Truncated cotton subtilase promoter directs guard cell-specific expression of foreign genes in tobacco and Arabidopsis

Functional characterization of 5'-regulatory region of flavonoid 3',5'-hydroxylase-1 gene of banana plants

Generation of crops with broad-spectrum tolerance to biotic and abiotic stress conditions depends upon availability of genetic elements suitable for varied situations and diverse genotypes. Here, we characterize the 5'-upstream regulatory region of flavonoid 3'5'-hydroxylase-1 (F3'5'H-1) gene from banana and analyzed its tissue-specific and stress-mediated activation in genetic background of tobacco plants. MusaF3'5'H-1 is a stress-responsive gene as its expression is induced in banana after application of salicylic acid and methyl jasmonate while its transcript levels were drastically reduced in response to drought, high salinity and abscisic acid. P_MusaF3'5'H-1 harbours cis-elements associated with stress conditions and those responsible for tissue-specific expression. Transgenic lines harbouring P_MusaF3'5'H-1-GUS displays strong GUS expression in guard cells of stomata indicating guard cell preferred activity of P_MusaF3'5'H-1 while its activity was undetectable in roots. Drought and high salinity induce strong expression of GUS in transgenic tobacco lines and exposure to abscisic acid, salicylic acid and methyl jasmonate revealed distinct profiles of GUS expression in transgenic lines confirming involvement of F3'5'H-1 in plant stress responses. Fluorescent β-galactosidase assay revealed induction profiles of P_MusaF3'5'H-1 at different time points in transgenic lines exposed to salicylic acid and abscisic acid while strong suppression in GUS expression was observed after application of methyl jasmonate. The guard cell preferred activity of P_MusaF3'5'H-1 and stress-mediated expression profiles of MusaF3'5'H-1 indicated the suitability of P_MusaF3'5'H-1 for generating stress-enduring crops and analyzing guard cell functions.

Optimizing the Protein Fluorescence Reporting System for Somatic Embryogenesis Regeneration Screening and Visual Labeling of Functional Genes in Cotton

Protein fluorescence reporting systems are of crucial importance to in-depth life science research, providing systematic labeling tools for visualization of microscopic biological activities in vivo and revolutionizing basic research. Cotton somatic cell regeneration efficiency is low, causing difficulty in cotton transformation. It is conducive to screening transgenic somatic embryo using the fluorescence reporting system. However, available fluorescence labeling systems in cotton are currently limited. To optimize the fluorescence reporting system of cotton with an expanded range of available fluorescent proteins, we selected 11 fluorescent proteins covering red, green, yellow, and cyan fluorescence colors and expressed them in cotton. Besides mRuby2 and G3GFP, the other nine fluorescent proteins (mCherry, tdTomato, sfGFP, Clover, EYFP, YPet, mVenus, mCerulean, and ECFP) were stably and intensely expressed in transgenic callus and embryo, and inherited in different cotton organs derive from the screened embryo. In addition, transgenic cotton expressing tdTomato appears pink under white light, not only for callus and embryo tissues but also various organs of mature plants, providing a visual marker in the cotton genetic transformation process, accelerating the evaluation of transgenic events. Further, we constructed transgenic cotton expressing mCherry-labeled organelle markers in vivo that cover seven specific subcellular compartments: plasma membrane, endoplasmic reticulum, tonoplast, mitochondrion, plastid, Golgi apparatus, and peroxisome. We also provide a simple and highly efficient strategy to quickly determine the subcellular localization of uncharacterized proteins in cotton cells using organelle markers. Lastly, we built the first cotton stomatal fluorescence reporting system using stomata-specific expression promoters (ProKST1, ProGbSLSP, and ProGC1) to drive Clover expression. The optimized fluorescence labeling system for transgenic somatic embryo screening and functional gene labeling in this study offers the potential to accelerating somatic cell regeneration efficiency and the in vivo monitoring of diverse cellular processes in cotton.

