HOMOLOGY-DEPENDENT GENE SILENCING IN PLANTS

Contiguous identity between entire coding regions of transgenic and native genes rather than special regions is essential for a strong co-suppression

The discovery of co-suppression in plants has greatly boosted the study of gene silencing mechanisms, but its triggering mechanism has remained a mystery. In this study, we explored its possible trigger mechanism by using Fatty acid desaturase 2 (FAD2) and Fatty acid elongase 1 (FAE1) strong co-suppression systems. Analysis of small RNAs in FAD2 co-suppression lines showed that siRNAs distributed throughout the coding region of FAD2 with an accumulated peak. However, mutations of the peak siRNA-matched site and siRNA derived site had not alleviated the co-suppression of its transgenic lines. Synthetic FAD2 (AtFAD2sm), which has synonymous mutations in the entire coding region, failed to trigger any co-suppression. Furthermore, 5' and 3' portions of AtFAD2 and AtFAD2sm were swapped to form two hybrid genes, AtFAD2-3sm and AtFAD2-5sm. 80 % and 92 % of their transgenic lines exhibited co-suppression, respectively. Finally, FAE1s with different degrees of the continuous sequence identity compared with AtFAE1 were tested in their Arabidopsis transgenic lines, and the results showed the co-suppression frequency was reduced as their continuous sequence identity stepped down. This work suggests that contiguous identity between the entire coding regions of transgenic and native genes rather than a special region is essential for a strong co-suppression.

Choreographing root architecture and rhizosphere interactions through synthetic biology

Climate change is driving extreme changes to the environment, posing substantial threats to global food security and bioenergy. Given the direct role of plant roots in mediating plant-environment interactions, engineering the form and function of root systems and their associated microbiota may mitigate these effects. Synthetic genetic circuits have enabled sophisticated control of gene expression in microbial systems for years and a surge of advances has heralded the extension of this approach to multicellular plant species. Targeting these tools to affect root structure, exudation, and microbe activity on root surfaces provide multiple strategies for the advancement of climate-ready crops.

Production of functional recombinant roseltide rT1 antimicrobial peptide in tobacco plants

Plant-derived peptides represent a promising group of natural compounds with broad industrial and pharmaceutical applications. Low-efficiency production level is the major obstacle to the commercial production of such bioactive peptides. Today, recombinant techniques have been developed for fast and cost-effective production of high-quality peptides for various applications in the chemical and food industries. The roseltide rT1 is a plant peptide with different antimicrobial properties and therapeutic applications in the prevention and treatment of inflammatory lung diseases by inhibiting human neutrophil elastases. Here, we report the expression of functional recombinant roseltide rT1 peptide in tobacco plants. Transgenic plants were generated by the Agrobacterium-mediated transformation method followed by molecular analysis of transgenic plants to demonstrate successful integration and expression of recombinant rT1 peptide. Protein extracts of transgenic plants expressing a single-copy rT1 gene showed efficient antimicrobial properties as verified by growth inhibition of different bacterial strains. Our results illustrate that plant-derived recombinant rT1 peptide is a promising alternative for rapid and cost-effective production of this important antimicrobial peptide for application in therapeutic and food industries.

Quantitative Investigation of FAD2 Cosuppression Reveals RDR6-Dependent and RDR6-Independent Gene Silencing Pathways

The frequency and extent of transgene-mediated cosuppression varies substantially among plant genes. However, the underlying mechanisms leading to strong cosuppression have received little attention. In previous studies, we showed that the expression of FAD2 in the seeds of Arabidopsis results in strong RDR6-mediated cosuppression, where both endogenous and transgenic FAD2 were silenced. Here, the FAD2 strong cosuppression system was quantitatively investigated to identify the genetic factors by the expression of FAD2 in their mutants. The involvement of DCL2, DCL4, AGO1, and EIN5 was first confirmed in FAD2 cosuppression. SKI2, a remover of 3' end aberrant RNAs, was newly identified as being involved in the cosuppression, while DCL3 was identified as antagonistic to DCL2 and DCL3. FAD2 cosuppression was markedly reduced in dcl2, dcl4, and ago1. The existence of an RDR6-independent cosuppression was revealed for the first time, which was demonstrated by weak gene silencing in rdr6 ein5 ski2. Further investigation of FAD2 cosuppression may unveil unknown genetic factor(s).

Simultaneous EYFP-CENH3/H2B-DsRed Expression Is Impaired Differentially in Meristematic and Differentiated Nuclei of Arabidopsis Double Transformants

Fluorescence live-cell microscopy is important in cell biology to perform artifact-free investigations. To analyze the dynamics of chromatin and centromeres at different stages of the cell cycle in nuclei and chromosomes, we performed simultaneous EYFP-CENH3/H2B-DsRed and single H2B-YFP transformations in Arabidopsis wild-type and cohesin T-DNA mutants. All constructs were under the control of the strong CaMV 35S promoter. While a strong silencing of fluorescence expression occurred differently in leaf and root tissues in the double transformants, nearly all single-transformed wild-type and most mutant cells showed H2B-YFP fluorescence. It seems that for an efficient co-expression of two fluorescence proteins, endogenous promoters and terminators should be used.

A Suite of Constitutive Promoters for Tuning Gene Expression in Plants

The need for convenient tools to express transgenes over a large dynamic range is pervasive throughout plant synthetic biology; however, current efforts are largely limited by the heavy reliance on a small set of strong promoters, precluding more nuanced and refined engineering endeavors in planta. To address this technical gap, we characterize a suite of constitutive promoters that span a wide range of transcriptional levels and develop a GoldenGate-based plasmid toolkit named PCONS, optimized for versatile cloning and rapid testing of transgene expression at varying strengths. We demonstrate how easy access to a stepwise gradient of expression levels can be used for optimizing synthetic transcriptional systems and the production of small molecules in planta. We also systematically investigate the potential of using PCONS as an internal standard in plant biology experimental design, establishing the best practices for signal normalization in experiments. Although our library has primarily been developed for optimizing expression in N. benthamiana, we demonstrate the translatability of our promoters across distantly related species using a multiplexed reporter assay with barcoded transcripts. Our findings showcase the advantages of the PCONS library as an invaluable toolkit for plant synthetic biology.

