Ectopic Expression of a Foreign Aquaporin Disrupts the Natural Expression Patterns of Endogenous Aquaporin Genes and Alters Plant Responses to Different Stress Conditions

De novo transcriptome assembly of Dalbergia sissoo Roxb. (Fabaceae) under Botryodiplodia theobromae-induced dieback disease

Dalbergia sissoo Roxb. (Shisham) is a timber-producing species of economic, cultural, and medicinal importance in the Indian subcontinent. In the past few decades, Shisham's dieback disease caused by the fungus Botryodiplodia theobromae has become an evolving issue in the subcontinent endangering its survival. To gain insights into this issue, a standard transcriptome assembly was deployed to assess the response of D. sissoo at the transcriptomic level under the stress of B. theobromae infection. For RNA isolation, the control and infected leaf tissue samples were taken from 1-year-old greenhouse-grown D. sissoo plants after 20 days of stem-base spore inoculation. cDNA synthesis was performed from these freshly isolated RNA samples that were then sent for sequencing. About 18.14 Gb (Giga base) of data was generated using the BGISEQ-500 sequencing platform. In terms of Unigenes, 513,821 were identified after a combined assembly of all samples and then filtering the abundance. The total length of Unigenes, their average length, N50, and GC-content were 310,523,693 bp, 604 bp, 1,101 bp, and 39.95% respectively. The Unigenes were annotated using 7 functional databases i.e., 200,355 (NR: 38.99%), 164,973 (NT: 32.11%), 123,733 (Swissprot: 24.08%), 142,580 (KOG: 27.75%), 139,588 (KEGG: 27.17%), 99,752 (GO: 19.41%), and 137,281 (InterPro: 26.72%). Furthermore, the Transdecoder detected 115,762 CDS. In terms of SSR (Simple Sequence Repeat) markers, 62,863 of them were distributed on 51,508 Unigenes and on the predicted 4673 TF (Transcription Factor) coding Unigenes. A total of 16,018 up- and 19,530 down-regulated Differentially Expressed Genes (DEGs) were also identified. Moreover, the Plant Resistance Genes (PRGs) had a count of 9230. We are hopeful that in the future, these identified Unigenes, SSR markers, DEGs and PRGs will provide the prerequisites for managing Shisham dieback disease, its breeding, and in tree improvement programs.

Overexpression of an aquaporin gene EsPIP1;4 enhances abiotic stress tolerance and promotes flowering in Arabidopsis thaliana

Aquaporins are water channel proteins that play an essential role in plant growth and development. Despite extensive functional characterization of aquaporins in model plants such as Arabidopsis, their contributions to abiotic stress tolerance in non-model plants are still poorly understood. As a close relative of Arabidopsis thaliana, Eutrema salsugineum is an excellent model for studying salt tolerance. Here, we identified and functionally characterized EsPIP1;4, a gene encoding a plasma membrane intrinsic protein (PIP) aquaporin in E. salsugineum. Overexpression of EsPIP1;4 in Arabidopsis improved seed germination and root growth of transgenic plants under abiotic stress, which was accompanied by an increase in proline accumulation, reduction in MDA, and decrease in the rate of ion leakage. Under abiotic stress, transgenic plants overexpressing EsPIP1;4 also showed increased antioxidant enzyme activity, and enhanced K⁺/Na⁺ ratio compared to control plants. Furthermore, overexpression of EsPIP1;4 promoted flowering by regulating genes in multiple flowering pathways. Together, our results demonstrated that an aquaporin from E. salsugineum improves abiotic stress tolerance and promotes flowering.

