 10°C root temperature for 5 d. However, low root temperature had no effect on growth in plants overexpressing PIP2;5. These results provide strong evidence for a link between growth at low root temperature and aquaporin-mediated root water transport in Arabidopsis.

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

BACKGROUND INFORMATION

Transmembrane water flow is aided by water-specific channel proteins, aquaporins. Plant genomes code for approx. 35 expressed and functional aquaporin isoforms. Plant aquaporins fall into four different subfamilies of which the PIPs (plasma membrane intrinsic proteins) constitute the largest and evolutionarily most conserved subfamily with 13 members in Arabidopsis and maize. Furthermore, the PIPs can be divided into two phylogenetic groups, PIP1 and PIP2, of which the PIP1 isoforms are most tightly conserved, sharing >90% amino acid sequence identity. As the nomenclature implies, the majority of PIPs have been shown to be localized at the plasma membrane. Recently, two highly abundant plasma membrane aquaporins, SoPIP2;1 and SoPIP1;2, have been purified and structurally characterized.

RESULTS

We report the cloning of a cDNA encoding SoPIP1;2 and show that there are at least five additional sequences homologous with SoPIP2;1 and SoPIP1;2 in the spinach genome. To understand their role in planta, we have investigated the cellular localization of the aquaporin homologues SoPIP1;2 and SoPIP1;1. By Western- and Northern-blot analyses and by immunocytochemical detection at the light and electron microscopic levels, we show that SoPIP1;2 is highly expressed in phloem sieve elements of leaves, roots and petioles and that SoPIP1;1 is present in stomatal guard cells.

CONCLUSIONS

Localization of the two abundant aquaporin isoforms suggests roles for specific PIPs of the PIP1 subgroup in phloem loading, transport and unloading, and in stomatal movements.

An auxin gradient and maximum in the Arabidopsis root apex shown by high-resolution cell-specific analysis of IAA distribution and synthesis

Local concentration gradients of the plant growth regulator auxin (indole-3-acetic acid [IAA]) are thought to instruct the positioning of organ primordia and stem cell niches and to direct cell division, expansion, and differentiation. High-resolution measurements of endogenous IAA concentrations in support of the gradient hypothesis are required to substantiate this hypothesis. Here, we introduce fluorescence-activated cell sorting of green fluorescent protein-marked cell types combined with highly sensitive mass spectrometry methods as a novel means for analyses of IAA distribution and metabolism at cellular resolution. Our results reveal the presence of IAA concentration gradients within the Arabidopsis thaliana root tip with a distinct maximum in the organizing quiescent center of the root apex. We also demonstrate that the root apex provides an important source of IAA and that cells of all types display a high synthesis capacity, suggesting a substantial contribution of local biosynthesis to auxin homeostasis in the root tip. Our results indicate that local biosynthesis and polar transport combine to produce auxin gradients and maxima in the root tip.

Roles of morphology, anatomy, and aquaporins in determining contrasting hydraulic behavior of roots

The contrasting hydraulic properties of wheat (Triticum aestivum), narrow-leafed lupin (Lupinus angustifolius), and yellow lupin (Lupinus luteus) roots were identified by integrating measurements of water flow across different structural levels of organization with anatomy and modeling. Anatomy played a major role in root hydraulics, influencing axial conductance (L(ax)) and the distribution of water uptake along the root, with a more localized role for aquaporins (AQPs). Lupin roots had greater L(ax) than wheat roots, due to greater xylem development. L(ax) and root hydraulic conductance (L(r)) were related to each other, such that both variables increased with distance from the root tip in lupin roots. L(ax) and L(r) were constant with distance from the tip in wheat roots. Despite these contrasting behaviors, the hydraulic conductivity of root cells (Lp(c)) was similar for all species and increased from the root surface toward the endodermis. Lp(c) was largely controlled by AQPs, as demonstrated by dramatic reductions in Lp(c) by the AQP blocker mercury. Modeling the root as a series of concentric, cylindrical membranes, and the inhibition of AQP activity at the root level, indicated that water flow in lupin roots occurred primarily through the apoplast, without crossing membranes and without the involvement of AQPs. In contrast, water flow across wheat roots crossed mercury-sensitive AQPs in the endodermis, which significantly influenced L(r). This study demonstrates the importance of examining root morphology and anatomy in assessing the role of AQPs in root hydraulics.

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.

