Analysis of aquaporins in northern grasses reveal functional importance of Puccinellia nuttalliana PIP2;2 in salt tolerance.

To better understand the roles of aquaporins in salt tolerance, we cloned PIP2;1, PIP2;2, PIP2;3, PIP1;1, PIP1;3, and TIP1;1 aquaporins from three northern grasses varying is salt tolerance including the halophytic grass Puccinellia nuttalliana, moderately salt tolerant Poa juncifolia, and relatively salt sensitive Poa pratensis. We analysed aquaporin expression in roots by exposing the plants to 0 and 150 mM for 6 days in hydroponic culture. NaCl treatment upregulated several PIP transcripts in P. nuttalliana while decreasing PnuTIP1;1. The PnuPIP2;2 transcripts increased by about six-fold in P. nuttalliana, two-fold in Poa juncifolia, and did not change in Poa pratensis. The NaCl treatment enhanced the rate of water transport in yeast expressing PnuPIP2;2 by 56% compared with control. PnuPIP2,2 expression also resulted in a higher Na+ uptake in yeast cells compared with an empty vector suggesting that PnuPIP2;2 may have both water and ion transporting functions. Structural analysis revealed that the transport properties of PnuPIP2;2 could be affected by its unique pore characteristics, which include a combination of hourglass, cylindrical, and increasing diameter conical entrance shape with pore hydropathy of -0.22.

Transcriptomic Analysis of Distal Parts of Roots Reveals Potentially Important Mechanisms Contributing to Limited Flooding Tolerance of Canola (Brassica napus) Plants.

Since most of the root metabolic activities as well as root elongation and the uptake of water and mineral nutrients take place in the distal parts of roots, we aimed to gain insight into the physiological and transcriptional changes induced by root hypoxia in the distal parts of roots in canola (Brassica napus) plants, which are relatively sensitive to flooding conditions. Plants were subject to three days of root hypoxia via lowering oxygen content in hydroponic medium, and various physiological and anatomical features were examined to characterize plant responses. Untargeted transcriptomic profiling approaches were also applied to investigate changes in gene expression that took place in the distal root tissues in response to hypoxia. Plants responded to three days of root hypoxia by reducing growth and gas exchange rates. These changes were accompanied by decreases in leaf water potential (Ψleaf) and root hydraulic conductivity (Lpr). Increased deposition of lignin and suberin was also observed in the root tissues of hypoxic plants. The transcriptomic data demonstrated that the effect of hypoxia on plant water relations involved downregulation of most BnPIPs in the root tissues with the exception of BnPIP1;3 and BnPIP2;7, which were upregulated. Since some members of the PIP1 subfamily of aquaporins are known to transport oxygen, the increase in BnPIP1;3 may represent an important hypoxia tolerance strategy in plants. The results also demonstrated substantial rearrangements of different signaling pathways and transcription factors (TFs), which resulted in alterations of genes involved in the regulation of Lpr, TCA (tricarboxylic acid) cycle-related enzymes, antioxidant enzymes, and cell wall modifications. An integration of these data enabled us to draft a comprehensive model of the molecular pathways involved in the responses of distal parts of roots in B. napus. The model highlights systematic transcriptomic reprogramming aimed at explaining the relative sensitivity of Brassica napus to root hypoxia.

Salinity Tolerance of Halophytic Grass Puccinellia nuttalliana Is Associated with Enhancement of Aquaporin-Mediated Water Transport by Sodium.

In salt-sensitive plants, root hydraulic conductivity is severely inhibited by NaCl, rapidly leading to the loss of water balance. However, halophytic plants appear to effectively control plant water flow under salinity conditions. In this study, we tested the hypothesis that Na+ is the principal salt factor responsible for the enhancement of aquaporin-mediated water transport in the roots of halophytic grasses, and this enhancement plays a significant role in the maintenance of water balance, gas exchange, and the growth of halophytic plants exposed to salinity. We examined the effects of treatments with 150 mM of NaCl, KCl, and Na2SO4 to separate the factors that affect water relations and, consequently, physiological and growth responses in three related grass species varying in salt tolerance. The grasses included relatively salt-sensitive Poa pratensis, moderately salt-tolerant Poa juncifolia, and the salt-loving halophytic grass Puccinellia nuttalliana. Our study demonstrated that sustained growth, chlorophyll concentrations, gas exchange, and water transport in Puccinellia nuttalliana were associated with the presence of Na in the applied salt treatments. Contrary to the other examined grasses, the root cell hydraulic conductivity in Puccinellia nuttalliana was enhanced by the 150 mM NaCl and 150 mM Na2SO4 treatments. This enhancement was abolished by the 50 µM HgCl2 treatment, demonstrating that Na was the factor responsible for the increase in mercury-sensitive, aquaporin-mediated water transport. The observed increases in root Ca and K concentrations likely played a role in the transcriptional and (or) posttranslational regulation of aquaporins that enhanced root water transport capacity in Puccinellia nuttalliana. The study demonstrates that Na plays a key role in the aquaporin-mediated root water transport of the halophytic grass Puccinellia nuttalliana, contributing to its salinity tolerance.