Using the Maize Nested Association Mapping (NAM) Population to Partition Arbuscular Mycorrhizal Effects on Drought Stress Tolerance into Hormonal and Hydraulic Components

In this study, a first experiment was conducted with the objective of determining how drought stress alters the radial water flow and physiology in the whole maize nested association mapping (NAM) population and to find out which contrasting maize lines should be tested in a second experiment for their responses to drought in combination with an arbuscular mycorrhizal (AM) fungus. Emphasis was placed on determining the role of plant aquaporins and phytohormones in the responses of these contrasting maize lines to cope with drought stress. Results showed that both plant aquaporins and hormones are altered by the AM symbiosis and are highly involved in the physiological responses of maize plants to drought stress. The regulation by the AM symbiosis of aquaporins involved in water transport across cell membranes alters radial water transport in host plants. Hormones such as IAA, SA, ABA and jasmonates must be involved in this process either by regulating the own plant-AM fungus interaction and the activity of aquaporins, or by inducing posttranscriptional changes in these aquaporins, which in turns alter their water transport capacity. An intricate relationship between root hydraulic conductivity, aquaporins and phytohormones has been observed, revealing a complex network controlling water transport in maize roots.

Genetic regulation of water and nutrient transport in water stress tolerance in roots

Drought stress is one of the major abiotic factors affecting the growth and development of crops. The primary effect of drought is the alteration of water and nutrient uptake and transport by roots, related essentially with aquaporins and ion transporters of the plasma membrane. Therefore, the efficiency of water and nutrient transport across cell layers is a main factor in tolerance mechanisms. The regulation of this transport under water stress - in relation to the differing degrees of tolerance of crops and the effect of arbuscular mycorrhizae, together with signaling mechanisms - is reviewed here. Three different phases in the response to stress (immediate, short-term and long-term), involving different signals and levels of gene regulation, are highlighted.

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.

Enhanced Drought Stress Tolerance by the Arbuscular Mycorrhizal Symbiosis in a Drought-Sensitive Maize Cultivar Is Related to a Broader and Differential Regulation of Host Plant Aquaporins than in a Drought-Tolerant Cultivar

The arbuscular mycorrhizal (AM) symbiosis has been shown to improve maize tolerance to different drought stress scenarios by regulating a wide range of host plants aquaporins. The objective of this study was to highlight the differences in aquaporin regulation by comparing the effects of the AM symbiosis on root aquaporin gene expression and plant physiology in two maize cultivars with contrasting drought sensitivity. This information would help to identify key aquaporin genes involved in the enhanced drought tolerance by the AM symbiosis. Results showed that when plants were subjected to drought stress the AM symbiosis induced a higher improvement of physiological parameters in drought-sensitive plants than in drought-tolerant plants. These include efficiency of photosystem II, membrane stability, accumulation of soluble sugars and plant biomass production. Thus, drought-sensitive plants obtained higher physiological benefit from the AM symbiosis. In addition, the genes ZmPIP1;1, ZmPIP1;3, ZmPIP1;4, ZmPIP1;6, ZmPIP2;2, ZmPIP2;4, ZmTIP1;1, and ZmTIP2;3 were down-regulated by the AM symbiosis in the drought-sensitive cultivar and only ZmTIP4;1 was up-regulated. In contrast, in the drought-tolerant cultivar only three of the studied aquaporin genes (ZmPIP1;6, ZmPIP2;2, and ZmTIP4;1) were regulated by the AM symbiosis, resulting induced. Results in the drought-sensitive cultivar are in line with the hypothesis that down-regulation of aquaporins under water deprivation could be a way to minimize water loss, and the AM symbiosis could be helping the plant in this regulation. Indeed, during drought stress episodes, water conservation is critical for plant survival and productivity, and is achieved by an efficient uptake and stringently regulated water loss, in which aquaporins participate. Moreover, the broader and contrasting regulation of these aquaporins by the AM symbiosis in the drought-sensitive than the drought-tolerant cultivar suggests a role of these aquaporins in water homeostasis or in the transport of other solutes of physiological importance in both cultivars under drought stress conditions, which may be important for the AM-induced tolerance to drought stress.

