A cotton gene encoding a plasma membrane aquaporin is involved in seedling development and in response to drought stress

Genome-wide analysis and expression of the aquaporin gene family in Avena sativa L

Background

Oat (Avena sativa L.) belongs to the early maturity grass subfamily of the Gramineae subfamily oats (Avena) and has excellent characteristics, such as tolerance to barrenness, salt, cold, and drought. Aquaporin (AQP) proteins belong to the major intrinsic protein (MIP) superfamily, are widely involved in plant growth and development, and play an important role in abiotic stress responses. To date, previous studies have not identified or analyzed the AsAQP gene family system, and functional studies of oat AQP genes in response to drought, cold, and salt stress have not been performed.

Methods

In this study, AQP genes (AsAQP) were identified from the oat genome, and various bioinformatics data on the AQP gene family, gene structure, gene replication, promoters and regulatory networks were analyzed. Quantitative real-time PCR technology was used to verify the expression patterns of the AQP gene family in different oat tissues under different abiotic stresses.

Results

In this study, a total of 45 AQP genes (AsAQP) were identified from the oat reference genome. According to a phylogenetic analysis, 45 AsAQP were divided into 4 subfamilies (PIP, SIP, NIP, and TIP). Among the 45 AsAQP, 23 proteins had interactions, and among these, 5AG0000633.1 had the largest number of interacting proteins. The 20 AsAQP genes were expressed in all tissues, and their expression varied greatly among different tissues and organs. All 20 AsAQP genes responded to salt, drought and cold stress. The NIP subfamily 6Ag0000836.1 gene was significantly upregulated under different abiotic stresses and could be further verified as a key candidate gene.

Conclusion

The findings of this study provide a comprehensive list of members and their sequence characteristics of the AsAQP protein family, laying a solid theoretical foundation for further functional analysis of AsAQP in oats. This research also offers valuable reference for the creation of stress-tolerant oat varieties through genetic engineering techniques.

Coronatine promotes maize water uptake by directly binding to the aquaporin ZmPIP2;5 and enhancing its activity

Water uptake is crucial for crop growth and development and drought stress tolerance. The water channel aquaporins (AQP) play important roles in plant water uptake. Here, we discovered that a jasmonic acid analog, coronatine (COR), enhanced maize (Zea mays) root water uptake capacity under artificial water deficiency conditions. COR treatment induced the expression of the AQP gene Plasma membrane intrinsic protein 2;5 (ZmPIP2;5). In vivo and in vitro experiments indicated that COR also directly acts on ZmPIP2;5 to improve water uptake in maize and Xenopus oocytes. The leaf water potential and hydraulic conductivity of roots growing under hyperosmotic conditions were higher in ZmPIP2;5-overexpression lines and lower in the zmpip2;5 knockout mutant, compared to wild-type plants. Based on a comparison between ZmPIP2;5 and other PIP2s, we predicted that COR may bind to the functional site in loop E of ZmPIP2;5. We confirmed this prediction by surface plasmon resonance technology and a microscale thermophoresis assay, and showed that deleting the binding motif greatly reduced COR binding. We identified the N241 residue as the COR-specific binding site, which may activate the channel of the AQP tetramer and increase water transport activity, which may facilitate water uptake under hyperosmotic stress.

Plant aquaporins alleviate drought tolerance in plants by modulating cellular biochemistry, root-architecture, and photosynthesis

Water is a vital resource for plants to grow, thrive, and complete their life cycle. In recent years, drastic changes in the climate, especially drought frequency and severity, have increased, which reduces agricultural productivity worldwide. Aquaporins are membrane channels belonging to the major intrinsic protein superfamily, which play an essential role in cellular water and osmotic homeostasis of plants under both control and water deficit conditions. A genome-wide search reveals the vast availability of aquaporin isoforms, phylogenetic relationships, different families, conserved residues, chromosomal locations, and gene structure of aquaporins. Furthermore, aquaporins gating and subcellular trafficking are commonly controlled by phosphorylation, cytosolic pH, divalent cations, reactive oxygen species, and stoichiometry. Researchers have identified their involvement in regulating hydraulic conductance, root system architecture, modulation of abiotic stress-related genes, seed viability and germination, phloem loading, xylem water exit, photosynthetic parameters, and post-drought recovery. Remarkable effects following the change in aquaporin activity and/or gene expression have been observed on root water transport properties, nutrient acquisition, physiology, transpiration, stomatal aperture, gas exchange, and water use efficiency. The present review highlights the role of different aquaporin homologs under water-deficit stress condition in model and crop plants. Moreover, the opportunity and challenges encountered to explore aquaporins for engineering drought-tolerant crop plants are also discussed here.

