Comparative functional analysis of three abiotic stress-inducible promoters in transgenic rice

Molecular cloning and functional characterization of the promoter of a novel Aspergillus flavus inducible gene (AhOMT1) from peanut

Peanut is an important oil and food legume crop grown in more than one hundred countries, but the yield and quality are often impaired by different pathogens and diseases, especially aflatoxins jeopardizing human health and causing global concerns. For better management of aflatoxin contamination, we report the cloning and characterization of a novel A. flavus inducible promoter of the O-methyltransferase gene (AhOMT1) from peanut. The AhOMT1 gene was identified as the highest inducible gene by A. flavus infection through genome-wide microarray analysis and verified by qRT-PCR analysis. AhOMT1 gene was studied in detail, and its promoter, fussed with the GUS gene, was introduced into Arabidopsis to generate homozygous transgenic lines. Expression of GUS gene was studied in transgenic plants under the infection of A. flavus. The analysis of AhOMT1 gene characterized by in silico assay, RNAseq, and qRT-PCR revealed minute expression in different organs and tissues with trace or no response to low temperature, drought, hormones, Ca2+, and bacterial stresses, but highly induced by A. flavus infection. It contains four exons encoding 297 aa predicted to transfer the methyl group of S-adenosyl-L-methionine (SAM). The promoter contains different cis-elements responsible for its expression characteristics. Functional characterization of AhOMT1P in transgenic Arabidopsis plants demonstrated highly inducible behavior only under A. flavus infection. The transgenic plants did not show GUS expression in any tissue(s) without inoculation of A. flavus spores. However, GUS activity increased significantly after inoculation of A. flavus and maintained a high level of expression after 48 hours of infection. These results provided a novel way for future management of peanut aflatoxins contamination through driving resistance genes in A. flavus inducible manner.

Genomics and transcriptomics to protect rice (Oryza sativa. L.) from abiotic stressors: -pathways to achieving zero hunger

More over half of the world's population depends on rice as a major food crop. Rice (Oryza sativa L.) is vulnerable to abiotic challenges including drought, cold, and salinity since it grown in semi-aquatic, tropical, or subtropical settings. Abiotic stress resistance has bred into rice plants since the earliest rice cultivation techniques. Prior to the discovery of the genome, abiotic stress-related genes were identified using forward genetic methods, and abiotic stress-tolerant lines have developed using traditional breeding methods. Dynamic transcriptome expression represents the degree of gene expression in a specific cell, tissue, or organ of an individual organism at a specific point in its growth and development. Transcriptomics can reveal the expression at the entire genome level during stressful conditions from the entire transcriptional level, which can be helpful in understanding the intricate regulatory network relating to the stress tolerance and adaptability of plants. Rice (Oryza sativa L.) gene families found comparatively using the reference genome sequences of other plant species, allowing for genome-wide identification. Transcriptomics via gene expression profiling which have recently dominated by RNA-seq complements genomic techniques. The identification of numerous important qtl,s genes, promoter elements, transcription factors and miRNAs involved in rice response to abiotic stress was made possible by all of these genomic and transcriptomic techniques. The use of several genomes and transcriptome methodologies to comprehend rice (Oryza sativa, L.) ability to withstand abiotic stress have been discussed in this review.

The MADS transcription factor GhFYF is involved in abiotic stress responses in upland cotton (Gossypium hirsutum L.)

Cotton is an important textile industry raw material crops, which plays a critical role in the development of society. MADS transcription factors (TFs) play a key role about the flowering time, flower development, and abiotic stress responses in plants, but little is known about their functions on abiotic stress in cotton. In this study, a MIKC^C subfamily gene from cotton, GhFYF (FOREVER YOUNG FLOWER), was isolated and characterized. Our data showed that GhFYF localized to the nucleus. A β-glucuronidase (GUS) activity assay revealed that the promoter of GhFYF was mainly expressed in the flower and seed of ProGhFYF::GUS transgenic A. thaliana plants. The GUS staining of flowers and seeds was deepened after drought, salt treatment, and the expression level of the GUS gene and corresponding stress genes AtERD10, AtAnnexin1 are up-regulated in the inflorescence. Overexpression GhFYF in A. thaliana could promote the seed germination and growth under different salt concentrations, and determin the proline content. Yeast two-hybrid (Y2H) assays showed that GhFYF interacted with the HAD-like protein GhGPP2, which has responds to abiotic stress. Our findings indicate that GhFYF is involved in abiotic stress responses, especially for salt stress. This work establishes a solid foundation for further functional analysis of the GhFYF gene in cotton.

