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

Functional and molecular characterization of fluoride exporter (FEX) from rice and its constitutive overexpression in Nicotiana benthamiana to promote fluoride tolerance

KEY MESSAGE

Early induction of OsFEX was insufficient for fluoride adaptation in IR-64. Overexpression of OsFEX in yeast and Nicotiana benthamiana enhanced fluoride tolerance. The present study delineates the regulation of fluoride exporter (FEX) in the fluoride-sensitive rice cultivar, IR-64 and its efficacy in generating high fluoride tolerance in transgenic Nicotiana benthamiana. Gene and protein expression profiling revealed that OsFEX exhibited early induction during fluoride stress in the vegetative and reproductive tissues of IR-64, although the expression was suppressed upon prolonged stress treatment. Analysis of OsFEX promoter in transgenic N. benthamiana, using β-glucuronidase reporter assay confirmed its early inducible nature, since the reporter expression and activity peaked at 12 h of NaF stress, after which it was lowered. OsFEX expression was up regulated in the presence of gibberellic acid (GA) and melatonin, while it was suppressed by abscisic acid (ABA). Complementation of ΔFEX1ΔFEX2 yeast mutants with OsFEX enabled high fluoride tolerance, thus validating the functional efficiency of the transgene. Bioassay of transgenic N. benthamiana lines, expressing OsFEX either under its own promoter or under CaMV35S promoter, established that constitutive overexpression, rather than early induction of OsFEX was essential and crucial for generating fluoride tolerance in the transgenics. Overall, the suppression of OsFEX in the later growth phases of stressed IR-64 due to enhanced ABA conservation and lowered synthesis of GA, as supported by the application of the respective phytohormone biosynthetic inhibitors, such as sodium tungstate and paclobutrazol, accounted for the fluoride-hyperaccumulative nature of the rice cultivar.

The Expression of CARK1 or RCAR11 Driven by Synthetic Promoters Increases Drought Tolerance in Arabidopsis thaliana

Drought stress hinders plant growth and development, and abscisic acid (ABA) stimulates plants to respond to drought. Here, to increase plant tolerance to drought, we designed three synthetic promoters (Ap, Dp, ANDp) to determine transcription activity and drought stress resistance in plants resulting from combinations of (1) synthetic promoters and (2) the functional genes CARK1 (cytosolic ABA receptor kinase 1) and RCAR11 (regulatory components of ABA receptor 11). Transient expression of eGFP and the dual-luciferase assay demonstrated that the basal transcriptional activities of Ap and ANDp were present at low levels under normal conditions, while the synthetic promoters were apparently induced upon either treatment of exogenous ABA or co-transformation with effector DREB2A (dehydration-responsive element binding protein 2A). Analysis of the transgenic plants (Ap:CARK1, Dp:CARK1, ANDp:CARK1, and Dp:RCAR11-Ap:CARK1) showed that the synthetic promoters Ap, Dp, and ANDp increased the expression of exogenous genes in transgenic plants upon treatment of ABA or d-mannitol. ANDp:CARK1 and Dp:RCAR11-Ap:CARK1 transgenic plants were sensitive to ABA and d-mannitol during cotyledon greening and root growth. A drought tolerance assay revealed that ANDp:CARK1 and Dp:RCAR11-Ap:CARK1 exhibited a higher survival rate than others upon drought stress. These results indicate that the combinations ANDp:CARK1 and Dp:RCAR11-Ap:CARK1 can be used to generate drought stress resistance in plants.

Rice Improvement Through Genome-Based Functional Analysis and Molecular Breeding in India

Rice is one of the main pillars of food security in India. Its improvement for higher yield in sustainable agriculture system is also vital to provide energy and nutritional needs of growing world population, expected to reach more than 9 billion by 2050. The high quality genome sequence of rice has provided a rich resource to mine information about diversity of genes and alleles which can contribute to improvement of useful agronomic traits. Defining the function of each gene and regulatory element of rice remains a challenge for the rice community in the coming years. Subsequent to participation in IRGSP, India has continued to contribute in the areas of diversity analysis, transcriptomics, functional genomics, marker development, QTL mapping and molecular breeding, through national and multi-national research programs. These efforts have helped generate resources for rice improvement, some of which have already been deployed to mitigate loss due to environmental stress and pathogens. With renewed efforts, Indian researchers are making new strides, along with the international scientific community, in both basic research and realization of its translational impact.