Synthetic sub-genomic transcript promoter from Horseradish Latent Virus (HRLV)

We characterized an efficient chimeric sub-genomic transcript promoter from Horseradish Latent Virus, FHS4, active in both dicot and monocot plants, and it could be a potential tool for plant biotechnology. Plant pararetroviruses are a rich source of novel plant promoters widely used for biotechnological applications. Here, we comprehensively characterized a unique sub-genomic transcript (Sgt) promoter of Horseradish Latent Virus (HRLV) and identified a fragment (HS4; - 340 to + 10; 351 bp) that showed the highest expression of reporter genes in both transient and transgenic assays as evidenced by biochemical, histochemical GUS reporter assay and transcript analysis of uidA gene by qRT-PCR. Phylogenetic analysis showed that the HSgt promoter was closely related to the sub-genomic promoter of the Cauliflower Mosaic Virus (CaMV19S). We found that the as-1 element and W-box played an important role in the transcriptional activity of the HS4 promoter. Furthermore, the HS4 promoter was also induced by salicylic acid. Alongside, we enhanced the activity of the HS4 promoter by coupling the enhancer region from Figwort Mosaic Virus (FMV) promoter to the upstream region of it. This hybrid promoter FHS4 was around 1.1 times stronger than the most commonly used promoter, 35S (Cauliflower Mosaic Virus full-length transcript promoter), and was efficient in driving reporter genes in both dicot and monocot plants. Subsequently, transgenic tobacco plants expressing an anti-microbial peptide BrLTP2.1 (Brassica rapa lipid transport protein 2.1), under the control of the FHS4 promoter, were developed. The in vitro anti-fungal assay revealed that the plant-derived BrLTP2.1 protein driven by an FHS4 promoter manifested increased resistance against an important plant fungal pathogen, Alternaria alternata. Finally, we concluded that the FHS4 promoter can be used as an alternative to the 35S promoter and has a high potential to become an efficient tool in plant biotechnology.

Functional analysis of soybean cyst nematode-inducible synthetic promoters and their regulation by biotic and abiotic stimuli in transgenic soybean (Glycine max)

We previously identified cis-regulatory motifs in the soybean (Glycine max) genome during interaction between soybean and soybean cyst nematode (SCN), Heterodera glycines. The regulatory motifs were used to develop synthetic promoters, and their inducibility in response to SCN infection was shown in transgenic soybean hairy roots. Here, we studied the functionality of two SCN-inducible synthetic promoters; 4 × M1.1 (TAAAATAAAGTTCTTTAATT) and 4 × M2.3 (ATATAATTAAGT) each fused to the -46 CaMV35S core sequence in transgenic soybean. Histochemical GUS analyses of transgenic soybean plants containing the individual synthetic promoter::GUS construct revealed that under unstressed condition, no GUS activity is present in leaves and roots. While upon nematode infection, the synthetic promoters direct GUS expression to roots predominantly in the nematode feeding structures induced by the SCN and by the root-knot nematode (RKN), Meloidogyne incognita. There were no differences in GUS activity in leaves between nematode-infected and non-infected plants. Furthermore, we examined the specificity of the synthetic promoters in response to various biotic (insect: fall armyworm, Spodoptera frugiperda; and bacteria: Pseudomonas syringe pv. glycinea, P. syringe pv. tomato, and P. marginalis) stresses. Additionally, we examined the specificity to various abiotic (dehydration, salt, cold, wounding) as well as to the signal molecules salicylic acid (SA), methyl jasmonate (MeJA), and abscisic acid (ABA) in the transgenic plants. Our wide-range analyses provide insights into the potential applications of synthetic promoter engineering for conditional expression of transgenes leading to transgenic crop development for resistance improvement in plant.

Current Status and Perspectives on the Application of CRISPR/Cas9 Gene-Editing System to Develop a Low-Gluten, Non-Transgenic Wheat Variety

Wheat gluten contains epitopes that trigger celiac disease (CD). A life-long strict gluten-free diet is the only treatment accepted for CD. However, very low-gluten wheat may provide an alternative treatment to CD. Conventional plant breeding methods have not been sufficient to produce celiac-safe wheat. RNA interference technology, to some extent, has succeeded in the development of safer wheat varieties. However, these varieties have multiple challenges in terms of their implementation. Clustered Regularly Interspaced Short Palindromic Repeats-associated nuclease 9 (CRISPR/Cas9) is a versatile gene-editing tool that has the ability to edit immunogenic gluten genes. So far, only a few studies have applied CRISPR/Cas9 to modify the wheat genome. In this article, we reviewed the published literature that applied CRISPR/Cas9 in wheat genome editing to investigate the current status of the CRISPR/Cas9 system to produce a low-immunogenic wheat variety. We found that in recent years, the CRISPR/Cas9 system has been continuously improved to edit the complex hexaploid wheat genome. Although some reduced immunogenic wheat varieties have been reported, CRISPR/Cas9 has still not been fully explored in terms of editing the wheat genome. We conclude that further studies are required to apply the CRISPR/Cas9 gene-editing system efficiently for the development of a celiac-safe wheat variety and to establish it as a "tool to celiac safe wheat".

Bioengineering potato plants to produce benzylglucosinolate for improved broad-spectrum pest and disease resistance

In traditional, small-scale agriculture in the Andes, potatoes are frequently co-cultivated with the Andean edible tuber Tropaeolum tuberosum, commonly known as mashua, which is believed to exert a pest and disease protective role due to its content of the phenylalanine-derived benzylglucosinolate (BGLS). We bioengineered the production of BGLS in potato by consecutive generation of stable transgenic events with two polycistronic constructs encoding for expression of six BGLS biosynthetic genes from Arabidopsis thaliana. First, we integrated a polycistronic construct coding for the last three genes of the pathway (SUR1, UGT74B1 and SOT16) into potato driven by the cauliflower mosaic virus 35S promoter. After identifying the single-insertion transgenic event with the highest transgene expression, we stacked a second polycistronic construct coding for the first three genes in the pathway (CYP79A2, CYP83B1 and GGP1) driven by the leaf-specific promoter of the rubisco small subunit from chrysanthemum. We obtained transgenic events producing as high as 5.18 pmol BGLS/mg fresh weight compared to the non-transgenic potato plant producing undetectable levels of BGLS. Preliminary bioassays suggest a possible activity against Phytophthora infestans, causing the late blight disease and Premnotrypes suturicallus, referred to as the Andean potato weevil. However, we observed altered leaf morphology, abnormally thick and curlier leaves, reduced growth and tuber production in five out of ten selected transgenic events, which indicates that the expression of BGLS biosynthetic genes has an undesirable impact on the potato. Optimization of the expression of the BGLS biosynthetic pathway in potato is required to avoid alterations of plant development.