The 5'-upstream region of WRKY18 transcription factor from banana is a stress-inducible promoter with strong expression in guard cells

Increasing crop productivity in an ever-changing environmental scenario is a major challenge for maintaining the food supply worldwide. Generation of crops having broad-spectrum pathogen resistance with the ability to cope with water scarcity is the only solution to feed the expanding world population. Stomatal closure has implications on pathogen colonization and drought tolerance. Recent studies have provided novel insights into networks involved in stomatal closure which is being used in biotechnological applications for improving crop endurance. Despite that genetic engineering of stomata requires guard cell preferred or specific regulatory regions to avoid undesirable side effects. In the present study, we describe the 5'-upstream regulatory region of the WRKY18 transcription factor of banana and functionally analyzed its stress meditated activation and strong guard cell preferred activity. Expression of MusaWRKY18 is augmented in leaves of banana cultivars Karibale Monthan, Rasthali and Grand Nain under multiple stress conditions suggesting its role in stress responses of banana plants. Transgenic tobacco lines harboring P_MusaWRKY18 -β-D-glucuronidase (GUS) were regenerated and GUS staining demonstrated substantial GUS expression in guard cells which corroborates with multiple Dof1 binding cis-elements in P_MusaWRKY18 . Fluorescent β-galactosidase assay demonstrated the stress-mediated strong induction profiles of P_MusaWRKY18 at different time points in transgenic tobacco lines exposed to drought, high-salinity, cold, and applications of abscisic acid, salicylic acid, methyl jasmonate, and ethephon. This study sheds novel insights into guard cell preferred expression of WRKY genes under stress and confirm the utility of P_MusaWRKY18 for exploring guard cell functions and guard cell engineering.

A banana NAC transcription factor (MusaSNAC1) impart drought tolerance by modulating stomatal closure and H2O2 content

MusaSNAC1 function in H₂O₂ mediated stomatal closure and promote drought tolerance by directly binding to CGT[A/G] motif in regulatory region of multiple stress-related genes. Drought is a abiotic stress-condition, causing reduced plant growth and diminished crop yield. Guard cells of the stomata control photosynthesis and transpiration by regulating CO₂ exchange and water loss, thus affecting growth and crop yield. Roles of NAC (NAM, ATAF1/2 and CUC2) protein in regulation of stress-conditions has been well documented however, their control over stomatal aperture is largely unknown. In this study we report a banana NAC protein, MusaSNAC1 which induced stomatal closure by elevating H₂O₂ content in guard cells during drought stress. Overexpression of MusaSNAC1 in banana resulted in higher number of stomata closure causing reduced water loss and thus elevated drought-tolerance. During drought, expression of GUS (β-glucuronidase) under P _MusaSNAC1 was remarkably elevated in guard cells of stomata which correlated with its function as a transcription factor regulating stomatal aperture closing. MusaSNAC1 is a transcriptional activator belonging to SNAC subgroup and its 5'-upstream region contain multiple Dof1 elements as well as stress-associated cis-elements. Moreover, MusaSNAC1 also regulate multiple stress-related genes by binding to core site of NAC-proteins CGT[A/G] in their 5'-upstream region. Results indicated an interesting mechanism of drought tolerance through stomatal closure by H₂O₂ generation in guard cells, regulated by a NAC-protein in banana.

The Solanum tuberosum KST1 partial promoter as a tool for guard cell expression in multiple plant species

To date, guard cell promoters have been examined in only a few species, primarily annual dicots. A partial segment of the potato (Solanum tuberosum) KST1 promoter (KST1 partial promoter, KST1ppro) has previously been shown to confer guard cell expression in potato, tomato (Solanum lycopersicum), citrus [Troyer citrange (C. sinensis×Poncirus trifoliata)], and Arabidopsis (Arabidopsis thaliana). Here, we describe an extensive analysis of the expression pattern of KST1ppro in eight (previously reported, as well as new) species from five different angiosperm families, including the Solanaceae and the Cucurbitaceae, Arabidopsis, the monocot barley (Hordeum vulgare), and two perennial species: grapevine (Vitis vinifera) and citrus. Using confocal imaging and three-dimensional movies, we demonstrate that KST1ppro drives guard cell expression in all of these species, making it the first dicot-originated guard cell promoter shown to be active in a monocot and the first promoter reported to confer guard cell expression in barley and cucumber (Cucumis sativus). The results presented here indicate that KST1ppro can be used to drive constitutive guard cell expression in monocots and dicots and in both annual and perennial plants. In addition, we show that the KST1ppro is active in guard cells shortly after the symmetric division of the guard mother cell and generates stable expression in mature guard cells. This allows us to follow the spatial and temporal distribution of stomata in cotyledons and true leaves.

