MpAsr encodes an intrinsically unstructured protein and enhances osmotic tolerance in transgenic Arabidopsis

HaASR2 from Haloxylon ammodendron confers drought and salt tolerance in plants

Abscisic acid (ABA), stress, and ripening-induced proteins (ASR), which belong to the ABA/WDS domain superfamily, are involved in the plant response to abiotic stresses. Haloxylon ammodendron is a succulent xerohalophyte species that exhibits strong resistance to abiotic stress. In this study, we isolated HaASR2 from H. ammodendron and demonstrated its detailed molecular function for drought and salt stress tolerance. HaASR2 interacted with the HaNHX1 protein, and its expression was significantly up-regulated under osmotic stress. Overexpression of HaASR2 improved drought and salt tolerance by enhancing water use efficiency and photosynthetic capacity in Arabidopsis thaliana. Overexpression of HaASR2 maintained the homeostasis of reactive oxygen species (ROS) and decreased sensitivity to exogenous ABA and endogenous ABA levels by down-regulating ABA biosynthesis genes under drought stress. Furthermore, a transcriptomic comparison between wild-type and HaASR2 transgenic Arabidopsis plants indicated that HaASR2 significantly induced the expression of 896 genes in roots and 406 genes in shoots under osmotic stress. Gene ontology (GO) enrichment analysis showed that those DEGs were mainly involved in ROS scavenging, metal ion homeostasis, response to hormone stimulus, etc. The results demonstrated that HaASR2 from the desert shrub, H. ammodendron, plays a critical role in plant adaptation to drought and salt stress and could be a promising gene for the genetic improvement of crop abiotic stress tolerance.

Isoprene-Emitting Tobacco Plants Are Less Affected by Moderate Water Deficit under Future Climate Change Scenario and Show Adjustments of Stress-Related Proteins in Actual Climate

Isoprene-emitting plants are better protected against thermal and oxidative stresses, which is a desirable trait in a climate-changing (drier and warmer) world. Here we compared the ecophysiological performances of transgenic isoprene-emitting and wild-type non-emitting tobacco plants during water stress and after re-watering in actual environmental conditions (400 ppm of CO₂ and 28 °C of average daily temperature) and in a future climate scenario (600 ppm of CO₂ and 32 °C of average daily temperature). Furthermore, we intended to complement the present knowledge on the mechanisms involved in isoprene-induced resistance to water deficit stress by examining the proteome of transgenic isoprene-emitting and wild-type non-emitting tobacco plants during water stress and after re-watering in actual climate. Isoprene emitters maintained higher photosynthesis and electron transport rates under moderate stress in future climate conditions. However, physiological resistance to water stress in the isoprene-emitting plants was not as marked as expected in actual climate conditions, perhaps because the stress developed rapidly. In actual climate, isoprene emission capacity affected the tobacco proteomic profile, in particular by upregulating proteins associated with stress protection. Our results strengthen the hypothesis that isoprene biosynthesis is related to metabolic changes at the gene and protein levels involved in the activation of general stress defensive mechanisms of plants.

Grape ASR Regulates Glucose Transport, Metabolism and Signaling

To decipher the mediator role of the grape Abscisic acid, Stress, Ripening (ASR) protein, VvMSA, in the pathways of glucose signaling through the regulation of its target, the promoter of hexose transporter VvHT1, we overexpressed and repressed VvMSA in embryogenic and non-embryogenic grapevine cells. The embryogenic cells with organized cell proliferation were chosen as an appropriate model for high sensitivity to the glucose signal, due to their very low intracellular glucose content and low glycolysis flux. In contrast, the non-embryogenic cells displaying anarchic cell proliferation, supported by high glycolysis flux and a partial switch to fermentation, appeared particularly sensitive to inhibitors of glucose metabolism. By using different glucose analogs to discriminate between distinct pathways of glucose signal transduction, we revealed VvMSA positioning as a transcriptional regulator of the glucose transporter gene VvHT1 in glycolysis-dependent glucose signaling. The effects of both the overexpression and repression of VvMSA on glucose transport and metabolism via glycolysis were analyzed, and the results demonstrated its role as a mediator in the interplay of glucose metabolism, transport and signaling. The overexpression of VvMSA in the Arabidopsis mutant abi8 provided evidence for its partial functional complementation by improving glucose absorption activity.

Abscisic Acid-Stress-Ripening Genes Involved in Plant Response to High Salinity and Water Deficit in Durum and Common Wheat

In the dry and hot Mediterranean regions wheat is greatly susceptible to several abiotic stresses such as extreme temperatures, drought, and salinity, causing plant growth to decrease together with severe yield and quality losses. Thus, the identification of gene sequences involved in plant adaptation to such stresses is crucial for the optimization of molecular tools aimed at genetic selection and development of stress-tolerant varieties. Abscisic acid, stress, ripening-induced (ASR) genes act in the protection mechanism against high salinity and water deficit in several plant species. In a previous study, we isolated for the first time the TtASR1 gene from the 4A chromosome of durum wheat in a salt-tolerant Tunisian landrace and assessed its involvement in plant response to some developmental and environmental signals in several organs. In this work, we focused attention on ASR genes located on the homoeologous chromosome group 4 and used for the first time a Real-Time approach to "in planta" to evaluate the role of such genes in modulating wheat adaptation to salinity and drought. Gene expression modulation was evaluated under the influence of different variables - kind of stress, ploidy level, susceptibility, plant tissue, time post-stress application, gene chromosome location. ASR response to abiotic stresses was found only slightly affected by ploidy level or chromosomal location, as durum and common wheat exhibited a similar gene expression profile in response to salt increase and water deficiency. On the contrary, gene activity was more influenced by other variables such as plant tissue (expression levels were higher in roots than in leaves), kind of stress [NaCl was more affecting than polyethylene glycol (PEG)], and genotype (transcripts accumulated differentially in susceptible or tolerant genotypes). Based on such experimental evidence, we confirmed Abscisic acid, stress, ripening-induced genes involvement in plant response to high salinity and drought and suggested the quantification of gene expression variation after long salt exposure (72 h) as a reliable parameter to discriminate between salt-tolerant and salt-susceptible genotypes in both Triticum aestivum and Triticum durum.

Grape ASR-Silencing Sways Nuclear Proteome, Histone Marks and Interplay of Intrinsically Disordered Proteins

In order to unravel the functions of ASR (Abscisic acid, Stress, Ripening-induced) proteins in the nucleus, we created a new model of genetically transformed grape embryogenic cells by RNAi-knockdown of grape ASR (VvMSA). Nuclear proteomes of wild-type and VvMSA-RNAi grape cell lines were analyzed by quantitative isobaric tagging (iTRAQ 8-plex). The most significantly up- or down-regulated nuclear proteins were involved in epigenetic regulation, DNA replication/repair, transcription, mRNA splicing/stability/editing, rRNA processing/biogenesis, metabolism, cell division/differentiation and stress responses. The spectacular up-regulation in VvMSA-silenced cells was that of the stress response protein VvLEA D-29 (Late Embryogenesis Abundant). Both VvMSA and VvLEA D-29 genes displayed strong and contrasted responsiveness to auxin depletion, repression of VvMSA and induction of VvLEA D-29. In silico analysis of VvMSA and VvLEA D-29 proteins highlighted their intrinsically disordered nature and possible compensatory relationship. Semi-quantitative evaluation by medium-throughput immunoblotting of eighteen post-translational modifications of histones H3 and H4 in VvMSA-knockdown cells showed significant enrichment/depletion of the histone marks H3K4me1, H3K4me3, H3K9me1, H3K9me2, H3K36me2, H3K36me3 and H4K16ac. We demonstrate that grape ASR repression differentially affects members of complex nucleoprotein structures and may not only act as molecular chaperone/transcription factor, but also participates in plant responses to developmental and environmental cues through epigenetic mechanisms.