Molecular cloning and characterization of high-affinity potassium transporter (AlHKT2;1) gene promoter from halophyte Aeluropus lagopoides

HKT subfamily II functions as Na⁺- K⁺ co-transporter and prevents plants from salinity stress. A 760 bp promoter region of AlHKT2;1 was isolated, sequenced and cloned. The full length promoter D1, has many cis-regulatory elements like MYB, MBS, W box, ABRE etc. involved in abiotic stress responses. D1 and subsequent 5' deletions were cloned into pCAMBIA1301 and studied for its efficacy in stress conditions in heterologous system. Blue colour staining was observed in flower petals, anther lobe, and dehiscence slit of anther in T₀ plants. The T₁ seedlings showed staining in leaf veins, shoot vasculature and root except root tip. T₁ seedlings were subjected to NaCl, KCl, NaCl + KCl and ABA stresses. GUS activity was quantified by 4-methylumbelliferyl glucuronide (4-MUG) assay under control and stress conditions. The smallest deletion- D4 also showed GUS expression but highest activity was observed in D2 as compared to full length promoter and other deletions. The electrophoretic mobility shift assay using stress-induced protein with different promoter deletions revealed more prominent binding in D2. These results suggest that AlHKT2;1 promoter is involved in abiotic stress response and deletion D2 might be sufficient to drive the stress-inducible expression of various genes involved in providing stress tolerance in plants.

Enhanced glutathione content improves lateral root development and grain yield in rice plants

Enhanced glutathione content improves lateral root development by positively regulating the transcripts of root development genes responsive to glutathione treatment, thereby increasing the overall productivity of rice plants. Glutathione is primarily known as a cellular antioxidant molecule, but its role in lateral root development in rice plants has not been elucidated. Here, we have investigated its role in lateral root development of rice Oryza sativa L. Exogenous glutathione (GSH) promoted both the number and length of lateral roots in rice, and the GSH biosynthesis inhibitor buthionine sulfoximine (BSO) significantly reduced these parameters, compared to untreated plants. The inhibition by BSO was reversed with exogenous GSH. Transcript profiling by RNA-seq revealed that expression of the transcription factor genes DREB and ERF and the hormone-related genes AOS, LOX, JAZ, and SAUR were significantly downregulated in the BSO-treated plants and, in contrast, upregulated in plants treated with GSH and with GSH and BSO together. We generated OsGS-overexpressing transgenic plants in which the transgene is controlled by the abiotic-stress-inducible OsRab21 promoter to study the effect of endogenously increased GSH levels. In cold stress, transgenic rice plants enhanced stress tolerance and lateral root development by maintaining redox homeostasis and improving upregulating the expression of transcription factors and hormone-related genes involved in lateral root development. We observed improved root growth of OsGS-overexpressing plants in paddy fields compared to the wild-type controls. These traits may have alleviated transplanting stress during early growth in the field and accounted for the increased productivity. These results provide information and perspectives on the role of GSH in gene expression, lateral root development, and grain yield in rice.

Cloning and functional analysis of AmDUF1517 promoter from Ammopiptanthus mongolicus

Domains of unknown function protein family 1517 (DUF1517) in Ammopiptanthus mongolicus, could be induced by abiotic stresses, whose upstream regulatory sequence might be an ideal source of abiotic-induced promoter. In this study, a 1026-bp promoter of AmDUF1517 from A. mongolicus was cloned. Five deletion fragments (Full, Q1-Q4) of different length of the AmDUF1517 promoter were fused with the β-glucuronidase (GUS) reporter and transformed into Arabidopsis thaliana. The deletion analysis showed that sequences Full, Q1 and Q3 responded well to mannitol, NaCl and 4 °C stresses, while Q2 and Q4 segments did not. The Q3 fragment (280 bp; -280 to -1 bp) showed the highest promoter activity under normal and mannitol, NaCl and 4 °C conditions. The result suggested that Q3 in the AmDUF1517 gene promoter could be a new source of induced promoters for abiotic resistance breeding in plant genetic engineering.

Identification of a 119-bp Promoter of the Maize Sulfite Oxidase Gene (ZmSO) That Confers High-Level Gene Expression and ABA or Drought Inducibility in Transgenic Plants

Drought adversely affects crop growth and yields. The cloning and characterization of drought- or abscisic acid (ABA)-inducible promoters is of great significance for their utilization in the genetic improvement of crop resistance. Our previous studies have shown that maize sulfite oxidase (SO) has a sulfite-oxidizing function and is involved in the drought stress response. However, the promoter of the maize SO gene has not yet been characterized. In this study, the promoter (ZmSOPro, 1194 bp upstream region of the translation initiation site) was isolated from the maize genome. The in-silico analysis of the ZmSOPro promoter identified several cis-elements responsive to the phytohormone ABA and drought stress such as ABA-responsive element (ABRE) and MYB binding site (MBS), besides a number of core cis-acting elements, such as TATA-box and CAAT-box. A 5′ RACE (rapid amplification of cDNA ends) assay identified an adenine residue as the transcription start site of the ZmSO. The ZmSOPro activity was detected by β-glucuronidase (GUS) staining at nearly all developmental stages and in most plant organs, except for the roots in transgenic Arabidopsis. Moreover, its activity was significantly induced by ABA and drought stress. The 5′-deletion mutant analysis of the ZmSOPro in tobacco plants revealed that a 119-bp fragment in the ZmSOPro (upstream of the transcription start site) is a minimal region, which is required for its high-level expression. Moreover, the minimal ZmSOPro was significantly activated by ABA or drought stress in transgenic plants. Further mutant analysis indicated that the MBS element in the minimal ZmSOPro region (119 bp upstream of the transcription start site) is responsible for ABA and drought-stress induced expression. These results improve our understanding of the transcriptional regulation mechanism of the ZmSO gene, and the characterized 119-bp promoter fragment could be an ideal candidate for drought-tolerant gene engineering in both monocot and dicot crops.