Synthetic promoters in planta

This paper reviews the importance, prospective and development of synthetic promoters reported in planta. A review of the synthetic promoters developed in planta would help researchers utilize the available resources and design new promoters to benefit fundamental research and agricultural applications. The demand for promoters for the improvement and application of transgenic techniques in research and agricultural production is increasing. Native/naturally occurring promoters have some limitations in terms of their induction conditions, transcription efficiency and size. The strength and specificity of native promoter can be tailored by manipulating its 'cis-architecture' by the use of several recombinant DNA technologies. Newly derived chimeric promoters with specific attributes are emerging as an efficient tool for plant molecular biology. In the last three decades, synthetic promoters have been used to regulate plant gene expression. To better understand synthetic promoters, in this article, we reviewed promoter structure, the scope of cis-engineering, strategies for their development, their importance in plant biology and the total number of such promoters (188) developed in planta to date; we then categorized them under different functional regimes as biotic stress-inducible, abiotic stress-inducible, light-responsive, chemical-inducible, hormone-inducible, constitutive and tissue-specific. Furthermore, we identified a set of 36 synthetic promoters that control multiple types of expression in planta. Additionally, we illustrated the differences between native and synthetic promoters and among different synthetic promoter in each group, especially in terms of efficiency and induction conditions. As a prospective of this review, the use of ideal synthetic promoters is one of the prime requirements for generating transgenic plants suitable for promoting sustainable agriculture and plant molecular farming.

Isolation, promoter analysis and expression profile of Dreb2 in response to drought stress in wheat ancestors

Drought is one of the most important abiotic stresses, constraining crop production seriously. The dehydration responsive element binding proteins (DREBs) are important plant-specific transcription factors that respond to various abiotic stresses and consequently induce abiotic stress-related genes that impart stress endurance in plants. Wild species are naturally exposed to various abiotic stresses and potentially harbor suitable alleles through natural selection. In this study we isolated and characterized Dreb2 from Triticum urartu (GenBank: KF731664), Aegilops speltoides (GenBank: KF731665) and Aegilops tauschii (GenBank: KF731663), the A, B and D genome ancestors of bread wheat, respectively. Analysis of over 1.3 kb upstream region of the gene revealed the presence of several conserved cis-acting regulatory elements including ABA-responsive elements, low temperature responsive elements, and several light and environmental signaling related motifs potentially vindicate Dreb2 responses to environmental signals. Moreover, the gene exhibited an alternative splicing, conserved among orthologous genes in grasses, and produced a non-functional isoform due to splicing in an exon resulted frame-shift creating an early stop codon before the functional domain. The expression analysis of Dreb2 under normal and different levels of dehydration stress conditions indicated that the two active spliced isoforms are upregulated when the plant exposed to drought stress whereas the non-functional isoform is downregulated in severe drought.

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.

Cross-talk between abscisic acid-dependent and abscisic acid-independent pathways during abiotic stress

Salinity, drought and low temperature are the common forms of abiotic stress encountered by land plants. To cope with these adverse environmental factors, plants execute several physiological and metabolic responses. Both osmotic stress (elicited by water deficit or high salt) and cold stress increase the endogenous level of the phytohormone abscisic acid (ABA). ABA-dependent stomatal closure to reduce water loss is associated with small signaling molecules like nitric oxide, reactive oxygen species and cytosolic free calcium, and mediated by rapidly altering ion fluxes in guard cells. ABA also triggers the expression of osmotic stress-responsive (OR) genes, which usually contain single/multiple copies of cis-acting sequence called abscisic acid-responsive element (ABRE) in their upstream regions, mostly recognized by the basic leucine zipper-transcription factors (TFs), namely, ABA-responsive element-binding protein/ABA-binding factor. Another conserved sequence called the dehydration-responsive element (DRE)/C-repeat, responding to cold or osmotic stress, but not to ABA, occurs in some OR promoters, to which the DRE-binding protein/C-repeat-binding factor binds. In contrast, there are genes or TFs containing both DRE/CRT and ABRE, which can integrate input stimuli from salinity, drought, cold and ABA signaling pathways, thereby enabling cross-tolerance to multiple stresses. A strong candidate that mediates such cross-talk is calcium, which serves as a common second messenger for abiotic stress conditions and ABA. The present review highlights the involvement of both ABA-dependent and ABA-independent signaling components and their interaction or convergence in activating the stress genes. We restrict our discussion to salinity, drought and cold stress.

Wheat chloroplast targeted sHSP26 promoter confers heat and abiotic stress inducible expression in transgenic Arabidopsis Plants

The small heat shock proteins (sHSPs) have been found to play a critical role in physiological stress conditions in protecting proteins from irreversible aggregation. To characterize the hloroplast targeted sHSP26 promoter in detail, deletion analysis of the promoter is carried out and analysed via transgenics in Arabidopsis. In the present study, complete assessment of the importance of CCAAT-box elements along with Heat shock elements (HSEs) in the promoter of sHSP26 was performed. Moreover, the importance of 5' untranslated region (UTR) has also been established in the promoter via Arabidopsis transgenics. An intense GUS expression was observed after heat stress in the transgenics harbouring a full-length promoter, confirming the heat-stress inducibility of the promoter. Transgenic plants without UTR showed reduced GUS expression when compared to transgenic plants with UTR as was confirmed at the RNA and protein levels by qRT-PCR and GUS histochemical assays, thus suggesting the possible involvement of some regulatory elements present in the UTR in heat-stress inducibility of the promoter. Promoter activity was also checked under different abiotic stresses and revealed differential expression in different deletion constructs. Promoter analysis based on histochemical assay, real-time qPCR and fluorimetric analysis revealed that HSEs alone could not transcribe GUS gene significantly in sHSP26 promoter and CCAAT box elements contribute synergistically to the transcription. Our results also provide insight into the importance of 5`UTR of sHsp26 promoter thus emphasizing the probable role of imperfect CCAAT-box element or some novel cis-element with respect to heat stress.