Differential fine-tuning of gene expression regulation in coffee leaves by CcDREB1D promoter haplotypes under water deficit

Despite the importance of the DREB1D gene (also known as CBF4) in plant responses to water deficit and cold stress, studies analysing its regulation by transgenic approaches are lacking. In the current work, a functional study of three CcDREB1D promoter haplotypes (named HP15, HP16 and HP17) isolated from drought-tolerant and drought-sensitive clones of Coffea canephora was carried out in plants of C. arabica stably transformed by Agrobacterium tumefaciens by analysing their ability to regulate the expression of the uidA reporter gene in response to water deficit mimicked by polyethylene glycol (-2.0 MPa) and low relative humidity treatments. A deletion analysis of their corresponding 5'-upstream regions revealed increased specificity of β-glucuronidase activity in the polyethylene glycol and low relative humidity treatments, with high expression in leaf mesophyll and guard cells in full-length constructs. RT-qPCR assays also revealed that the HP16 haplotype (specific to clone tolerant to water deficit) had stronger and earlier activity compared with the HP15 and HP17 haplotypes. As most of the cis-regulatory elements involved in ABA-dependent and -independent networks, tissue specificity and light regulation are common to these haplotypes, we propose that their organization, as well as the nucleic acid polymorphisms present outside these boxes, may play a role in modulating activities of DREB1D promoters in guard cells.

Cytosolic and Nuclear Co-localization of Betalain Biosynthetic Enzymes in Tobacco Suggests that Betalains Are Synthesized in the Cytoplasm and/or Nucleus of Betalainic Plant Cells

Betalains replace anthocyanins as color pigments in most families of Caryophyllales. Unlike anthocyanins, betalains are derived from tyrosine via three enzymatic steps: hydroxylation of L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA; step 1), and conversion of L-DOPA to betalamic acid (step 2), and to cyclo-DOPA (cDOPA; step 3). The principal enzymes responsible for these reactions have been elucidated at the molecular level, but their subcellular localizations have not been explored; hence, the intracellular compartments wherein betalains are biosynthesized remain unknown. Here, we report on the subcellular localization of these principal enzymes. Bioinformatic predictors and N- and C-terminal GFP tagging in transgenic tobacco, showed that Beta vulgaris CYP76AD1 which mediates both steps 1 and 3, DODA1 that catalyzes step 2, and CYP76AD6 which also mediates step 1, were similarly localized to the cytoplasm and nucleus (although the P450s were also weakly present in the endoplasmic reticulum). These two compartments were also the principal locations of Mirabilis jalapa cDOPA5GT. The cytoplasmic and nuclear co-localization of these key enzymes in tobacco suggests that betalains are biosynthesized in the cytoplasm and/or nucleus of betalain-containing plant cells. Elucidation of the subcellular compartmentation of betalain biosynthesis will facilitate the bioengineering of the betalain biosynthetic pathway in non-betalain-containing plants.

Subcellular Localization of a Plant Catalase-Phenol Oxidase, AcCATPO, from Amaranthus and Identification of a Non-canonical Peroxisome Targeting Signal

AcCATPO is a plant catalase-phenol oxidase recently identified from red amaranth. Its physiological function remains unexplored. As the starting step of functional analysis, here we report its subcellular localization and a non-canonical targeting signal. Commonly used bioinformatics programs predicted a peroxisomal localization for AcCATPO, but failed in identification of canonical peroxisomal targeting signals (PTS). The C-terminal GFP tagging led the fusion protein AcCATPO-GFP to the cytosol and the nucleus, but N-terminal tagging directed the GFP-AcCATPO to peroxisomes and nuclei, in transgenic tobacco. Deleting the tripeptide (PTM) at the extreme C-terminus almost ruled out the peroxisomal localization of GFP-AcCATPOΔ3, and removing the C-terminal decapeptide completely excluded peroxisomes as the residence of GFP-AcCATPOΔ10. Furthermore, this decapeptide as a targeting signal could import GFP-10aa to the peroxisome exclusively. Taken together, these results demonstrate that AcCATPO is localized to the peroxisome and the nucleus, and its peroxisomal localization is attributed to a non-canonical PTS1, the C-terminal decapeptide which contains an internal SRL motif and a conserved tripeptide P-S/T-I/M at the extreme of C-terminus. This work may further the study as to the physiological function of AcCATPO, especially clarify its involvement in betalain biosynthesis, and provide a clue to elucidate more non-canonic PTS.