Genome-Wide Identification and Comparative Analysis of the ASR Gene Family in the Rosaceae and Expression Analysis of PbrASRs During Fruit Development

The members of the Abscisic Acid (ABA) Stress and Ripening gene family (ASR) encode a class of plant-specific proteins with ABA/WDS domains that play important roles in fruit ripening, abiotic stress tolerance and biotic stress resistance in plants. The ASR gene family has been widely investigated in the monocotyledons and dicotyledons. Although the genome sequence is already available for eight fruit species of the Rosaceae, there is far less information about the evolutionary characteristics and the function of the ASR genes in the Rosaceae than in other plant families. Twenty-seven ASR genes were identified from species in the Rosaceae and divided into four subfamilies (I, II, III, and IV) on the basis of structural characteristics and phylogenetic analysis. Purifying selection was the primary force for ASR family gene evolution in eight Rosaceae species. qPCR experiments showed that the expression pattern of PbrASR genes from Pyrus bretschneideri was organ-specific, being mainly expressed in flower, fruit, leaf, and root. During fruit development, the mRNA abundance levels of different PbrASR genes were either down- or up-regulated, and were also induced by exogenous ABA. Furthermore, subcellular localization results showed that PbrASR proteins were mainly located in the nucleus and cytoplasm. These results provide a theoretical foundation for investigation of the evolution, expression, and functions of the ASR gene family in commercial fruit species of the Rosaceae family.

Multiomics analyses reveal the roles of the ASR1 transcription factor in tomato fruits

The transcription factor ASR1 (ABA, STRESS, RIPENING 1) plays multiple roles in plant responses to abiotic stresses as well as being involved in the regulation of central metabolism in several plant species. However, despite the high expression of ASR1 in tomato fruits, large scale analyses to uncover its function in fruits are still lacking. In order to study its function in the context of fruit ripening, we performed a multiomics analysis of ASR1-antisense transgenic tomato fruits at the transcriptome and metabolome levels. Our results indicate that ASR1 is involved in several pathways implicated in the fruit ripening process, including cell wall, amino acid, and carotenoid metabolism, as well as abiotic stress pathways. Moreover, we found that ASR1-antisense fruits are more susceptible to the infection by the necrotrophic fungus Botrytis cinerea. Given that ASR1 could be regulated by fruit ripening regulators such as FRUITFULL1/FRUITFULL2 (FUL1/FUL2), NON-RIPENING (NOR), and COLORLESS NON-RIPENING (CNR), we positioned it in the regulatory cascade of red ripe tomato fruits. These data extend the known range of functions of ASR1 as an important auxiliary regulator of tomato fruit ripening.

Genome-Wide Analysis of the Late Embryogenesis Abundant (LEA) and Abscisic Acid-, Stress-, and Ripening-Induced (ASR) Gene Superfamily from Canavalia rosea and Their Roles in Salinity/Alkaline and Drought Tolerance

Canavalia rosea (bay bean), distributing in coastal areas or islands in tropical and subtropical regions, is an extremophile halophyte with good adaptability to seawater and drought. Late embryogenesis abundant (LEA) proteins typically accumulate in response to various abiotic stresses, including dehydration, salinity, high temperature, and cold, or during the late stage of seed development. Abscisic acid-, stress-, and ripening-induced (ASR) genes are stress and developmentally regulated plant-specific genes. In this study, we reported the first comprehensive survey of the LEA and ASR gene superfamily in C. rosea. A total of 84 CrLEAs and three CrASRs were identified in C. rosea and classified into nine groups. All CrLEAs and CrASRs harbored the conserved motif for their family proteins. Our results revealed that the CrLEA genes were widely distributed in different chromosomes, and all of the CrLEA/CrASR genes showed wide expression features in different tissues in C. rosea plants. Additionally, we introduced 10 genes from different groups into yeast to assess the functions of the CrLEAs/CrASRs. These results contribute to our understanding of LEA/ASR genes from halophytes and provide robust candidate genes for functional investigations in plant species adapted to extreme environments.

Structural and Functional Characterization of the ABA-Water Deficit Stress Domain from Wheat and Barley: An Intrinsically Disordered Domain behind the Versatile Functions of the Plant Abscissic Acid, Stress and Ripening Protein Family

The ASR protein family has been discovered thirty years ago in many plant species and is involved in the tolerance of various abiotic stresses such as dehydration, salinity and heat. Despite its importance, nothing is known about the conserved ABA-Water Deficit Stress Domain (ABA-WDS) of the ASR gene family. In this study, we characterized two ABA-WDS domains, isolated from durum wheat (TtABA-WDS) and barley (HvABA-WDS). Bioinformatics analysis shows that they are both consistently predicted to be intrinsically disordered. Hydrodynamic and circular dichroism analysis indicate that both domains are largely disordered but belong to different structural classes, with HvABA-WDS and TtABA-WDS adopting a PreMolten Globule-like (PMG-like) and a Random Coil-like (RC-like) conformation, respectively. In the presence of the secondary structure stabilizer trifluoroethanol (TFE) or of increasing glycerol concentrations, which mimics dehydration, the two domains acquire an α-helical structure. Interestingly, both domains are able to prevent heat- and dehydration-induced inactivation of the enzyme lactate dehydrogenase (LDH). Furthermore, heterologous expression of TtABA-WDS and HvABA-WDS in the yeast Saccharomyces cerevisiae improves its tolerance to salt, heat and cold stresses. Taken together our results converge to show that the ABA-WDS domain is an intrinsically disordered functional domain whose conformational plasticity could be instrumental to support the versatile functions attributed to the ASR family, including its role in abiotic stress tolerance. Finally, and after validation in the plant system, this domain could be used to improve crop tolerance to abiotic stresses.

Novel ASR isolated from drought stress responsive SSH library in pearl millet confers multiple abiotic stress tolerance in PgASR3 transgenic Arabidopsis

A genomic resource of drought stress responsive genes/ESTs was generated using Suppression Subtractive Hybridization (SSH) approach in a drought stress tolerant Pennisetum glaucum genotype 841B. Fifty five days old plants were subjected to drought stress after withholding water for different time intervals (10 days, 15 days, 20 days and 25 days). A forward subtractive cDNA library was prepared from isolated RNA of leaf tissue. Differential gene expression under drought stress was validated for selected nine contigs by RT-qPCR. A transcript homologous to Setaria italica ASR3 upregulated under drought stress was isolated from genotype 841B and characterized. Heterologous expression of PgASR3 was validated in Arabidopsis and confirmed under multiple abiotic stress conditions. A total of four independent transgenic lines overexpressing gene PgASR3 were analyzed by Southern blot at T₁ stage. For drought stress tolerance, three independent lines (T₂ stage) were analyzed by biochemical and physiological assays at seedling stage. The growth rate (shoot and root length) of transgenic seedlings improved as compared to WT seedling under differenct abiotic stress conditions. The three transgenic lines were also validated for drought stress tolerance and RT-qPCR analysis, at maturity stage. Under drought stress conditions, the mature transgenic lines showed higher levels of RWC, chlorophyll and proline but lower levels of MDA as compared to WT plants. PgASR3 gene isolated and validated in this study can be utilized for developing abiotic stress tolerant crops.

PheASR2, a novel stress-responsive transcription factor from moso bamboo (Phyllostachys edulis), enhances drought tolerance in transgenic rice via increased sensitivity to abscisic acid

Abscisic acid, stress and ripening (ASR) transcription factors comprise a small family of proteins that play a key role in stress responses in plants. ASR genes involved in drought tolerance in moso bamboo (Phyllostachys edulis) are largely unknown. In our study, an ASR gene, PheASR2, was isolated and characterized. The expression of PheASR2 was up-regulated under various abiotic stresses, including drought, salt and abscisic acid (ABA). PheASR2 was localized in the nucleus in tobacco cells, and displayed transactivation activity in yeast. Ectopic expression of PheASR2 in rice conferred enhanced tolerance to drought stress, as determined through physiological analyses of germination rate, plant height, water loss and survival rate. The PheASR2-overexpressing transgenic plants showed an increase in reactive oxygen species (ROS), electrolyte leakage and malondialdehyde levels, reduced enzyme (CAT and SOD) activities, and higher expression of genes encoding ROS-scavenging enzymes. Consequently, the transgenic plants exhibited increased tolerance to oxidative stress compared with wild-type plants. Moreover, following ABA treatment, the seed germination rate and plant height of the PheASR2-overexpressing lines were inhibited, and stomatal closure was reduced. The expression of marker genes, including, OsAREB, OsP5CS1, OsLEA, and OsNCED2, was up-regulated in the PheASR2-overexpressing lines when subjected to drought treatment. Together, these results indicate that PheASR2 functions in drought stress tolerance through ABA signaling.

