Aquaporins constitute a large and highly divergent protein family in maize

Multifaceted role and regulation of aquaporins for efficient stomatal movements

Stomata are micropores on the leaf epidermis that allow carbon dioxide (CO2) uptake for photosynthesis at the expense of water loss through transpiration. Stomata coordinate the plant gas exchange of carbon and water with the atmosphere through their opening and closing dynamics. In the context of global climate change, it is essential to better understand the mechanism of stomatal movements under different environmental stimuli. Aquaporins (AQPs) are considered important regulators of stomatal movements by contributing to membrane diffusion of water, CO2 and hydrogen peroxide. This review compiles the most recent findings and discusses future directions to update our knowledge of the role of AQPs in stomatal movements. After highlighting the role of subsidiary cells (SCs), which contribute to the high water use efficiency of grass stomata, we explore the expression of AQP genes in guard cells and SCs. We then focus on the cellular regulation of AQP activity at the protein level in stomata. After introducing their post-translational modifications, we detail their trafficking as well as their physical interaction with various partners that regulate AQP subcellular dynamics towards and within specific regions of the cell membranes, such as microdomains and membrane contact sites.

TIP aquaporins in Cyperus esculentus: genome-wide identification, expression profiles, subcellular localizations, and interaction patterns

BACKGROUND

Tonoplast intrinsic proteins (TIPs), which typically mediate water transport across vacuolar membranes, play an essential role in plant growth, development, and stress responses. However, their characterization in tigernut (Cyperus esculentus L.), an oil-bearing tuber plant of the Cyperaceae family, is still in the infancy.

RESULTS

In this study, a first genome-wide characterization of the TIP subfamily was conducted in tigernut, resulting in ten members representing five previously defined phylogenetic groups, i.e., TIP1-5. Although the gene amounts are equal to that present in two model plants Arabidopsis and rice, the group composition and/or evolution pattern were shown to be different. Except for CeTIP1;3 that has no counterpart in both Arabidopsis and rice, complex orthologous relationships of 1:1, 1:2, 1:3, 2:1, and 2:2 were observed. Expansion of the CeTIP subfamily was contributed by whole-genome duplication (WGD), transposed, and dispersed duplications. In contrast to the recent WGD-derivation of CeTIP3;1/-3;2, synteny analyses indicated that TIP4 and - 5 are old WGD repeats of TIP2, appearing sometime before monocot-eudicot divergence. Expression analysis revealed that CeTIP genes exhibit diverse expression profiles and are subjected to developmental and diurnal fluctuation regulation. Moreover, when transiently overexpressed in tobacco leaves, CeTIP1;1 was shown to locate in the vacuolar membrane and function in homo/heteromultimer, whereas CeTIP2;1 is located in the cell membrane and only function in heteromultimer. Interestingly, CeTIP1;1 could mediate the tonoplast-localization of CeTIP2;1 via protein interaction, implying complex regulatory patterns.

CONCLUSIONS

Our findings provide a global view of CeTIP genes, which provide valuable information for further functional analysis and genetic improvement through manipulating key members in tigernut.

Aquaporin ZmTIP2;3 Promotes Drought Resistance of Maize through Symbiosis with Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi symbiosis plays important roles in enhancing plant tolerance to biotic and abiotic stresses. Aquaporins have also been linked to improved drought tolerance in plants and the regulation of water transport. However, the mechanisms that underlie this association remain to be further explored. In this study, we found that arbuscular mycorrhiza fungi symbiosis could induce the gene expression of the aquaporin ZmTIP2;3 in maize roots. Moreover, compared with the wild-type plants, the maize zmtip2;3 mutant also showed a lower total biomass, colonization rate, relative water content, and POD and SOD activities after arbuscular mycorrhiza fungi symbiosis under drought stress. qRT-PCR assays revealed reduced expression levels of stress genes including LEA3, P5CS4, and NECD1 in the maize zmtip2;3 mutant. Taken together, these data suggest that ZmTIP2;3 plays an important role in promoting maize tolerance to drought stress during arbuscular mycorrhiza fungi symbiosis.

AtPIP1;4 and AtPIP2;4 Cooperatively Mediate H2O2 Transport to Regulate Plant Growth and Disease Resistance

The rapid production of hydrogen peroxide (H2O2) is a hallmark of plants' successful recognition of pathogen infection and plays a crucial role in innate immune signaling. Aquaporins (AQPs) are membrane channels that facilitate the transport of small molecular compounds across cell membranes. In plants, AQPs from the plasma membrane intrinsic protein (PIP) family are utilized for the transport of H2O2, thereby regulating various biological processes. Plants contain two PIP families, PIP1s and PIP2s. However, the specific functions and relationships between these subfamilies in plant growth and immunity remain largely unknown. In this study, we explore the synergistic role of AtPIP1;4 and AtPIP2;4 in regulating plant growth and disease resistance in Arabidopsis. We found that in plant cells treated with H2O2, AtPIP1;4 and AtPIP2;4 act as facilitators of H2O2 across membranes and the translocation of externally applied H2O2 from the apoplast to the cytoplasm. Moreover, AtPIP1;4 and AtPIP2;4 collaborate to transport bacterial pathogens and flg22-induced apoplastic H2O2 into the cytoplasm, leading to increased callose deposition and enhanced defense gene expression to strengthen immunity. These findings suggest that AtPIP1;4 and AtPIP2;4 cooperatively mediate H2O2 transport to regulate plant growth and immunity.

Overexpression of MsNIP2 improves salinity tolerance in Medicago sativa

Alfalfa (Medicago sativa) is one of the most widely cultivated forage crops in the world. However, alfalfa yield and quality are adversely affected by salinity stress. Nodulin 26-like intrinsic proteins (NIPs) play essential roles in water and small molecules transport and response to salt stress. Here, we isolated a salt stress responsive MsNIP2 gene and demonstrated its functions by overexpression in alfalfa. The open reading frame of MsNIP2 is 816 bp in length, and it encodes 272 amino acids. It has six transmembrane domains and two NPA motifs. MsNIP2 showed high identity to other known NIP proteins, and its tertiary model was similar to the crystal structure of OsNIP2-1 (7cjs) tetramer. Subcellular localization analysis showed that MsNIP2 protein fused with green fluorescent protein (GFP) was localized to the plasma membrane. Transgenic alfalfa lines overexpressing MsNIP2 showed significantly higher height and branch number compared with the non-transgenic control. The POD and CAT activity of the transgenic alfalfa lines was significantly increased and their MDA content was notably reduced compared with the control group under the treatment of NaCl. The transgenic lines showed higher capability in scavenging oxygen radicals with lighter NBT staining than the control under salt stress. The transgenic lines showed relative lower water loss rate and electrolyte leakage, but relatively higher Na+ content than the control line under salt stress. The relative expression levels of abiotic-stress-related genes (MsHSP23, MsCOR47, MsATPase, and MsRD2) in three transgenic lines were compared with the control, among them, only the expression of MsCOR47 was up-regulated. Consequently, this study offers a novel perspective for exploring the function of MsNIP2 in improving salt tolerance of alfalfa.

A new method to measure aquaporin-facilitated membrane diffusion of hydrogen peroxide and cations in plant suspension cells

Plant aquaporins (AQPs) facilitate the membrane diffusion of water and small solutes, including hydrogen peroxide (H2 O2 ) and, possibly, cations, essential signalling molecules in many physiological processes. While the determination of the channel activity generally depends on heterologous expression of AQPs in Xenopus oocytes or yeast cells, we established a genetic tool to determine whether they facilitate the diffusion of H2 O2 through the plasma membrane in living plant cells. We designed genetic constructs to co-express the fluorescent H2 O2 sensor HyPer and AQPs, with expression controlled by a heat shock-inducible promoter in Nicotiana tabacum BY-2 suspension cells. After induction of ZmPIP2;5 AQP expression, a HyPer signal was recorded when the cells were incubated with H2 O2 , suggesting that ZmPIP2;5 facilitates H2 O2 transmembrane diffusion; in contrast, the ZmPIP2;5W85A mutated protein was inactive as a water or H2 O2 channel. ZmPIP2;1, ZmPIP2;4 and AtPIP2;1 also facilitated H2 O2 diffusion. Incubation with abscisic acid and the elicitor flg22 peptide induced the intracellular H2 O2 accumulation in BY-2 cells expressing ZmPIP2;5. We also monitored cation channel activity of ZmPIP2;5 using a novel fluorescent photo-switchable Li+ sensor in BY-2 cells. BY-2 suspension cells engineered for inducible expression of AQPs as well as HyPer expression and the use of Li+ sensors constitute a powerful toolkit for evaluating the transport activity and the molecular determinants of PIPs in living plant cells.