Promoter isolation and characterization of GhAO-like1, a Gossypium hirsutum gene similar to multicopper oxidases that is highly expressed in reproductive organs

Cotton is one of the most economically important cultivated crops. It is the major source of natural fiber for the textile industry and an important target for genetic modification for both biotic stress and herbicide tolerance. Therefore, the characterization of genes and regulatory regions that might be useful for genetic transformation is indispensable. The isolation and characterization of new regulatory regions is of great importance to drive transgene expression in genetically modified crops. One of the major drawbacks in cotton production is pest damage; therefore, the most promising, cost-effective, and sustainable method for pest control is the development of genetically resistant cotton lines. Considering this scenario, our group isolated and characterized the promoter region of a MCO (multicopper oxidase) from Gossypium hirsutum, named GhAO-like1 (ascorbate oxidase-like1). The quantitative expression, together with the in vivo characterization of the promoter region reveals that GhAO-like1 has a flower- and fruit-specific expression pattern. The GUS activity is mainly observed in stamens, as expected considering that the GhAO-like1 regulatory sequence is enriched in cis elements, which have been characterized as a target of reproductive tissue specific transcription factors. Both histological and quantitative analyses in Arabidopsis thaliana have confirmed flower (mainly in stamens) and fruit expression of GhAO-like1. In the present paper, we isolated and characterized both in silico and in vivo the promoter region of the GhAO-like1 gene. The regulatory region of GhAO-like1 might be useful to confer tissue-specific expression in genetically modified plants.

A PPO Promoter from Betalain-Producing Red Swiss Chard, Directs Petiole- and Root-Preferential Expression of Foreign Gene in Anthocyanins-Producing Plants

A 1670 bp 5'-flanking region of the polyphenol oxidase (PPO) gene was isolated from red Swiss chard, a betalain-producing plant. This region, named promoter BvcPPOP, and its 5'-truncated versions were fused with the GUS gene and introduced into Arabidopsis, an anthocyanins-producing plant. GUS histochemical staining and quantitative analysis of transgenic plants at the vegetative and reproductive stages showed that BvcPPOP could direct GUS gene expression in vegetative organs with root- and petiole-preference, but not in reproductive organs including inflorescences shoot, inflorescences leaf, flower, pod and seed. This promoter was regulated by developmental stages in its driving strength, but not in expression pattern. It was also regulated by the abiotic stressors tested, positively by salicylic acid (SA) and methyl jasmonate (MeJA) but negatively by abscisic acid (ABA), gibberellin (GA), NaCl and OH(-). Its four 5'-truncated versions varied in the driving strength, but not obviously in expression pattern, and even the shortest version (-225 to +22) retained the root- and petiole- preference. This promoter is, to our knowledge, the first PPO promoter cloned and functionally elucidated from the betalain-producing plant, and thus provides not only a useful tool for expressing gene(s) of agricultural interest in vegetative organs, but also a clue to clarify the function of metabolism-specific PPO in betalain biosynthesis.

Cloning and characterization of a multifunctional promoter from maize (Zea mays L.)

The use of tissue-specific promoters to drive the expression of target genes during certain developmental stages or in specific organs can prevent unnecessary gene expression caused by constitutive promoters. Utilizing heterologous promoters to regulate the expression of genes in transgenic receptors can help prevent gene silencing. Here, we engineered heterologous maize promoters that regulate gene-specific expression in rice plant receptors. We performed a histochemical and quantitative β-glucuronidase (GUS) analysis of the Zea mays legumin1 (ZM-LEGF) gene promoter and detailed detection of stably transformed rice expressing the GUS gene under the control of the promoter of ZM-LEGF (pZM-LEGF) and its truncated promoters throughout development. When the promoter sequence was truncated, the location and intensity of GUS expression changed. The results suggest that the sequence from -140 to +41 is a critical region that confers the expression of the entire promoter. Truncation of pZM-LEG (3'-deleted region of pZM-LEGF) markedly increased the GUS activity, with the core cis-elements located in the -273 to -140 regions, namely pZM-LEG6. Detailed analysis of pZM-LEG6::GUS T2 transformant rice seeds and plant tissues at different developmental stages indicated that this promoter is an ideal vegetative tissue-specific promoter that can serve as a valuable tool for transgenic rice breeding and genetic engineering studies.