Kurup S, Cheruth AJ, Subramaniam S, Hassan Tawfik N, M.A. Amiri K. Transcriptome Profiling of Haloxylon persicum (Bunge ex Boiss and Buhse) an Endangered Plant Species under PEG-Induced Drought Stress

Haloxylon persicum is an endangered western Asiatic desert plant species, which survives under extreme environmental conditions. In this study, we focused on transcriptome analysis of H. persicum to understand the molecular mechanisms associated with drought tolerance. Two different periods of polyethylene glycol (PEG)-induced drought stress (48 h and 72 h) were imposed on H. persicum under in vitro conditions, which resulted in 18 million reads, subsequently assembled by de novo method with more than 8000 transcripts in each treatment. The N50 values were 1437, 1467, and 1524 for the control sample, 48 h samples, and 72 h samples, respectively. The gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis resulted in enrichment of mitogen-activated protein kinase (MAPK) and plant hormone signal transduction pathways under PEG-induced drought conditions. The differential gene expression analysis (DGEs) revealed significant changes in the expression pattern between the control and the treated samples. The KEGG analysis resulted in mapping transcripts with 138 different pathways reported in plants. The differential expression of drought-responsive transcription factors depicts the possible signaling cascades involved in drought tolerance. The present study provides greater insight into the fundamental transcriptome reprogramming of desert plants under drought.

Genome-wide characterization of the abscisic acid-, stress- and ripening-induced (ASR) gene family in wheat (Triticum aestivum L.)

BACKGROUND

Abscisic acid-, stress-, and ripening-induced (ASR) genes are a class of plant specific transcription factors (TFs), which play important roles in plant development, growth and abiotic stress responses. The wheat ASRs have not been described in genome-wide yet.

METHODS

We predicted the transmembrane regions and subcellular localization using the TMHMM server, and Plant-mPLoc server and CELLO v2.5, respectively. Then the phylogeny tree was built by MEGA7. The exon-intron structures, conserved motifs and TFs binding sites were analyzed by GSDS, MEME program and PlantRegMap, respectively.

RESULTS

In wheat, 33ASR genes were identified through a genome-wide survey and classified into six groups. Phylogenetic analyses revealed that the TaASR proteins in the same group tightly clustered together, compared with those from other species. Duplication analysis indicated that the TaASR gene family has expanded mainly through tandem and segmental duplication events. Similar gene structures and conserved protein motifs of TaASRs in wheat were identified in the same groups. ASR genes contained various TF binding cites associated with the stress responses in the promoter region. Gene expression was generally associated with the expected group-specific expression pattern in five tissues, including grain, leaf, root, spike and stem, indicating the broad conservation of ASR genes function during wheat evolution. The qRT-PCR analysis revealed that several ASRs were up-regulated in response to NaCl and PEG stress.

CONCLUSION

We identified ASR genes in wheat and found that gene duplication events are the main driving force for ASR gene evolution in wheat. The expression of wheat ASR genes was modulated in responses to multiple abiotic stresses, including drought/osmotic and salt stress. The results provided important information for further identifications of the functions of wheat ASR genes and candidate genes for high abiotic stress tolerant wheat breeding.

Abscisic Acid, Stress, and Ripening (TtASR1) Gene as a Functional Marker for Salt Tolerance in Durum Wheat

In semiarid Mediterranean agroecosystems, drought and salinity are the main abiotic stresses hampering wheat productivity and yield instability. Abscisic acid, stress, and ripening (ASR) are small plant proteins and play important roles in different biological processes. In the present study, the TtASR1 gene was isolated and characterized for the first time from durum wheat (Tritucum turgidum L. subsp. durum). TtASR1 is a small gene, about 684 bp long, located on chromosome 4AL, encoding a protein of 136 amino acid residues consisting of a histidine-rich N terminus and C-terminal conserved ABA-WDS domain (Pfam PF02496). Our results showed that TtASR1 protein could function as a chaperone-like protein and improve the viability of E. coli under heat and cold stress and increase the Saccharomyces cerevisiae tolerance under salt and osmotic stress. Transcript expression patterns of TtASR1 revealed that ASRs play important roles in abiotic stress responses in diverse organs. Indeed, TtASR1 was upregulated in leaves by different developmental (ABA) and environmental signals (PEG, salt). In cv. Mahmoudi (salt-tolerant Tunisian durum landraces) roots, TtASR1 was upregulated by salt stress, while it was downregulated in cv. Azizi (salt-sensitive Tunisian durum landraces), supporting the implication of this gene in the salt tolerance mechanism. Taken together and after validation in the plant system, the TtASR1 gene may provide a potential functional marker for marker-assisted selection in a durum wheat breeding program for salt tolerance.

CRISPR/Cas9-Induced Mutagenesis of Semi-Rolled Leaf1,2 Confers Curled Leaf Phenotype and Drought Tolerance by Influencing Protein Expression Patterns and ROS Scavenging in Rice (Oryza sativa L.)

Rice leaf morphology is an essential agronomic trait to develop drought-tolerant genotypes for adequate and stable crop production in drought-prone areas. Here, rolled leaf mutant plants were acquired by CRISPR/Cas9-based mutagenesis of Semi-rolled leaf1,2 (SRL1 and SRL2) genes, and isobaric tags for relative and absolute quantification (iTRAQ) based proteomic analysis was performed to analyze the subsequent proteomic regulation events. Homozygous mutants exhibit decreased chlorophyll content, transpiration rate, stomatal conductance, vascular bundles (VB), stomatal number, and agronomic traits with increased panicle number and bulliform cells (BCs). Under drought stress, mutant plants displayed lower malondialdehyde (MDA) content while higher survival rate, abscisic acid (ABA) content, superoxide dismutase (SOD), catalase (CAT) activities, and grain filling percentage compare with their wild type (WT). Proteomic results revealed that 270 proteins were significantly downregulated, and 107 proteins were upregulated in the mutant line compared with WT. Proteins related to lateral organ boundaries’ (LOB) domain (LBD) were downregulated, whereas abiotic stress-responsive proteins were upregulated in the CRISPR mutant. LBD proteins (Q5KQR7, Q6K713, Q7XGL4, Q8LQH4), probable indole-3-acetic acid-amido synthetase (Q60EJ6), putative auxin transporter-like protein 4 (Q53JG7), Monoculm 1 (Q84MM9) and AP2 (Apetala2) domain-containing protein (Q10A97) were found to be hub-proteins. The hybrids developed from mutant restorers showed a semi-rolled leaf phenotype with increased panicle number, grain number per panicle, and yield per plant. Our findings reveal the intrinsic value of genome editing and expand the knowledge about the network of proteins for leaf rolling and drought avoidance in rice.