Characterization of wheat TaSnRK2.7 promoter in Arabidopsis

Expression of TaSnRK2.7 promoter is strongly induced under abiotic stress and could be used as a valuable tool for improving plant stress resistance via transgenic techniques. The sucrose non-fermenting 1-related protein kinase 2 (SnRK2) gene family plays pivotal roles in response to abiotic stresses (drought, salinity and cold). Here, we studied the expression of five wheat TaSnRK2.7 promoter-5'-deletion constructs (- 2547, - 1621, - 806, - 599, and - 254) fused to beta-glucuronidase (GUS) in Arabidopsis. Tissue-expression analysis revealed that the - 254 to ATG fragment was sufficient for inducing GUS expression in hypocotyls. Additionally, the - 806 to - 599 and - 2547 to - 1621 fragments contained leaf- and root-specific elements, respectively. Deletion analysis showed that these fragments were unresponsive to ABA treatment, suggesting that TaSnRK2.7 participates in an ABA-independent signaling pathway. Assays examining stress responses of constructs demonstrated that the - 599 to - 254 and - 806 to - 599 fragments contained elements responsive to abiotic and osmotic stress, respectively. The TaSnRK2.7 promoter contained enhancers from - 806 to - 254 and - 2547 to - 1621, while the - 1621 to - 806 fragment contained negative regulatory elements that restrict root and leaf gene expression in response to abiotic stress. Furthermore, under drought and salt stress, the TaSnRK2.7 promoter conferred greater gene expression in leaves than the rd29A promoter, even though both were induced by abiotic stress. These findings enhance our understanding of the molecular mechanisms behind TaSnRK2.7 action, which should prove useful in transgenic studies investigating stress-induced gene expression.

Multiple genomic regions influence root morphology and seedling growth in cultivated sunflower (Helianthus annuus L.) under well-watered and water-limited conditions

With climate change and an ever-increasing human population threatening food security, developing a better understanding of the genetic basis of crop performance under stressful conditions has become increasingly important. Here, we used genome-wide association studies to genetically dissect variation in seedling growth traits in cultivated sunflower (Helianthus annuus L.) under well-watered and water-limited (i.e., osmotic stress) conditions, with a particular focus on root morphology. Water limitation reduced seedling size and produced a shift toward deeper rooting. These effects varied across genotypes, and we identified 13 genomic regions that were associated with traits of interest across the two environments. These regions varied in size from a single marker to 186.2 Mbp and harbored numerous genes, some of which are known to be involved in the plant growth/development as well as the response to osmotic stress. In many cases, these associations corresponded to growth traits where the common allele outperformed the rare variant, suggesting that selection for increased vigor during the evolution of cultivated sunflower might be responsible for the relatively high frequency of these alleles. We also found evidence of pleiotropy across multiple traits, as well as numerous environmentally independent genetic effects. Overall, our results indicate the existence of genetic variation in root morphology and allocation and further suggest that the majority of alleles associated with these traits have consistent effects across environments.

Stress Inducible Overexpression of AtHDG11 Leads to Improved Drought and Salt Stress Tolerance in Peanut (Arachis hypogaea L.)

Peanut is an important oilseed and food legume cultivated as a rain-fed crop in semi-arid tropics. Drought and high salinity are the major abiotic stresses limiting the peanut productivity in this region. Development of drought and salt tolerant peanut varieties with improved yield potential using biotechnological approach is highly desirable to improve the peanut productivity in marginal geographies. As abiotic stress tolerance and yield represent complex traits, engineering of regulatory genes to produce abiotic stress-resilient transgenic crops appears to be a viable approach. In the present study, we developed transgenic peanut plants expressing an Arabidopsis homeodomain-leucine zipper transcription factor (AtHDG11) under stress inducible rd29A promoter. A stress-inducible expression of AtHDG11 in three independent homozygous transgenic peanut lines resulted in improved drought and salt tolerance through up-regulation of known stress responsive genes (LEA, HSP70, Cu/Zn SOD, APX, P5CS, NCED1, RRS5, ERF1, NAC4, MIPS, Aquaporin, TIP, ELIP) in the stress gene network, antioxidative enzymes, free proline along with improved water use efficiency traits such as longer root system, reduced stomatal density, higher chlorophyll content, increased specific leaf area, improved photosynthetic rates, and increased intrinsic instantaneous WUE. Transgenic peanut plants displayed high yield compared to non-transgenic plants under both drought and salt stress conditions. Holistically, our study demonstrates the potentiality of stress-induced expression of AtHDG11 to improve the drought, salt tolerance in peanut.