Reduction of Allergenic Potential in Bread Wheat RNAi Transgenic Lines Silenced for CM3, CM16 and 0.28 ATI Genes

Although wheat is used worldwide as a staple food, it can give rise to adverse reactions, for which the triggering factors have not been identified yet. These reactions can be caused mainly by kernel proteins, both gluten and non-gluten proteins. Among these latter proteins, α-amylase/trypsin inhibitors (ATI) are involved in baker’s asthma and realistically in Non Celiac Wheat Sensitivity (NCWS). In this paper, we report characterization of three transgenic lines obtained from the bread wheat cultivar Bobwhite silenced by RNAi in the three ATI genes CM3, CM16 and 0.28. We have obtained transgenic lines showing an effective decrease in the activity of target genes that, although showing a higher trypsin inhibition as a pleiotropic effect, generate a lower reaction when tested with sera of patients allergic to wheat, accounting for the important role of the three target proteins in wheat allergies. Finally, these lines show unintended differences in high molecular weight glutenin subunits (HMW-GS) accumulation, involved in technological performances, but do not show differences in terms of yield. The development of new genotypes accumulating a lower amount of proteins potentially or effectively involved in allergies to wheat and NCWS, not only offers the possibility to use them as a basis for the production of varieties with a lower impact on adverse reaction, but also to test if these proteins are actually implicated in those pathologies for which the triggering factor has not been established yet.

Recent Advances of In Vitro Culture for the Application of New Breeding Techniques in Citrus

Citrus is one of the most important fruit crops in the world. This review will discuss the recent findings related to citrus transformation and regeneration protocols of juvenile and adult explants. Despite the many advances that have been made in the last years (including the use of inducible promoters and site-specific recombination systems), transformation efficiency, and regeneration potential still represent a bottleneck in the application of the new breeding techniques in commercial citrus varieties. The influence of genotype, explant type, and other factors affecting the regeneration and transformation of the most used citrus varieties will be described, as well as some examples of how these processes can be applied to improve fruit quality and resistance to various pathogens and pests, including the potential of using genome editing in citrus. The availability of efficient regeneration and transformation protocols, together with the availability of the source of resistance, is made even more important in light of the fast diffusion of emerging diseases, such as Huanglongbing (HLB), which is seriously challenging citriculture worldwide.

CaLecRK-S.5, a pepper L-type lectin receptor kinase gene, accelerates Phytophthora elicitin-mediated defense response

Innate immunity in plants relies on the recognition of pathogen-associated molecular patterns (PAMPs) by pattern-recognition receptors (PRRs) located on the plant cell surface. CaLecRK-S.5, a pepper L-type lectin receptor kinase, has been shown to confer broad-spectrum resistance through priming activation. To further elucidate the molecular mechanism of CaLecRK-S.5, transgenic tobacco plants were generated in this study. Interestingly, hemizygous transgenic plants exhibited a high accumulation of CaLecRK-S.5, but this accumulation was completely abolished in homozygous transgenic plants by a cosuppression mechanism. Gain-of-function and loss-of-function analyses revealed that CaLecRK-S.5 plays a positive role in Phytophthora elicitin-mediated defense responses.

Characterizing standard genetic parts and establishing common principles for engineering legume and cereal roots

Plant synthetic biology and cereal engineering depend on the controlled expression of transgenes of interest. Most engineering in plant species to date has relied heavily on the use of a few, well-established constitutive promoters to achieve high levels of expression; however, the levels of transgene expression can also be influenced by the use of codon optimization, intron-mediated enhancement and varying terminator sequences. Most of these alternative approaches for regulating transgene expression have only been tested in small-scale experiments, typically testing a single gene of interest. It is therefore difficult to interpret the relative importance of these approaches and to design engineering strategies that are likely to succeed in different plant species, particularly if engineering multigenic traits where the expression of each transgene needs to be precisely regulated. Here, we present data on the characterization of 46 promoters and 10 terminators in Medicago truncatula, Lotus japonicus, Nicotiana benthamiana and Hordeum vulgare, as well as the effects of codon optimization and intron-mediated enhancement on the expression of two transgenes in H. vulgare. We have identified a core set of promoters and terminators of relevance to researchers engineering novel traits in plant roots. In addition, we have shown that combining codon optimization and intron-mediated enhancement increases transgene expression and protein levels in barley. Based on our study, we recommend a core set of promoters and terminators for broad use and also propose a general set of principles and guidelines for those engineering cereal species.

Groundnut AhcAPX conferred abiotic stress tolerance in transgenic banana through modulation of the ascorbate-glutathione pathway

A stress inducible cytosolic ascorbate peroxidase gene (AhcAPX) was ectopically expressed in banana (cv. Grand naine) plants to strengthen their antioxidant capacity. High level of AhcAPX gene transcripts and enzyme suggested constitutive and functional expression of candidate gene in transgenic (TR) plants. The tolerance level of in vitro and in vivo grown TR banana plantlets were assessed against salt and drought stress. The TR banana plants conferred tolerance against the abiotic stresses by maintaining a high redox state of ascorbate and glutathione, which correlated with lower accumulation of H₂O₂, O₂ ^⋅- and higher level of antioxidant enzyme (SOD, APX, CAT, GR, DHAR and MDHAR) activities. The efficacy of AhcAPX over-expression was also investigated in terms of different physiochemical attributes of TR and untransformed control plants, such as, proline content, membrane stability, electrolyte leakage and chlorophyll retention. The TR plants showed higher photochemical efficiency of PSII (Fv/Fm), and stomatal attributes under photosynthesis generated reactive oxygen species (ROS) stress. The outcome of present investigation suggest that ectopic expression of AhcAPX gene in banana enhances the tolerance to drought and salt stress by annulling the damage caused by ROS.

Functional characterization of a strong promoter of the early light-inducible protein gene from tomato

The tomato ELIP gene promoter is mainly active in the ripening fruit. Considering its high activity, the promoter could be used for molecular breeding of plants in the future. The ability to obtain new varieties of transgenic plants with economically valuable traits relies on a high level of target gene expression, which is largely controlled by a gene promoter. Hence, research aimed at finding and characterizing new tissue-specific promoters that direct gene expression in specific plant tissues or at certain developmental stages has become the most important field of plant biotechnology. Here, we cloned and characterized the promoter of the early light-inducible protein (ELIP) gene from tomato (Solanum lycopersicum cv. Yalf). ELIPs are produced in the presence of light and putatively function in the chloroplast-to-chromoplast conversion, playing a photorepairing role in the photosynthetic system. Analysis of the promoter sequence revealed multiple cis-acting elements related to light responsiveness, and other motifs involved in plant hormone response and circadian control. To determine the functionality of the promoter, seven 5'-deletion variants were fused with the β-glucuronidase (GUS) reporter gene and introduced into tomato. Histochemical analysis of transgenic tomato plants revealed different levels of GUS activity in most analyzed tissues, depending on the promoter fragment used. The intensity of staining was considerably higher in ripening fruits than in unripe and non-fruit tissues. Quantitative analysis indicated that the level of GUS activity with the longest (full-length) version of the ELIP promoter in ripened fruits was comparable to that in plants expressing the constitutive CaMV35S promoter. Further, the location of both negative and positive regulatory motifs was identified. The described ELIP promoter is a potential tool for various applications in plant biotechnology.