Transcriptome analysis reveals potential roles of a barley ASR gene that confers stress tolerance in transgenic rice

Control of gene expression and induction of cellular protection mechanisms are two important processes that plants employ to protect themselves against abiotic stresses. ABA-, stress, and ripening-induced (ASR) proteins have been identified to participate in such responses. Previous studies have proposed that these proteins can act as transcription factors and as molecular chaperones protecting transgenic plants against stresses; however a gene network regulated by ASRs has not been explored. To expand our knowledge on the function of these proteins in cereals, we present the functional characterization of a barley ASR gene. Expression of HvASR5 was almost ubiquitous in different organs and responded to ABA and to different stress treatments. When expressed ectopically, HvASR5 was able to confer drought and salt stress tolerance to Arabidopsis thaliana and to improve growth performance of rice plants under stress conditions. A transcriptomic analysis with two transgenic rice lines overexpressing HvASR5 helped to identify potential downstream targets and understand ASR-regulated cellular processes. HvASR5 up-regulated the expression of a distinct set of genes associated with stress responses, metabolic processes (particularly carbohydrate metabolism), as well as reproduction and development. These data, together with the confirmed nuclear and cytoplasmic localization of HvASR5, further support the hypothesis that HvASR5 can also carry out roles as molecular protector and transcriptional regulator.

Comprehensive Analysis of the Cadmium Tolerance of Abscisic Acid-, Stress- and Ripening-Induced Proteins (ASRs) in Maize

In plants, abscisic acid-, stress-, and ripening-induced (ASR) proteins have been shown to impart tolerance to multiple abiotic stresses such as drought and salinity. However, their roles in metal stress tolerance are poorly understood. To screen plant Cd-tolerance genes, the yeast-based gene hunting method which aimed to screen Cd-tolerance colonies from maize leaf cDNA library hosted in yeast was carried out. Here, maize ZmASR1 was identified to be putative Cd-tolerant through this survival screening strategy. In silico analysis of the functional domain organization, phylogenetic classification and tissue-specific expression patterns revealed that maize ASR1 to ASR5 are typical ASRs with considerable expression in leaves. Further, four of them were cloned for testifying Cd tolerance using yeast complementation assay. The results indicated that they all confer Cd tolerance in Cd-sensitive yeast. Then they were transiently expressed in tobacco leaves for subcellular localization analysis and for Cd-challenged lesion assay, continuously. The results demonstrated that all 4 maize ASRs tested are localized to the cell nucleus and cytoplasm in tobacco leaves. Moreover, they were confirmed to be Cd-tolerance genes in planta through lesion analysis in Cd-infiltrated leaves transiently expressing them. Taken together, our results demonstrate that maize ASRs play important roles in Cd tolerance, and they could be used as promising candidate genes for further functional studies toward improving the Cd tolerance in plants.

Physiological and proteome studies of maize (Zea mays L.) in response to leaf removal under high plant density

BACKGROUND

Under high plant density, intensifying competition among individual plants led to overconsumption of energy and nutrients and resulted in an almost dark condition in the lower strata of the canopy, which suppressed the photosynthetic potential of the shaded leaves. Leaf removal could help to ameliorate this problem and increase crop yields. To reveal the mechanism of leaf removal in maize, tandem mass tags label-based quantitative analysis coupled with liquid chromatography-tandem mass spectrometry were used to capture the differential protein expression profiles of maize subjected to the removal of the two uppermost leaves (S₂), the four uppermost leaves (S₄), and with no leaf removal as control (S₀).

RESULTS

Excising leaves strengthened the light transmission rate of the canopy and increased the content of malondialdehyde, whereas decreased the activities of superoxide dismutase and peroxidase. Two leaves removal increased the photosynthetic capacity of ear leaves and the grain yield significantly, whereas S₄ decreased the yield markedly. Besides, 239 up-accumulated proteins and 99 down-accumulated proteins were identified between S₂ and S₀, which were strongly enriched into 30 and 23 functional groups; 71 increased proteins and 42 decreased proteins were identified between S₄ and S₀, which were strongly enriched into 22 and 23 functional groups, for increased and decreased proteins, respectively.

CONCLUSIONS

Different defoliation levels had contrastive effects on maize. The canopy light transmission rate was strengthened and proteins related to photosynthetic electron-transfer reaction were up-regulated significantly for treatment S₂, which improved the leaf photosynthetic capacity, and obtained a higher grain yield consequently. In contrast, S₄ decreased the grain yield and increased the expressions of proteins and genes associated with fatty acid metabolism. Besides, both S₂ and S₄ exaggerated the defensive response of maize in physiological and proteomic level. Although further studies are required, the results in our study provide new insights to the further improvement in maize grain yield by leaf removal.

Ipomoea pes-caprae IpASR Improves Salinity and Drought Tolerance in Transgenic Escherichia coli and Arabidopsis

Ipomoea pes-caprae L. is an extremophile halophyte with strong adaptability to seawater and drought. It is widely used in the ecological restoration of coastal areas or degraded islands in tropical and subtropical regions. In this study, a new abscisic acid, stressandripening (ASR) gene, IpASR, was reported, and is mainly associated with biological functions involved in salt and drought tolerance. Sequence analysis of IpASR showed that this protein contains an ABA/WDS (abscisic acid/water deficit stress) domain, which is a common feature of all plant ASR members. Overexpression of IpASR improved Escherichia coli growth performance compared with the control under abiotic stress treatment. The transgenic overexpressing IpASR Arabidopsis showed higher tolerance to salt and drought stress than the wild type and lower accumulation of hydrogen peroxide (H₂O₂) and superoxide (O₂⁻) accompanied by increased antioxidant enzyme activity in vivo. IpASR exhibits transcription factor’s activity. Therefore, the overexpression of IpASR in Arabidopsis is supposed to influence the expression of some genes involved in anti-oxidative and abiotic stresses. The results indicate that IpASR is involved in the plant response to salt and drought and probably acts as a reactive oxygen species scavenger or transcription factor, and therefore influences physiological processes associated with various abiotic stresses in plants.

OsASR2 regulates the expression of a defence-related gene, Os2H16, by targeting the GT-1 cis-element

The GT-1 cis-element widely exists in many plant gene promoters. However, the molecular mechanism that underlies the response of the GT-1 cis-element to abiotic and biotic stresses remains elusive in rice. We previously isolated a rice short-chain peptide-encoding gene, Os2H16, and demonstrated that it plays important roles in both disease resistance and drought tolerance. Here, we conducted a promoter assay of Os2H16 and identified GT-1 as an important cis-element that mediates Os2H16 expression in response to pathogen attack and osmotic stress. Using the repeated GT-1 as bait, we characterized an abscisic acid, stress and ripening 2 (ASR2) protein from yeast-one hybridization screening. Sequence alignments showed that the carboxy-terminal domain of OsASR2 containing residues 80-138 was the DNA-binding domain. Furthermore, we identified that OsASR2 was specifically bound to GT-1 and activated the expression of the target gene Os2H16, as well as GFP driven by the chimeric promoter of 2 × GT-1-35S mini construct. Additionally, the expression of OsASR2 was elevated by pathogens and osmotic stress challenges. Overexpression of OsASR2 enhanced the resistance against Xanthomonas oryzae pv. oryzae and Rhizoctonia solani, and tolerance to drought in rice. These results suggest that the interaction between OsASR2 and GT-1 plays an important role in the crosstalk of the response of rice to biotic and abiotic stresses.

Structural disorder and induced folding within two cereal, ABA stress and ripening (ASR) proteins

Abscisic acid (ABA), stress and ripening (ASR) proteins are plant-specific proteins involved in plant response to multiple abiotic stresses. We previously isolated the ASR genes and cDNAs from durum wheat (TtASR1) and barley (HvASR1). Here, we show that HvASR1 and TtASR1 are consistently predicted to be disordered and further confirm this experimentally. Addition of glycerol, which mimics dehydration, triggers a gain of structure in both proteins. Limited proteolysis showed that they are highly sensitive to protease degradation. Addition of 2,2,2-trifluoroethanol (TFE) however, results in a decreased susceptibility to proteolysis that is paralleled by a gain of structure. Mass spectrometry analyses (MS) led to the identification of a protein fragment resistant to proteolysis. Addition of zinc also induces a gain of structure and Hydrogen/Deuterium eXchange-Mass Spectrometry (HDX-MS) allowed identification of the region involved in the disorder-to-order transition. This study is the first reported experimental characterization of HvASR1 and TtASR1 proteins, and paves the way for future studies aimed at unveiling the functional impact of the structural transitions that these proteins undergo in the presence of zinc and at achieving atomic-resolution conformational ensemble description of these two plant intrinsically disordered proteins (IDPs).