Expanding roles of plant aquaporins in plasma membranes and cell organelles

Aquaporins facilitate water transport across biomembranes in a manner dependent on osmotic pressure and water-potential gradient. The discovery of aquaporins has facilitated research on intracellular and whole-plant water transport at the molecular level. Aquaporins belong to a ubiquitous family of membrane intrinsic proteins (MIP). Plants have four subfamilies: plasma-membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), and small basic intrinsic protein (SIP). Recent research has revealed a diversity of plant aquaporins, especially their physiological functions and intracellular localisation. A few PIP members have been reported to be involved in carbon dioxide permeability of cells. Newly identified transport substrates for NIP members of rice and Arabidopsis thaliana have been demonstrated to transport silicon and boron, respectively. Ammonia, glycerol, and hydrogen peroxide have been identified as substrates for plant aquaporins. The intracellular localisation of plant aquaporins is diverse; for example, SIP members are localised on the ER membrane. There has been much progress in the research on the functional regulation of water channel activity of PIP members including phosphorylation, formation of hetero-oligomer, and protonation of histidine residues under acidic condition. This review provides a broad overview of the range of potential aquaporins, which are now believed to participate in the transport of several small molecules in various membrane systems in model plants, crops, flowers and fruits.

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.

Osmotic water permeability of plasma and vacuolar membranes in protoplasts II: theoretical basis

Water permeability of the plasma membrane (PM) and the vacuolar membrane (VM) is important for intracellular and transcellular water movement in plants, because mature plant cells have large central vacuoles. We have developed a new method for measuring the osmotic water permeability of the PM and VM (P ( f1) and P ( f2), respectively) in individual plant cells. Here, the theoretical basis and procedure of the method are discussed. Protoplasts isolated from higher plant tissues are used to measure P ( f1) and P ( f2). Because of the semi-permeability (selective permeability) of cellular membranes, protoplasts swell or shrink under hypotonic or hypertonic conditions. A theoretical three-compartment model is presented for simulating time-dependent volume changes in the vacuolar and cytoplasmic spaces in a protoplast during osmotic excursions. The model describes the theoretical relationships between P ( f1), P ( f2) and the bulk osmotic water permeability of protoplasts (P ( f(bulk))). The procedure for measuring the osmotic water permeability is: (1) P ( f(bulk)) is calculated from the time when half of the total change in protoplast volume is completed, by assuming that the protoplast has a single barrier to water movement across it (two-compartment model); (2) P ( f2) of vacuoles isolated from protoplasts is obtained in the same manner; and (3) P ( f1) is determined from P ( f(bulk)) and P ( f2) according to the three-compartment model. The theoretical relationship between P ( fl ) (m s(-1)) and L ( Pl ) (hydraulic conductivity, l=1, 2) (m s(-1) Pa(-1)) is also discussed.

Osmotic water permeability of plasma and vacuolar membranes in protoplasts I: high osmotic water permeability in radish (Raphanus sativus) root cells as measured by a new method

Intra- and transcellular water movements in plants are regulated by the water permeability of the plasma membrane (PM) and vacuolar membrane (VM) in plant cells. In the present study, we investigated the osmotic water permeability of both PM (P ( f1)) and VM (P ( f2)), as well as the bulk osmotic water permeability of a protoplast (P ( f(bulk))) isolated from radish (Raphanus sativus) roots. The values of P ( f(bulk)) and P ( f2) were determined from the swelling/shrinking rate of protoplasts and isolated vacuoles under hypo- or hypertonic conditions. In order to minimize the effect of unstirred layer, we monitored dropping or rising protoplasts (vacuoles) in sorbitol solutions as they swelled or shrunk. P ( f1) was calculated from P ( f(bulk)) and P ( f2) by using the 'three-compartment model', which describes the theoretical relationship between P ( f1), P ( f2) and P ( f(bulk)) (Kuwagata and Murai-Hatano in J Plant Res, 2007). The time-dependent changes in the volume of protoplasts and isolated vacuoles fitted well to the theoretical curves, and solute permeation of PM and VM was able to be neglected for measuring the osmotic water permeability. High osmotic water permeability of more than 500 mum s(-1), indicating high activity of aquaporins (water channels), was observed in both PM and VM in radish root cells. This method has the advantage that P ( f1) and P ( f2) can be measured accurately in individual higher plant cells.