Sugarcane mosaic virus employs 6K2 protein to impair ScPIP2;4 transport of H2O2 to facilitate virus infection

Sugarcane mosaic virus (SCMV), one of the main pathogens causing sugarcane mosaic disease, is widespread in sugarcane (Saccharum spp. hybrid) planting areas and causes heavy yield losses. RESPIRATORY BURST OXIDASE HOMOLOG (RBOH) NADPH oxidases and plasma membrane intrinsic proteins (PIPs) have been associated with the response to SCMV infection. However, the underlying mechanism is barely known. In the present study, we demonstrated that SCMV infection upregulates the expression of ScRBOHs and the accumulation of hydrogen peroxide (H2O2), which inhibits SCMV replication. All eight sugarcane PIPs (ScPIPs) interacted with SCMV-encoded protein 6K2, whereby two PIP2s (ScPIP2;1 and ScPIP2;4) were verified as capable of H2O2 transport. Furthermore, we revealed that SCMV-6K2 interacts with ScPIP2;4 via transmembrane domain 5 to interfere with the oligomerization of ScPIP2;4, subsequently impairing ScPIP2;4 transport of H2O2. This study highlights a mechanism adopted by SCMV to employ 6K2 to counteract the host resistance mediated by H2O2 to facilitate virus infection and provides potential molecular targets for engineering sugarcane resistance against SCMV.

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.

Plasma membrane aquaporins regulate root hydraulic conductivity in the model plant Setaria viridis

The high rate of productivity observed in panicoid crops is in part due to their extensive root system. Recently, green foxtail (Setaria viridis) has emerged as a genetic model system for panicoid grasses. Natural accessions of S. viridis originating from different parts of the world, with differential leaf physiological behavior, have been identified. This work focused on understanding the physiological and molecular mechanisms controlling root hydraulic conductivity and root-to-shoot gas exchange signaling in S. viridis. We identified 2 accessions, SHA and ZHA, with contrasting behavior at the leaf, root, and whole-plant levels. Our results indicated a role for root aquaporin (AQP) plasma membrane (PM) intrinsic proteins in the differential behavior of SHA and ZHA. Moreover, a different root hydraulic response to low levels of abscisic acid between SHA and ZHA was observed, which was associated with root AQPs. Using cell imaging, biochemical, and reverse genetic approaches, we identified PM intrinsic protein 1;6 (PIP1;6) as a possible PIP1 candidate that regulates radial root hydraulics and root-to-shoot signaling of gas exchange in S. viridis. In heterologous systems, PIP1;6 localized in the endoplasmic reticulum, and upon interaction with PIP2s, relocalization to the PM was observed. PIP1;6 was predominantly expressed at the root endodermis. Generation of knockout PIP1;6 plants (KO-PIP1;6) in S. viridis showed altered root hydraulic conductivity, altered gas exchange, and alteration of root transcriptional patterns. Our results indicate that PIPs are essential in regulating whole-plant water homeostasis in S. viridis. We conclude that root hydraulic conductivity and gas exchange are positively associated and are regulated by AQPs.

Research advances in mechanisms of arsenic hyperaccumulation of Pteris vittata: Perspectives from plant physiology, molecular biology, and phylogeny

Pteris vittata, as the firstly discovered arsenic (As) hyperaccumulator, has great application value in As-contaminated soil remediation. Currently, the genes involved in As hyperaccumulation in P. vittata have been mined continuously, while they have not been used in practice to enhance phytoremediation efficiency. Aiming to better assist the practice of phytoremediation, this review collects 130 studies to clarify the progress in research into the As hyperaccumulation process in P. vittata from multiple perspectives. Antioxidant defense, rhizosphere activities, vacuolar sequestration, and As efflux are important physiological activities involved in As hyperaccumulation in P. vittata. Among related 19 genes, PHT, TIP, ACR3, ACR2 and HAC family genes play essential roles in arsenate (AsⅤ) transport, arsenite (AsⅢ) transport, vacuole sequestration of AsⅢ, and the reduction of AsⅤ to AsⅢ, respectively. Gene ontology enrichment analysis indicated it is necessary to further explore genes that can bind to related ions, with transport activity, or with function of transmembrane transport. Phylogeny analysis results implied ACR2, HAC and ACR3 family genes with rapid evolutionary rate may be the decisive factors for P. vittata as an As hyperaccumulator. A deeper understanding of the As hyperaccumulation network and key gene components could provide useful tools for further bio-engineered phytoremediation.

From Homeostasis to Pathology: Decoding the Multifaceted Impact of Aquaporins in the Central Nervous System

Aquaporins (AQPs), integral membrane proteins facilitating selective water and solute transport across cell membranes, have been the focus of extensive research over the past few decades. Particularly noteworthy is their role in maintaining cellular homeostasis and fluid balance in neural compartments, as dysregulated AQP expression is implicated in various degenerative and acute brain pathologies. This article provides an exhaustive review on the evolutionary history, molecular classification, and physiological relevance of aquaporins, emphasizing their significance in the central nervous system (CNS). The paper journeys through the early studies of water transport to the groundbreaking discovery of Aquaporin 1, charting the molecular intricacies that make AQPs unique. It delves into AQP distribution in mammalian systems, detailing their selective permeability through permeability assays. The article provides an in-depth exploration of AQP4 and AQP1 in the brain, examining their contribution to fluid homeostasis. Furthermore, it elucidates the interplay between AQPs and the glymphatic system, a critical framework for waste clearance and fluid balance in the brain. The dysregulation of AQP-mediated processes in this system hints at a strong association with neurodegenerative disorders such as Parkinson's Disease, idiopathic normal pressure hydrocephalus, and Alzheimer's Disease. This relationship is further explored in the context of acute cerebral events such as stroke and autoimmune conditions such as neuromyelitis optica (NMO). Moreover, the article scrutinizes AQPs at the intersection of oncology and neurology, exploring their role in tumorigenesis, cell migration, invasiveness, and angiogenesis. Lastly, the article outlines emerging aquaporin-targeted therapies, offering a glimpse into future directions in combatting CNS malignancies and neurodegenerative diseases.

Genome-Wide Identification and Gene Expression Analysis of Sweet Cherry Aquaporins (Prunus avium L.) under Abiotic Stresses

Aquaporins (AQPs) are integral transmembrane proteins well known as channels involved in the mobilization of water, small uncharged molecules and gases. In this work, the main objective was to carry out a comprehensive study of AQP encoding genes in Prunus avium (cv. Mazzard F12/1) on a genome-wide scale and describe their transcriptional behaviors in organs and in response to different abiotic stresses. A total of 28 non-redundant AQP genes were identified in Prunus spp. Genomes, which were phylogenetically grouped into five subfamilies (seven PIPs, eight NIPs, eight TIPs, three SIPs and two XIPs). Bioinformatic analyses revealed a high synteny and remarkable conservation of structural features among orthologs of different Prunus genomes. Several cis-acting regulatory elements (CREs) related to stress regulation were detected (ARE, WRE3, WUN, STRE, LTR, MBS, DRE, AT-rich and TC-rich). The above could be accounting for the expression variations associated with plant organs and, especially, each abiotic stress analyzed. Gene expressions of different PruavAQPs were shown to be preferentially associated with different stresses. PruavXIP2;1 and PruavXIP1;1 were up-regulated in roots at 6 h and 72 h of hypoxia, and in PruavXIP2;1 a slight induction of expression was also detected in leaves. Drought treatment strongly down-regulated PruavTIP4;1 but only in roots. Salt stress exhibited little or no variation in roots, except for PruavNIP4;1 and PruavNIP7;1, which showed remarkable gene repression and induction, respectively. Interestingly, PruavNIP4;1, the AQP most expressed in cherry roots subjected to cold temperatures, also showed this pattern in roots under high salinity. Similarly, PruavNIP4;2 consistently was up-regulated at 72 h of heat and drought treatments. From our evidence is possible to propose candidate genes for the development of molecular markers for selection processes in breeding programs for rootstocks and/or varieties of cherry.