An embryo lethal transgenic line manifests global expression changes and elevated protein/oil ratios in heterozygous soybean plants

Understanding the molecular processes of seed development is important especially in agronomic crops that produce large amounts of nutrient reserves. Because soybean is a vital source of vegetable protein worldwide, producers are concerned about increasing the total amount of protein in the seed without substantially lowering the amount of oil, another economically important product. Here we describe a transgenic soybean line with increased protein and protein/oil ratio, containing an average of 42.2% protein vs. 38.5% in controls and with a protein/oil ratio of 2.02 vs. 1.76 in controls over several generations of greenhouse growth. Other phenotypic data show that the seeds are heavier, although there are overall lower yields per plant. We postulate these effects result from insertion site mutagenesis by the transgenic construct. As this line never achieves homozygosity and appears to be embryo lethal when homozygous, one functional copy of the gene is most likely essential for normal seed development. Global transcript analyses using RNA-Seq for 88,000 gene models over two stages of cotyledon development revealed that more genes are over-expressed in the transgenic line including ribosomal protein related genes and those in the membrane protein and transporters families. Localization of the insertion site should reveal the genes and developmental program that has been perturbed by the transgenic construct, resulting in this economically interesting increase in protein and the protein/oil ratio.

CO2 enrichment enhanced drought resistance by regulating growth, hydraulic conductivity and phytohormone contents in the root of cucumber seedlings

The coordinated effects of CO2 enrichment and drought stress on cucumber leaves have attracted increasing research attention, but few studies have investigated the effects of CO2 enrichment on the root system under drought stress. So we analyzed the morphological parameters, hydraulic conductivity, aquaporin-related gene expression, and endogenous phytohormone contents in roots of cucumber seedlings cultured under different CO2 concentrations (approximately 400 and 800 ± 40 μmol mol-1) and drought stresses simulated by polyethylene glycol 6000 (0%, 5%, and 10%). The results showed that under drought stress, regardless of the CO2 concentration, the root biomass and hydraulic conductivity decreased, the contents of auxin (IAA), zeatin nucleoside (ZR), and gibberellin (GA) decreased, the abscisic acid (ABA) content and the transcript levels of the aquaporin-related genes CsPIP2-4 increased, and the transcript levels of the aquaporin-related genes CsPIP2-5 and CsPIP2-7 decreased compared with no drought stress. Under moderate drought stress, CO2 enrichment decreased ABA content and the transcript level of CsPIP2-4, increased root biomass and GA content and the transcript level of CsPIP2-7, improved contribution rate of cell-to-cell water transport (mediated by aquaporins) and roots hydraulic conductivity. In summary, drought stress changed the water transport capacity of the roots and inhibited the growth of cucumber seedlings. CO2 enrichment regulated phytohormone contents and aquaporin-related gene expression, maintained the normal contribution rate of cell-to-cell water transport, and improved the root biomass and hydraulic conductivity, thereby alleviated the negative effects of drought stress on cucumber seedlings.

Genome-wide identification and expression analysis of aquaporin gene family related to abiotic stress in watermelon

The plant aquaporins (AQPs) are highly conserved integral membrane proteins that participate in multiple developmental processes and responses to various stresses. In this study, a total of 35 AQP genes were identified in the watermelon genome. The phylogenetic analysis showed that these AQPs can be divided into five types, including 16 plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight nodulin 26-like intrinsic proteins (NIPs), two small basic intrinsic proteins (SIPs), and one uncategorized X intrinsic protein (XIP). A number of cis-elements related to plant responses to hormones and stresses were detected in the promoter sequences of ClAQP genes. Chromosome distribution analysis revealed that the genes are unevenly distributed on eight chromosomes, with chromosomes 1 and 4 possessing the most genes. Expression analysis at different developmental stages in flesh and rind indicated that most of ClAQPs have tissue-specific expression. Meanwhile, some other AQP genes showed differential expression in response to cold, salt, and ABA treatments, which is consistent with the organization of the stress-responsive cis-elements detected in the promoter regions. Our results lay a foundation for understanding the specific functions of ClAQP genes to help the genetic improvement of watermelon.