OsASR5 enhances drought tolerance through a stomatal closure pathway associated with ABA and H2 O2 signalling in rice

Drought is one of the major abiotic stresses that directly implicate plant growth and crop productivity. Although many genes in response to drought stress have been identified, genetic improvement to drought resistance especially in food crops is showing relatively slow progress worldwide. Here, we reported the isolation of abscisic acid, stress and ripening (ASR) genes from upland rice variety, IRAT109 (Oryza sativa L. ssp. japonica), and demonstrated that overexpression of OsASR5 enhanced osmotic tolerance in Escherichia coli and drought tolerance in Arabidopsis and rice by regulating leaf water status under drought stress conditions. Moreover, overexpression of OsASR5 in rice increased endogenous ABA level and showed hypersensitive to exogenous ABA treatment at both germination and postgermination stages. The production of H₂ O₂ , a second messenger for the induction of stomatal closure in response to ABA, was activated in overexpression plants under drought stress conditions, consequently, increased stomatal closure and decreased stomatal conductance. In contrast, the loss-of-function mutant, osasr5, showed sensitivity to drought stress with lower relative water content under drought stress conditions. Further studies demonstrated that OsASR5 functioned as chaperone-like protein and interacted with stress-related HSP40 and 2OG-Fe (II) oxygenase domain containing proteins in yeast and plants. Taken together, we suggest that OsASR5 plays multiple roles in response to drought stress by regulating ABA biosynthesis, promoting stomatal closure, as well as acting as chaperone-like protein that possibly prevents drought stress-related proteins from inactivation.

Abscisic acid and sucrose regulate tomato and strawberry fruit ripening through the abscisic acid-stress-ripening transcription factor

Although great progress has been made towards understanding the role of abscisic acid (ABA) and sucrose in fruit ripening, the mechanisms underlying the ABA and sucrose signalling pathways remain elusive. In this study, transcription factor ABA-stress-ripening (ASR), which is involved in the transduction of ABA and sucrose signalling pathways, was isolated and analysed in the nonclimacteric fruit, strawberry and the climacteric fruit, tomato. We have identified four ASR isoforms in tomato and one in strawberry. All ASR sequences contained the ABA stress- and ripening-induced proteins and water-deficit stress-induced proteins (ABA/WDS) domain and all ASR transcripts showed increased expression during fruit development. The expression of the ASR gene was influenced not only by sucrose and ABA, but also by jasmonic acid (JA) and indole-3-acetic acid (IAA), and these four factors were correlated with each other during fruit development. ASR bound the hexose transporter (HT) promoter, which contained a sugar box that activated downstream gene expression. Overexpression of the ASR gene promoted fruit softening and ripening, whereas RNA interference delayed fruit ripening, as well as affected fruit physiological changes. Change in ASR gene expression influenced the expression of several ripening-related genes such as CHS, CHI, F3H, DFR, ANS, UFGT, PG, PL, EXP1/2, XET16, Cel1/2 and PME. Taken together, this study may provide new evidence on the important role of ASR in cross-signalling between ABA and sucrose to regulate tomato and strawberry fruit ripening. The findings of this study also provide new insights into the regulatory mechanism underlying fruit development.

Genome-wide identification and expression analysis of the apple ASR gene family in response to Alternaria alternata f. sp. mali

The ABA/water stress/ripening-induced (ASR) gene family exists universally in higher plants, and many ASR genes are up-regulated during periods of environmental stress and fruit ripening. Although a considerable amount of research has been performed investigating ASR gene response to abiotic stresses, relatively little is known about their roles in response to biotic stresses. In this report, we identified five ASR genes in apple (Malus × domestica) and explored their phylogenetic relationship, duplication events, and selective pressure. Five apple ASR genes (Md-ASR) were divided into two clades based on phylogenetic analysis. Species-specific duplication was detected in M. domestica ASR genes. Leaves of 'Golden delicious' and 'Starking' were infected with Alternaria alternata f. sp. mali, which causes apple blotch disease, and examined for the expression of the ASR genes in lesion areas during the first 72 h after inoculation. Md-ASR genes showed different expression patterns at different sampling times in 'Golden delicious' and 'Starking'. The activities of stress-related enzymes, peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), phenylalanine ammonia lyase (PAL), and polyphenoloxidase (PPO), and the content of malondialdehyde (MDA) were also measured in different stages of disease development in two cultivars. The ASR gene expression patterns and theses physiological indexes for disease resistance suggested that Md-ASR genes are involved in biotic stress responses in apple.

Identification of the ASR gene family from Brachypodium distachyon and functional characterization of BdASR1 in response to drought stress

A genome-wide investigation identified five B. distachyon ASR genes. BdASR1 may be a transcription factor that confers drought resistance by activating antioxidant systems involving ROS-scavenging enzymes and non-enzymatic antioxidants. Abscisic acid-, stress-, and ripening-induced (ASR) proteins belong to a family of plant-specific, small, and hydrophilic proteins with important roles in responses to abiotic stresses. Although several ASR genes involved in drought tolerance have been characterized in various plant species, the mechanisms regulating ASR activities are still uncharacterized. Additionally, no research on Brachypodium distachyon ASR proteins have been completed. In this study, five B. distachyon BdASR genes were identified through genome-wide analyses. Phylogenetic analyses revealed that BdASR genes originated from tandem and whole genome duplications. Expression analyses revealed the BdASR genes responded to various abiotic stresses, including cold, drought, and salinity, as well as signaling molecules such as abscisic acid, ethylene, and H2O2. BdASR1, which localizes to the nucleus and is transcriptionally active, was functionally characterized. BdASR1 overexpression considerably enhanced drought tolerance in transgenic tobacco plants, which was accompanied by increased superoxide dismutase, catalase, and peroxidase activities, as well as an increased abundance of antioxidants such as ascorbate, tocopherols, and glutathione. BdASR1 may function as a transcription factor that provides drought stress resistance by inducing the production of reactive oxygen species-scavenging enzymes and non-enzymatic antioxidants.

Molecular cloning and characterization of drought stress responsive abscisic acid-stress-ripening (Asr 1) gene from wild jujube, Ziziphus nummularia (Burm.f.) Wight & Arn

Drought is a calamitous abiotic stress hampering agricultural productivity all over the world and its severity is likely to increase further. Abscisic acid-stress-ripening proteins (ASR), are a group of small hydrophilic proteins which are induced by abscisic acid, stress and ripening in many plants. In the present study, ZnAsr 1 gene was fully characterized for the first time from Ziziphus nummularia, which is one of the most low water forbearing plant. Full length ZnAsr 1 gene was characterised and in silico analysis of ZnASR1 protein was done for predicting its phylogeny and physiochemical properties. To validate transcriptional pattern of ZnAsr 1 in response to drought stress, expression profiling in polyethylene glycol (PEG) induced Z. nummularia seedlings was studied by RT-qPCR analysis and heterologous expression of the recombinant ZnAsr1 in Escherichia coli. The nucleotide sequence analysis revealed that the complete open reading frame of ZnAsr 1 is 819 bp long encoding a protein of 273 amino acid residues, consisting of a histidine rich N terminus with an abscisic acid/water deficit stress domain and a nuclear targeting signal at the C terminus. In expression studies, ZnAsr 1 gene was found to be highly upregulated under drought stress and recombinant clones of E. coli cells expressing ZnASR1 protein showed better survival in PEG containing media. ZnAsr1 was proven to enhance drought stress tolerance in the recombinant E.coli cells expressing ZnASR1. The cloned ZnAsr1 after proper validation in a plant system, can be used to develop drought tolerant transgenic crops.