Enhanced Drought Stress Tolerance by the Arbuscular Mycorrhizal Symbiosis in a Drought-Sensitive Maize Cultivar Is Related to a Broader and Differential Regulation of Host Plant Aquaporins than in a Drought-Tolerant Cultivar

The arbuscular mycorrhizal (AM) symbiosis has been shown to improve maize tolerance to different drought stress scenarios by regulating a wide range of host plants aquaporins. The objective of this study was to highlight the differences in aquaporin regulation by comparing the effects of the AM symbiosis on root aquaporin gene expression and plant physiology in two maize cultivars with contrasting drought sensitivity. This information would help to identify key aquaporin genes involved in the enhanced drought tolerance by the AM symbiosis. Results showed that when plants were subjected to drought stress the AM symbiosis induced a higher improvement of physiological parameters in drought-sensitive plants than in drought-tolerant plants. These include efficiency of photosystem II, membrane stability, accumulation of soluble sugars and plant biomass production. Thus, drought-sensitive plants obtained higher physiological benefit from the AM symbiosis. In addition, the genes ZmPIP1;1, ZmPIP1;3, ZmPIP1;4, ZmPIP1;6, ZmPIP2;2, ZmPIP2;4, ZmTIP1;1, and ZmTIP2;3 were down-regulated by the AM symbiosis in the drought-sensitive cultivar and only ZmTIP4;1 was up-regulated. In contrast, in the drought-tolerant cultivar only three of the studied aquaporin genes (ZmPIP1;6, ZmPIP2;2, and ZmTIP4;1) were regulated by the AM symbiosis, resulting induced. Results in the drought-sensitive cultivar are in line with the hypothesis that down-regulation of aquaporins under water deprivation could be a way to minimize water loss, and the AM symbiosis could be helping the plant in this regulation. Indeed, during drought stress episodes, water conservation is critical for plant survival and productivity, and is achieved by an efficient uptake and stringently regulated water loss, in which aquaporins participate. Moreover, the broader and contrasting regulation of these aquaporins by the AM symbiosis in the drought-sensitive than the drought-tolerant cultivar suggests a role of these aquaporins in water homeostasis or in the transport of other solutes of physiological importance in both cultivars under drought stress conditions, which may be important for the AM-induced tolerance to drought stress.

Aquaporins and plant transpiration

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

An aquaporin gene from halophyte Sesuvium portulacastrum, SpAQP1, increases salt tolerance in transgenic tobacco

KEY MESSAGE

SpAQP1 was strongly induced by salt in an ABA-independent way, promoted seed germination and root growth in transgenic tobaccos and increased salt tolerance by increasing the activities of antioxidative enzymes. Aquaporin (AQP) plays crucial roles in the responses of plant to abiotic stresses such as drought, salt and cold. Compared to glycophytes, halophytes often have excellent salt and drought tolerances. To uncover the molecular mechanism of halophyte Sesuvium portulacastrum tolerance to salt, in this study, an AQP gene, SpAQP1, from S. portulacastrum was isolated and characterized. The amino acid sequence of SpAQP1 shared high homology with that of plant plasma membrane intrinsic proteins (PIPs) and contained the distinct molecular features of PIPs. In the phylogenic tree, SpAQP1 was evidently classified as the PIP2 subfamily. SpAQP1 is expressed in roots, stems and leaves, and was significantly induced by NaCl treatment and inhibited by abscisic acid (ABA) treatment. When heterologously expressed in yeast and tobacco, SpAQP1 enhanced the salt tolerance of yeast strains and tobacco plants and promoted seed germination and root growth under salt stress in transgenic plants. The activity of antioxidative enzymes including superoxide dismutase, peroxidase and catalase was increased in transgenic plants overexpressing SpAQP1. Taken together, our studies suggested that SpAQP1 functioned in the responses of S. portulacastrum to salt stress and could increase salt tolerance by enhancing the antioxidative activity of plants.

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.