Exogenous Promoter Triggers APETALA3 Silencing through RNA-Directed DNA Methylation Pathway in Arabidopsis

The development of floral organs plays a vital role in plant reproduction. In our research, the APETALA3 (AP3) promoter-transgenic lines showed abnormal developmental phenotypes in stamens and petals. The aim of this study is to understand the molecular mechanisms of the morphological defects in transgenic plants. By performing transgenic analysis, it was found that the AP3-promoted genes and the vector had no relation to the morphological defects. Then, we performed the expression analysis of the class A, B, and C genes. A dramatic reduction of transcript levels of class B genes (AP3 and PISTILLATA) was observed. Additionally, we also analyzed the methylation of the promoters of class B genes and found that the promoter of AP3 was hypermethylated. Furthermore, combining mutations in rdr2-2, drm1/2, and nrpd1b-11 with the AP3-silencing lines rescued the abnormal development of stamens and petals. The expression of AP3 was reactivated and the methylation level of AP3 promoter was also reduced in RdDM-defective AP3-silencing lines. Our results showed that the RdDM pathway contributed to the transcriptional silencing in the transgenic AP3-silencing lines. Moreover, the results revealed that fact that the exogenous fragment of a promoter could trigger the methylation of homologous endogenous sequences, which may be ubiquitous in transgenic plants.

Generation of a selectable marker free, highly expressed single copy locus as landing pad for transgene stacking in sugarcane

A selectable marker free, highly expressed single copy locus flanked by insulators was created as landing pad for transgene stacking in sugarcane. These events displayed superior transgene expression compared to single-copy transgenic lines lacking insulators. Excision of the selectable marker gene from transgenic sugarcane lines was supported by FLPe/FRT site-specific recombination. Sugarcane, a tropical C4 grass in the genus Saccharum (Poaceae), accounts for nearly 80% of sugar produced worldwide and is also an important feedstock for biofuel production. Generating transgenic sugarcane with predictable and stable transgene expression is critical for crop improvement. In this study, we generated a highly expressed single copy locus as landing pad for transgene stacking. Transgenic sugarcane lines with stable integration of a single copy nptII expression cassette flanked by insulators supported higher transgene expression along with reduced line to line variation when compared to single copy events without insulators by NPTII ELISA analysis. Subsequently, the nptII selectable marker gene was efficiently excised from the sugarcane genome by the FLPe/FRT site-specific recombination system to create selectable marker free plants. This study provides valuable resources for future gene stacking using site-specific recombination or genome editing tools.

Optimizing photorespiration for improved crop productivity

In C3 plants, photorespiration is an energy-expensive process, including the oxygenation of ribulose-1,5-bisphosphate (RuBP) by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the ensuing multi-organellar photorespiratory pathway required to recycle the toxic byproducts and recapture a portion of the fixed carbon. Photorespiration significantly impacts crop productivity through reducing yields in C3 crops by as much as 50% under severe conditions. Thus, reducing the flux through, or improving the efficiency of photorespiration has the potential of large improvements in C3 crop productivity. Here, we review an array of approaches intended to engineer photorespiration in a range of plant systems with the goal of increasing crop productivity. Approaches include optimizing flux through the native photorespiratory pathway, installing non-native alternative photorespiratory pathways, and lowering or even eliminating Rubisco-catalyzed oxygenation of RuBP to reduce substrate entrance into the photorespiratory cycle. Some proposed designs have been successful at the proof of concept level. A plant systems-engineering approach, based on new opportunities available from synthetic biology to implement in silico designs, holds promise for further progress toward delivering more productive crops to farmer's fields.

Molecular farming of antimicrobial peptides: available platforms and strategies for improving protein biosynthesis using modified virus vectors

The constant demand for new antibiotic drugs has driven efforts by the scientific community to prospect for peptides with a broad spectrum of action. In this context, antimicrobial peptides (AMPs) have acquired great scientific importance in recent years due to their ability to possess antimicrobial and immunomodulatory activity. In the last two decades, plants have attracted the interest of the scientific community and industry as regards their potential as biofactories of heterologous proteins. One of the most promising approaches is the use of viral vectors to maximize the transient expression of drugs in the leaves of the plant Nicotiana benthamiana. Recently, the MagnifectionTM expression system was launched. This sophisticated commercial platform allows the assembly of the viral particle in leaf cells and the systemic spread of heterologous protein biosynthesis in green tissues caused by Agrobacterium tumefaciens "gene delivery method". The system also presents increased gene expression levels mediated by potent viral expression machinery. These characteristics allow the mass recovery of heterologous proteins in the leaves of N. benthamiana in 8 to 10 days. This system was highly efficient for the synthesis of different classes of pharmacological proteins and contains enormous potential for the rapid and abundant biosynthesis of AMPs.

Isolation, characterization, and evaluation of three Citrus sinensis-derived constitutive gene promoters

Regulatory sequences from the citrus constitutive genes cyclophilin (CsCYP), glyceraldehyde-3-phosphate dehydrogenase C2 (CsGAPC2), and elongation factor 1-alpha (CsEF1) were isolated, fused to the uidA gene, and qualitatively and quantitatively evaluated in transgenic sweet orange plants. The 5' upstream region of a gene (the promoter) is the most important component for the initiation and regulation of gene transcription of both native genes and transgenes in plants. The isolation and characterization of gene regulatory sequences are essential to the development of intragenic or cisgenic genetic manipulation strategies, which imply the use of genetic material from the same species or from closely related species. We describe herein the isolation and evaluation of the promoter sequence from three constitutively expressed citrus genes: cyclophilin (CsCYP), glyceraldehyde-3-phosphate dehydrogenase C2 (CsGAPC2), and elongation factor 1-alpha (CsEF1). The functionality of the promoters was confirmed by a histochemical GUS assay in leaves, stems, and roots of stably transformed citrus plants expressing the promoter-uidA construct. Lower uidA mRNA levels were detected when the transgene was under the control of citrus promoters as compared to the expression under the control of the CaMV35S promoter. The association of the uidA gene with the citrus-derived promoters resulted in mRNA levels of up to 60-41.8% of the value obtained with the construct containing CaMV35S driving the uidA gene. Moreover, a lower inter-individual variability in transgene expression was observed amongst the different transgenic lines, where gene constructs containing citrus-derived promoters were used. In silico analysis of the citrus-derived promoter sequences revealed that their activity may be controlled by several putative cis-regulatory elements. These citrus promoters will expand the availability of regulatory sequences for driving gene expression in citrus gene-modification programs.