Inoculation with Ericoid Mycorrhizal Associations Alleviates Drought Stress in Lowland and Upland Velvetleaf Blueberry (Vaccinium myrtilloides) Seedlings.

Although velvetleaf blueberry (Vaccinium myrtilloides) is usually associated with sandy (upland) areas of the North American boreal forest, lowland populations can be also found in bogs, suggesting possible adaptations to different site conditions. In this study, we examined the role of ericoid mycorrhizal (ERM) fungi in conferring drought resistance to the upland and lowland velvetleaf blueberry seedlings. The seedlings were inoculated with four ERM fungi (Pezicula ericae, Pezoloma ericae, Meliniomyces variabilis, and Oidiodendron maius) isolated from the roots of ericaceous plants and grown under controlled environmental conditions in sterilized soil. The inoculated and non-inoculated (inoculation control) plants were subsequently subjected to three cycles of drought stress by withdrawing watering. Lowland plants appeared to benefit relatively more from mycorrhizal colonization, compared with the upland plants, in terms of plant growth and drought survival. After three weeks of treatments, the dry weights of non-inoculated well-watered upland plants were higher compared to the non-inoculated lowland plants. However, these differences were offset by the inoculation of plants with ERM fungi, some of which also significantly improved drought resistance characteristics of the upland and lowland plants. There were no major differences in the effects of different ERM fungal species on drought responses of upland and lowland plants. Of the examined ericoid mycorrhizal fungi, inoculation with Pezicula ericae was the most effective in conferring drought resistance characteristics to both upland and lowland seedlings and helped maintain higher shoot water potentials, net photosynthetic, and transpiration rates in plants subjected to drought stress.

Transcriptome and Metabolome Analyses Reveal Potential Salt Tolerance Mechanisms Contributing to Maintenance of Water Balance by the Halophytic Grass Puccinellia nuttalliana.

Elevated soil salinity exacerbated by human activities and global climate change poses serious threats to plant survival. Although halophytes provide many important clues concerning salt tolerance in plants, some unanswered questions remain to be addressed, including the processes of water and solute transport regulation. We performed high-throughput RNA-sequencing in roots and metabolome characterizations in roots and leaves of Puccinellia nuttalliana halophytic grass subjected to 0 (control) and 150 mM NaCl. In RNAseq, a total of 31 Gb clean bases generated were de novo assembled into 941,894 transcripts. The PIP2;2 and HKT1;5 transcript levels increased in response to the NaCl treatment implying their roles in water and ion homeostasis. Several transcription factors, including WRKY39, DEK3, HY5, and ABF2, were also overexpressed in response to NaCl. The metabolomic analysis revealed that proline and dopamine significantly increased due to the upregulation of the pathway genes under salt stress, likely contributing to salt tolerance mechanisms. Several phosphatidylcholines significantly increased in roots suggesting that the alterations of membrane lipid composition may be an important strategy in P. nuttalliana for maintaining cellular homeostasis and membrane integrity under salt stress. In leaves, the TCA cycle was enriched suggesting enhanced energy metabolism to cope with salt stress. Other features contributing to the ability of P. nuttalliana to survive under high salinity conditions include salt secretion by the salt glands and enhanced cell wall lignification of the root cells. While most of the reported transcriptomic, metabolomics, and structural alterations may have consequences to water balance maintenance by plants under salinity stress, the key processes that need to be further addressed include the role of the changes in the aquaporin gene expression profiles in the earlier reported enhancement of the aquaporin-mediated root water transport.

Ethylene enhances root water transport and aquaporin expression in trembling aspen (Populus tremuloides) exposed to root hypoxia.