Regulation of drought tolerance in Arabidopsis involves the PLATZ4-mediated transcriptional repression of plasma membrane aquaporin PIP2;8

Plant A/T-rich protein and zinc-binding protein (PLATZ) transcription factors play important roles in plant growth, development and abiotic stress responses. However, how PLATZ influences plant drought tolerance remains poorly understood. The present study showed that PLATZ4 increased drought tolerance in Arabidopsis thaliana by causing stomatal closure. Transcriptional profiling analysis revealed that PLATZ4 affected the expression of a set of genes involved in water and ion transport, antioxidant metabolism, small peptides and abscisic acid (ABA) signaling. Among these genes, the direct binding of PLATZ4 to the A/T-rich sequences in the plasma membrane intrinsic protein 2;8 (PIP2;8) promoter was identified. PIP2;8 consistently reduced drought tolerance in Arabidopsis through inhibiting stomatal closure. PIP2;8 was localized in the plasma membrane, exhibited water channel activity in Xenopus laevis oocytes and acted epistatically to PLATZ4 in regulating the drought stress response in Arabidopsis. PLATZ4 increased ABA sensitivity through upregulating the expression of ABSCISIC ACID INSENSITIVE 3 (ABI3), ABI4 and ABI5. The transcripts of PLATZ4 were induced to high levels in vegetative seedlings under drought and ABA treatments within 6 and 3 h, respectively. Collectively, these findings reveal that PLATZ4 positively influences plant drought tolerance through regulating the expression of PIP2;8 and genes involved in ABA signaling.

Fast X-ray fluorescence microscopy provides high-throughput phenotyping of element distribution in seeds

The concentration, chemical speciation, and spatial distribution of essential and toxic mineral elements in cereal seeds have important implications for human health. To identify genes responsible for element uptake, translocation, and storage, high-throughput phenotyping methods are needed to visualize element distribution and concentration in seeds. Here, we used X-ray fluorescence microscopy (μ-XRF) as a method for rapid and high-throughput phenotyping of seed libraries and developed an ImageJ-based pipeline to analyze the spatial distribution of elements. Using this method, we nondestructively scanned 4,190 ethyl methanesulfonate (EMS)-mutagenized M1 rice (Oryza sativa) seeds and 533 diverse rice accessions in a genome-wide association study (GWAS) panel to simultaneously measure concentrations and spatial distribution of elements in the embryo, endosperm, and aleurone layer. A total of 692 putative mutants and 65 loci associated with the spatial distribution of elements in rice seed were identified. This powerful method provides a basis for investigating the genetics and molecular mechanisms controlling the accumulation and spatial variations of mineral elements in plant seeds.

Dual Inoculation with Rhizophagus irregularis and Bacillus megaterium Improves Maize Tolerance to Combined Drought and High Temperature Stress by Enhancing Root Hydraulics, Photosynthesis and Hormonal Responses

Climate change is leading to combined drought and high temperature stress in many areas, drastically reducing crop production, especially for high-water-consuming crops such as maize. This study aimed to determine how the co-inoculation of an arbuscular mycorrhizal (AM) fungus (Rhizophagus irregularis) and the PGPR Bacillus megaterium (Bm) alters the radial water movement and physiology in maize plants in order to cope with combined drought and high temperature stress. Thus, maize plants were kept uninoculated or inoculated with R. irregularis (AM), with B. megaterium (Bm) or with both microorganisms (AM + Bm) and subjected or not to combined drought and high temperature stress (D + T). We measured plant physiological responses, root hydraulic parameters, aquaporin gene expression and protein abundances and sap hormonal content. The results showed that dual AM + Bm inoculation was more effective against combined D + T stress than single inoculation. This was related to a synergistic enhancement of efficiency of the phytosystem II, stomatal conductance and photosynthetic activity. Moreover, dually inoculated plants maintained higher root hydraulic conductivity, which was related to regulation of the aquaporins ZmPIP1;3, ZmTIP1.1, ZmPIP2;2 and GintAQPF1 and levels of plant sap hormones. This study demonstrates the usefulness of combining beneficial soil microorganisms to improve crop productivity under the current climate-change scenario.

Ice plant root plasma membrane aquaporins are regulated by clathrin-coated vesicles in response to salt stress

The regulation of root Plasma membrane (PM) Intrinsic Protein (PIP)-type aquaporins (AQPs) is potentially important for salinity tolerance. However, the molecular and cellular details underlying this process in halophytes remain unclear. Using free-flow electrophoresis and label-free proteomics, we report that the increased abundance of PIPs at the PM of the halophyte ice plant (Mesembryanthemum crystallinum L.) roots under salinity conditions is regulated by clathrin-coated vesicles (CCV). To understand this regulation, we analyzed several components of the M. crystallinum CCV complexes: clathrin light chain (McCLC) and subunits μ1 and μ2 of the adaptor protein (AP) complex (McAP1μ and McAP2μ). Co-localization analyses revealed the association between McPIP1;4 and McAP2μ and between McPIP2;1 and McAP1μ, observations corroborated by mbSUS assays, suggesting that AQP abundance at the PM is under the control of CCV. The ability of McPIP1;4 and McPIP2;1 to form homo- and hetero-oligomers was tested and confirmed, as well as their activity as water channels. Also, we found increased phosphorylation of McPIP2;1 only at the PM in response to salt stress. Our results indicate root PIPs from halophytes might be regulated through CCV trafficking and phosphorylation, impacting their localization, transport activity, and abundance under salinity conditions.

Analysis of QTL mapping for germination and seedling response to drought stress in sunflower (Helianthus annuus L.)

Sunflower is an important oilseed crop across the world. It is considered as a moderately drought tolerant plant, however, its yield is still negatively affected by drought stress. Improving drought tolerance is of the outmost important for breeding. Although several studies have documented the relationship between the sunflower phenotype and genotype under drought stress, but relatively few studies have simultaneously investigated the molecular mechanisms of drought tolerance in the sunflower at different growth stages. In this study, we conducted quantitative trait locus (QTL) analysis for different sunflower traits during the germination and seedling stages. Eighteen phenotypic traits were evaluated under well-watered and drought stress conditions. We determined that the germination rate, germination potential, germination index, and root-to-shoot ratio can be used as effective indexes for drought tolerance selection and breeding. A total of 33 QTLs were identified on eight chromosomes (PVE: 0.016%-10.712% with LOD: 2.017-7.439). Within the confidence interval of the QTL, we identified 60 putative drought-related genes. Four genes located on chromosome 13 may function in both germination and seedling stages for drought response. Genes LOC110898128, LOC110898092, LOC110898071, and LOC110898072 were annotated as aquaporin SIP1-2-like, cytochrome P450 94C1, GABA transporter 1-like, and GABA transporter 1-like isoform X2, respectively. These genes will be used for further functional validation. This study provides insight into the molecular mechanisms of the sunflower's in response to drought stress. At the same time, it lays a foundation for sunflower drought tolerance breeding and genetic improvement.

High aquaporin expression correlates with increased translocation of quinclorac from shoots to roots in resistant Echinochloa crus-galli var. zelayensis

BACKGROUND

Echinochloa crus-galli var. zelayensis is a troublesome weed in rice fields and can be controlled by using quinclorac. However, over-reliance on quinclorac has resulted in resistant (R) barnyardgrass, which differs significantly in its ability to transport quinclorac compared to susceptible (S) barnyardgrass. This study aimed to investigate the underlying mechanisms for this different translocation between R and S barnyardgrass.

RESULTS

Larger amount of quinclorac was transferred from shoots to roots in R compared to S barnyardgrass. After 1 day of quinclorac [300 g active ingredient (a.i.) ha^-1 ] foliar treatment, its content in shoots of R was 81.92% of that in S barnyardgrass; correspondingly, in roots of R was 1.17 fold of that in S barnyardgrass. RNA-sequencing and quantitative real-time polymerase chain reaction (qRT-PCR) confirmed the expression levels of PIPs belonging to aquaporins (AQPs) in R were higher than in S barnyardgrass, with or without quinclorac treatment. With co-application of quinclorac and AQPs inhibitors [mercury(II) chloride (HgCl₂ )] treatment, even though the expression levels of PIPs and the transport rates of quinclorac were both suppressed in R and S barnyardgrass, this process was less pronounced in R than in S barnyardgrass.

CONCLUSION

This report provides clear evidence that higher PIPs expression results in rapid quinclorac translocation from shoots to roots and reduces the quinclorac accumulation in the shoot meristems in R barnyardgrass, thus reducing the control efficacy of quinclorac. © 2022 Society of Chemical Industry.

The TaGSK1, TaSRG, TaPTF1, and TaP5CS Gene Transcripts Confirm Salinity Tolerance by Increasing Proline Production in Wheat (Triticum aestivum L.)

