Structural and Functional Analysis of a Bidirectional Promoter from Gossypium hirsutum in Arabidopsis

The non-canonically organized members of MIR395 gene family in Brassica juncea are associated with developmentally regulated, sulfate-stress responsive bidirectional promoters that exhibit orientation-dependent differential transcriptional activity

Several MICRORNA genes belonging to same family or different families are often found in homologous or non-homologous clusters. Among the various classes, head-to-head arranged genes form one of the largest categories of non-canonically organized genes. Such head-to-head arranged, non-canonically organized genes possibly share cis-regulatory region with the intergenic sequence having the potential to function as bi-directional promoter (BDP). The transcriptional regulation of head-to-head arranged genes, especially with bidirectional promoters, remains an enigma. In the past, bidirectional promoters have been characterized for a small set of protein-coding gene pairs in plants; however, to the best of our knowledge, no such study has been carried so far for MICRORNA genes. The present study thus functionally characterizes bidirectional promoters associated with members of MIR395 family, which is evolutionary conserved and is most frequently occurring cluster across plant kingdom. In Arabidopsis thaliana, the MIR395 gene family contains six members with two head-to-head arranged gene pairs- MIR395A-B and MIR395E-F. This organization was found to be conserved at seven loci for MIR395A-B, and eleven loci for MIR395E-F in five Brassica sps. Sequence analysis of the putative bidirectional promoters revealed variation in length, GC content and distribution of strict TATA-box. Comparatively higher level of conservation at both the ends of the bidirectional promoters, corresponding to ca. 250 bp upstream of 5'end of the respective MIRNA precursor, was observed. These conserved regions harbour several abiotic stress (nutrient, salt, drought) and hormone (ABA, ethylene) responsive cis-motifs. Functional characterization of putative bidirectional promoters associated with MIR395A-B and MIR395E-F from Arabidopsis and their respective orthologs from Brassica juncea (Bj_A08 MIR395A-B, Bj_B03 MIR395A-B, Bj_A07.1 MIR395E-F and Bj_A07.2 MIR395E-F) was carried out using a dual-reporter vector with β-glucuronidase (GUS) and Green Fluorescent Protein (GFP). Analysis of transcriptional regulation of the two reporter genes - GUS and GFP during developmental stages confirmed their bidirectional nature. Orientation-dependent differential reporter activity indicated asymmetric nature of the promoters. Comparison of the reporter activity amongst orthologs, paralogs and homeologs revealed regulatory diversification, an outcome expected in polyploid genomes. Interestingly, reporter gene activities driven by selected bidirectional promoters were also observed in anther and siliques apart vegetative tissues indicating role of miR395 in anther and fruit development. Finally, we evaluated the activity of reporter genes driven under transcriptional regulation of bidirectional promoters under normal and sulfate-deprived conditions which revealed asymmetric inducibility under sulfate-starvation, in agreement with the known role of miR395 in sulfate homeostasis.

Identification and functional characterization of bidirectional gene pairs and their intergenic regions in cotton

BACKGROUND

In research to improve the quality of transgenic crops, it is often necessary to introduce multiple functionally related genes into recipient plants simultaneously to improve crop genetic traits effectively. Compared with unidirectional promoters, bidirectional promoters simultaneously regulate the expression of multiple genes and improve the efficiency of biotechnology. Therefore, in this study, bidirectional gene pairs were systematically analyzed in Gossypium hirsutum TM-1, and the structure, function and evolutionary relationships of the bidirectional genes were analyzed. The endogenous bidirectional promoters of cotton were mined, and their specific regulatory elements and biological functions were explored to provide useful promoter resources and a theoretical basis for cultivating new cotton germplasms with excellent fiber quality.

RESULTS

Using an improved search model, a total of 1,383 bidirectional transcript pairs were identified in the Gossypium hirsutum TM-1 genome, and their gene structure and functional annotations were systematically analyzed. Thirty bidirectional intergenic sequences were randomly screened for promoter activity analysis via a transient expression system, and 25 intergenic sequences were found to have bidirectional promoter activity. Comparative analysis of the bidirectional gene profiles of the four cotton subspecies revealed that these subspecies presented abundant bidirectional gene pairs with high homology and that the bidirectional genes in the cotton subspecies were more similar in terms of their molecular functions, cellular components and biological processes. In addition, parallel analysis of bidirectional genes in dicotyledons and monocotyledons revealed that abundant bidirectional gene pairs exist in different species. Although the total number of orthologous bidirectional genes was similar, there was a significant difference in the number of orthologous bidirectional gene pairs between dicotyledons and monocotyledons. This evolutionary analysis of the function and structure of homologous bidirectional gene pairs in different varieties and different subspecies of the same species revealed potential pathways by which these gene pairs originated, which may be necessary for the evolution of a new species.

CONCLUSION

In this study, many bidirectional gene pairs in Gossypium hirsutum TM-1 were identified using computer programming, and systematic analysis was conducted to explore their functions and evolutionary relationships. In addition, the promoter activity of the bidirectional intergenic sequences was verified. The combination of computer programming screening, experimental validation and other methods is expected to provide preferred bidirectional promoters for transgenic breeding work via multigene cotransformation methods, and this information is valuable for genetic engineering research and applications.