BACKGROUND: Root hypoxia has detrimental effects on physiological processes and growth in most plants. The effects of hypoxia can be partly alleviated by ethylene. However, the tolerance mechanisms contributing to the ethylene-mediated hypoxia tolerance in plants remain poorly understood. RESULTS: In this study, we examined the effects of root hypoxia and exogenous ethylene treatments on leaf gas exchange, root hydraulic conductance, and the expression levels of several aquaporins of the plasma membrane intrinsic protein group (PIP) in trembling aspen (Populus tremuloides) seedlings. Ethylene enhanced net photosynthetic rates, transpiration rates, and root hydraulic conductance in hypoxic plants. Of the two subgroups of PIPs (PIP1 and PIP2), the protein abundance of PIP2s and the transcript abundance of PIP2;4 and PIP2;5 were higher in ethylene-treated trembling aspen roots compared with non-treated roots under hypoxia. The increases in the expression levels of these aquaporins could potentially facilitate root water transport. The enhanced root water transport by ethylene was likely responsible for the increase in leaf gas exchange of the hypoxic plants. CONCLUSIONS: Exogenous ethylene enhanced root water transport and the expression levels of PIP2;4 and PIP2;5 in hypoxic roots of trembling aspen. The results suggest that ethylene facilitates the aquaporin-mediated water transport in plants exposed to root hypoxia.

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

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

Ericoid mycorrhizal fungi enhance salt tolerance in ericaceous plants.

To examine the effects of ericoid mycorrhizal (ERM) fungi on salt tolerance of ericaceous plants, we inoculated roots of velvetleaf blueberry (Vaccinium myrtilloides), Labrador tea (Rhododendron groenlandicum), and lingonberry (Vaccinium vitis-idaea) with ericoid mycorrhizal fungi Oidiodendron maius and Meliniomyces variabilis. Plants were subjected to 0 (NaCl control) and 30 mM NaCl treatments, and plant dry weights, gas exchange, and leaf chlorophyll concentrations were compared in inoculated and non-inoculated plants. M. variabilis increased root dry weights in all three species of NaCl-treated plants, and O. maius enhanced root dry weights of lingonberry plants treated with NaCl. Both fungal species were especially effective in enhancing root and shoot dry weights in control (0 mM NaCl) and NaCl-treated lingonberry seedlings. Leaf chlorophyll concentrations were enhanced by fungal inoculation in all three plant species, and this effect persisted under salt stress in Labrador tea and lingonberry. Salt treatment drastically reduced transpiration rates (E) and lowered net photosynthesis (Pn) to the negative values in all three species of non-inoculated plants, and this effect was partly or almost completely reversed by the inoculation with O. maius and M. variabilis. Fungal inoculation was especially effective in reducing NaCl effects on Pn in lingonberry. Oidiodendron maius and M. variabilis were also equally effective in reversing NaCl-induced declines of E in velvetleaf blueberry and lingonberry. However, in Labrador tea, O. maius reversed the decline of E in NaCl-treated plants less compared with M. variabilis resulting in high photosynthetic water use efficiency values. The results support the hypothesis that, similarly to arbuscular mycorrhizal and ectomycorrhizal associations, ERM association increases salt tolerance of plants.

Effects of pH and Mineral Nutrition on Growth and Physiological Responses of Trembling Aspen (Populus tremuloides), Jack Pine (Pinus banksiana), and White Spruce (Picea glauca) Seedlings in Sand Culture.

Responses of trembling aspen (Populus tremuloides), jack pine (Pinus banksiana), and white spruce (Picea glauca) seedlings to root zone pH ranging from 5 to 9 were studied in sand culture in the presence of two mineral nutrition levels. After eight weeks of treatments, effects of pH on plant dry weights varied between the plant species and were relatively minor in white spruce. Higher nutrient supply significantly increased dry weights only in trembling aspen subjected to pH 5 treatment. There was little effect of pH and nutrition level on net photosynthesis and transpiration rates in white spruce and jack pine, but net photosynthesis markedly declined in aspen at high pH. Chlorophyll concentrations in young foliage decreased the most in trembling aspen and jack pine. The effects of high pH treatments on the concentrations of Mg, P, Ca, Mn, Zn, and Fe in young foliage varied between the plant species with no significant decreases of Fe and Zn recorded in trembling aspen and white spruce, respectively. This was in contrast to earlier reports from the studies carried out in hydroponic culture. The sand culture system that we developed could be a more suitable alternative to hydroponics to study plant responses to pH in the root zone. Plant responses to high pH appear to involve complex events with a likely contribution of nutritional effects and altered water transport processes.

Fungal Aquaporins in Ectomycorrhizal Root Water Transport.