Expression of a finger millet transcription factor, EcNAC1, in tobacco confers abiotic stress-tolerance

NAC (NAM, ATAF1-2, and CUC2) proteins constitute one of the largest families of plant-specific transcription factors and have been shown to be involved in diverse plant processes including plant growth, development, and stress-tolerance. In this study, a stress-responsive NAC gene, EcNAC1, was isolated from the subtracted stress cDNA library generated from a drought adapted crop, finger millet, and characterized for its role in stress-tolerance. The expression analysis showed that EcNAC1 was highly induced during water-deficit and salt stress. EcNAC1 shares high amino acid similarity with rice genes that have been phylogenetically classified into stress-related NAC genes. Our results demonstrated that tobacco transgenic plants expressing EcNAC1 exhibit tolerance to various abiotic stresses like simulated osmotic stress, by polyethylene glycol (PEG) and mannitol, and salinity stress. The transgenic plants also showed enhanced tolerance to methyl-viologen (MV) induced oxidative stress. Reduced levels of reactive oxygen species (ROS) and ROS-induced damage were noticed in pot grown transgenic lines under water-deficit and natural high light conditions. Root growth under stress and recovery growth after stress alleviation was more in transgenic plants. Many stress-responsive genes were found to be up-regulated in transgenic lines expressing EcNAC1. Our results suggest that EcNAC1 overexpression confers tolerance against abiotic stress in susceptible species, tobacco.

Overexpression of Arabidopsis and rice stress genes' inducible transcription factor confers drought and salinity tolerance to rice

Rice yield is greatly affected by environmental stresses such as drought and salinity. In response to the challenge of producing rice plants tolerant to these stresses, we introduced cDNA encoding the transcription factors DREB1A and DREB1B under the control of the stress inducible rd29 promoter. Two different indica rice cultivars were used, BR29, an improved commercially cultivated variety from Bangladesh and IR68899B, an IRRI bred maintainer line for hybrid rice. Agrobacterium mediated transformation of BR29 was done independently with DREB1A isolated from rice and Arabidopsis and DREB1B isolated from rice, whereas biolistic transformation was done with rice- DREB1B in the case of IR68899B. Initial genetic integration was confirmed by PCR and Southern blot analysis. Salinity tolerance was assayed in very young seedlings. Drought stress tests were found to be more reliable when they were carried out at the pre-flowering booting stage. RNA gel blot analysis as well as quantitative PCR analysis was performed to estimate the transcription level under stressed and unstressed conditions. Agronomic performance studies were done with stressed and unstressed plants to compare the yield losses due to dehydration and salt loading stresses. Noticeably enhanced tolerance to dehydration was observed in the plants transformed with DREB1A isolated from Arabidopsis while DREB1B was found to be more effective for salt tolerance.

Overexpression of Rab16A gene in indica rice variety for generating enhanced salt tolerance

We report here the overexpression of Rab16A full length gene (promoter + ORF), from the salt-tolerant indica rice Pokkali, in the salt-susceptible indica rice variety Khitish, via particle bombardment. Molecular analysis of the transgenics revealed stable integration of the transgene upto T2 generation. High level of expression of the transgene (driven by its own stress-inducible promoter), as well as the protein, was detectable in the leaves under simulated salinity stress (250 mM NaCl, 24 h); the expression level being higher than wild type (WT) plants. The Rab16A transcript also increased gradually with seed maturity, with its maximal accumulation at 30 d after pollination (DAP) i.e., fully matured seeds, explaining the protective role of Rab16A gene during seed maturation. Enhanced tolerance to salinity was observed in the plants transformed with Rab16A. The superior physiological performances of the transgenics under salt treatment were also reflected in lesser shoot or root length inhibition, reduced chlorophyll damages, lesser accumulation of Na(+) and reduced loss of K(+), increased proline content as compared with the WT plants. All these results indicated that the overproduction of RAB16A protein in the transgenics enable them to display enhanced tolerance to salinity stress with improved physiological traits. Our work demonstrates the profound potential of Group 2 LEA proteins (to which RAB16A belongs to) in conferring stress tolerance in crop plants through their genetic manipulation.