Measuring the osmotic water permeability of the plant protoplast plasma membrane: implication of the nonosmotic volume

Starting from the original theoretical descriptions of osmotically induced water volume flow in membrane systems, a convenient procedure to determine the osmotic water permeability coefficient (P (os)) and the relative nonosmotic volume (beta) of individual protoplasts is presented. Measurements performed on protoplasts prepared from pollen grains and pollen tubes of Lilium longiflorum cv. Thunb. and from mesophyll cells of Nicotiana tabacum L. and Arabidopsis thaliana revealed low values for the osmotic water permeability coefficient in the range 5-20 microm.s(-1) with significant differences in P (os), depending on whether beta is considered or not. The value of beta was determined using two different methods: by interpolation from Boyle-van't Hoff plots or by fitting a solution of the theoretical equation for water volume flow to the whole volume transients measured during osmotic swelling. The values determined with the second method were less affected by the heterogeneity of the protoplast samples and were around 30% of the respective isoosmotic protoplast volume. It is therefore important to consider nonosmotic volume in the calculation of P (os) as plant protoplasts behave as nonideal osmometers.

Immunochemical analysis of aquaporin isoforms in Arabidopsis suspension-cultured cells

Aquaporins mediate the movement of water across biomembranes. Arabidopsis thaliana contains 35 aquaporins that belong to four subfamilies (PIP, TIP, SIP, and NIP). We investigated their expression profiles immunochemically in suspension-cultured Arabidopsis thaliana cells during growth and in response to salt and osmotic stresses. Protein amounts of all aquaporins were much lower in cultured cells than in the plant tissues. This is consistent with the low water permeability of protoplasts from cultured cells. After treatment with NaCl, the protein amounts of PIP2;1, PIP2;2, and PIP2;3 in the cells increased several-fold, and those of TIP1;1 and TIP1;2, 15- and 3-fold respectively. PIP1 did not change under the stress. Cell death began after 19 d in culture, accompanied by marked accumulation of PIPs and TIPs and a gradual decrease in SIPs. Our results suggest the followings: (i) Accumulation of aquaporin isoforms was individually regulated at low levels in single cells. (ii) At least PIP2;2, PIP2;3, TIP1;1, and TIP1;2 are stress-responsive aquaporins in suspension cells. (iii) A sudden increment of several members of PIP2 and TIP1 subfamilies might be related to cell death.

Water relations and an expression analysis of plasma membrane intrinsic proteins in sensitive and tolerant rice during chilling and recovery

A symptom of chilling injury is development of water deficit in shoots, resulting from an imbalance of water transport and transpiration. In this work, two rice varieties (Oryza sativa L. var. Wasetoitsu and Somewake) seedlings were chilled at 7 degrees C, followed by recovery at 28 degrees C. Based on the growth phenotype and electrolyte leakage tests, Somewake was shown to be a chilling-tolerant variety, and Wasetoitsu a chilling-sensitive one. The chilling stress reduced markedly the relative water content (RWC) of leaves, accumulative transpiration and osmotic root hydraulic conductivity (Lp) in both varieties. But when returned to 28 degrees C, the water relation balance of Somewake recovered better. The mRNA expression profile of all the 11 plasma membrane intrinsic proteins (PIPs), a subgroup of aquaporins, was subsequently determined by real-time reverse transcription (RT)-PCR with TaqMan-minor grove binder (MGB) probes derived from rice var. Nipponbare during chilling treatment and recovery. Most of the PIP genes was down-regulated at the low temperature, and recovered at the warm temperature. The relative expression of some PIPs in both Somewake and Wasetoitsu decreased in parallel during the chilling. However during the recovery, the relative expression of OsPIP1;1, OsPIP2;1, OsPIP2;7 in shoots and OsPIP1;1, OsPIP2;1 in roots were significantly higher in Somewake than Wasetoitsu. This supports the role of PIPs in re-establishing water balance after chilling conditions. We discuss the diversified roles played by members of the aquaporin PIP subfamily in plant chilling tolerance depending on aquaporin isoforms, plant tissue and the stage of chilling duration.