Improved stress tolerance and productivity in transgenic rice plants constitutively expressing the Oryza sativa glutathione synthetase OsGS under paddy field conditions

Reactive oxygen species, which increase under various environmental stresses, have deleterious effects on plants. An important antioxidant, glutathione, is used to detoxify reactive oxygen species in plant cells and is mainly produced by two enzymes: gamma-glutamylcysteine synthetase (γ-ECS) and glutathione synthetase (GS). To evaluate the functional roles of the glutathione synthetase gene (OsGS) in rice, we generated four independent transgenic rice plants (TG1-TG4) that overexpressed OsGS under the control of the constitutively expressed OsCc1 promoter. When grown under natural paddy field conditions, the TG rice plants exhibited greater growth development, higher chlorophyll content, and higher GSH/GSSH ratios than control wild-type (WT) rice plants. Subsequently, the TG rice plants enhanced redox homeostasis by preventing hydroperoxide-mediated membrane damage, which improved their adaptation to environmental stresses. As a result, TG rice plants improved rice grain yield and total biomass following increases in panicle number and number of spikelets per panicle, despite differences in climate during the cultivation periods of 2014 and 2015. Overall, our results indicate that OsGS overexpression improved redox homeostasis by enhancing the glutathione pool, which resulted in greater tolerance to environmental stresses in the paddy fields.

Functional Characterization of TaSnRK2.8 Promoter in Response to Abiotic Stresses by Deletion Analysis in Transgenic Arabidopsis

Drought, salinity, and cold are the major factors limiting wheat quality and productivity; it is thus highly desirable to characterize the abiotic-stress-inducible promoters suitable for the genetic improvement of plant resistance. The sucrose non-fermenting 1-related protein kinase 2 (SnRK2) family genes show distinct regulatory properties in response to abiotic stresses. The present study characterized the approximately 3000-bp upstream sequence (the 313 bp upstream of the ATG was the transcription start site) of the Triticum aestivum TaSnRK2.8 promoter under abscisic acid (ABA) and abiotic stresses. Four different-length 5' deletion fragments of TaSnRK2.8 promoter were fused with the GUS reporter gene and transformed into Arabidopsis. Tissue expression analysis showed that the TaSnRK2.8 promoter region from position -1481 to -821 contained the stalk-specific elements, and the region from position -2631 to -1481 contained the leaf- and root-specific elements. In the ABA-treated seedlings, the deletion analysis showed that the TaSnRK2.8 promoter region from position -821 to -2631 contained ABA response elements. The abiotic stress responses of the TaSnRK2.8 promoter derivatives demonstrated that they harbored abiotic-stress response elements: the region from position -821 to -408 harbored the osmotic-stress response elements, whereas the region from position -2631 to -1481 contained the positive regulatory motifs and the region from position -1481 to -821 contained the leaf- and stalk-specific enhancers. Further deletion analysis of the promoter region from position -821 to -408 indicated that a 125-bp region from position -693 to -568 was required to induce an osmotic-stress response. These results contribute to a better understanding of the molecular mechanisms of TaSnRK2.8 in response to abiotic stresses, and the TaSnRK2.8 promoter seems to be a candidate for regulating the expression of abiotic stress response genes in transgenic plants.

Overexpression of SbAP37 in rice alleviates concurrent imposition of combination stresses and modulates different sets of leaf protein profiles

SbAP37 transcription factor contributes to a combination of abiotic stresses when applied simultaneously in rice. It modulates a plethora of proteins that might regulate the downstream pathways to impart salt stress tolerance. APETALA type of transcription factor was isolated from Sorghum bicolor (SbAP37), overexpressed in rice using a salt inducible abscisic acid 2 (ABA2) promoter through Agrobacterium tumefaciens following in planta method. Transgenics were confirmed by PCR amplification of SbAP37, hygromycin phosphotransferase (hptII) marker and ABA2 promoter and DNA blot analysis. Plants were exposed to 150 mM NaCl coupled with high day/night 36 ± 2/25 ± 2 °C temperatures and also drought stress by withholding water for 1-week separately at the booting stage. SbAP37 expression was 2.8- to 4.7-folds higher in transgenic leaf under salt, but 1.8- to 3.3-folds higher in roots under drought stress. Native gene expression analysis showed that it is expressed more in stem than in roots and leaves under 150 mM NaCl and 6% PEG stress. In the present study, proteomic analysis of transgenics exposed to 150 mM NaCl coupled with elevated temperatures was taken up using quadrupole time-of-flight (Q-TOF) mass spectrometry (MS). The leaf proteome revealed 11 down regulated, 26 upregulated, 101 common (shared), 193 newly synthesized proteins in transgenics besides 368 proteins in untransformed plants. Some of these protein sets appeared different and unique to combined stresses. Our results suggest that the SbAP37 has the potential to improve combined stress tolerance without causing undesirable phenotypic characters when used under the influence of ABA2 promoter.