Conferring virus resistance in tomato by independent RNA silencing of three tomato homologs of Arabidopsis TOM1

The TOM1/TOM3 genes from Arabidopsis are involved in the replication of tobamoviruses. Tomato homologs of these genes, LeTH1, LeTH2 and LeTH3, are known. In this study, we examined transgenic tomato lines where inverted repeats of either LeTH1, LeTH2 or LeTH3 were introduced by Agrobacterium. Endogenous mRNA expression for each gene was detected in non-transgenic control plants, whereas a very low level of each of the three genes was found in the corresponding line. Small interfering RNA was detected in the transgenic lines. Each silenced line showed similar levels of tobamovirus resistance, indicating that each gene is similarly involved in virus replication.

Expression of a novel bi-directional Brassica napus promoter in soybean

The expression profile of a natural bi-directional promoter, derived from the Brassica napus EPSPS-A gene, was studied in transgenic soybean (Glycine max C.V. Maverick) lines. Two constructs, pDAB100331 and pDAB100333, were assembled to test the bi-directionality of the promoter. Two reporter genes, gfp and gusA, were employed and they were interchangeably placed in both constructs, one on each end of the promoter such that both proteins expressed divergently in each construct. In the T0 generation, GUS expression was more uniform throughout the leaf of pDAB100333 transgenic plants, where the gusA gene was expressed from the downstream or EPSPS-A end of the bi-directional promoter. Comparatively, GUS expression was more localized in the midrib and veins of the leaf of pDAB100331 transgenic plants, where the gusA gene was expressed from the upstream end of the bi-directional promoter. These observations indicated a unique expression pattern from each end of the promoter and consistently higher expression in genes expressed from the downstream end (e.g., EPSPS-A end) of the promoter in the tissues examined. The GFP expression pattern followed that of GUS when placed in the same position relative to the promoter. In the T1 generation, transcript analysis also showed higher expression of both gusA and gfp when those genes were located at the downstream end of the promoter. Accordingly, the pDAB100331 events exhibited a higher gfp/gusA transcript ratio, while pDAB100333 events produced a higher gusA/gfp transcript ratio consistent with the observations in T0 plants. These results demonstrated that the EPSPS-A gene bidirectional promoter can be effectively utilized to drive expression of two transgenes for the desired traits.

Developing transgenic wheat to encounter rusts and powdery mildew by overexpressing barley chi26 gene for fungal resistance

BACKGROUND

The main aim of this study was to improve fungal resistance in bread wheat via transgenesis. Transgenic wheat plants harboring barley chitinase (chi26) gene, driven by maize ubi promoter, were obtained using biolistic bombardment, whereas the herbicide resistance gene, bar, driven by the CaMV 35S promoter was used as a selectable marker.

RESULTS

Molecular analysis confirmed the integration, copy number, and the level of expression of the chi26 gene in four independent transgenic events. Chitinase enzyme activity was detected using a standard enzymatic assay. The expression levels of chi26 gene in the different transgenic lines, compared to their respective controls, were determined using qRT-PCR. The transgene was silenced in some transgenic families across generations. Gene silencing in the present study seemed to be random and irreversible. The homozygous transgenic plants of T4, T5, T6, T8, and T9 generations were tested in the field for five growing seasons to evaluate their resistance against rusts and powdery mildew. The results indicated high chitinase activity at T0 and high transgene expression levels in few transgenic families. This resulted in high resistance against wheat rusts and powdery mildew under field conditions. It was indicated by proximate and chemical analyses that one of the transgenic families and the non-transgenic line were substantially equivalent.

CONCLUSION

Transgenic wheat with barley chi26 was found to be resistant even after five generations under artificial fungal infection conditions. One transgenic line was proved to be substantially equivalent as compared to the non-transgenic control.

Towards combinatorial transcriptional engineering

The modular nature of the transcriptional unit makes it possible to design robust modules with predictable input-output characteristics using a ‘parts- off a shelf’ approach. Customized regulatory circuits composed of multiple such transcriptional units have immense scope for application in diverse fields of basic and applied research. Synthetic transcriptional engineering seeks to construct such genetic cascades. Here, we discuss the three principle strands of transcriptional engineering: promoter and transcriptional factor engineering, and programming inducibilty into synthetic modules. In this context, we review the scope and limitations of some recent technologies that seek to achieve these ends. Our discussion emphasizes a requirement for rational combinatorial engineering principles and the promise this approach holds for the future development of this field.

The pharmaceutics from the foreign empire: the molecular pharming of the prokaryotic staphylokinase in Arabidopsis thaliana plants

Here, we present the application of microbiology and biotechnology for the production of recombinant pharmaceutical proteins in plant cells. To the best of our knowledge and belief it is one of few examples of the expression of the prokaryotic staphylokinase (SAK) in the eukaryotic system. Despite the tremendous progress made in the plant biotechnology, most of the heterologous proteins still accumulate to low concentrations in plant tissues. Therefore, the composition of expression cassettes to assure economically feasible level of protein production in plants remains crucial. The aim of our research was obtaining a high concentration of the bacterial anticoagulant factor—staphylokinase, in Arabidopsis thaliana seeds. The coding sequence of staphylokinase was placed under control of the β-phaseolin promoter and cloned between the signal sequence of the seed storage protein 2S2 and the carboxy-terminal KDEL signal sequence. The engineered binary vector pATAG-sak was introduced into Arabidopsis thaliana plants via Agrobacterium tumefaciens-mediated transformation. Analysis of the subsequent generations of Arabidopsis seeds revealed both presence of the sak and nptII transgenes, and the SAK protein. Moreover, a plasminogen activator activity of staphylokinase was observed in the protein extracts from seeds, while such a reaction was not observed in the leaf extracts showing seed-specific activity of the β-phaseolin promoter.

Green tissue-specific co-expression of chitinase and oxalate oxidase 4 genes in rice for enhanced resistance against sheath blight

Green tissue-specific simultaneous overexpression of two defense-related genes ( OsCHI11 & OsOXO4 ) in rice leads to significant resistance against sheath blight pathogen ( R. solani ) without distressing any agronomically important traits. Overexpressing two defense-related genes (OsOXO4 and OsCHI11) cloned from rice is effective at enhancing resistance against sheath blight caused by Rhizoctonia solani. These genes were expressed under the control of two different green tissue-specific promoters, viz. maize phosphoenolpyruvate carboxylase gene promoter, PEPC, and rice cis-acting 544-bp DNA element, immediately upstream of the D54O translational start site, P D54O-544 . Putative T0 transgenic rice plants were screened by PCR and integration of genes was confirmed by Southern hybridization of progeny (T1) rice plants. Successful expression of OsOXO4 and OsCHI11 in all tested plants was confirmed. Expression of PR genes increased significantly following pathogen infection in overexpressing transgenic plants. Following infection, transgenic plants exhibited elevated hydrogen peroxide levels, significant changes in activity of ROS scavenging enzymes and reduced membrane damage when compared to their wild-type counterpart. In a Rhizoctonia solani toxin assay, a detached leaf inoculation test and an in vivo plant bioassay, transgenic plants showed a significant reduction in disease symptoms in comparison to non-transgenic control plants. This is the first report of overexpression of two different PR genes driven by two green tissue-specific promoters providing enhanced sheath blight resistance in transgenic rice.