Ectomycorrhizal fungi influence root water transport of host plants. To delineate the exact mechanisms of how fungal partner alters root water relations, it is important to understand the functions of fungal transmembrane water channels, i.e., aquaporins, the key component in the symplastic pathways. In this paper, we discussed what roles the fungal aquaporins may play in root water transport. We also highlighted the opportunities of using integrated approaches to address rising questions in future hotspots of aquaporin and root water relations research.

Growth and physiological responses of tree seedlings to oil sands non-segregated tailings.

Bitumen recovery from oil sands in northeastern Alberta, Canada produces large volumes of tailings, which are deposited in mining areas that must be reclaimed upon mine closure. A new technology of non-segregated tailings (NST) developed by Canadian Natural Resources Limited (CNRL) was designed to accelerate the process of oil sands fine tailings consolidation. However, effects of these novel tailings on plants used for the reclamation of oil sands mining areas remain to be determined. In the present study, we investigated the effects of NST on seedlings of three species of plants commonly planted in oil sands reclamation sites including paper birch (Betula papyrifera), white spruce (Picea glauca) and green alder (Alnus viridis). In the controlled-environment study, we grew seedlings directly in NST and in the two types of reclamation soils with and without added NST and we measured seedling growth, gas exchange parameters, as well as tissue concentrations of selected elements and foliar chlorophyll. White spruce seedlings suffered from severe mortality when grown directly in NST and their needles contained high concentrations of Na. The growth and physiological processes were also inhibited by NST in green alder and paper birch. However, the addition of top soil and peat mineral soil mix to NST significantly improved the growth of plants, possibly due to a more balanced nutrient uptake. It appears that NST may offer some advantages in terms of site revegetation compared with the traditional oil sands tailings that were used in the past. The results also suggest that, white spruce may be less suitable for planting at reclamation sites containing NST compared with the two studied deciduous tree species.

Role of urban ectomycorrhizal fungi in improving the tolerance of lodgepole pine (Pinus contorta) seedlings to salt stress.

With large forested urban areas, the city of Edmonton, Alberta, Canada, faces high annual costs of replacing trees injured by deicing salts that are commonly used for winter road maintenance. Ectomycorrhizal fungi form symbiotic associations with tree roots that allow trees to tolerate the detrimental effects of polluted soils. Here, we examined mycorrhizal colonization of Pinus contorta by germinating seeds in soils collected from different locations: (1) two urban areas within the city of Edmonton, and (2) an intact pine forest just outside Edmonton. We then tested the responses of seedlings to 0-, 60-, and 90-mM NaCl. Our results showed lower abundance and diversity of ectomycorrhizal fungi in seedlings colonized with the urban soils compared to those from the pine forest soil. However, when subsequently exposed to NaCl treatments, only seedlings inoculated with one of the urban soils containing fungi from the genera Tuber, Suillus, and Wilcoxina, showed reduced shoot Na accumulation and higher growth rates. Our results indicate that local ectomycorrhizal fungi that are adapted to challenging urban sites may offer a potential suitable source for inoculum for conifer trees designated for plating in polluted urban environments.

Regulation of water transport in Arabidopsis by methyl jasmonate.

Following a stress event, jasmonate-dependent signaling pathway triggers a shift from growth to defense responses that are accompanied by the cessation of growth in many plants. However, the processes leading to this growth inhibition remain obscure. In this study, we provide evidence for a rapid inhibition of cell hydraulic conductivity (Lp) by methyl jasmonate (MeJA) in the roots of wild-type Arabidopsis within 0.5 h of 20 and 50 µM MeJA treatments. We also demonstrate that MeJA did not affect Lp in fad3-2 and fad7-2 Arabidopsis mutants that are deficient in jasmonate precursor, linolenic acid. The reductions of Lp in wild-type plants were accompanied by the down-regulation of several plasma membrane intrinsic protein (PIP) isoforms, and dephosphorylation. Treatments with HgCl2 did not further reduce Lp in the wild-type plants, but significantly reduced Lp in the fad3-2 and fad7-2 that had been first treated with MeJA. Continuous prolonged exposure to exogenous 50 µM MeJA inhibited the relative growth rates (RGR) of shoots and net photosynthesis (Pn) in the Arabidopsis wild-type and fad7-2 plants, but had no effect on the RGR of roots. The results demonstrated that a reduction of aquaporin (AQP)-mediated water transport was the initial target of MeJA exposure, and may contribute to the processes of growth inhibition by MeJA.