Ectopic overexpression of a novel Glycine soja stress-induced plasma membrane intrinsic protein increases sensitivity to salt and dehydration in transgenic Arabidopsis thaliana plants

Plasma membrane intrinsic proteins (PIPs) belong to the aquaporin family and facilitate water movement across plasma membranes. Existing data indicate that PIP genes are associated with the abilities of plants to tolerate certain stress conditions. A review of our Glycine soja expressed sequence tag (EST) dataset revealed that abiotic stress stimulated expression of a PIP, herein designated as GsPIP2;1 (GenBank_Accn: FJ825766). To understand the roles of this PIP in stress tolerance, we generated a coding sequence for GsPIP2;1 by in silico elongation and cloned the cDNA by 5'-RACE. Semiquantitative RT-PCR showed that GsPIP2;1 expression was stimulated in G. soja leaves by cold, salt, or dehydration stress, whereas the same stresses suppressed GsPIP2;1 expression in the roots. Transgenic Arabidopsis thaliana plants overexpressing GsPIP2;1 grew normally under unstressed and cold conditions, but exhibited depressed tolerance to salt and dehydration stresses. Moreover, greater changes in water potential were detected in the transgenic A. thaliana shoots, implying that GsPIP2;1 may negatively impact stress tolerance by regulating water potential. These results, deviating from those obtained in previous reports, provide new insights into the relationship between PIPs and abiotic stress tolerance in plants.

New challenges in plant aquaporin biotechnology

Recent advances concerning genetic manipulation provide new perspectives regarding the improvement of the physiological responses in herbaceous and woody plants to abiotic stresses. The beneficial or negative effects of these manipulations on plant physiology are discussed, underlining the role of aquaporin isoforms as representative markers of water uptake and whole plant water status. Increasing water use efficiency and the promotion of plant water retention seem to be critical goals in the improvement of plant tolerance to abiotic stress. However, newly uncovered mechanisms, such as aquaporin functions and regulation, may be essential for the beneficial effects seen in plants overexpressing aquaporin genes. Under distinct stress conditions, differences in the phenotype of transgenic plants where aquaporins were manipulated need to be analyzed. In the development of nano-technologies for agricultural practices, multiple-walled carbon nanotubes promoted plant germination and cell growth. Their effects on aquaporins need further investigation.

The Thellungiella salsuginea tonoplast aquaporin TsTIP1;2 functions in protection against multiple abiotic stresses

Examination of aquaporin (AQP) membrane channels in extremophile plants may increase our understanding of plant tolerance to high salt, drought or other conditions. Here, we cloned a tonoplast AQP gene (TsTIP1;2) from the halophyte Thellungiella salsuginea and characterized its biological functions. TsTIP1;2 transcripts accumulate to high levels in several organs, increasing in response to multiple external stimuli. Ectopic overexpression of TsTIP1;2 in Arabidopsis significantly increased plant tolerance to drought, salt and oxidative stresses. TsTIP1;2 had water channel activity when expressed in Xenopus oocytes. TsTIP1;2 was also able to conduct H₂O₂ molecules into yeast cells in response to oxidative stress. TsTIP1;2 was not permeable to Na(+) in Xenopus oocytes, but it could facilitate the entry of Na(+) ions into plant cell vacuoles by an indirect process under high-salinity conditions. Collectively, these data showed that TsTIP1;2 could mediate the conduction of both H₂O and H₂O₂ across membranes, and may act as a multifunctional contributor to survival of T. salsuginea in highly stressful habitats.

Aquaporin OsPIP1;1 promotes rice salt resistance and seed germination

OsPIP1;1 is one of the most abundant aquaporins in rice leaves and roots and is highly responsible to environmental stresses. However, its biochemical and physiological functions are still largely unknown. The oocyte assay data showed OsPIP1;1 had lower water channel activity in contrast to OsPIP2;1. EGFP and immunoelectron microscopy studies revealed OsPIP1;1 was predominantly localized in not only plasma membrane but also in some ER-like intracellular compartments in the cells. OsPIP1;1 exhibited low water channel activity in Xenopus oocytes but coexpression of OsPIP2;1 significantly enhanced its water permeability. Stop-flow assay indicated that 10His-OsPIP1;1-reconstituted proteoliposomes had significantly higher water permeability than the control liposomes. Overexpression of OsPIP1;1 greatly altered many physiological features of transgenic plants in a dosage-dependent manner. Moderate expression of OsPIP1;1 increased rice seed yield, salt resistance, root hydraulic conductivity, and seed germination rate. This work suggests OsPIP1;1 functions as an active water channel and plays important physiological roles.