The Fate of Integrated Ri T-DNA rol Genes during Regeneration via Somatic Embryogenesis in Tylophora indica

The fate of integrated Ri T-DNA rol genes during regeneration via indirect somatic embryogenesis and stability of its effect on morphology and tylophorine content of Ri-transformed plants have been studied in Tylophora indica. Integration and expression of Ri T-DNA genes in transformed embryogenic callus lines derived from transformed root lines, 300 Ri-transformed somatic embryos, and 23 Ri-transformed plant lines were analysed. Fifty root lines studied showed integration and expression of four rol genes of TL-DNA. Spontaneous regeneration via indirect somatic embryogenesis was obtained from root lines that were TL+/TR−. Stable integration and expression of rol genes were observed in root lines, embryogenic callus lines, and the spontaneously induced somatic embryos. Nineteen out of the 23 Ri-transformed plant lines and their clones showed phenotypic and genetic stability over the period of 3 years. Four Ri-transformed plants were morphologically similar to nontransformed plants but showed variation with the integration and expression of the rolA gene and absence of other rol genes. Variant Ri-transformed plant line A428#1-V showed highest tylophorine content (2.93±0.03 mg gDW−1) among plant lines studied. The effects of T-DNA genes on growth, morphology, and tylophorine content of the Ri-transformed plants were stable in the long term culture.

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.

Transgenic Citrus Expressing an Arabidopsis NPR1 Gene Exhibit Enhanced Resistance against Huanglongbing (HLB; Citrus Greening)

Commercial sweet orange cultivars lack resistance to Huanglongbing (HLB), a serious phloem limited bacterial disease that is usually fatal. In order to develop sustained disease resistance to HLB, transgenic sweet orange cultivars ‘Hamlin’ and ‘Valencia’ expressing an Arabidopsis thaliana NPR1 gene under the control of a constitutive CaMV 35S promoter or a phloem specific Arabidopsis SUC2 (AtSUC2) promoter were produced. Overexpression of AtNPR1 resulted in trees with normal phenotypes that exhibited enhanced resistance to HLB. Phloem specific expression of NPR1 was equally effective for enhancing disease resistance. Transgenic trees exhibited reduced diseased severity and a few lines remained disease-free even after 36 months of planting in a high-disease pressure field site. Expression of the NPR1 gene induced expression of several native genes involved in the plant defense signaling pathways. The AtNPR1 gene being plant derived can serve as a component for the development of an all plant T-DNA derived consumer friendly GM tree.

Epigenetic silencing in transgenic plants

Epigenetic silencing is a natural phenomenon in which the expression of genes is regulated through modifications of DNA, RNA, or histone proteins. It is a mechanism for defending host genomes against the effects of transposable elements and viral infection, and acts as a modulator of expression of duplicated gene family members and as a silencer of transgenes. A major breakthrough in understanding the mechanism of epigenetic silencing was the discovery of silencing in transgenic tobacco plants due to the interaction between two homologous promoters. The molecular mechanism of epigenetic mechanism is highly complicated and it is not completely understood yet. Two different molecular routes have been proposed for this, that is, transcriptional gene silencing, which is associated with heavy methylation of promoter regions and blocks the transcription of transgenes, and post-transcriptional gene silencing (PTGS), the basic mechanism is degradation of the cytosolic mRNA of transgenes or endogenous genes. Undesired transgene silencing is of major concern in the transgenic technologies used in crop improvement. A complete understanding of this phenomenon will be very useful for transgenic applications, where silencing of specific genes is required. The current status of epigenetic silencing in transgenic technology is discussed and summarized in this mini-review.

Strategies to improve low copy transgenic events in Agrobacterium-mediated transformation of maize

Transgenic plants containing low copy transgene insertion free of vector backbone are highly desired for many biotechnological applications. We have investigated two different strategies for increasing the percentage of low copy events in Agrobacterium-mediated transformation experiments in maize. One of the strategies is to use a binary vector with two separate T-DNAs, one T-DNA containing an intact E.coli manA gene encoding phosphomannose isomerase (PMI) as selectable marker gene cassette and another T-DNA containing an RNAi cassette of PMI sequences. By using this strategy, low copy transgenic events containing the transgenes were increased from 43 to 60 % in maize. An alternate strategy is using selectable marker gene cassettes containing regulatory or coding sequences derived from essential plant genes such as 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) or MADS box transcription factor. In this paper we demonstrate that higher percentage of low copy transgenic events can be obtained in Agrobacterium-mediated maize transformation experiments using both strategies. We propose that the above two strategies can be used independently or in combination to increase transgenic events that contain low copy transgene insertion in Agrobacterium-mediated transformation experiments.

Production of polyhydroxybutyrate in oil palm (Elaeis guineensis Jacq.) mediated by microprojectile bombardment of PHB biosynthesis genes into embryogenic calli

Biodegradable plastics, mainly polyhydroxybutyrate (PHB), which are traditionally produced by bacterial cells, have been produced in the cells of more than 15 plant species. Since the production of biodegradable plastics and the synthesis of oil in plants share the same substrate, acetyl-coenzyme A (acetyl-CoA), producing PHB in oil bearing crops, such as oil palm, will be advantageous. In this study, three bacterial genes, bktB, phaB, and phaC, which are required for the synthesis of PHB and selectable marker gene, bar, for herbicide Basta resistant, were transformed into embryogenic calli. A number of transformed embryogenic lines resistant to herbicide Basta were obtained and were later regenerated to produce few hundred plantlets. Molecular analyses, including polymerase chain reaction (PCR), Southern blot, and real-time PCR have demonstrated stable integration and expression of the transgenes in the oil palm genome. HPLC and Nile blue A staining analyses confirmed the synthesis of PHB in some of the plantlets.