Salinity is an abiotic stress factor that reduces yield and threatens food security in the world's arid and semi-arid regions. The development of salt-tolerant genotypes is critical for mitigating yield losses, and this journey begins with the identification of sensitive and tolerant plants. Numerous physiologic and molecular markers for detecting salt-tolerant wheat genotypes have been developed. One of them is proline, which has been used for a long time but has received little information about proline-related genes in wheat genotypes. In this study, proline content and the expression levels of proline-related genes (TaPTF1, TaDHN, TaSRG, TaSC, TaPIMP1, TaMIP, TaHKT1;4, TaGSK, TaP5CS, and TaMYB) were examined in sensitive, moderate, and tolerant genotypes under salt stress (0, 50, 150, and 250 mM NaCl) for 0, 12, and 24 h. Our results show that salt stress increased the proline content in all genotypes, but it was found higher in salt-tolerant genotypes than in moderate and sensitive genotypes. The salinity stress increased gene expression levels in salt-tolerant and moderate genotypes. While salt-stress exposure for 12 and 24 h had a substantial effect on gene expression in wheat, TaPTF1, TaPIMP1, TaMIP, TaHKT1;4, and TaMYB genes were considerably upregulated in 24 h. The salt-tolerant genotypes showed a higher positive interaction than a negative interaction. The TaPTF1, TaP5CS, TaGSK1, and TaSRG genes were found to be more selective than the other analyzed genes under salt-stress conditions. Despite each gene's specific function, increasing proline biosynthesis functioned as a common mechanism for separating salt tolerance from sensitivity.

PIPs from Fragaria vesca: A structural analysis of native and mutated protein

Aquaporins are an ancient family of membrane channel proteins present in all eukaryotes and most prokaryotes, and apart from water, allow the transport of neutral solutes and organic compounds through the pore. These proteins are essential role differentially expressed during ripening in Fragaria vesca fruits. Fv PIP2-1a is intensively expressed in fruits, inclusive several other proteins member are differentially expressed in fruit but also in other plant tissues. Phylogenetic analysis shows that Fv PIP2-1a grouped with other Fragaria proteins and far apart from other F. vesca PIP proteins. A structural model for Fv PIP2-1a protein was built by comparative modelling methodology, which was validated and refined by molecular dynamics simulation. Fv PIP2-1a structure consists of 6 transmembrane regions and two NPA domains. The mobilization of water was analyzed by molecular docking simulations in wildtype and two mutants. Interestingly, the mutant FvPIP2-1a_H214G allowed the prediction of an increment in the flux of water molecules. On contrary, structural analysis predicted that H214E mutation blocked passage of water associated to constriction of the pore.

Promoter of Vegetable Soybean GmTIP1;6 Responds to Diverse Abiotic Stresses and Hormone Signals in Transgenic Arabidopsis

Tonoplast intrinsic proteins (TIPs), a sub-family of aquaporins (AQPs), are known to play important roles in plant abiotic stress responses. However, evidence for the promoters of TIPs involvement in abiotic stress processes remains scarce. In this study, the promoter of the vegetable soybean GmTIP1;6 gene, which had the highest similarity to TIP1-type AQPs from other plants, was cloned. Expression pattern analyses indicated that the GmTIP1;6 gene was dramatically induced by drought, salt, abscisic acid (ABA), and methyl jasmonate (MeJA) stimuli. Promoter analyses revealed that the GmTIP1;6 promoter contained drought, ABA, and MeJA cis-acting elements. Histochemical staining of the GmTIP1;6 promoter in transgenic Arabidopsis corroborated that it was strongly expressed in the vascular bundles of leaves, stems, and roots. Beta-glucuronidase (GUS) activity assays showed that the activities of the GmTIP1;6 promoter were enhanced by different concentrations of polyethylene glycol 6000 (PEG 6000), NaCl, ABA, and MEJA treatments. Integrating these results revealed that the GmTIP1;6 promoter could be applied for improving the tolerance to abiotic stresses of the transgenic plants by promoting the expression of vegetable soybean AQPs.

Assessment of root-specific promoters in banana and tobacco and identification of a banana TIP2 promoter with strong root activity

Genetic modification is one possible strategy to generate bananas (Musa spp.) with resistance to the soil-borne pathogen causing Fusarium wilt. The availability of banana root-specific promoters to target transgene expression to the sites of infection would be beneficial. We have assessed 18 promoter sequences derived from a range of plant species for their expression profiles in banana tissues to identify those with root-specific activity. Promoter sequences were isolated and fused to the β-glucuronidase (GUS) gene to assess their expression levels and tissue specificity in both banana and the model plant tobacco. Two heterologous promoters conferring high root expression levels in banana were identified, including a β-glucosidase 1 (GLU1) promoter from maize and the RB7-type tonoplast intrinsic protein (TIP)-2 promoter from strawberry. Further, a novel Musa TIP2-2 promoter sequence was isolated and characterized which, when fused to the GUS gene, conferred very high GUS expression levels in banana roots. These promoters will expand the options for the control of gene expression in genetically modified bananas, providing a tool to develop plants with resistance not only to soil-borne diseases such as Fusarium wilt, but also for the improvement of other traits, such as nematode resistance, nutrition or abiotic stress resistance.

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.

Combined use of specific length amplified fragment sequencing (SLAF-seq) and bulked segregant analysis (BSA) for rapid identification of genes influencing fiber content of hemp (Cannabis sativa L.)

Hemp (Cannabis sativa L.), an ancient crop, is a significant source of high-quality fiber that primarily caters to the textile industry worldwide. Fiber content is a crucial quantitative trait for evaluating fiber yield in hemp. Understanding the genetic mechanisms involved in hemp breeding is essential for improving yield. In this study, we developed 660 F1 plants from a cross between Jindao-15 (high fiber content fiber-use variety) and Fire No.1 (low fiber content fiber-use variety), and thirty plants each with high and low fiber content were selected from 305 monoecious plants of this population according to 5%-10% of population size for quantitative traits. The DNA from these plants was extracted to establish two bulk DNA pools and then subjected to the restriction digestion by the enzymes RsaI and HaeIII to obtain 314-364 bp digestion fragments and subjected to sequencing using specific length amplified fragment sequencing (SLAF-seq). Finally, we successfully developed 368,404 SLAF tags, which led to the detection of 25,133 high-quality SNPs. Combing with the resequencing results of parents, the SNPs of mixed pools were then subjected to the SNP-Index correlation algorithm, which revealed four candidate regions related to fiber content traits on Chromosome 1, with a length of 8.68 Mb and containing 389 annotated genes. The annotation information and the comparison results identified 15 genes that were highly likely to modulate the fiber content of hemp. Further, qPCR validation identified six genes (LOC115705530, LOC115705875, LOC115704794, LOC115705371, LOC115705688 and LOC115707511) that were highly positively correlated with influencing the hemp fiber content. These genes were involved in the transcription regulation, auxin and water transportion, one carbon and sugar metabolism. And non-synnoumous mutation SNPs which may play vital role in influencing the fiber content were detected in LOC115705875, LOC115704794, LOC115705688 and LOC115707511. Thus, our study highlights the importance of the combined use of SLAF-Seq and Bulked Segregant analysis (BSA) to locate genes related to hemp fiber content rapidly. Hence, our study provides novel mechanistic inputs for the fast identification of genes related to important agronomic traits of hemp and other crops catering to the textile industry.

Genome-Wide Identification and Expression Analyses of the Aquaporin Gene Family in Passion Fruit (Passiflora edulis), Revealing PeTIP3-2 to Be Involved in Drought Stress

Aquaporins (AQPs) in plants can transport water and small molecules, and they play an important role in plant development and abiotic stress response. However, to date, a comprehensive study on AQP family members is lacking. In this study, 27 AQP genes were identified from the passion fruit genome and classified into four groups (NIP, PIP, TIP, SIP) on the basis of their phylogenetic relationships. The prediction of protein interactions indicated that the AQPs of passion fruit were mainly associated with AQP family members and boron protein family genes. Promoter cis-acting elements showed that most PeAQPs contain light response elements, hormone response elements, and abiotic stress response elements. According to collinear analysis, passion fruit is more closely related to Arabidopsis than rice. Furthermore, three different fruit ripening stages and different tissues were analyzed on the basis of the transcriptome sequencing results for passion fruit AQPs under drought, high-salt, cold and high-temperature stress, and the results were confirmed by qRT-PCR. The results showed that the PeAQPs were able to respond to different abiotic stresses, and some members could be induced by and expressed in response to multiple abiotic stresses at the same time. Among the three different fruit ripening stages, 15 AQPs had the highest expression levels in the first stage. AQPs are expressed in all tissues of the passion fruit. One of the passion fruit aquaporin genes, PeTIP3-2, which was induced by drought stress, was selected and transformed into Arabidopsis. The survival rate of transgenic plants under drought stress treatment is higher than that of wild-type plants. The results indicated that PeTIP3-2 was able to improve the drought resistance of plants. Our discovery lays the foundation for the functional study of AQPs in passion fruit.