Stable expression of aquaporins and hypoxia-responsive genes in adventitious roots are linked to maintaining hydraulic conductance in tobacco (Nicotiana tabacum) exposed to root hypoxia.

Formation of adventitious roots in plants is a common response to hypoxia caused by flooding. In tobacco, after one week of root hypoxia treatment, plants produced twice as many adventitious roots as the aerated plants, but their maximum length was reduced. Hypoxia severely reduced net photosynthesis, transpiration rates, and photosynthetic light responses. Relative transcript abundance of the examined aquaporins in lateral roots was reduced by hypoxia, but in adventitious roots it remained unchanged. This apparent lack of an effect of root hypoxia on the aquaporin expression likely contributed to maintenance of high hydraulic conductance in adventitious roots. Lateral roots had lower porosity compared with adventitious roots and the expression of the ACS (1-aminocyclopropane-1-carboxylate synthase) gene was induced in hypoxic lateral roots, but not in adventitious roots, providing additional evidence that lateral roots were more affected by hypoxia compared with adventitious roots. ATP concentrations were markedly lower in both hypoxic lateral and adventitious roots compared with aerated roots, while the expression of fermentation-related genes, ADH1 (alcohol dehydrogenase 1) and PDC1 (pyruvate decarboxylase 1), was higher in lateral roots compared with adventitious roots. Since root porosity was greater in adventitious compared with lateral roots, the results suggest that the improved O2 delivery and stable root aquaporin expression in adventitious roots were likely the key factors helping flooded tobacco plants maintain high rates of root hydraulic conductance and, consequently, shoot gas exchange.

Variation in Aquaporin and Physiological Responses Among Pinus contorta Families Under Different Moisture Conditions.

A population of eight open pollinated families of Pinus contorta was selected from sites varying in precipitation regimes and elevation to examine the possible role of aquaporins in adaptation to different moisture conditions. Five Pinus contorta aquaporins encoding PiconPIP2;1, PiconPIP2;2, PiconPIP2;3, PiconPIP1;2, and PiconTIP1;1 were cloned and detailed structural analyses were conducted to provide essential information that can explain their biological and molecular function. All five PiconAQPs contained hydrophilic aromatic/arginine selective filters to facilitate the transport of water. Transcript abundance patterns of PiconAQPs varied significantly across the P. contorta families under varying soil moisture conditions. The transcript abundance of five PiconPIPs remained unchanged under control and water-stress conditions in two families that originated from the sites with lower precipitation levels. These two families also displayed a different adaptive strategy of photosynthesis to cope with drought stress, which was manifested by reduced sensitivity in photosynthesis (maintaining the same rate) while exhibiting a reduction in stomatal conductance. In general, root:shoot ratios were not affected by drought stress, but some variation was observed between families. The results showed variability in drought coping mechanisms, including the expression of aquaporin genes and plant biomass allocation among eight families of Pinus contorta.

Water transport properties of root cells contribute to salt tolerance in halophytic grasses Poa juncifolia and Puccinellia nuttalliana.

Plant water uptake and aquaporin-mediated root water transport are among the most salt-sensitive processes in most plants, but even relatively high salt concentrations do not appear to impair water transport processes in halophytes. To develop better understanding of these processes in halophytic plants, we compared the responses to NaCl of the two halophytic grasses varying in salt tolerance, Puccinellia nuttalliana and Poa juncifolia, with the glycophytic grass Poa pratensis. The plants were hydroponically grown and subjected to different NaCl concentrations for up to 10 days. At the lower NaCl concentrations, shoot and root dry weights were drastically reduced in Poa pratensis, but increased in Puccinellia nuttalliana and Poa juncifolia. The examined treatment concentrations of up to 300 mM NaCl had either no effect (Puccinellia nuttalliana) or little effect (Poa juncifolia) on the net photosynthesis and transpiration rates in plants, but severely decreased the gas exchange parameters in Poa pratensis. Similarly, to growth and gas exchange, leaf water content in Puccinellia nuttalliana was not affected even by the highest, 300 mM NaCl concentration, while Poa pratensis showed decreased shoot water content in all examined NaCl treatments and Poa juncifolia in 150 and 300 mM NaCl. Cell hydraulic conductivity in roots of Poa pratensis also showed high sensitivity to NaCl and was drastically reduced in all examined NaCl concentrations. Cell hydraulic conductivity in Poa juncifolia roots was less affected by NaCl compared with Poa pratensis and in Puccinellia nuttalliana, cell hydraulic conductivity increased in response to NaCl treatments. Both Puccinellia nuttalliana and Poa juncifolia accumulated less Na in their shoot tissues compared with Poa pratensis. The concentrations of K in the roots of Poa pratensis sharply decreased with increasing NaCl treatment concentrations while in Puccinellia nuttalliana, K root concentrations remained high in all NaCl treatments and in Poa juncifoila, root K decreased only in the 300 mM NaCl treatment. Since K efflux from the cytoplasm can contribute to the acidification of the cytoplasm, this process could potentially lead to the inhibition of aquaporin function and reduction of root hydraulic conductivity. The, significance of stable K root concentrations in the roots of halophytes should be further investigated as a possible salt tolerance mechanism that could contribute to the maintenance of aquaporin function and root water transport under salt stress conditions.