ASR5 is involved in the regulation of miRNA expression in rice

The work describes an ASR knockdown transcriptomic analysis by deep sequencing of rice root seedlings and the transactivation of ASR cis-acting elements in the upstream region of a MIR gene. MicroRNAs are key regulators of gene expression that guide post-transcriptional control of plant development and responses to environmental stresses. ASR (ABA, Stress and Ripening) proteins are plant-specific transcription factors with key roles in different biological processes. In rice, ASR proteins have been suggested to participate in the regulation of stress response genes. This work describes the transcriptomic analysis by deep sequencing two libraries, comparing miRNA abundance from the roots of transgenic ASR5 knockdown rice seedlings with that of the roots of wild-type non-transformed rice seedlings. Members of 59 miRNA families were detected, and 276 mature miRNAs were identified. Our analysis detected 112 miRNAs that were differentially expressed between the two libraries. A predicted inverse correlation between miR167abc and its target gene (LOC_Os07g29820) was confirmed using RT-qPCR. Protoplast transactivation assays showed that ASR5 is able to recognize binding sites upstream of the MIR167a gene and drive its expression in vivo. Together, our data establish a comparative study of miRNAome profiles and is the first study to suggest the involvement of ASR proteins in miRNA gene regulation.

Introgression of the SbASR-1 gene cloned from a halophyte Salicornia brachiate enhances salinity and drought endurance in transgenic groundnut (arachis hypogaea)and acts as a transcription factor [corrected]

The SbASR-1 gene, cloned from a halophyte Salicornia brachiata, encodes a plant-specific hydrophilic and stress responsive protein. The genome of S. brachiata has two paralogs of the SbASR-1 gene (2549 bp), which is comprised of a single intron of 1611 bp, the largest intron of the  abscisic acid stress ripening [ASR] gene family yet reported. In silico analysis of the 843-bp putative promoter revealed the presence of ABA, biotic stress, dehydration, phytohormone, salinity, and sugar responsive cis-regulatory motifs. The SbASR-1 protein belongs to Group 7 LEA protein family with different amino acid composition compared to their glycophytic homologs. Bipartite Nuclear Localization Signal (NLS) was found on the C-terminal end of protein and localization study confirmed that SbASR-1 is a nuclear protein. Furthermore, transgenic groundnut (Arachis hypogaea) plants over-expressing the SbASR-1 gene constitutively showed enhanced salinity and drought stress tolerance in the T1 generation. Leaves of transgenic lines exhibited higher chlorophyll and relative water contents and lower electrolyte leakage, malondialdehyde content, proline, sugars, and starch accumulation under stress treatments than wild-type (Wt) plants. Also, lower accumulation of H2O2 and O2.- radicals was detected in transgenic lines compared to Wt plants under stress conditions. Transcript expression of APX (ascorbate peroxidase) and CAT (catalase) genes were higher in Wt plants, whereas the SOD (superoxide dismutase) transcripts were higher in transgenic lines under stress. Electrophoretic mobility shift assay (EMSA) confirmed that the SbASR-1 protein binds at the consensus sequence (C/G/A)(G/T)CC(C/G)(C/G/A)(A/T). Based on results of the present study, it may be concluded that SbASR-1 enhances the salinity and drought stress tolerance in transgenic groundnut by functioning as a LEA (late embryogenesis abundant) protein and a transcription factor.

Constitutive and stress-inducible overexpression of a native aquaporin gene (MusaPIP2;6) in transgenic banana plants signals its pivotal role in salt tolerance

High soil salinity constitutes a major abiotic stress and an important limiting factor in cultivation of crop plants worldwide. Here, we report the identification and characterization of a aquaporin gene, MusaPIP2;6 which is involved in salt stress signaling in banana. MusaPIP2;6 was firstly identified based on comparative analysis of stressed and non-stressed banana tissue derived EST data sets and later overexpression in transgenic banana plants was performed to study its tangible functions in banana plants. The overexpression of MusaPIP2;6 in transgenic banana plants using constitutive or inducible promoter led to higher salt tolerance as compared to equivalent untransformed control plants. Cellular localization assay performed using transiently transformed onion peel cells indicated that MusaPIP2;6 protein tagged with green fluorescent protein was translocated to the plasma membrane. MusaPIP2;6-overexpressing banana plants displayed better photosynthetic efficiency and lower membrane damage under salt stress conditions. Our results suggest that MusaPIP2;6 is involved in salt stress signaling and tolerance in banana.

ASR1 transcription factor and its role in metabolism

Asr1 (ABA, stress, ripening) is a plant gene widely distributed in many species which was discovered by differential induction levels in tomato plants subjected to drought stress conditions. ASR1 also regulates the expression of a hexose transporter in grape and is involved in sugar and amino acid accumulation in some species like maize and potato. The control that ASR1 exerts on hexose transport is interesting from a biotechnological perspective because both sugar partitioning and content in specific organs affect the yield and the quality of many agronomically important crops. ASR1 affect plant metabolism by its dual activity as a transcription factor and as a chaperone-like protein. In this paper, we review possible mechanisms by which ASR1 affects metabolism, the differences observed among tissues and species, and the possible physiological implications of its role in metabolism.

Tomato ABSCISIC ACID STRESS RIPENING (ASR) gene family revisited

Tomato ABSCISIC ACID RIPENING 1 (ASR1) was the first cloned plant ASR gene. ASR orthologs were then cloned from a large number of monocot, dicot and gymnosperm plants, where they are mostly involved in response to abiotic (drought and salinity) stress and fruit ripening. The tomato genome encodes five ASR genes: ASR1, 2, 3 and 5 encode low-molecular-weight proteins (ca. 110 amino acid residues each), whereas ASR4 encodes a 297-residue polypeptide. Information on the expression of the tomato ASR gene family is scarce. We used quantitative RT-PCR to assay the expression of this gene family in plant development and in response to salt and osmotic stresses. ASR1 and ASR4 were the main expressed genes in all tested organs and conditions, whereas ASR2 and ASR3/5 expression was two to three orders of magnitude lower (with the exception of cotyledons). ASR1 is expressed in all plant tissues tested whereas ASR4 expression is limited to photosynthetic organs and stamens. Essentially, ASR1 accounted for most of ASR gene expression in roots, stems and fruits at all developmental stages, whereas ASR4 was the major gene expressed in cotyledons and young and fully developed leaves. Both ASR1 and ASR4 were expressed in flower organs, with ASR1 expression dominating in stamens and pistils, ASR4 in sepals and petals. Steady-state levels of ASR1 and ASR4 were upregulated in plant vegetative organs following exposure to salt stress, osmotic stress or the plant abiotic stress hormone abscisic acid (ABA). Tomato plants overexpressing ASR1 displayed enhanced survival rates under conditions of water stress, whereas ASR1-antisense plants displayed marginal hypersensitivity to water withholding.

Oligo-dT anchored cDNA-SCoT: a novel differential display method for analyzing differential gene expression in response to several stress treatments in mango (Mangifera indica L.)

Differential display is a powerful technique for analyzing differences in gene expression. Oligo-dT cDNAstart codon targeted marker (cDNA-SCoT) technique is a novel, simple, cheap, rapid, and efficient method for differential gene expression research. In the present study, the oligo-dT anchored cDNA-SCoT technique was exploited to identify differentially expressed genes during several stress treatments in mango. A total of 37 primers combined with oligo-dT anchor primers 3side amplified approximately 150 fragments of 150 bp to 1500 bp in length. Up to 100 fragments were differentially expressed among the stress treatments and control samples, among which 92 were obtained and sequenced. Out of the 92 transcript derived fragments (TDFs), 70% were highly homologous to known genes, and 30% encoded unclassified proteins with unknown functions. The expression pattern of nine genes with known functions involved in several abiotic stresses in other species was confirmed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) under cold (4 °C), salinity (NaCl), polyethylene glycol (PEG, MW 6000), and heavy metal treatments in leaves and stems at different time points (0, 24, 48, and 72 h). The expression patterns of the genes (TDF4, TDF7, TDF23, TDF45, TDF49, TDF50, TDF57, TDF91 and TDF92) that had direct or indirect relationships with cold, salinity, drought and heavy metal stress response were analyzed through qRT-PCR. The possible roles of these genes are discussed. This study suggests that the oligo-dT anchored cDNA-SCoT differential display method is a useful tool to serve as an initial step for characterizing transcriptional changes induced by abiotic stresses and provide gene information for further study and application in genetic improvement and breeding in mango.