Hypoxia-Induced Aquaporins and Regulation of Redox Homeostasis by a Trans-Plasma Membrane Electron Transport System in Maize Roots

In plants, flooding-induced oxygen deficiency causes severe stress, leading to growth reduction and yield loss. It is therefore important to understand the molecular mechanisms for adaptation to hypoxia. Aquaporins at the plasma membrane play a crucial role in water uptake. However, their role during hypoxia and membrane redox changes is still not fully understood. The influence of 24 h hypoxia induction on hydroponically grown maize (Zea mays L.) was investigated using an oil-based setup. Analyses of physiological parameters revealed typical flooding symptoms such as increased ethylene and H₂O₂ levels, an increased alcohol dehydrogenase activity, and an increased redox activity at the plasma membrane along with decreased oxygen of the medium. Transcriptomic analysis and shotgun proteomics of plasma membranes and soluble fractions were performed to determine alterations in maize roots. RNA-sequencing data confirmed the upregulation of genes involved in anaerobic metabolism, biosynthesis of the phytohormone ethylene, and its receptors. Transcripts of several antioxidative systems and other oxidoreductases were regulated. Mass spectrometry analysis of the plasma membrane proteome revealed alterations in redox systems and an increased abundance of aquaporins. Here, we discuss the importance of plasma membrane aquaporins and redox systems in hypoxia stress response, including the regulation of plant growth and redox homeostasis.

Identification and Characterization of Salt- and Drought-Responsive AQP Family Genes in Medicagosativa L

Aquaporins (AQP) are distributed ubiquitously in plants, and they play important roles in multiple aspects of plant growth and development, as well as in plant resistance to various environmental stresses. In this study, 43 MsAQP genes were identified in the forage crop Medicago sativa. All the MsAQP proteins were clustered into four subfamilies based on sequence similarity and phylogenetic relationship, including 17 TIPs, 14 NIPs, 9 PIPs and 3 SIPs. Analyses of gene structure and conserved domains indicated that the majority of the deduced MsAQP proteins contained the signature transmembrane domains and the NPA motifs. Analyses on cis-acting elements in the promoter region of MsAQP genes revealed the presence of multiple and diverse stress-responsive and hormone-responsive cis-acting elements. In addition, by analyzing the available and comprehensive gene expression data of M. truncatula, we screened ten representative MtAQP genes that were responsive to NaCl or drought stress. By analyzing the sequence similarity and phylogenetic relationship, we finally identified the corresponding ten salt- or drought-responsive AQP genes in M. sativa, including three MsTIPs, three MsPIPs and four MsNIPs. The qPCRs showed that the relative expression levels of these ten selected MsAQP genes responded differently to NaCl or drought treatment in M. sativa. Gene expression patterns showed that most MsAQP genes were preferentially expressed in roots or in leaves, which may reflect their tissue-specific functions associated with development. Our results lay an important foundation for the future characterization of the functions of MsAQP genes, and provide candidate genes for stress resistance improvement through genetic breeding in M. sativa.

Pearl Millet Aquaporin Gene PgPIP2;6 Improves Abiotic Stress Tolerance in Transgenic Tobacco

Pearl millet [Pennisetum glaucum (L) R. Br.] is an important cereal crop of the semiarid tropics, which can withstand prolonged drought and heat stress. Considering an active involvement of the aquaporin (AQP) genes in water transport and desiccation tolerance besides several basic functions, their potential role in abiotic stress tolerance was systematically characterized and functionally validated. A total of 34 AQP genes from P. glaucum were identified and categorized into four subfamilies, viz., plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin-26-like intrinsic proteins (NIPs), and small basic intrinsic proteins (SIPs). Sequence analysis revealed that PgAQPs have conserved characters of AQP genes with a closer relationship to sorghum. The PgAQPs were expressed differentially under high vapor pressure deficit (VPD) and progressive drought stresses where the PgPIP2;6 gene showed significant expression under high VPD and drought stress. Transgenic tobacco plants were developed by heterologous expression of the PgPIP2;6 gene and functionally characterized under different abiotic stresses to further unravel their role. Transgenic tobacco plants in the T2 generations displayed restricted transpiration and low root exudation rates in low- and high-VPD conditions. Under progressive drought stress, wild-type (WT) plants showed a quick or faster decline of soil moisture than transgenics. While under heat stress, PgPIP2;6 transgenics showed better adaptation to heat (40°C) with high canopy temperature depression (CTD) and low transpiration; under low-temperature stress, they displayed lower transpiration than their non-transgenic counterparts. Cumulatively, lower transpiration rate (Tr), low root exudation rate, declined transpiration, elevated CTD, and lower transpiration indicate that PgPIP2;6 plays a role under abiotic stress tolerance. Since the PgPIP2;6 transgenic plants exhibited better adaptation against major abiotic stresses such as drought, high VPD, heat, and cold stresses by virtue of enhanced transpiration efficiency, it has the potential to engineer abiotic stress tolerance for sustained growth and productivity of crops.

Label-free quantitative proteomics of maize roots from different root zones provides insight into proteins associated with enhance water uptake

Background

Maize is one of the most important food crops worldwide. Roots play important role in maize productivity through water and nutrient uptake from the soil. Improving maize root traits for efficient water uptake will help to optimize irrigation and contribute to sustainable maize production. Therefore, we investigated the protein profiles of maize cv. Anyu308 root system divided into Upper root zone (UR), Middle root (MR), and Lower root (LR), by label free quantitative shotgun proteomic approach (LFQ). The aim of our study was to identify proteins and mechanisms associated with enhanced water uptake in different maize root zones under automatic irrigation system.

Results

At field capacity, MR had the highest water uptake than the UR and LR. We identified a total of 489 differentially abundant proteins (DAPs) by pairwise comparison of MR, LR, and UR. Cluster analysis of DAPs revealed MR and UR had similar protein abundance patterns different from LR. More proteins were differentially abundant in MR/UR compared to LR/MR and LR/UR. Comparisons of protein profiles indicate that the DAPs in MR increased in abundance, compared to UR and LR which had more downregulated DAPs. The abundance patterns, functional category, and pathway enrichment analyses highlight chromatin structure and dynamics, ribosomal structures, polysaccharide metabolism, energy metabolism and transport, induction of water channels, inorganic ion transport, intracellular trafficking, and vesicular transport, and posttranslational modification as primary biological processes related to enhanced root water uptake in maize. Specifically, the abundance of histones, ribosomal proteins, and aquaporins, including mitochondrion electron transport proteins and the TCA cycle, underpinned MR’s enhanced water uptake. Furthermore, proteins involved in folding and vascular transport supported the radial transport of solute across cell membranes in UR and MR. Parallel reaction monitoring analysis was used to confirmed profile of the DAPs obtained by LFQ-based proteomics.

Conclusion

The list of differentially abundant proteins identified in MR are interesting candidates for further elucidation of their role in enhanced water uptake in maize root. Overall, the current results provided an insight into the mechanisms of maize root water uptake.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08394-y.

Identification of Aquaporin Gene Family in Response to Natural Cold Stress in Ligustrum × vicaryi Rehd

Plants are susceptible to a variety of abiotic stresses during the growing period, among which low temperature is one of the more frequent stress factors. Maintaining water balance under cold stress is a difficult and critical challenge for plants. Studies have shown that aquaporins located on the cytomembrane play an important role in controlling water homeostasis under cold stress, and are involved in the tolerance mechanism of plant cells to cold stress. In addition, the aquaporin gene family is closely related to the cold resistance of plants. As a major greening tree species in urban landscaping, Ligustrum× vicaryi Rehd. is more likely to be harmed by low temperature after a harsh winter and a spring with fluctuating temperatures. Screening the target aquaporin genes of Ligustrum × vicaryi responding to cold resistance under natural cold stress will provide a scientific theoretical basis for cold resistance breeding of Ligustrum × vicaryi. In this study, the genome-wide identification of the aquaporin gene family was performed at four different overwintering periods in September, November, January and April, and finally, 58 candidate Ligustrum × vicaryi aquaporin (LvAQP) genes were identified. The phylogenetic analysis revealed four subfamilies of the LvAQP gene family: 32 PIPs, 11 TIPs, 11 NIPs and 4 SIPs. The number of genes in PIPs subfamily was more than that in other plants. Through the analysis of aquaporin genes related to cold stress in other plants and LvAQP gene expression patterns identified 20 LvAQP genes in response to cold stress, and most of them belonged to the PIPs subfamily. The significantly upregulated LvAQP gene was Cluster-9981.114831, and the significantly downregulated LvAQP genes were Cluster-9981.112839, Cluster-9981.107281, and Cluster-9981.112777. These genes might play a key role in responding to cold tolerance in the natural low-temperature growth stage of Ligustrum × vicaryi.