Plant water transport and aquaporins in oxygen-deprived environments.

Oxygen deprivation commonly affects plants exposed to flooding and soil compaction. The resulting root hypoxia has an immediate effect on plant water relations and upsets water balance. Hypoxia inhibits root water transport and triggers stomatal closure. The processes contributing to the inhibition of root hydraulic conductivity and conductance (hydraulic conductivity of the whole root system) are complex and involve changes in root morphology and the functions of aquaporins. Aquaporins (AQPs) comprise a group of membrane intrinsic proteins that are responsible for the transport of water, as well as some small neutral solutes and ions. They respond to a wide range of environmental stresses including O2 deprivation, but the underlying functional mechanisms are still elusive. The aquaporin-mediated water transport is affected by the acidification of the cytoplasm and depletion of ATP that is required for aquaporin phosphorylation and membrane functions. Cytoplasmic pH, phosphorylation, and intracellular Ca2+ concentration directly control AQP gating, all of which are related to O2 deprivation. This review addresses the structural determinants that are essential for pore conformational changes in AQPs, to highlight the underlying mechanisms triggered by O2 deprivation stress. Gene expression of AQPs is modified in hypoxic plants, which may constitute an important, yet little explored, mechanism of hypoxia tolerance. In addition to water transport, AQPs may contribute to hypoxia tolerance by transporting O2, H2O2, and lactic acid. Responses of plants to O2 deprivation, and especially those that contribute to maintenance of water transport, are highly complex and entail the signals originating in roots and shoots that lead to and follow the stomatal closure. These complex responses may involve ethylene, abscisic acid, and possibly other hormonal factors and signaling molecules in ways that remain to be elucidated.

Regulation of aquaporins in plants under stress.

Aquaporins (AQP) are channel proteins belonging to the Major Intrinsic Protein (MIP) superfamily that play an important role in plant water relations. The main role of aquaporins in plants is transport of water and other small neutral molecules across cellular biological membranes. AQPs have remarkable features to provide an efficient and often, specific water flow and enable them to transport water into and out of the cells along the water potential gradient. Plant AQPs are classified into five main subfamilies including the 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). AQPs are localized in the cell membranes and are found in all living cells. However, most of the AQPs that have been described in plants are localized to the tonoplast and plasma membranes. Regulation of AQP activity and gene expression, are also considered as a part of the adaptation mechanisms to stress conditions and rely on complex processes and signaling pathways as well as complex transcriptional, translational and posttranscriptional factors. Gating of AQPs through different mechanisms, such as phosphorylation, tetramerization, pH, cations, reactive oxygen species, phytohormones and other chemical agents, may play a key role in plant responses to environmental stresses by maintaining the uptake and movement of water in the plant body.

Regulation of aquaporins in plants under stress

Abstract Aquaporins (AQP) are channel proteins belonging to the Major Intrinsic Protein (MIP) superfamily that play an important role in plant water relations. The main role of aquaporins in plants is transport of water and other small neutral molecules across cellular biological membranes. AQPs have remarkable features to provide an efficient and often, specific water flow and enable them to transport water into and out of the cells along the water potential gradient. Plant AQPs are classified into five main subfamilies including the 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). AQPs are localized in the cell membranes and are found in all living cells. However, most of the AQPs that have been described in plants are localized to the tonoplast and plasma membranes. Regulation of AQP activity and gene expression, are also considered as a part of the adaptation mechanisms to stress conditions and rely on complex processes and signaling pathways as well as complex transcriptional, translational and posttranscriptional factors. Gating of AQPs through different mechanisms, such as phosphorylation, tetramerization, pH, cations, reactive oxygen species, phytohormones and other chemical agents, may play a key role in plant responses to environmental stresses by maintaining the uptake and movement of water in the plant body.