Combining genome-wide and transcriptome-wide analyses reveal the evolutionary conservation and functional diversity of aquaporins in cotton

BACKGROUND

Aquaporins (AQPs) are integral membrane proteins from a larger family of major intrinsic proteins (MIPs) and function in a huge variety of processes such as water transport, plant growth and stress response. The availability of the whole-genome data of different cotton species allows us to study systematic evolution and function of cotton AQPs on a genome-wide level.

RESULTS

Here, a total of 53, 58, 113 and 111 AQP genes were identified in G. arboreum, G. raimondii, G. hirsutum and G. barbadense, respectively. A comprehensive analysis of cotton AQPs, involved in exon/intron structure, functional domains, phylogenetic relationships and gene duplications, divided these AQPs into five subfamilies (PIP, NIP, SIP, TIP and XIP). Comparative genome analysis among 30 species from algae to angiosperm as well as common tandem duplication events in 24 well-studied plants further revealed the evolutionary conservation of AQP family in the organism kingdom. Combining transcriptome analysis and Quantitative Real-time PCR (qRT-PCR) verification, most AQPs exhibited tissue-specific expression patterns both in G. raimondii and G. hirsutum. Meanwhile, a bias of time to peak expression of several AQPs was also detected after treating G. davidsonii and G. hirsutum with 200 mM NaCl. It is interesting that both PIP1;4 h/i/j and PIP2;2a/e showed the highly conserved tandem structure, but differentially contributed to tissue development and stress response in different cotton species.

CONCLUSIONS

These results demonstrated that cotton AQPs were structural conservation while experienced the functional differentiation during the process of evolution and domestication. This study will further broaden our insights into the evolution and functional elucidation of AQP gene family in cotton.

Investigation of the AQP Family in Soybean and the Promoter Activity of TIP2;6 in Heat Stress and Hormone Responses

Aquaporins (AQPs) are one diverse family of membrane channel proteins that play crucial regulatory roles in plant stress physiology. However, the heat stress responsiveness of AQP genes in soybean remains poorly understood. In this study, 75 non-redundant AQP encoding genes were identified in soybean. Multiple sequence alignments showed that all GmAQP proteins possessed the conserved regions, which contained 6 trans-membrane domains (TM1 to TM6). Different GmAQP members consisted of distinct Asn-Pro-Ala (NPA) motifs, aromatic/arginine (ar/R) selectivity filters and Froger's positions (FPs). Phylogenetic analyses distinguished five sub-families within these GmAQPs: 24 GmPIPs, 24 GmTIPs, 17 GmNIPs, 8 GmSIPs, and 2 GmXIPs. Promoter cis-acting elements analyses revealed that distinct number and composition of heat stress and hormone responsive elements existed in different promoter regions of GmAQPs. QRT-PCR assays demonstrated that 12 candidate GmAQPs with relatively extensive expression in various tissues or high expression levels in root or leaf exhibited different expression changes under heat stress and hormone cues (abscisic acid (ABA), l-aminocyclopropane-l-carboxylic acid (ACC), salicylic acid (SA) and methyl jasmonate (MeJA)). Furthermore, the promoter activity of one previously functionally unknown AQP gene-GmTIP2;6 was investigated in transgenic Arabidopsis plants. The beta-glucuronidase (GUS) activity driven by the promoter of GmTIP2;6 was strongly induced in the heat- and ACC-treated transgenic plants and tended to be accumulated in the hypocotyls, vascular bundles, and leaf trichomes. These results will contribute to uncovering the potential functions and molecular mechanisms of soybean GmAQPs in mediating heat stress and hormone signal responses.

Plasma Membrane Intrinsic Proteins SlPIP2;1, SlPIP2;7 and SlPIP2;5 Conferring Enhanced Drought Stress Tolerance in Tomato

The function of aquaporin (AQP) protein in transporting water is crucial for plants to survive in drought stress. With 47 homologues in tomato (Solanum lycopersicum) were reported, but the individual and integrated functions of aquaporins involved in drought response remains unclear. Here, three plasma membrane intrinsic protein genes, SlPIP2;1, SlPIP2;7 and SlPIP2;5, were identified as candidate aquaporins genes because of highly expressed in tomato roots. Assay on expression in Xenopus oocytes demonstrated that SlPIP2s protein displayed water channel activity and facilitated water transport into the cells. With real-time PCR and in situ hybridization analysis, SlPIP2s were considered to be involved in response to drought treatment. To test its function, transgenic Arabidopsis and tomato lines overexpressing SlPIP2;1, SlPIP2;7 or SlPIP2;5 were generated. Compared with wild type, the over-expression of SlPIP2;1, SlPIP2;7 or SlPIP2;5 transgenic Arabidopsis and tomato plants all showed significantly higher hydraulic conductivity levels and survival rates under both normal and drought conditions. Taken together, this study concludes that aquaporins (SlPIP2;1, SlPIP2;7 and SlPIP2;5) contribute substantially to root water uptake in tomato plants through improving plant water content and maintaining osmotic balance.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