Overexpression of a Cotton Aquaporin Gene GhTIP1;1-like Confers Cold Tolerance in Transgenic Arabidopsis

Cold stress can significantly affect the development, yield, and quality of crops and restrict the geographical distribution and growing seasons of plants. Aquaporins are the main channels for water transport in plant cells. Abiotic stresses such as cold and drought dehydrate cells by changing the water potential. In this study, we cloned a gene GhTIP1;1-like encodes tonoplast aquaporin from the transcriptome database of cotton seedlings after cold stress. Expression analysis showed that GhTIP1;1-like not only responds to cold stress but was also induced by heat, drought and salt stress. Subcellular localization showed that the protein was anchored to the vacuole membrane. Promoter deletion analysis revealed that a MYC motif within the promoter region of GhTIP1;1-like were the core cis-elements in response to low temperature. Virus-induced gene silencing (VIGS) and histochemical staining indicate that GhTIP1;1-like plays a positive role in plant cold tolerance. Overexpression of GhTIP1;1-like in Arabidopsis delayed the senescence process and enhanced the cold tolerance of transgenic plants. Compared with the wild type, the soluble protein concentration and peroxidase activity of the transgenic lines under cold stress were higher, while the malondialdehyde content was lower. In addition, the expression levels of cold-responsive genes were significantly increased in transgenic plants under cold stress. Our results indicate that GhTIP1;1-like could respond to different abiotic stresses and be positively involved in regulating the cold tolerance of cotton.

Combined Transcriptomics and Metabolomics Analysis Reveals the Molecular Mechanism of Salt Tolerance of Huayouza 62, an Elite Cultivar in Rapeseed (Brassica napus L.)

Soil salinity is one of the most significant abiotic stresses affecting crop yield around the world. To explore the molecular mechanism of salt tolerance in rapeseed (Brassica napus L.), the transcriptome analysis and metabolomics analysis were used to dissect the differentially expressed genes and metabolites in two rapeseed varieties with significant differences in salt tolerance; one is an elite rapeseed cultivar, Huayouza 62. A total of 103 key differentially expressed metabolites (DEMs) and 53 key differentials expressed genes (DEGs) that might be related to salt stress were identified through metabolomics and transcriptomics analysis. GO and KEGG analysis revealed that the DEGs were mainly involved in ion transport, reactive oxygen scavenging, osmotic regulation substance synthesis, and macromolecular protein synthesis. The DEMs were involved in TCA cycle, proline metabolism, inositol metabolism, carbohydrate metabolic processes, and oxidation-reduction processes. In addition, overexpression of BnLTP3, which was one of the key DEGs, could increase tolerance to salt stress in Arabidopsis plants. This study reveals that the regulation mechanism of salt tolerance in rapeseed at the transcriptome and metabolism level and provides abundant data for further in-depth identification of essential salt tolerance genes.

Genome-Wide Identification of Maize Aquaporin and Functional Analysis During Seed Germination and Seedling Establishment

Water uptake facilitates the initiation of seed germination. It is presumed that aquaporin (AQP)-mediated water inflow contributes to seed germination, but the genetic evidence is still lacking. This study aimed at genome-wide identification of ZmAQPs and further determined the physiological functions. Following a comprehensive search, a total of 41 ZmAQPs were identified according to the latest genome database. Through bioinformatic approaches, the physicochemical characteristics, phylogenetic relationships, and structural features of ZmAQPs were analyzed. The gene expression analysis of 20 high-resolution and multi-tissues samples showed that ZmAQPs had distinct spatiotemporal and tissue-specific expression profiles during seed germination and early seedling development. We then focused on the aquaporin of maize tonoplast intrinsic protein 3 (ZmTIP3), which is specifically expressed in germinating seed. A mutant zmtip3-1 with disruption of the ZmTIP3-1 gene showed shorter shoot and root length, and decreased seedling dry weight compared with the control (W22). The result revealed that ZmTIP3-1 improved the absolute content of seed protein and promoted storage reserves mobilization, suggesting that ZmTIP3 may be a positive regulator of seed vigor. This work provides valuable clues for understanding the function and possible regulatory mechanism of ZmAQPs in seed germination and seedling growth.

Overexpression of an aquaporin protein from Aspergillus glaucus confers salt tolerance in transgenic soybean

Salt stress is an important abiotic factor that causes severe losses in soybean yield and quality. Therefore, breeding salt-tolerant soybean germplasm resources via genetic engineering has gained importance. Aspergillus glaucus, a halophilic fungus that exhibits significant tolerance to salt, carries the gene AgGlpF. In this study, we used the soybean cotyledonary node transformation method to transfer the AgGlpF gene into the genome of the soybean variety Williams 82 to generate salt-tolerant transgenic soybean varieties. The results of PCR, Southern blot, ddPCR, and RT-PCR indicated that AgGlpF was successfully integrated into the soybean genome and stably expressed. When subjected to salt stress conditions via treatment with 250 mM NaCl for 3 d, the transgenic soybean plants showed significant tolerance compared with wild-type plants, which exhibited withering symptoms and leaf abscission after 9 d. The results of this study indicated that the transfer of AgGlpF into the genome of soybean plants produced transgenic soybean with significantly improved salt stress tolerance.

Expression of a CO2-permeable aquaporin enhances mesophyll conductance in the C4 species Setaria viridis

A fundamental limitation of photosynthetic carbon fixation is the availability of CO₂. In C₄ plants, primary carboxylation occurs in mesophyll cytosol, and little is known about the role of CO₂ diffusion in facilitating C₄ photosynthesis. We have examined the expression, localization, and functional role of selected plasma membrane intrinsic aquaporins (PIPs) from Setaria italica (foxtail millet) and discovered that SiPIP2;7 is CO₂-permeable. When ectopically expressed in mesophyll cells of Setaria viridis (green foxtail), SiPIP2;7 was localized to the plasma membrane and caused no marked changes in leaf biochemistry. Gas exchange and C¹⁸O¹⁶O discrimination measurements revealed that targeted expression of SiPIP2;7 enhanced the conductance to CO₂ diffusion from the intercellular airspace to the mesophyll cytosol. Our results demonstrate that mesophyll conductance limits C₄ photosynthesis at low pCO₂ and that SiPIP2;7 is a functional CO₂ permeable aquaporin that can improve CO₂ diffusion at the airspace/mesophyll interface and enhance C₄ photosynthesis.

Genome-Wide Identification and Function of Aquaporin Genes During Dormancy and Sprouting Periods of Kernel-Using Apricot (Prunus armeniaca L.)

Aquaporins (AQPs) are essential channel proteins that play a major role in plant growth and development, regulate plant water homeostasis, and transport uncharged solutes across biological membranes. In this study, 33 AQP genes were systematically identified from the kernel-using apricot (Prunus armeniaca L.) genome and divided into five subfamilies based on phylogenetic analyses. A total of 14 collinear blocks containing AQP genes between P. armeniaca and Arabidopsis thaliana were identified by synteny analysis, and 30 collinear blocks were identified between P. armeniaca and P. persica. Gene structure and conserved functional motif analyses indicated that the PaAQPs exhibit a conserved exon-intron pattern and that conserved motifs are present within members of each subfamily. Physiological mechanism prediction based on the aromatic/arginine selectivity filter, Froger's positions, and three-dimensional (3D) protein model construction revealed marked differences in substrate specificity between the members of the five subfamilies of PaAQPs. Promoter analysis of the PaAQP genes for conserved regulatory elements suggested a greater abundance of cis-elements involved in light, hormone, and stress responses, which may reflect the differences in expression patterns of PaAQPs and their various functions associated with plant development and abiotic stress responses. Gene expression patterns of PaAQPs showed that PaPIP1-3, PaPIP2-1, and PaTIP1-1 were highly expressed in flower buds during the dormancy and sprouting stages of P. armeniaca. A PaAQP coexpression network showed that PaAQPs were coexpressed with 14 cold resistance genes and with 16 cold stress-associated genes. The expression pattern of 70% of the PaAQPs coexpressed with cold stress resistance genes was consistent with the four periods [Physiological dormancy (PD), ecological dormancy (ED), sprouting period (SP), and germination stage (GS)] of flower buds of P. armeniaca. Detection of the transient expression of GFP-tagged PaPIP1-1, PaPIP2-3, PaSIP1-3, PaXIP1-2, PaNIP6-1, and PaTIP1-1 revealed that the fusion proteins localized to the plasma membrane. Predictions of an A. thaliana ortholog-based protein-protein interaction network indicated that PaAQP proteins had complex relationships with the cold tolerance pathway, PaNIP6-1 could interact with WRKY6, PaTIP1-1 could interact with TSPO, and PaPIP2-1 could interact with ATHATPLC1G. Interestingly, overexpression of PaPIP1-3 and PaTIP1-1 increased the cold tolerance of and protein accumulation in yeast. Compared with wild-type plants, PaPIP1-3- and PaTIP1-1-overexpressing (OE) Arabidopsis plants exhibited greater tolerance to cold stress, as evidenced by better growth and greater antioxidative enzyme activities. Overall, our study provides insights into the interaction networks, expression patterns, and functional analysis of PaAQP genes in P. armeniaca L. and contributes to the further functional characterization of PaAQPs.