Isolation and Functional Validation of Salinity and Osmotic Stress Inducible Promoter from the Maize Type-II H+-Pyrophosphatase Gene by Deletion Analysis in Transgenic Tobacco Plants

Salinity and drought severely affect both plant growth and productivity, making the isolation and characterization of salinity- or drought-inducible promoters suitable for genetic improvement of crop resistance highly desirable. In this study, a 1468-bp sequence upstream of the translation initiation codon ATG of the promoter for ZmGAPP (maize Type-II H+-pyrophosphatase gene) was cloned. Nine 5´ deletion fragments (D1-D9) of different lengths of the ZmGAPP promoter were fused with the GUS reporter and translocated into tobacco. The deletion analysis showed that fragments D1-D8 responded well to NaCl and PEG stresses, whereas fragment D9 and CaMV 35S did not. The D8 segment (219 bp; -219 to -1 bp) exhibited the highest promoter activity of all tissues, with the exception of petals among the D1-D9 transgenic tobacco, which corresponds to about 10% and 25% of CaMV 35S under normal and NaCl or PEG stress conditions, respectively. As such, the D8 segment may confer strong gene expression in a salinity and osmotic stress inducible manner. A 71-bp segment (-219 to -148 bp) was considered as the key region regulating ZmGAPP response to NaCl or PEG stress, as transient transformation assays demonstrated that the 71-bp sequence was sufficient for the salinity or osmotic stress response. These results enhance our understanding of the molecular mechanisms regulating ZmGAPP expression, and that the D8 promoter would be an ideal candidate for moderating expression of drought and salinity response genes in transgenic plants.

Identification of a 467 bp Promoter of Maize Phosphatidylinositol Synthase Gene (ZmPIS) Which Confers High-Level Gene Expression and Salinity or Osmotic Stress Inducibility in Transgenic Tobacco

Salinity and drought often affect plant growth and crop yields. Cloning and identification of salinity and drought stress inducible promoters is of great significance for their use in the genetic improvement of crop resistance. Previous studies showed that phosphatidylinositol synthase is involved in plant salinity and drought stress responses but its promoter has not been characterized by far. In the study, the promoter (pZmPIS, 1834 bp upstream region of the translation initiation site) was isolated from maize genome. To functionally validate the promoter, eight 5' deletion fragments of pZmPIS in different lengths were fused to GUS to produce pZmPIS::GUS constructs and transformed into tobacco, namely PZ1-PZ8. The transcription activity and expression pattern obviously changed when the promoter was truncated. Previous studies have demonstrated that NaCl and PEG treatments are usually used to simulate salinity and drought treatments. The results showed that PZ1-PZ7 can respond well upon NaCl and PEG treatments, while PZ8 not. PZ7 (467 bp) displayed the highest transcription activity in all tissues of transgenic tobacco amongst 5' deleted promoter fragments, which corresponds to about 20 and 50% of CaMV35S under normal and NaCl or PEG treatment, respectively. This implied that PZ7 is the core region of pZmPIS which confers high-level gene expression and NaCl or PEG inducible nature. The 113 bp segment between PZ7 and PZ8 (-467 to -355 bp) was considered as the key sequence for ZmPIS responding to NaCl or PEG treatment. GUS transient assay in tobacco leaves showed that this segment was sufficient for the NaCl or PEG stress response. Bioinformatic analysis revealed that the 113 bp sequence may contain new elements that are crucial for ZmPIS response to NaCl or PEG stress. These results promote our understanding on transcriptional regulation mechanism of ZmPIS and the characterized PZ7 promoter fragment would be an ideal candidate for the overexpression of drought and salinity responsive gene to improve crop resistance.

Rice tissue-specific promoters and condition-dependent promoters for effective translational application

Rice (Oryza sativa) is one of the most important staple food crops for more than half of the world's population. The demand is increasing for food security because of population growth and environmental challenges triggered by climate changes. This scenario has led to more interest in developing crops with greater productivity and sustainability. The process of genetic transformation, a major tool for crop improvement, utilizes promoters as one of its key elements. Those promoters are generally divided into three types: constitutive, spatiotemporal, and condition-dependent. Transcriptional control of a constitutive promoter often leads to reduced plant growth, due to a negative effect of accumulated molecules during cellular functions or energy consumption. To maximize the effect of a transgene on transgenic plants, it is better to use condition-dependent or tissue-specific promoters. However, until now, those types have not been as widely applied in crop biotechnology. In this review, we introduce and discuss four groups of tissue-specific promoters (50 promoters in total) and six groups of condition-dependent promoters (27 promoters). These promoters can be utilized to fine-tune desirable agronomic traits and develop crops with tolerance to various stresses, enhanced nutritional value, and advanced productivity.