Seasonal courses of maximum hydraulic conductance in shoots of six temperate deciduous tree species

The seasonal course of maximum hydraulic conductance of leaf laminae (K _lamina) of shoots correlated strongly with the seasonal course of the maximum hydraulic conductance of leaf laminae of HgCl₂-treated shoots (K _{lamina(HgCl2)}), and with the seasonal course of the difference (dK _lamina) between K _lamina and K _{lamina(HgCl2)}. However, it did not correlate strongly with the seasonal course of the hydraulic conductance of stem and petioles of the shoot (K _stpt) in six temperate deciduous tree species. The species ranked according to K _lamina as follows: Populus tremula L. > Salix caprea L. > Padus avium Mill. > Quercus robur L. > Tilia cordata Mill. > Acer platanoides L. The species-specific maxima of K _lamina correlated positively with the simultaneous values of K _{lamina(HgCl2)}, dK _lamina and K _stpt; the correlation was strongest with K _{lamina(HgCl2)}. It was concluded that the seasonal dynamics of maximum hydraulic conductance of leaf laminae was determined almost equally by the seasonal dynamics of the hydraulic conductance of foliar protoplasts and apoplast, but the inter-specific differences in K _lamina were mainly caused by the different apoplastic hydraulic conductance in leaves of these species. The relative contribution of dK _lamina (in K _lamina) was highest in slow-growing species (~55% in A. platanoides) and the lowest in fast-growers (~30% in S. caprea).

Plasma membrane of Beta vulgaris storage root shows high water channel activity regulated by cytoplasmic pH and a dual range of calcium concentrations

Plasma membrane vesicles isolated by two-phase partitioning from the storage root of Beta vulgaris show atypically high water permeability that is equivalent only to those reported for active aquaporins in tonoplast or animal red cells (Pf=542 microm s(-1)). The values were determined from the shrinking kinetics measured by stopped-flow light scattering. This high Pf was only partially inhibited by mercury (HgCl2) but showed low activation energy (Ea) consistent with water permeation through water channels. To study short-term regulation of water transport that could be the result of channel gating, the effects of pH, divalent cations, and protection against dephosphorylation were tested. The high Pf observed at pH 8.3 was dramatically reduced by medium acidification. Moreover, intra-vesicular acidification (corresponding to the cytoplasmic face of the membrane) shut down the aquaporins. De-phosphorylation was discounted as a regulatory mechanism in this preparation. On the other hand, among divalent cations, only calcium showed a clear effect on aquaporin activity, with two distinct ranges of sensitivity to free Ca2+ concentration (pCa 8 and pCa 4). Since the normal cytoplasmic free Ca2+ sits between these ranges it allows for the possibility of changes in Ca2+ to finely up- or down-regulate water channel activity. The calcium effect is predominantly on the cytoplasmic face, and inhibition corresponds to an increase in the activation energy for water transport. In conclusion, these findings establish both cytoplasmic pH and Ca2+ as important regulatory factors involved in aquaporin gating.

Regulation of plant aquaporin activity

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

The novel herbicide oxaziclomefone inhibits cell expansion in maize cell cultures without affecting turgor pressure or wall acidification

Oxaziclomefone [OAC; IUPAC name 3-(1-(3,5-dichlorophenyl)-1-methylethyl)-3,4-dihydro-6-methyl-5-phenyl-2H-1,3-oxazin-4-one] is a new herbicide that inhibits cell expansion in grass roots. Its effects on cell cultures and mode of action were unknown. In principle, cell expansion could be inhibited by a decrease in either turgor pressure or wall extensibility. Cell expansion was estimated as settled cell volume; cell division was estimated by cell counting. Membrane permeability to water was measured by a novel method involving simultaneous assay of the efflux of (3)H(2)O and [(14)C]mannitol from a 'bed' of cultured cells. Osmotic potential was measured by depression of freezing point. OAC inhibited cell expansion in cultures of maize (Zea mays), spinach (Spinacia oleracea) and rose (Rosa sp.), with an ID(50) of 5, 30 and 250 nm, respectively. In maize cultures, OAC did not affect cell division for the first 40 h. It did not affect the osmotic potential of cell sap or culture medium, nor did it impede water transport across cell membranes. It did not affect cells' ability to acidify the apoplast (medium), which may be necessary for 'acid growth'. As OAC did not diminish turgor pressure, its ability to inhibit cell expansion must depend on changes in wall extensibility. It could be a valuable tool for studies on cell expansion.