Cold Stress in Citrus: A Molecular, Physiological and Biochemical Perspective

Due to climate change, we are forced to face new abiotic stress challenges like cold and heat waves that currently result from global warming. Losses due to frost and low temperatures force us to better understand the physiological, hormonal, and molecular mechanisms of response to such stress to face losses, especially in tropical and subtropical crops like citrus fruit, which are well adapted to certain weather conditions. Many of the responses to cold stress that are found are also conserved in citrus. Hence, this review also intends to show the latest work on citrus. In addition to basic research, there is a great need to employ and cultivate new citrus rootstocks to better adapt to environmental conditions.

Overexpression of a Banana Aquaporin Gene MaPIP1;1 Enhances Tolerance to Multiple Abiotic Stresses in Transgenic Banana and Analysis of Its Interacting Transcription Factors

Aquaporins can improve the ability of plants to resist abiotic stresses, but the mechanism is still not completely clear. In this research, overexpression of MaPIP1;1 in banana improved tolerance to multiple stresses. The transgenic plants resulted in lower ion leakage and malondialdehyde content, while the proline, chlorophyll, soluble sugar, and abscisic acid (ABA) contents were higher. In addition, under high salt and recovery conditions, the content of Na⁺ and K⁺ is higher, also under recovery conditions, the ratio of K⁺/Na⁺ is higher. Finally, under stress conditions, the expression levels of ABA biosynthesis and response genes in the transgenic lines are higher than those of the wild type. In previous studies, we proved that the MaMADS3 could bind to the promoter region of MaPIP1;1, thereby regulating the expression of MaPIP1;1 and affecting the drought tolerance of banana plants. However, the mechanism of MaPIP1;1 gene response to stress under different adversity conditions might be regulated differently. In this study, we proved that some transcription factor genes, including MaERF14, MaDREB1G, MaMYB1R1, MaERF1/39, MabZIP53, and MaMYB22, showed similar expression patterns with MaPIP1;1 under salt or cold stresses, and their encoded proteins could bind to the promoter region of MaPIP1;1. Here we proposed a novel MaPIP1;1-mediated mechanism that enhanced salt and cold tolerance in bananas. The results of this study have enriched the stress-resistant regulatory network of aquaporins genes and are of great significance for the development of molecular breeding strategies for stress-resistant fruit crops.

Genome-Wide Identification of Aquaporin Gene Family in Pitaya Reveals an HuNIP6;1 Involved in Flowering Process

Aquaporins (AQPs) are essential membrane proteins involved in seed maturation and germination, stomata movement, photosynthesis, and regulation of plant flowering processes. Pitaya flowers are open at night and wither at daybreak, which shows an obvious circadian rhythm. In this study, a comprehensive genome-wide analysis of AQPs in Hylocereus undantus was conducted to screen key genes associated with flowering processes. A total of 33 HuAQP genes were identified from the H. undantus genome. The 33 HuAQPs were grouped into four subfamilies: 10 PIPs, 13 TIPs, 8 NIPs, and 2 SIPs, which were distributed on 9 out of 11 pitaya chromosomes (Chr) (except for Chr7 and Chr10). Results from expression profiles showed that HuNIP6;1 may be involved in pitaya's floral opening. HuNIP6;1 was localized exclusively in the cell membrane. Overexpression of HuNIP6;1 in Arabidopsis thaliana significantly promoted early flowering through regulating negative flowering regulators of MJM30, COL9, and PRR5, suggesting that HuNIP6;1 plays key roles in regulating flowering time. The present study provides the first genome-wide analysis of the AQP gene family in pitaya and valuable information for utilization of HuAQPs.

Genome-wide identification of candidate aquaporins involved in water accumulation of pomegranate outer seed coat

Aquaporins (AQPs) are a class of highly conserved integral membrane proteins that facilitate the uptake and transport of water and other small molecules across cell membranes. However, little is known about AQP genes in pomegranate (Punica granatum L.) and their potential role in water accumulation of the outer seed coat. We identified 38 PgrAQP genes in the pomegranate genome and divided them into five subfamilies based on a comparative analysis. Purifying selection played a role in the evolution of PgrAQP genes and a whole-genome duplication event in Myrtales may have contributed to the expansion of PgrTIP, PgrSIP, and PgrXIP genes. Transcriptome data analysis revealed that the PgrAQP genes exhibited different tissue-specific expression patterns. Among them, the transcript abundance of PgrPIPs were significantly higher than that of other subfamilies. The mRNA transcription levels of PgrPIP1.3, PgrPIP2.8, and PgrSIP1.2 showed a significant linear relationship with water accumulation in seed coats, indicating that PgrPIP1.3/PgrPIP2.8 located in the plasma membrane and PgrSIP1.2 proteins located on the tonoplast may be involved in water accumulation and contribute to the cell expansion of the outer seed coat, which then develops into juicy edible flesh. Overall, our results provided not only information on the characteristics and evolution of PgrAQPs, but also insights on the genetic improvement of outer seed coats.

Genome-wide identification and expression analysis of aquaporin family in Canavalia rosea and their roles in the adaptation to saline-alkaline soils and drought stress

BACKGROUND

Canavalia rosea (Sw.) DC. (bay bean) is an extremophile halophyte that is widely distributed in coastal areas of the tropics and subtropics. Seawater and drought tolerance in this species may be facilitated by aquaporins (AQPs), channel proteins that transport water and small molecules across cell membranes and thereby maintain cellular water homeostasis in the face of abiotic stress. In C. rosea, AQP diversity, protein features, and their biological functions are still largely unknown.

RESULTS

We describe the action of AQPs in C. rosea using evolutionary analyses coupled with promoter and expression analyses. A total of 37 AQPs were identified in the C. rosea genome and classified into five subgroups: 11 plasma membrane intrinsic proteins, 10 tonoplast intrinsic proteins, 11 Nod26-like intrinsic proteins, 4 small and basic intrinsic proteins, and 1 X-intrinsic protein. Analysis of RNA-Seq data and targeted qPCR revealed organ-specific expression of aquaporin genes and the involvement of some AQP members in adaptation of C. rosea to extreme coral reef environments. We also analyzed C. rosea sequences for phylogeny reconstruction, protein modeling, cellular localizations, and promoter analysis. Furthermore, one of PIP1 gene, CrPIP1;5, was identified as functional using a yeast expression system and transgenic overexpression in Arabidopsis.

CONCLUSIONS

Our results indicate that AQPs play an important role in C. rosea responses to saline-alkaline soils and drought stress. These findings not only increase our understanding of the role AQPs play in mediating C. rosea adaptation to extreme environments, but also improve our knowledge of plant aquaporin evolution more generally.

Citrus NIP5;1 aquaporin regulates cell membrane water permeability and alters PIPs plasma membrane localization

The ER or donut-like structures localized aquaporin NIP5;1, which interacts with PIPs and alters their localization from plasma membrane to donut-like structures, regulates water permeability. NOD26-like intrinsic proteins (NIPs) play important roles in nutrient uptake and response to various stresses. However, there have been few studies of their functions in water transportation in citrus. Here, we demonstrate the functions of a novel citrus NIP aquaporin (CsNIP5;1) via multiple physiological and biochemical experiments. CsNIP5;1 showed high water permeability when expressed in Xenopus laevis oocytes and yeast. However, subcellular localization assays showed that this protein was localized in the endoplasmic reticulum (ER) or donut-like structures in citrus callus and tobacco leaf. Meanwhile, overexpression of CsNIP5;1 led to a reduction in the water permeability of citrus callus. Protein-protein interaction experiments and subcellular localization assays further revealed that CsNIP5;1 physically interacted with PIPs (CsPIP1;1 and AtPIP2;1), which altered their subcellular localization from the plasma membrane to donut-like structures. Together, CsNIP5;1 was identified as a good water channel when expressed in oocytes and yeast. Meanwhile, CsNIP5;1 participated in the regulation of water permeability of citrus callus, which may be associated with CsNIP5;1-induced re-localization of water channels PIPs. In summary, these results provide new insights into the regulatory mechanism of AQPs-mediated water diffusion.