Twenty years of research on Asr (ABA-stress-ripening) genes and proteins

Investigating how plants cope with different abiotic stresses-mainly drought and extreme temperatures-is pivotal for both understanding the underlying signaling pathways and improving genetically engineered crops. Plant cells are known to react defensively to mild and severe dehydration by initiating several signal transduction pathways that result in the accumulation of different proteins, sugar molecules and lipophilic anti-oxidants. Among the proteins that build up under these adverse conditions are members of the ancestral ASR (ABA-stress-ripening) family, which is conserved in the plant kingdom but lacks orthologs in Arabidopsis. This review provides a comprehensive summary of the state of the art regarding ASRs, going back to the original description and cloning of the tomato ASR cDNA. That seminal discovery sparked worldwide interest amongst research groups spanning multiple fields: biochemistry, cell biology, evolution, physiology and epigenetics. As these proteins function as both chaperones and transcription factors; this review also covers the progress made on relevant molecular features that account for these dual roles-including the recent identification of their target genes-which may inspire future basic research. In addition, we address reports of drought-tolerant ASR-transgenic plants of different species, highlighting the influential work of authors taking more biotechnological approaches.

Organ- and stress-specific expression of the ASR genes in rice

Rice ASR genes respond distinctly to abscisic acid, dehydration and cold stress. Their tissue-specific expression provides new hints about their possible roles in plant responses to stress. Plant ASR proteins have emerged as an interesting distinct group of proteins with apparent roles in protecting cellular structures as well as putative regulators of gene expression, both important responses of plants to environmental stresses. Regardless of the possible functions proposed by different studies, little is known about their role in cereals. To further understand the function of these proteins in the Gramineae, we investigated the expression pattern of the six ASR genes present in the rice genome in response to ABA, stress conditions and in different organs. Although transcription of most OsASRs is transiently enhanced by ABA treatment, the genes present a differential response under cold and drought stress as well as specific expression in certain tissues and organs. Analysis of their promoters reveals regulatory cis-elements associated to hormonal, sugar and stress responses. The promoters of two genes, OsASR1 and OsASR5, direct the expression of the GUS reporter gene especially to leaf vascular tissue in response to dehydration and low temperature. In control conditions, a GUS reporter assay also indicates specific expression of these two genes in roots, anthers and seed scutellar tissues. These results provide new clues about the possible role of ASRs in plant stress responses and development.

Transgenic banana plants overexpressing a native plasma membrane aquaporin MusaPIP1;2 display high tolerance levels to different abiotic stresses

Water transport across cellular membranes is regulated by a family of water channel proteins known as aquaporins (AQPs). As most abiotic stresses like suboptimal temperatures, drought or salinity result in cellular dehydration, it is imperative to study the cause-effect relationship between AQPs and the cellular consequences of abiotic stress stimuli. Although plant cells have a high isoform diversity of AQPs, the individual and integrated roles of individual AQPs in optimal and suboptimal physiological conditions remain unclear. Herein, we have identified a plasma membrane intrinsic protein gene (MusaPIP1;2) from banana and characterized it by overexpression in transgenic banana plants. Cellular localization assay performed using MusaPIP1;2::GFP fusion protein indicated that MusaPIP1;2 translocated to plasma membrane in transformed banana cells. Transgenic banana plants overexpressing MusaPIP1;2 constitutively displayed better abiotic stress survival characteristics. The transgenic lines had lower malondialdehyde levels, elevated proline and relative water content and higher photosynthetic efficiency as compared to equivalent controls under different abiotic stress conditions. Greenhouse-maintained hardened transgenic plants showed faster recovery towards normal growth and development after cessation of abiotic stress stimuli, thereby underlining the importance of these plants in actual environmental conditions wherein the stress stimuli is often transient but severe. Further, transgenic plants where the overexpression of MusaPIP1;2 was made conditional by tagging it with a stress-inducible native dehydrin promoter also showed similar stress tolerance characteristics in in vitro and in vivo assays. Plants developed in this study could potentially enable banana cultivation in areas where adverse environmental conditions hitherto preclude commercial banana cultivation.

Abiotic stress responsive rice ASR1 and ASR3 exhibit different tissue-dependent sugar and hormone-sensitivities

The expression of the six rice ASR genes is differentially regulated in a tissue-dependent manner according to environmental conditions and reproductive stages. OsASR1 and OsASR3 are the most abundant and are found in most tissues; they are enriched in the leaves and roots, respectively. Coexpression analysis of OsASR1 and OsASR3 and a comparison of the cis-acting elements upstream of OsASR1 and OsASR3 suggested that their expression is regulated in common by abiotic stresses but differently regulated by hormone and sugar signals. The results of quantitative real-time PCR analyses of OsASR1 and OsASR3 expression under various conditions further support this model. The expression of both OsASR1 and OsASR3 was induced by drought stress, which is a major regulator of the expression of all ASR genes in rice. In contrast, ABA is not a common regulator of the expression of these genes. OsASR1 transcription was highly induced by ABA, whereas OsASR3 transcription was strongly induced by GA. In addition, OsASR1 and OsASR3 expression was significantly induced by sucrose and sucrose/glucose treatments, respectively. The induction of gene expression in response to these specific hormone and sugar signals was primarily observed in the major target tissues of these genes (i.e., OsASR1 in leaves and OsASR3 in roots). Our data also showed that the overexpression of either OsASR1 or OsASR3 in transgenic rice plants increased their tolerance to drought and cold stress. Taken together, our results revealed that the transcriptional control of different rice ASR genes exhibit different tissue-dependent sugar and hormone-sensitivities.

Effects of Fe3+ and Zn2+ on the structural and thermodynamic properties of a soybean ASR protein

Abscisic acid-, stress-, and ripening-induced (ASR) protein play important roles in protecting plants from abiotic stress. The functions of some ASR proteins are known to be modulated by binding to metal ions. In this study, we demonstrated that the non-tagged full-length soybean (Glycine max) ASR protein (GmASR) can bind Fe(3+), Ni(2+), Cu(2+), and Zn(2+). The direct binding properties of GmASR to Fe(3+) and Zn(2+) were further confirmed by intrinsic fluorescence assays. The GmASR protein was found to have three Fe(3+) binding sites but only two Zn(2+) binding sites. Natively disordered in aqueous solution, GmASR remained disordered in the presence of Fe(3+), but was found to aggregate in the presence of Zn(2+). The aggregated GmASR protein was partially resolubilized after Zn(2+) was chelated by EDTA. GmASR exhibited Fe(3+)-binding-dependent antioxidant activity in vitro. We speculate that GmASR thus protects against oxidation damage by buffering metal ions, thus alleviating metal toxicity in plant cells under stressed conditions.

ASR1 mediates glucose-hormone cross talk by affecting sugar trafficking in tobacco plants

Asr (for ABA, stress, ripening) genes are exclusively found in the genomes of higher plants, and the encoded proteins have been found localized both to the nucleus and cytoplasm. However, before the mechanisms underlying the activity of ASR proteins can be determined, the role of these proteins in planta should be deciphered. Results from this study suggest that ASR is positioned within the signaling cascade of interactions among glucose, abscisic acid, and gibberellins. Tobacco (Nicotiana tabacum) transgenic lines with reduced levels of ASR protein showed impaired glucose metabolism and altered abscisic acid and gibberellin levels. These changes were associated with dwarfism, reduced carbon dioxide assimilation, and accelerated leaf senescence as a consequence of a fine regulation exerted by ASR to the glucose metabolism. This regulation resulted in an impact on glucose signaling mediated by Hexokinase1 and Snf1-related kinase, which would subsequently have been responsible for photosynthesis, leaf senescence, and hormone level alterations. It thus can be postulated that ASR is not only involved in the control of hexose uptake in heterotrophic organs, as we have previously reported, but also in the control of carbon fixation by the leaves mediated by a similar mechanism.