The activities of four constitutively expressed promoters in single-copy transgenic rice plants for two homozygous generations

We have characterized four novel constitutive promoters ARP1, H3F3, HSP and H2BF3 that are active in all tissues/stages of transgenic plants and stable over two homozygous generations. Gene promoters that are active and stable over several generations in transgenic plants are valuable tools for plant research and biotechnology. In this study, we characterized four putative constitutive promoters (ARP1, H3F3, HSP and H2BF3) in transgenic rice plants. Promoter regions were fused to the green fluorescence protein (GFP) reporter gene and transformed into rice. Single-copy transgenic lines were then selected and promoter activity was analyzed in various organs and tissues of two successive homozygous generations. All four promoters showed a broad expression profile in most tissues and developmental stages, and indeed the expression of the ARP1 and H3F3 promoters was even greater than that of the PGD1 promoter, a previously described constitutive promoter that has been used in transgenic rice. This observation was based on expression levels in leaves, roots, dry seeds and flowers in both the T2 and T3 generations. Each promoter exhibited comparable levels of activity over two homozygous generations with no sign of transgene silencing, which is an important characteristic of promoters to be used in crop biotechnology applications. These promoters therefore have considerable potential value for the stable and constitutive expression of transgenes in monocotyledonous crops.

Maize transformation technology development for commercial event generation

Maize is an important food and feed crop in many countries. It is also one of the most important target crops for the application of biotechnology. Currently, there are more biotech traits available on the market in maize than in any other crop. Generation of transgenic events is a crucial step in the development of biotech traits. For commercial applications, a high throughput transformation system producing a large number of high quality events in an elite genetic background is highly desirable. There has been tremendous progress in Agrobacterium-mediated maize transformation since the publication of the Ishida et al. (1996) paper and the technology has been widely adopted for transgenic event production by many labs around the world. We will review general efforts in establishing efficient maize transformation technologies useful for transgenic event production in trait research and development. The review will also discuss transformation systems used for generating commercial maize trait events currently on the market. As the number of traits is increasing steadily and two or more modes of action are used to control key pests, new tools are needed to efficiently transform vectors containing multiple trait genes. We will review general guidelines for assembling binary vectors for commercial transformation. Approaches to increase transformation efficiency and gene expression of large gene stack vectors will be discussed. Finally, recent studies of targeted genome modification and transgene insertion using different site-directed nuclease technologies will be reviewed.

Artificial trans-acting small interfering RNA: a tool for plant biology study and crop improvements

Completion of whole genome sequencing in many plant species including economically important crop species not only opens up new opportunities but also imposes challenges for plant science research community. Functional validation and utilization of these enormous DNA sequences necessitate new or improved tools with high accuracy and efficiency. Of various tools, small RNA-mediated gene silencing platform plays an important and unique role in functional verification of plant genes and trait improvements. Artificial trans-acting small interfering RNA (atasiRNA) has emerged as a potent and specific gene silencing platform which overcomes major limitations of other small RNA silencing approaches including double-stranded RNA, artificial microRNA (amiRNA), and microRNA-induced gene silencing. To best utilize atasiRNA platform, it is essential to be able to test candidate atasiRNAs efficiently through either in vivo or in vitro validation approach. Very recently, a breakthrough has been made in developing a new method for in vitro screen of amiRNA candidates, named "epitope-tagged protein-based amiRNA screens". Such a screen can be readily employed to validate atasiRNA candidates and thus accelerate the deployment of atasiRNA technology. Therefore, atasiRNA as an emerging tool shall accelerate both plant biology study and crop genetic improvements including trait stacking.

Multiple host-cell recombination pathways act in Agrobacterium-mediated transformation of plant cells

Using floral-dip, tumorigenesis and root callus transformation assays of both germline and somatic cells, we present here results implicating the four major non-homologous and homologous recombination pathways in Agrobacterium-mediated transformation of Arabidopsis thaliana. All four single mutant lines showed similar mild reductions in transformability, but knocking out three of four pathways severely compromised Agrobacterium-mediated transformation. Although integration of T-DNA into the plant genome is severely compromised in the absence of known DNA double-strand break repair pathways, it does still occur, suggesting the existence of other pathways involved in T-DNA integration. Our results highlight the functional redundancy of the four major plant recombination pathways in transformation, and provide an explanation for the lack of strong effects observed in previous studies on the roles of plant recombination functions in transformation.

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.

Inhibition of TMV multiplication by siRNA constructs against TOM1 and TOM3 genes of Capsicum annuum

The host proteins TOM1 and TOM3 associated with tonoplast membrane are shown to be required for efficient multiplication of Tobamoviruses. In this study, homologous of TOM1 and TOM3 genes were identified in pepper (Capsicum annuum) using specific primers. Their gene sequences have similarity to Nicotiana tabacum NtTOM1 and NtTOM3. Sequence alignment showed that CaTOM1 and CaTOM3 are closely related to TOM1 and TOM3 of N. tabacum and Solanum lycopersicum with 90% and 70% nucleotide sequence identities, respectively. RNA interference approach was used to suppress the TOM1 and TOM3 gene expression which in turn prevented Tobacco mosaic virus replication in tobacco. Nicotiana plants agro-infiltrated with siRNA constructs of TOM1 or TOM3 showed no mosaic or necrotic infection symptoms upon inoculation with TMV. The results indicated that silencing of TOM1 and TOM3 of pepper using the siRNA constructs is an efficient method for generating TMV-resistant plants.

Recombinant hHscFv-RC-RNase protein derived from transgenic tobacco acts as a bifunctional molecular complex against hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common clinical primary malignant tumor; however, efficient drugs for the treatment of HCC are still lacking at the present time. To develop a new approach for liver cancer therapy, we designed a chimeric gene (his-HR) encoding a single-chain variable fragment of human HAb25 (hHscFv) fused to a cytotoxic ribonuclease from Rana catesbeiana (RC-RNase) and expressed the corresponding fusion protein in transgenic tobacco (Nicotiana tabacum). Eleven positive transgenic plant lines were identified from 204 regenerated tobacco plants by PCR and Southern blot analysis, and the immunocompetence of the recombinant his-HR protein was confirmed by Western blotting. The expression levels of his-HR protein ranged from 0.75 to 1.99 µg/g in the fresh tobacco leaves. To characterize the bifunction of the expressed his-HR protein in tobacco, binding specificity and cell toxicity to several cell lines were examined by the indirect immunocytochemical streptavidin-biotin complex method and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide assay. Data indicated that the his-HR protein had stronger specific binding affinity to HepG2 (human liver HCC cell line) than to the other tumor cell lines and normal liver cell line, and the capacity to kill the HCC cell lines SMMC7721 and HepG2 with an half maximal inhibiting concentration of 2.0 and 2.4 nM, respectively. The results suggest that recombinant bifunctional his-HR protein derived from transgenic plants may provide a novel strategy to treat HCC in the future.