Overexpression of a wheat aquaporin gene, TaAQP8, enhances salt stress tolerance in transgenic tobacco

Aquaporin (AQP) proteins have been shown to transport water and other small molecules through biological membranes, which is crucial for plants to combat salt stress. However, the precise role of AQP genes in salt stress response is not completely understood in plants. In this study, a PIP1 subgroup AQP gene, designated TaAQP8, was cloned and characterized from wheat. Transient expression of TaAQP8-green fluorescent protein (GFP) fusion protein revealed its localization in the plasma membrane. TaAQP8 exhibited water channel activity in Xenopus laevis oocytes. TaAQP8 transcript was induced by NaCl, ethylene and H(2)O(2). Further investigation showed that up-regulation of TaAQP8 under salt stress involves ethylene and H(2)O(2) signaling, with ethylene causing a positive effect and H(2)O(2) acting as a negative factor. Overexpression of TaAQP8 in tobacco increased root elongation compared with controls under salt stress. The roots of transgenic plants also retained a high K(+)/Na(+) ratio and Ca(2+) content, but reduced H(2)O(2) accumulation by an enhancement of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities under salt stress. Further investigation showed that whole seedlings from transgenic lines displayed higher SOD, CAT and POD activities, increased NtSOD and NtCAT transcript levels, and decreased H(2)O(2) accumulation and membrane injury under salt stress. Taken together, our results demonstrate that TaAQP8 confers salt stress tolerance not only by retaining high a K(+)/Na(+) ratio and Ca(2+) content, but also by reducing H(2)O(2) accumulation and membrane damage by enhancing the antioxidant system.

Identification of genes involved in the ACC-mediated control of root cell elongation in Arabidopsis thaliana

BACKGROUND

Along the root axis of Arabidopsis thaliana, cells pass through different developmental stages. In the apical meristem repeated cycles of division increase the numbers of cells. Upon leaving the meristem, these cells pass the transition zone where they are physiologically and mechanically prepared to undergo subsequent rapid elongation. During the process of elongation epidermal cells increase their length by 300% in a couple of hours. When elongation ceases, the cells acquire their final size, shape and functions (in the differentiation zone). Ethylene administered as its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is capable of inhibiting elongation in a concentration-dependent way. Using a microarray analysis, genes and/or processes involved in this elongation arrest are identified.

RESULTS

Using a CATMA-microarray analysis performed on control and 3h ACC-treated roots, 240 differentially expressed genes were identified. Quantitative Real-Time RT-PCR analysis of the 10 most up and down regulated genes combined with literature search confirmed the accurateness of the analysis. This revealed that inhibition of cell elongation is, at least partly, caused by restricting the events that under normal growth conditions initiate elongation and by increasing the processes that normally stop cellular elongation at the end of the elongation/onset of differentiation zone.

CONCLUSIONS

ACC interferes with cell elongation in the Arabidopsis thaliana roots by inhibiting cells from entering the elongation process and by immediately stimulating the formation of cross-links in cell wall components, diminishing the remaining elongation capacity. From the analysis of the differentially expressed genes, it becomes clear that many genes identified in this response, are also involved in several other kind of stress responses. This suggests that many responses originate from individual elicitors, but that somewhere in the downstream signaling cascade, these are converged to a 'common pathway'. Furthermore, several potential keyplayers, such as transcription factors and auxin-responsive genes, were identified by the microarray analysis. They await further analysis to reveal their exact role in the control of cell elongation.