New insights into desiccation-associated gene regulation by Lilium longiflorum ASR during pollen maturation and in transgenic Arabidopsis

LLA23, a member of the abscisic acid-, stress-, and ripening-induced (ASR) protein family, was previously isolated from lily (Lilium longiflorum) pollen. The lily ASR is induced through desiccation-associated ABA signaling transduction in the pollen. ASRs are highly hydrophilic and intrinsically unstructured proteins with molecular masses generally less than 18 kDa. LLA23 is abundant in the cytoplasm and nuclei of both vegetative and generative cells of pollen grains. The protein in the nucleus and in the cytoplasm is partly regulated by dehydration. A dual role is proposed for LLA23, as a regulator and a protective molecule, upon exposure to water deficits. This chapter reviews the current state of literature on Asr genes, protein structure, function, and their responses to various stresses. In a study, a genome-wide microarray was used to monitor the expression of LLA23-regulated genes, focusing on the relationship between ASR-, glucose-, and drought-inducible genes, and outlined the difference and cross talk of gene expression among these signaling networks. A strong association was observed in the expression of stress-responsive genes and found 25 genes that respond to all three treatments. Highly inducible genes were also found in each specific stress treatment. Promoter sequence analysis of LLA23-inducible genes enabled us not only to identify possible known cis-acting elements in the promoter regions but also to expect the existence of novel cis-acting elements involved in ASR-responsive gene expression. ASR can be used to improve crops and economically important plants against various environmental stresses.

The SbASR-1 gene cloned from an extreme halophyte Salicornia brachiata enhances salt tolerance in transgenic tobacco

Salinity severely affects plant growth and development. Plants evolved various mechanisms to cope up stress both at molecular and cellular levels. Halophytes have developed better mechanism to alleviate the salt stress than glycophytes, and therefore, it is advantageous to study the role of different genes from halophytes. Salicornia brachiata is an extreme halophyte, which grows luxuriantly in the salty marshes in the coastal areas. Earlier, we have isolated SbASR-1 (abscisic acid stress ripening-1) gene from S. brachiata using cDNA subtractive hybridisation library. ASR-1 genes are abscisic acid (ABA) responsive, whose expression level increases under abiotic stresses, injury, during fruit ripening and in pollen grains. The SbASR-1 transcript showed up-regulation under salt stress conditions. The SbASR-1 protein contains 202 amino acids of 21.01-kDa molecular mass and has 79 amino acid long signatures of ABA/WDS gene family. It has a maximum identity (73 %) with Solanum chilense ASR-1 protein. The SbASR-1 has a large number of disorder-promoting amino acids, which make it an intrinsically disordered protein. The SbASR-1 gene was over-expressed under CaMV 35S promoter in tobacco plant to study its physiological functions under salt stress. T(0) transgenic tobacco seeds showed better germination and seedling growth as compared to wild type (Wt) in a salt stress condition. In the leaf tissues of transgenic lines, Na(+) and proline contents were significantly lower, as compared to Wt plant, under salt treatment, suggesting that transgenic plants are better adapted to salt stress.

MusaSAP1, a A20/AN1 zinc finger gene from banana functions as a positive regulator in different stress responses

A20/AN1 zinc finger domain containing Stress Associated Proteins (SAP) are involved in diverse stress response pathways in plants. In the present study, a novel banana SAP gene, MusaSAP1, was identified from banana EST database and was subsequently characterized by overexpression in transgenic banana plants. Expression profiling in native banana plants showed that MusaSAP1 was up-regulated by drought, salt, cold, heat and oxidative stress as well as by treatment with abscisic acid. Cellular localization assay carried out by making a MusaSAP1::GFP fusion protein indicated that MusaSAP1 is incompletely translocated to nucleus. Copy number analysis performed using real time PCR and Southern blotting indicated that MusaSAP1 occurs in the banana genome in a single copy per 11 chromosome set. Transgenic banana plants constitutively overexpressing MusaSAP1 displayed better stress endurance characteristics as compared to controls in both in vitro and ex vivo assays. Lesser membrane damage as indicated by reduced malondialdehyde levels in transgenic leaves subjected to drought, salt or oxidative stress pointed towards significant role for MusaSAP1 in stress amelioration pathways of banana. Strong up-regulation of a polyphenol oxidase (PPO) coding transcript in MusaSAP1 overexpressing plants together with induction of MusaSAP1 by wounding and methyl jasmonate treatment indicated possible involvement of MusaSAP1 in biotic stress responses where PPOs perform major functions in multiple defense pathways.

Lily ASR protein-conferred cold and freezing resistance in Arabidopsis

The lily LLA23 protein is a member of the abscisic acid, stress and ripening-induced (ASR) protein family. Constitutive overexpression of LLA23 under the cauliflower mosaic virus 35S promoter confers cold and freezing tolerance in Arabidopsis. The phenotypical growth and survival percentage of the two transgenic 35S::LLA23 plants showed higher resistance to cold and freezing conditions than those of wild-type (WT) plants. The electrolyte leakage in WT leaves increased by approximately fourfold at -2 °C relative to that at 22 °C whereas both transgenic leaves showed little ion leakage under the same conditions. A microarray analysis of LLA23-overexpressing transgenic line, 35S::LLA23E, under normal growing conditions was previously conducted by Yang et al. (Protoplasma, 2008, 233:241-254). Microarray analysis showed that 12 cold-responsive genes are upregulated and 25 cold-responsive genes are downregulated by lily ASR. Many ASR-regulated genes encode proteins involved in the classes of defense/stress-related, transcription, and metabolism. Quantitative polymerase chain reaction analysis confirms the changes in mRNA levels observed in the microarray analysis. Thus, our results provide in vivo evidence implying that LLA23 mediates cold/freezing stress-responsive signaling. To gain further insight into the functions of LLA23 protein, an in vitro enzyme protection assay was used in which lactate dehydrogenase and malate dehydrogenase were subjected to unfavorable conditions. The assay revealed that both enzyme activities were significantly retained with the addition of LLA23, which was superior to either trehalose or BSA, suggesting that the LLA23 protein can protect enzymatic activities against freeze-thaw cycles. The 35S::LLA23 seedlings also exhibited enzyme activity superior to WT at -4 °C. These results suggest that LLA23 may act as an osmoprotectant as well as a transcription factor to confer 35S::LLA23 plants enhanced cold and freezing resistance.

Cu2+ triggers reversible aggregation of a disordered His-rich dehydrin MpDhn12 from Musa paradisiaca

Copper is an essential nutrient, but it is toxic in excess. Here, we cloned and characterized a His-rich low molecular weight dehydrin from Musa paradisiaca, MpDhn12. Analysis by circular dichroism (CD) spectra and a thermal stability assay showed that MpDhn12 is an intrinsically disordered protein, and immobilized-metal affinity chromatography (IMAC) analysis revealed that MpDhn12 can bind Cu(2+) both in vitro and in vivo. Interestingly, MpDhn12 aggregated under excess Cu(2+) conditions, and the aggregation was reversible and impaired by histidine modification with diethylpyrocarbonate (DEPC), while the disordered structure of another dehydrin ERD14 (as a control) was not changed. Furthermore, MpDhn12 could complement the copper-sensitive phenotype of yeast mutant Δsod1. These results together suggested that MpDhn12 may take part in buffering copper levels through chelation and formation of aggregates in excess Cu(2+) conditions. To the best of our knowledge, it is the first report that a dehydrin interchanged between disordered and aggregated state triggered by copper.