The Camelina aquaporin CsPIP2;1 is regulated by phosphorylation at Ser273, but not at Ser277, of the C-terminus and is involved in salt- and drought-stress responses

Aquaporin (AQP) proteins are involved in water homeostasis in cells at all taxonomic levels of life. Phosphorylation of some AQPs has been proposed to regulate water permeability via gating of the channel itself. We analyzed plasma membrane intrinsic proteins (PIP) from Camelina and characterized their biological functions under both stressful and favorable conditions. A three-dimensional theoretical model of the Camelina AQP proteins was built by homology modeling which could prove useful in further functional characterization of AQPs. CsPIP2;1 was strongly and constitutively expressed in roots and leaves of Camelina, suggesting that this gene is related to maintenance of homeostasis during salt and drought stresses. CsPIP2s exhibited water channel activity in Xenopus oocytes. We then examined the roles of CsPIP2;1 phosphorylation at Ser273 and Ser277 in the regulation of water permeability using phosphorylation mutants. A single deletion strain of CsPIP2;1 was generated to serve as the primary host for testing AQP expression constructs. A Ser277 to alanine mutation (to prevent phosphorylation) did not change CsPIP2;1 water permeability while a Ser273 mutation to alanine did affect water permeability. Furthermore, a CsPIP2;1 point mutation when ectopically expressed in yeast resulted in lower growth in salt and drought conditions compared with controls, and confirmation of Ser273 as the phosphorylation site. Our results support the idea that post-translational modifications in the Ser273 regulatory domains of the C-terminus fine tune water flux through CsPIP2;1.

Plant aquaporins: roles in plant physiology

BACKGROUND

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms.

SCOPE OF REVIEW

Here, we present comprehensive insights made on plant aquaporins in recent years, pointing to their molecular and physiological specificities with respect to animal or microbial counterparts.

MAJOR CONCLUSIONS

In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations and various physiological substrates in addition to water. Of particular relevance for plants is the transport by aquaporins of dissolved gases such as carbon dioxide or metalloids such as boric or silicic acid. The mechanisms that determine the gating and subcellular localization of plant aquaporins are extensively studied. They allow aquaporin regulation in response to multiple environmental and hormonal stimuli. Thus, aquaporins play key roles in hydraulic regulation and nutrient transport in roots and leaves. They contribute to several plant growth and developmental processes such as seed germination or emergence of lateral roots.

GENERAL SIGNIFICANCE

Plants with genetically altered aquaporin functions are now tested for their ability to improve plant resistance to stresses. This article is part of a Special Issue entitled Aquaporins.

Heterologous expression of the wheat aquaporin gene TaTIP2;2 compromises the abiotic stress tolerance of Arabidopsis thaliana

Aquaporins are channel proteins which transport water across cell membranes. We show that the bread wheat aquaporin gene TaTIP2;2 maps to the long arm of chromosome 7b and that its product localizes to the endomembrane system. The gene is expressed constitutively in both the root and the leaf, and is down-regulated by salinity and drought stress. Salinity stress induced an increased level of C-methylation within the CNG trinucleotides in the TaTIP2;2 promoter region. The heterologous expression of TaTIP2;2 in Arabidopsis thaliana compromised its drought and salinity tolerance, suggesting that TaTIP2;2 may be a negative regulator of abiotic stress. The proline content of transgenic A. thaliana plants fell, consistent with the down-regulation of P5CS1, while the expression of SOS1, SOS2, SOS3, CBF3 and DREB2A, which are all stress tolerance-related genes acting in an ABA-independent fashion, was also down-regulated. The supply of exogenous ABA had little effect either on TaTIP2;2 expression in wheat or on the phenotype of transgenic A. thaliana. The expression level of the ABA signalling genes ABI1, ABI2 and ABF3 remained unaltered in the transgenic A. thaliana plants. Thus TaTIP2;2 probably regulates the response to stress via an ABA-independent pathway(s).