Plant potassium content modifies the effects of arbuscular mycorrhizal symbiosis on root hydraulic properties in maize plants

It is well known that the arbuscular mycorrhizal (AM) symbiosis helps the host plant to overcome several abiotic stresses including drought. One of the mechanisms for this drought tolerance enhancement is the higher water uptake capacity of the mycorrhizal plants. However, the effects of the AM symbiosis on processes regulating root hydraulic properties of the host plant, such as root hydraulic conductivity and plasma membrane aquaporin gene expression, and protein abundance, are not well defined. Since it is known that K(+) status is modified by AM and that it regulates root hydraulic properties, it has been tested how plant K(+) status could modify the effects of the symbiosis on root hydraulic conductivity and plasma membrane aquaporin gene expression and protein abundance, using maize (Zea mays L.) plants and Glomus intraradices as a model. It was observed that the supply of extra K(+) increased root hydraulic conductivity only in AM plants. Also, the different pattern of plasma membrane aquaporin gene expression and protein abundance between AM and non-AM plants changed with the application of extra K(+). Thus, plant K(+) status could be one of the causes of the different observed effects of the AM symbiosis on root hydraulic properties. The present study also highlights the critical importance of AM fungal aquaporins in regulating root hydraulic properties of the host plant.

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.

A novel Glycine soja tonoplast intrinsic protein gene responds to abiotic stress and depresses salt and dehydration tolerance in transgenic Arabidopsis thaliana

Tonoplast intrinsic protein (TIP) is a subfamily of the aquaporin (AQP), also known as major intrinsic protein (MIP) family, and regulates water movement across vacuolar membranes. Some reports have implied that TIP genes are associated with plant tolerance to some abiotic stresses that cause water loss, such as drought and high salinity. In our previous work, we found that an expressed sequence tag (EST) representing a TIP gene in our Glycine soja EST library was inducible by abiotic stresses. This TIP was subsequently isolated from G. soja with cDNA library screening, EST assembly and PCR, and named as GsTIP2;1. The expression patterns of GsTIP2;1 in G. soja under low temperature, salt and dehydration stress were different in leaves and roots. Though GsTIP2;1 is a stress-induced gene, overexpression of GsTIP2;1 in Arabidopsis thaliana depressed tolerance to salt and dehydration stress, but did not affect seedling growth under cold or favorable conditions. Higher dehydration speed was detected in Arabidopsis plants overexpressing GsTIP2;1, implying GsTIP2;1 might mediate stress sensitivity by enhancing water loss in the plant. Such a result is not identical to previous reports, providing some new information about the relationship between TIP and plant abiotic stress tolerance.

Root-targeted biotechnology to mediate hormonal signalling and improve crop stress tolerance

Since plant root systems capture both water and nutrients essential for the formation of crop yield, there has been renewed biotechnological focus on root system improvement. Although water and nutrient uptake can be facilitated by membrane proteins known as aquaporins and nutrient transporters, respectively, there is a little evidence that root-localised overexpression of these proteins improves plant growth or stress tolerance. Recent work suggests that the major classes of phytohormones are involved not only in regulating aquaporin and nutrient transporter expression and activity, but also in sculpting root system architecture. Root-specific expression of plant and bacterial phytohormone-related genes, using either root-specific or root-inducible promoters or grafting non-transformed plants onto constitutive hormone producing rootstocks, has examined the role of root hormone production in mediating crop stress tolerance. Root-specific traits such as root system architecture, sensing of edaphic stress and root-to-shoot communication can be exploited to improve resource (water and nutrients) capture and plant development under resource-limited conditions. Thus, root system engineering provides new opportunities to maintain sustainable crop production under changing environmental conditions.

Aquaporins: a family of highly regulated multifunctional channels

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

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.

Transgenic increases in seed oil content are associated with the differential expression of novel Brassica-specific transcripts

Background

Seed oil accumulates primarily as triacylglycerol (TAG). While the biochemical pathway for TAG biosynthesis is known, its regulation remains unclear. Previous research identified microsomal diacylglycerol acyltransferase 1 (DGAT1, EC 2.3.1.20) as controlling a rate-limiting step in the TAG biosynthesis pathway. Of note, overexpression of DGAT1 results in substantial increases in oil content and seed size. To further analyze the global consequences of manipulating DGAT1 levels during seed development, a concerted transcriptome and metabolome analysis of transgenic B. napus prototypes was performed.