The promoter of the AlSAP gene from the halophyte grass Aeluropus littoralis directs a stress-inducible expression pattern in transgenic rice plants

When fused to " Pr AlSAP " promoter, transcripts of gusA exhibited similar accumulation patterns in transgenic rice as AlSAP transcripts in A. littoralis. Pr AlSAP can be used for engineering abiotic stress tolerance. We previously showed that ectopic expression of a stress-associated protein gene from Aeluropus littoralis (AlSAP) enhances tolerance to multiple abiotic stresses in tobacco, wheat and rice. The ortholog of AlSAP in rice is OsSAP9. Here, we demonstrate that AlSAP transcripts accumulate in Aeleuropus in response to multiple abiotic stresses and at a higher level in roots, while those of OsSAP9 are preferentially induced by cold and heat treatments and accumulate preferentially in leaves of rice. In silico analysis of the AlSAP promoter "Pr AlSAP " predicted several cis-acting elements responsible for gene regulation by dehydration, salt, heat, ABA, SA, wounding and tissue-specific expression. The Pr AlSAP promoter was fused to the gusA gene and used to produce transgenic rice plants. Transcripts of gusA exhibited similar accumulation patterns in transgenic rice as AlSAP transcripts in A. littoralis. Indeed, accumulation of gusA transcripts was higher in roots than in leaves and induced by salt, drought, cold and heat treatments. GUS activity was confirmed in roots, coleoptiles, leaves and glumes, but absent in the root cell elongation zone and in dry seeds. A wound treatment strongly induced GUS accumulation in leaves and imbibed seeds. Altogether, these results indicate that the regulatory regions of two ortholog genes "AlSAP" and "OsSAP9" have diverged in the specificity of the signals promoting their induction, but that the trans-acting elements allowing the correct spatiotemporal regulation and stress induction of Pr AlSAP exist in rice. Therefore, the AlSAP promoter appears to be an interesting candidate for engineering abiotic stress tolerance in cereals.

The transcriptional regulatory network in the drought response and its crosstalk in abiotic stress responses including drought, cold, and heat

Drought negatively impacts plant growth and the productivity of crops around the world. Understanding the molecular mechanisms in the drought response is important for improvement of drought tolerance using molecular techniques. In plants, abscisic acid (ABA) is accumulated under osmotic stress conditions caused by drought, and has a key role in stress responses and tolerance. Comprehensive molecular analyses have shown that ABA regulates the expression of many genes under osmotic stress conditions, and the ABA-responsive element (ABRE) is the major cis-element for ABA-responsive gene expression. Transcription factors (TFs) are master regulators of gene expression. ABRE-binding protein and ABRE-binding factor TFs control gene expression in an ABA-dependent manner. SNF1-related protein kinases 2, group A 2C-type protein phosphatases, and ABA receptors were shown to control the ABA signaling pathway. ABA-independent signaling pathways such as dehydration-responsive element-binding protein TFs and NAC TFs are also involved in stress responses including drought, heat, and cold. Recent studies have suggested that there are interactions between the major ABA signaling pathway and other signaling factors in stress responses. The important roles of these TFs in crosstalk among abiotic stress responses will be discussed. Control of ABA or stress signaling factor expression can improve tolerance to environmental stresses. Recent studies using crops have shown that stress-specific overexpression of TFs improves drought tolerance and grain yield compared with controls in the field.

Comparative functional analysis of six drought-responsive promoters in transgenic rice

Rice production is greatly affected by environmental stresses such as drought and high salinity. Transgenic rice plants tolerant to such stresses are expected to be produced. Stress-responsive promoters with low expression under normal growth conditions are needed to minimize the adverse effects of stress-tolerance genes on rice growth. We performed expression analyses of drought-responsive genes in rice plants using a microarray, and selected LIP9, OsNAC6, OsLEA14a, OsRAB16D, OsLEA3-1, and Oshox24 for promoter analysis. Transient assays using the promoters indicated that AREB/ABF (abscisic acid (ABA)-responsive element-binding protein/ABA-binding factor) transcription factors enhanced expressions of these genes. We generated transgenic rice plants containing each promoter and the β-glucuronidase (GUS) reporter gene. GUS assays revealed that the LIP9 and OsNAC6 promoters were induced by drought, high salinity, and ABA treatment, and both promoters showed strong activity under normal growth conditions in the root. The other promoters were strongly induced by stresses and ABA, but showed low activity under normal growth conditions. In seeds, GUS staining showed that Oshox24 expression was low and expressions of the other genes were high. Transgenic rice plants overexpressing OsNAC6 under the control of the Oshox24 promoter showed increased tolerance to drought and high salinity, and no growth defects. These data suggest that the Oshox24 promoter is useful to overexpress stress-tolerance genes without adversely affecting growth.

MicroRNA-mediated gene regulation: potential applications for plant genetic engineering

Food security is one of the most important issues challenging the world today. Any strategies to solve this problem must include increasing crop yields and quality. MicroRNA-based genetic modification technology (miRNA-based GM tech) can be one of the most promising solutions that contribute to agricultural productivity directly by developing superior crop cultivars with enhanced biotic and abiotic stress tolerance and increased biomass yields. Indirectly, the technology may increase usage of marginal soils and decrease pesticide use, among other benefits. This review highlights the most recent progress of transgenic studies utilizing various miRNAs and their targets for plant trait modifications, and analyzes the potential of miRNA-mediated gene regulation for use in crop improvement. Strategies for manipulating miRNAs and their targets in transgenic plants including constitutive, stress-induced, or tissue-specific expression of miRNAs or their targets, RNA interference, expressing miRNA-resistant target genes, artificial target mimic and artificial miRNAs were discussed. We also discussed potential risks of utilizing miRNA-based GM tech. In general, miRNAs and their targets not only provide an invaluable source of novel transgenes, but also inspire the development of several new GM strategies, allowing advances in breeding novel crop cultivars with agronomically useful characteristics.