Gradient NMR Method for Studies of Water Translational Diffusion in Plants

The review of a retrospective nature shows the stages of development of the spin-echo NMR method with constant and pulsed gradient of the magnetic field (gradient NMR) for the study of water diffusion in plant roots. The history of the initial use of gradient NMR for plants, in which it was not possible to experimentally confirm the bound state of water in cells, is described. The work presents the main ideas on which the technology of measuring diffusion by the spin-echo NMR method is built. Special attention is paid to the manifestations and record of the restricted diffusion phenomenon, permeability of membranes, along with the finite formulae used in real experiments. As examples, it gives the non-trivial results of studies of water transfer in roots through the symplastic system, from cell to cell through intercellular contacts with plasmodesmata, through aquaporins, transfer under the influence of changes in external pressure, and the composition of the gas atmosphere.

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.

Understanding aquaporin transport system, silicon and other metalloids uptake and deposition in bottle gourd (Lagenaria siceraria)

Aquaporins (AQPs) facilitates the transport of small solutes like water, urea, carbon dioxide, boron, and silicon (Si) and plays a critical role in important physiological processes. In this study, genome-wide characterization of AQPs was performed in bottle gourd. A total of 36 AQPs were identified in the bottle gourd, which were subsequently analyzed to understand the pore-morphology, exon-intron structure, subcellular-localization. In addition, available transcriptome data was used to study the tissue-specific expression. Several AQPs showed tissue-specific expression, more notably the LsiTIP3-1 having a high level of expression in flowers and fruits. Based on the in-silico prediction of solute specificity, LsiNIP2-1 was predicted to be a Si transporter. Silicon was quantified in different tissues, including root, young leaves, mature leaves, tendrils, and fruits of bottle gourd plants. More than 1.3% Si (d.w.) was observed in bottle gourd leaves, testified the in-silico predictions. Silicon deposition evaluated with an energy-dispersive X-ray coupled with a scanning electron microscope showed a high Si accumulation in the shaft of leaf trichomes. Similarly, co-localization of Si with arsenic and antimony was observed. Expression profiling performed with real-time quantitative PCR showed differential expression of AQPs in response to Si supplementation. The information provided in the present study will be helpful to better understand the AQP transport mechanism, particularly Si and other metalloids transport and localization in plants.

Decreases in arsenic accumulation by the plasma membrane intrinsic protein PIP2;2 in Arabidopsis and yeast

Arsenic (As) is a toxic pollutant that mainly enters the human body via plants. Therefore, understanding the strategy for reducing arsenic accumulation in plants is important to human health and the environment. Aquaporins are ubiquitous water channel proteins that bidirectionally transport water across cell membranes and play a role in the transportation of other molecules, such as glycerol, ammonia, boric acid, and arsenic acid. Previously, we observed that Arabidopsis PIP2;2, encoding a plasma membrane intrinsic protein, is highly expressed in NtCyc07-expressing Arabidopsis, which shows a higher tolerance to arsenite (As(III)). In this study, we report that the overexpression of AtPIP2;2 enhanced As(III) tolerance and reduced As(III) levels in yeast. Likewise, AtPIP2;2-overexpressing Arabidopsis exhibited improved As(III) tolerance and lower accumulation of As(III). In contrast, atpip2;2 knockout Arabidopsis showed reduced As(III) tolerance but no significant change in As(III) levels. Interestingly, the AtPIP2;2 transcript and protein levels were increased in roots and shoots of Arabidopsis in response to As(III). Furthermore, As(III) efflux was enhanced and As(III) influx/accumulation was reduced in AtPIP2;2-expressing plants. The expression of AtPIP2;2 rescued the As(III)-sensitive phenotype of acr3 mutant yeast by reducing As levels and slightly reduced the As(III)-tolerant phenotype of fps1 mutant yeast by enhancing As content, suggesting that AtPIP2; 2 functions as a bidirectional channel of As(III), while the As(III) exporter activity is higher than the As(III) importer activity. All these results indicate that AtPIP2;2 expression promotes As(III) tolerance by decreasing As(III) accumulation through enhancing As(III) efflux in Arabidopsis. This finding can be applied to the generation of low arsenic crops for human health.

Targeting aquaporins to alleviate hazardous metal(loid)s imposed stress in plants

Uptake of hazardous metal(loid)s adversely affects plants and imposes a threat to the entire food chain. Here, the role of aquaporins (AQPs) providing tolerance against hazardous metal(loid)s in plants is discussed to provide a perspective on the present understanding, knowledge gaps, and opportunities. Plants adopt complex molecular and physiological mechanisms for better tolerance, adaptability, and survival under metal(loid)s stress. Water conservation in plants is one such primary strategies regulated by AQPs, a family of channel-forming proteins facilitating the transport of water and many other solutes. The strategy is more evident with reports suggesting differential expression of AQPs adopted by plants to cope with the heavy metal stress. In this regard, numerous studies showing enhanced tolerance against hazardous elements in plants due to AQPs activity are discussed. Consequently, present understanding of various aspects of AQPs, such as tertiary-structure, transport activity, solute-specificity, differential expression, gating mechanism, and subcellular localization, are reviewed. Similarly, various tools and techniques are discussed in detail aiming at efficient utilization of resources and knowledge to combat metal(loid)s stress. The scope of AQP transgenesis focusing on heavy metal stresses is also highlighted. The information provided here will be helpful to design efficient strategies for the development of metal(loid)s stress-tolerant crops.

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

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

Zinc regulates the hydraulic response of maize root under water stress conditions

Zinc (Zn) is involved in plant growth and stress resistance and is known to increase crop yield. Here, we investigated the effect of Zn on water absorption in the roots of maize (Zea mays L.), a crop which is sensitive to Zn deficiency, during water stress conditions. Seedlings of the maize variety "Zhengdan 958" were cultivated with 0.1 or 6 μM ZnSO₄·7H₂O. To simulate drought stress, three-week-old seedlings were exposed to 15% polyethylene glycol (PEG). Root growth parameters, root antioxidant enzyme activity, root hydraulic conductivity, root aquaporin gene expression, root and leaf anatomy structure, leaf water potential, chlorophyll content, leaf area, and gas exchange parameters were measured. Under water stress, moderate Zn treatment promoted root growth; maintained root and leaf anatomy structural integrity. Moderate Zn significantly increased roots hydraulic conductivity (51%) and decreased roots antioxidant enzyme activity (POD: -11.1%, CAT: -35.1%, SOD: -3.1%) compared with low-level Zn under water stress. The expression of ZmPIP1;1, ZmPIP1;2, and ZmPIP2;2 was significantly higher with moderate Zn treatment than that of low-level Zn treatment. The leaf water potential, chlorophyll content, leaf area, and gas exchange parameters with moderate Zn treatment increased significantly under water stress compared with low-level Zn treatment. The moderate concentration of Zn improved root hydraulic conductivity in maize and increased resistance to simulated drought conditions by maintaining root structural integrity, decreasing antioxidant enzyme activity, and increasing aquaporin gene expression. Moderate Zn application increased root water absorption and leaf transpiration, thereby maintaining maize water balance under water stress conditions.

Two Aquaporin Genes, GhPIP2;7 and GhTIP2;1, Positively Regulate the Tolerance of Upland Cotton to Salt and Osmotic Stresses

Aquaporins (AQPs) facilitate the transport of water and small molecules across intrinsic membranes and play a critical role in abiotic stresses. In this study, 111, 54, and 56 candidate AQP genes were identified in Gossypium hirsutum (AD₁), Gossypium arboreum (A₂), and Gossypium raimondii (D₅), respectively, and were further classified into five subfamilies, namely, plasma intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), small basic intrinsic protein (SIP), and uncategorized X intrinsic protein (XIP). Transcriptome analysis and quantitative real-time PCR (qRT-PCR) revealed some high-expression GhPIPs and GhTIPs (PIP and TIP genes in G. hirsutum, respectively) in drought and salt stresses. GhPIP2;7-silenced plants decreased in the chlorophyll content, superoxide dismutase (SOD) activity, and peroxidase (POD) activity comparing the mock control (empty-vector) under 400 mM NaCl treatment, which indicated a positive regulatory role of GhPIP2;7 in salt tolerance of cotton. The GhTIP2;1-silenced cotton plants were more sensitive to osmotic stress. GhTIP2;1-overexpressed plants exhibited less accumulation of H₂O₂ and malondialdehyde but higher proline content under osmotic stress. In summary, our study elucidates the positive regulatory roles of two GhAQPs (GhPIP2;7 and GhTIP2;1) in salt and osmotic stress responses, respectively, and provides a new gene resource for future research.