Results

Using a targeted Brassica cDNA microarray, about 200 genes were differentially expressed in two independent transgenic lines analyzed. Interestingly, 24–33% of the targets showing significant changes have no matching gene in Arabidopsis although these represent only 5% of the targets on the microarray. Further analysis of some of these novel transcripts indicated that several are inducible by ABA in microspore-derived embryos. Of the 200 Arabidopsis genes implicated in lipid biology present on the microarray, 36 were found to be differentially regulated in DGAT transgenic lines. Furthermore, kinetic reverse transcriptase Polymerase Chain Reaction (k-PCR) analysis revealed up-regulation of genes encoding enzymes of the Kennedy pathway involved in assembly of TAGs. Hormone profiling indicated that levels of auxins and cytokinins varied between transgenic lines and untransformed controls, while differences in the pool sizes of ABA and catabolites were only observed at later stages of development.

Conclusion

Our results indicate that the increased TAG accumulation observed in transgenic DGAT1 plants is associated with modest transcriptional and hormonal changes during seed development that are not limited to the TAG biosynthesis pathway. These might be associated with feedback or feed-forward effects due to altered levels of DGAT1 activity. The fact that a large fraction of significant amplicons have no matching genes in Arabidopsis compromised our ability to draw concrete inferences from the data at this stage, but has led to the identification of novel genes of potential interest.

Characterization of OsPIP2;7, a water channel protein in rice

Aquaporins are water channel proteins that facilitate passage of water and other small neutral molecules across biological membranes. There are usually a large number of members of this family in higher plants, which exhibit various physiological functions and are regulated in a time-specific and particular mode. We have previously shown that a rice gene, OsPIP2;7, was generally up-regulated in roots but down-regulated in shoots at the early stage of chilling stress. Here, OsPIP2;7 was cloned and proved to be an aquaporin with high activity in Xenopus oocytes. OsPIP2;7 was localized mainly in mesophyll cells of leaves. In roots it was detected in the vascular tissues, epidermis cells and exodermis cells at the elongation zone, as well as in the epidermis cells, exodermis cells and root hair at the maturation zone. Yeast cells overexpressing OsPIP2;7 showed a higher survival rate after freeze-thaw stress. Furthermore, OsPIP2;7 enhanced the transpiration rate and tolerance to low temperature when overexpressed in rice. These results indicated that OsPIP2;7 was involved in rapid water transport and maintenance of the water balance in cells, and ultimately improves the tolerance of yeast and rice to low temperature stress.

Rhododendron catawbiense plasma membrane intrinsic proteins are aquaporins, and their over-expression compromises constitutive freezing tolerance and cold acclimation ability of transgenic Arabidopsis plants

Extracellular freezing results in cellular dehydration caused by water efflux, which is likely regulated by aquaporins (AQPs). In a seasonal cold acclimation (CA) study of Rhododendron catawbiense, two AQP cDNAs, RcPIP2;1 and RcPIP2;2, were down-regulated as the leaf freezing tolerance (FT) increased from -7 to approximately -50 degrees C. We hypothesized this down-regulation to be an adaptive component of CA process allowing cells to resist freeze-induced dehydration. Here, we characterize full-length cDNAs of the two Rhododendron PIPs, and demonstrate that RcPIP2s have water channel activity. Moreover, RcPIP2s were over-expressed in Arabidopsis, and FT of transgenic plants was compared with that of wild-type (WT) controls. Data indicated a significantly lower constitutive FT and CA ability of RcPIP2-OXP plants (compared with WT) due, presumably, to their lower ability to resist freeze desiccation. A relatively higher dehydration rate of RcPIP2-OXP leaves (than WT) supports this notion. Phenotypic and microscopic observations revealed bigger leaf size and mesophyll cells of RcPIP2-OXP plants than WT. It is proposed that lower FT of transgenic plants may be associated with their leaf cells' propensity to greater mechanical stress, that is, volume strain per unit surface, during freeze-thaw-induced contraction or expansion. Additionally, greater freeze injury in RcPIP2-OXP plants could also be attributed to their susceptibility to potentially faster rehydration (than WT) during a thaw.

Aquaporins and plant water balance

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