Activity of the Arabidopsis RD29A and RD29B promoter elements in soybean under water stress

The constitutive and drought-induced activities of the Arabidopsis thaliana RD29A and RD29B promoters were monitored in soybean (Glycine max (L.) Merr.] via fusions with the visual marker gene β-glucuronidase (GUS). Physiological responses of soybean plants were monitored over 9 days of water deprivation under greenhouse conditions. Data were used to select appropriate time points to monitor the activities of the respective promoter elements. Qualitative and quantitative assays for GUS expression were conducted in root and leaf tissues, from plants under well-watered and dry-down conditions. Both RD29A and RD29B promoters were significantly activated in soybean plants subjected to dry-down conditions. However, a low level of constitutive promoter activity was also observed in both root and leaves of plants under well-watered conditions. GUS expression was notably higher in roots than in leaves. These observations suggest that the respective drought-responsive regulatory elements present in the RD29X promoters may be useful in controlling targeted transgenes to mitigate abiotic stress in soybean, provided the transgene under control of these promoters does not invoke agronomic penalties with leaky expression when no abiotic stress is imposed.

Characterization of the stress-inducible OsNCED3 promoter in different transgenic rice organs and over three homozygous generations

To be effective in crop biotechnology applications, gene promoters need to be stably active over sequential generations in a population of single-copy transgenic lines. Most of the stress-inducible promoters characterized in plants thus far have been analyzed at early (T₀, T₁ or T₂) generations and/or by testing only a small number of transgenic lines. In our current study, we report our analysis of OsNCED3, a stress-inducible rice promoter involved in ABA biosynthesis, in various organs and tissues of transgenic rice plants over the T(2-4) homozygous generations. The transgene copy numbers in the lines harboring the OsNCED3:gfp construct were determined and six single- and two double-copy transgenic lines were analyzed for promoter activity in comparison with the Wsi18, a stress-inducible promoter previously characterized. The exogenous promoter activities were found to be significantly enhanced in the roots and leaves, whereas zero or low levels of activity were evident in grains and flowers, under drought and high-salinity conditions. The highest induction levels of gfp transcripts in the OsNCED3:gfp plants upon drought treatments were 161- and 93-fold in leaves and roots, respectively, and these levels were comparable with those of gfp transcripts in the Wsi18:gfp plants. A comparison of the promoter activities between the T₂-T₄ plants revealed that comparable activity levels were maintained over these three homozygous generations with no evidence of silencing. Thus, our results provide the OsNCED3 promoter that is stress-inducible in a whole rice plant except for in the aleurones and endosperm and stably active over three generations.

Induced over-expression of the transcription factor OsDREB2A improves drought tolerance in rice

The DREB and CBF transcription factors play a critical role in plant development and abiotic stress responses and, therefore, represent attractive targets for a molecular plant breeding approach. In this study, the rice OsDREB2A gene was isolated and expressed under the control of a stress-inducible promoter (4ABRC) to improve the abiotic stress tolerance of japonica rice variety TNG67. T₂ and T₃ transgenic lines over-expressing OsDREB2A were found to have improved survival rates under severe drought and salt stress conditions relative to non-transgenic rice plants or rice plants transformed with the empty vector control. OsDREB2A expression was found to be markedly induced by drought and ABA treatment. The results indicate that the induced over-expression of OsDREB2A driven by the 4ABRC promoter in engineered rice plants may protect cells during stress.

Inducibility of three salinity/abscisic acid-regulated promoters in transgenic rice with gusA reporter gene

The present study evaluates the pattern of stress inducibility of one natural promoter (from rice Rab16A) and two synthetically designed promoters, viz., 4X ABRE (abscisic acid-responsive element, having four tandem repeats of ABRE) and 2X ABRC (abscisic acid-responsive complex, having two tandem repeats of ABRE and two copies of coupling elements), in response to varying concentrations of NaCl and abscisic acid (ABA). Each promoter, independently linked to gusA (that encodes β glucuronidase, GUS), was introduced into rice (cv. Khitish) through particle bombardment. The T(2) progenies showed integration of gusA in their genome. The accumulation of gusA transcript, driven by each promoter in T(2) transgenics, increased with increasing salt/ABA concentration, with ABA being the better activator of each promoter. Induction in GUS expression, driven by different promoters, was noted on exogenous salt/ABA treatments in a concentration-dependent manner. The maximum induction was observed with 2X ABRC promoter. All the three promoters could drive stress-inducible GUS expression in both vegetative and floral organs. However, prominent GUS expression was noted in the whole seed (both embryo and aleurone layer of endosperm) only by 2X ABRC, whereas it was localized only in the embryo for the other two promoters. Thus, our observation characterizes three efficient salinity/ABA-inducible promoters that have the potentiality in crop biotechnology to drive transgene expression for stress tolerance, whenever abiotic stress is encountered.