Plant protoplasts: status and biotechnological perspectives

Plant protoplasts ("naked" cells) provide a unique single cell system to underpin several aspects of modern biotechnology. Major advances in genomics, proteomics, and metabolomics have stimulated renewed interest in these osmotically fragile wall-less cells. Reliable procedures are available to isolate and culture protoplasts from a range of plants, including both monocotyledonous and dicotyledonous crops. Several parameters, particularly the source tissue, culture medium, and environmental factors, influence the ability of protoplasts and protoplast-derived cells to express their totipotency and to develop into fertile plants. Importantly, novel approaches to maximise the efficiency of protoplast-to-plant systems include techniques already well established for animal and microbial cells, such as electrostimulation and exposure of protoplasts to surfactants and respiratory gas carriers, especially perfluorochemicals and hemoglobin. However, despite at least four decades of concerted effort and technology transfer between laboratories worldwide, many species still remain recalcitrant in culture. Nevertheless, isolated protoplasts are unique to a range of experimental procedures. In the context of plant genetic manipulation, somatic hybridisation by protoplast fusion enables nuclear and cytoplasmic genomes to be combined, fully or partially, at the interspecific and intergeneric levels to circumvent naturally occurring sexual incompatibility barriers. Uptake of isolated DNA into protoplasts provides the basis for transient and stable nuclear transformation, and also organelle transformation to generate transplastomic plants. Isolated protoplasts are also exploited in numerous miscellaneous studies involving membrane function, cell structure, synthesis of pharmaceutical products, and toxicological assessments. This review focuses upon the most recent developments in protoplast-based technologies.

Dynamic changes in the osmotic water permeability of protoplast plasma membrane

The osmotic water permeability coefficient (P(f)) of plasma membrane of maize (Zea mays) Black Mexican Sweet protoplasts changed dynamically during a hypoosmotic challenge, as revealed using a model-based computational approach. The best-fitting model had three free parameters: initial P(f), P(f) rate-of-change (slope(P(f))), and a delay, which were hypothesized to reflect changes in the number and/or activity of aquaporins in the plasma membrane. Remarkably, the swelling response was delayed 2 to 11 s after start of the noninstantaneous (but accounted for) bath flush. The P(f) during the delay was < or =1 microm s(-1). During the swelling period following the delay, P(f) changed dynamically: within the first 15 s P(f) either (1) increased gradually to approximately 8 microm s(-1) (in the majority population of low-initial-P(f) cells) or (2) increased abruptly to 10 to 20 microm s(-1) and then decreased gradually to 3 to 6 microm s(-1) (in the minority population of high-initial-P(f) cells). We affirmed the validity of our computational approach by the ability to reproduce previously reported initial P(f) values (including the absence of delay) in control experiments on Xenopus oocytes expressing the maize aquaporin ZmPIP2;5. Although mercury did not affect the P(f) in swelling Black Mexican Sweet cells, phloretin, another aquaporin inhibitor, inhibited swelling in a predicted manner, prolonging the delay and slowing P(f) increase, thereby confirming the hypothesis that P(f) dynamics, delay included, reflected the varying activity of aquaporins.

Water channel activity of radish plasma membrane aquaporins heterologously expressed in yeast and their modification by site-directed mutagenesis

Plants contain a number of aquaporin isoforms. We developed a method for determining the water channel activity of individual isoforms of aquaporin. Six plasma membrane aquaporins (RsPIPs) and two vacuolar membrane aquaporins (RsTIPs) of radish (Raphanus sativus) were expressed heterologously in Saccharomyces cerevisiae BJ5458, which is deficient in endogenous functional aquaporin. Aquaporins were detected by immunoblot analysis with corresponding antibodies. Water permeability of membranes from yeast transformants was assayed by stopped-flow spectrophotometry. The water channel activity of members of the RsPIP2 and RsTIP subfamilies was about 10 times and 5 times greater, respectively, than that of the control; however, RsPIP1s had little (RsPIP1-2 and RsPIP1-3) or no activity (RsPIP1-1). Site-directed mutation of several residues conserved in RsPIP1s or RsPIP2s markedly altered the water transport activity. Exchange of Ile244 of RsPIP1-3 with valine increased the activity to 250% of the wild type RsPIP1-3. On the other hand, exchange of Val235 of RsPIP2-2, which corresponds to RsPIP1-3 Ile244, with isoleucine caused a marked inactivation to 45% of the original RsPIP2-2. Mutation at possible phosphorylation sites at the N- and C-terminal tails also altered the activity. These results suggest that these residues in the half-helix loop E and the tails are involved in the water transport and the functional regulation of RsPIP1 and RsPIP2.