Analysis of the APX, PGD1 and R1G1B constitutive gene promoters in various organs over three homozygous generations of transgenic rice plants

We have previously characterized the constitutively active promoters of the APX, PGD1 and R1G1B genes in rice (Park et al. 2010 in J Exp Bot 61:2459-2467). To have potential crop biotechnology applications, gene promoters must be stably active over many generations. In our current study, we report our further detailed analysis of the APX, PGD1 and R1G1B gene promoters in various organs and tissues of transgenic rice plants for three (T₃₋₅) homozygous generations. The copy numbers in 37 transgenic lines that harbor promoter:gfp constructs were determined and promoter activities were measured by real-time qPCR. Analysis of the 37 lines revealed that 15 contained a single copy of one of the three promoter:gfp chimeric constructs. The promoter activity levels were generally higher in multi-copy lines, whereas variations in these levels over the T₃₋₅ generations studied were observed to be smaller in single-copy than in multi-copy lines. The three promoters were further found to be highly active in the whole plant body at both the vegetative and reproductive stages of plant growth, with the exception of the APX in the ovary and R1G1B in the pistil and filaments where zero or very low levels of activity were detected. Of note, the spatial activities of the PGD1 promoter were found to be strikingly similar to those of the ZmUbi1, a widely used constitutive promoter. Our comparison of promoter activities between T₃, T₄ and T₅ plants revealed that the APX, PGD1 and R1G1B promoters maintained their activities at comparable levels in leaves and roots over three homozygous generations and are therefore potentially viable alternative promoters for crop biotechnology applications.

Expression of rice thaumatin-like protein gene in transgenic banana plants enhances resistance to fusarium wilt

The possibility of controlling Fusarium wilt--caused by Fusarium oxysporum sp. cubensec (race 4)--was investigated by genetic engineering of banana plants for constitutive expression of rice thaumatin-like protein (tlp) gene. Transgene was introduced to cauliflower-like bodies' cluster, induced from meristemic parts of male inflorescences, using particle bombardment with plasmid carrying a rice tlp gene driving by the CaMV 35S promoter. Hygromycin B was used as the selection reagent. The presence and integration of rice tlp gene in genomic DNA confirmed by PCR and Southern blot analyses. RT-PCR revealed the expression of transgene in leaf and root tissues in transformants. Bioassay of transgenic banana plants challenged with Fusarium wilt pathogen showed that expression of TLP enhanced resistance to F. oxysporum sp. cubensec (race 4) compared to control plants.

Development of useful recombinant promoter and its expression analysis in different plant cells using confocal laser scanning microscopy

BACKGROUND

Designing functionally efficient recombinant promoters having reduced sequence homology and enhanced promoter activity will be an important step toward successful stacking or pyramiding of genes in a plant cell for developing transgenic plants expressing desired traits(s). Also basic knowledge regarding plant cell specific expression of a transgene under control of a promoter is crucial to assess the promoter's efficacy.

METHODOLOGY/PRINCIPAL FINDINGS

We have constructed a set of 10 recombinant promoters incorporating different up-stream activation sequences (UAS) of Mirabilis mosaic virus sub-genomic transcript (MS8, -306 to +27) and TATA containing core domains of Figwort mosaic virus sub-genomic transcript promoter (FS3, -271 to +31). Efficacies of recombinant promoters coupled to GUS and GFP reporter genes were tested in tobacco protoplasts. Among these, a 369-bp long hybrid sub-genomic transcript promoter (MSgt-FSgt) showed the highest activity in both transient and transgenic systems. In a transient system, MSgt-FSgt was 10.31, 2.86 and 2.18 times more active compared to the CaMV35S, MS8 and FS3 promoters, respectively. In transgenic tobacco (Nicotiana tabaccum, var. Samsun NN) and Arabidopsis plants, the MSgt-FSgt hybrid promoter showed 14.22 and 7.16 times stronger activity compared to CaMV35S promoter respectively. The correlation between GUS activity and uidA-mRNA levels in transgenic tobacco plants were identified by qRT-PCR. Both CaMV35S and MSgt-FSgt promoters caused gene silencing but the degree of silencing are less in the case of the MSgt-FSgt promoter compared to CaMV35S. Quantification of GUS activity in individual plant cells driven by the MSgt-FSgt and the CaMV35S promoter were estimated using confocal laser scanning microscopy and compared.

CONCLUSION AND SIGNIFICANCE

We propose strong recombinant promoter MSgt-FSgt, developed in this study, could be very useful for high-level constitutive expression of transgenes in a wide variety of plant cells.

Expression of functional recombinant human growth hormone in transgenic soybean seeds

We produced human growth hormone (hGH), a protein that stimulates growth and cell reproduction, in genetically engineered soybean [Glycine max (L.) Merrill] seeds. Utilising the alpha prime (α') subunit of β-conglycinin tissue-specific promoter from soybean and the α-Coixin signal peptide from Coix lacryma-jobi, we obtained transgenic soybean lines that expressed the mature form of hGH in their seeds. Expression levels of bioactive hGH up to 2.9% of the total soluble seed protein content (corresponding to approximately 9 g kg(-1)) were measured in mature dry soybean seeds. The results of ultrastructural immunocytochemistry assays indicated that the recombinant hGH in seed cotyledonary cells was efficiently directed to protein storage vacuoles. Specific bioassays demonstrated that the hGH expressed in the soybean seeds was fully active. The recombinant hGH protein sequence was confirmed by mass spectrometry characterisation. These results demonstrate that the utilisation of tissue-specific regulatory sequences is an attractive and viable option for achieving high-yield production of recombinant proteins in stable transgenic soybean seeds.

Approaches for gene targeting and targeted gene expression in plants

Transgenic science and technology are fundamental to state-of-the-art plant molecular genetics and crop improvement. The new generation of technology endeavors to introduce genes 'stably' into 'site-specific' locations and in 'single copy' without the integration of extraneous vector 'backbone' sequences or selectable markers and with a 'predictable and consistent' expression. Several similar strategies and technologies, which can push the development of 'smart' genetically modified plants with desirable attributes, as well as enhance their consumer acceptability, are discussed in this review.

Designer promoter: an artwork of cis engineering

Advances in systematic computational biology and rapid elucidation of synergistic interplay between cis and trans factors governing transcriptional control have facilitated functional annotation of gene networks. The generation of data through deconstructive, reconstructive and database assisted promoter studies, and its integration to principles of synthetic engineering has started an era of designer promoters. Exploration of natural promoter architecture and the concept of cis engineering have not only enabled fine tuning of single or multiple transgene expression in response to perturbations in the chemical, physiological and environmental stimuli but also provided researchers with a unique answer to various problems in crop improvement in the form of bidirectional promoters.