Analysis of transcriptional and upstream regulatory sequence activity of two environmental stress-inducible genes, NBS-Str1 and BLEC-Str8, of rice

Two abiotic stress-inducible upstream regulatory sequences (URSs) from rice have been identified and functionally characterized in rice. NBS-Str1 and BLEC-Str8 genes have been identified, by analysing the transcriptome data of cold, salt and desiccation stress-treated 7-day-old rice (Oryza sativa L. var. IR64) seedling, to be preferentially responsive to desiccation and salt stress, respectively. NBS-Str1 and BLEC-Str8 genes code for putative NBS (nucleotide binding site)-LRR (leucine rich repeat) and β-lectin domain protein, respectively. NBS-Str1 URS is induced in root tissue, preferentially in vascular bundle, during 3 and 24 h of desiccation stress condition in transgenic 7-day-old rice seedling. In mature transgenic plants, this URS shows induction in root and shoot tissue under desiccation stress as well as under prolonged (1 and 2 day) salt stress. BLEC-Str8 URS shows basal activity under un-stressed condition, however, it is inducible under salt stress condition in both root and leaf tissues in young seedling and mature plants. Activity of BLEC-Str8 URS has been found to be vascular tissue preferential, however, under salt stress condition its activity is also found in the mesophyll tissue. NBS-Str1 and BLEC-Str8 URSs are inducible by heavy metal, copper and manganese. Interestingly, both the URSs have been found to be non responsive to ABA treatment, implying them to be part of ABA-independent abiotic stress response pathway. These URSs could prove useful for expressing a transgene in a stress responsive manner for development of stress tolerant transgenic systems.

[Plant stress-inducible promoters and their function]

Promoter is an important cis-regulatory element for gene expression and plays an important role in the process of plant gene expression and regulation. Constitutive promoters are being used to drive alien gene expression in most transgenic engineering. Although constitutive promoters can improve resistance of transgenic plants to abiotic stresses, over- and constitutive-expression of the alien genes have been shown to cause stunted growth and reduction of yield in transgenic plants. Therefore, inducible promoters, which are expressed only when exposed to stresses, are of importance. This paper reviews the types and functions of plant gene promoters induced by bio- and abio-stresses. The prospect of stress-induced promoters was discussed.

Functional analysis of six drought-inducible promoters in transgenic rice plants throughout all stages of plant growth

There are few efficient promoters for use with stress-inducible gene expression in plants, and in particular for monocotyledonous crops. Here, we report the identification of six genes, Rab21, Wsi18, Lea3, Uge1, Dip1, and R1G1B that were induced by drought stress in rice microarray experiments. Gene promoters were linked to the gfp reporter and their activities were analyzed in transgenic rice plants throughout all stages of plant growth, from dry seeds to vegetative tissues to flowers, both before and after drought treatments. In fold induction levels, Rab21 and Wsi18 promoters ranged from 65- and 36-fold in leaves to 1,355- and 492-fold in flowers, respectively, whereas Lea3 and Uge1 were higher in leaves, but lower in roots and flowers, as compared with Rab21 and Wsi18. Dip1 and R1G1B promoters had higher basal levels of activity under normal growth conditions in all tissues, resulting in smaller fold-induction levels than those of the others. In drought treatment time course, activities of Dip1 and R1G1B promoters rapidly increased, peaked at 2 h, and remained constant until 8 h, while that of Lea3 slowly yet steadily increased until 8 h. Interestingly, Rab21 activity increased rapidly and steadily in response to drought stress until expression peaked at 8 h. Thus, we have isolated and characterized six rice promoters that are all distinct in fold induction, tissue specificity, and induction kinetics under drought conditions, providing a variety of drought-inducible promoters for crop biotechnology.

Characterization of a novel stilbene synthase promoter involved in pathogen- and stress-inducible expression from Chinese wild Vitis pseudoreticulata

Stilbene synthase is a plant-specific polyketide synthase, and plays important roles in diverse metabolic processes. The genomic stilbene synthase gene was cloned from accession "Baihe-35-1" of Chinese wild Vitis pseudoreticulata, and a stilbene synthase of V. pseudoreticulata (VpSTS) transcripts expressed in the grape-powdery mildew interaction were determined by semi-quantitative RT-PCR. To monitor VpSTS expression in plant, the promoter region flanking the 5' VpSTS coding region was isolated from the genomic DNA of Chinese wild V. pseudoreticulata accession Baihe-35-1. Alignment of the VpSTS promoter sequence showed a 56.4% identity to Vitis vinifera. To identify the upstream region of the VpSTS gene required for promoter activity, a series of VpSTS promoter deletion derivatives was constructed. Each deletion construct was analyzed by Agrobacterium-mediated transient transformation in grapevine and tobacco leaves after infection by Uncinula necator and Alternaria alternata. In transiently transformed grapevine leaves, GUS activity was also determined after treatment with salicylic acid (SA) and 4 degrees C cold. Analysis of a series of 5' deletions of the VpSTS promoter in grapevine leaves indicated that the proximal 162 bp from the transcription initiation site was proved to be necessary for establishing both the constitutive and induced pattern of expression.