Specificity Testing for NGT PCR-Based Detection Methods in the Context of the EU GMO Regulations

The term new genomic techniques (NGTs) is an umbrella term used to describe a variety of techniques that can alter the genetic material of an organism and that have emerged or have been developed since 2001, when the existing genetically modified organism (GMO) legislation was adopted. The analytical framework used to detect GMOs in Europe is an established single harmonized procedure that is mandatory for the authorization of GM food and feed, thus generating a reliable, transparent, and effective labeling scheme for GMO products. However, NGT products can challenge the implementation and enforcement of the current regulatory system in the EU, relating in particular to the detection of NGT products that contain no foreign genetic material. Consequently, the current detection methods might fail to meet the minimum performance requirements. Although existing detection methods may be able to detect and quantify even small alterations in the genome, this does not necessarily confirm the distinction between products resulting from NGTs subject to the GMO legislation and other products. Therefore, this study provides a stepwise approach for the in silico prediction of PCR systems' specificity by testing a bioinformatics pipeline for amplicon and primer set searches in current genomic databases. In addition, it also empirically tested the PCR system evaluated during the in silico analysis. Two mutant genotypes produced by CRISPR-Cas9 in Arabidopsis thaliana were used as a case study. Overall, our results demonstrate that the single PCR system developed for identifying a nucleotide insertion in the grf1-3 genotype has multiple matches in the databases, which do not enable the discrimination of this mutated event. Empirical assays further support this demonstration. In contrast, the second mutated genotype, grf8-61, which contains a -3 bp deletion, did not yield any matches in the sequence variant database. However, the primer sequences were not efficient during the empirical assay. Our approach represents a first step in decision making for analytical methods for NGT detection, identification, and quantification in light of the European labeling regulations.

Complex origin, evolution, and diversification of non-canonically organized OVATE-OFP and OVATE-Like OFP gene pair across Embryophyta

Ovate Family Proteins (OFP) is a plant-specific gene family of negative transcriptional regulators. Till-date, a handful of in-silico studies have provided glimpses into family size, expansion patterns, and genic features across all major plant lineages. A major lacuna exists in understanding origin of organisation complexity of members such as those arranged in a head-to-head manner which may lead to transcriptional co-regulation via a common bi-directional promoter. To address this gap, we investigated the origin, organization and evolution of two head-to-head arranged gene pairs of homologs of AtOFP2-AtOFP17, and, AtOFP4-AtOFP20 across Archaeplastida. The ancestral forms of AtOFP2, AtOFP4, AtOFP17, and AtOFP20 are likely to have evolved in last common ancestors of Embryophyta (land plants) given their complete absence in Rhodophyta and Chlorophyta. The OFP gene family originated and expanded in Bryophyta, including protein variants with complete (OVATE-OFP) or partial (OVATE-Like OFP) OVATE domain; with head-to-head organization present only in Spermatophyta (gymnosperms and angiosperms). Ancestral State Reconstruction revealed the origin of head-to-head organized gene pair in gymnosperms, with both genes being OVATE-OFP (homologs of AtOFP2/4). Phylogenetic reconstruction and copy number analysis suggests the presence of a single copy of the head-to-head arranged pair of OFP2/4 (OVATE)-OFP17/20 (OVATE-Like) in all angiosperms except Brassicaceae, and a duplication event in last common ancestor of core Brassicaceae approximately 32-54 MYA leading to origin of AtOFP2-AtOFP17 and AtOFP4-AtOFP20 as paralogs. Synteny analysis of genomic regions harbouring homologs of AtOFP2-AtOFP17, AtOFP4-AtOFP20 and AtOFP2/4-AtOFP17/20 across angiosperms suggested ancestral nature of AtOFP2-AtOFP17 gene pair. The present study thus establishes the orthology and evolutionary history of two non-canonically organised gene pairs with variation in their OVATE domain. The non-canonical organisation, atleast in Brassicaceae, has the potential of generating complex transcriptional regulation mediated via a common bi-directional promoter. The study thus lays down a framework to understand evolution of gene and protein structure, transcriptional regulation and function across a phylogenetic lineage through comparative analyses.

Plant synthetic biology: from inspiration to augmentation

Although it is still in its infancy, synthetic biology has the capacity to face scientific and societal problems related to modern agriculture. Innovations in cloning toolkits and genetic parts allow increased precision over gene expression in planta. We review the vast spectrum of available technologies providing a practical list of toolkits that take advantage of combinatorial power to introduce/alter metabolic pathways. We highlight that rational design is inspired by deep knowledge of natural and biochemical mechanisms. Finally, we provide several examples in which modern technologies have been applied to address these critical topics. Future applications in plants include not only pathway modifications but also prospects of augmenting plant anatomical features and developmental processes.

Functional characterization of wheat myo-inositol oxygenase promoter under different abiotic stress conditions in Arabidopsis thaliana

The production of wheat is severely affected by abiotic stresses such as cold, drought, salinity, and high temperature. Although constitutive promoters are frequently used to regulate the expression of alien genes, these may lead to undesirable side-effects in transgenic plants. Therefore, identification and characterization of an inducible promoter that can express transgene only when exposed to stresses are of great importance in the genetic engineering of crop plants. Previous studies have indicated the abiotic stress-responsive behavior of myo-inositol oxygenase (MIOX) gene in different plants. Here, we isolated the MIOX gene promoter from wheat (TaMIOX). The in-silico analysis revealed the presence of various abiotic stress-responsive cis-elements in the promoter region. The TaMIOX promoter was fused with the UidA reporter gene and transformed into Arabidopsis thaliana. The T₃ single-copy homozygous lines were analyzed for GUS activity using histochemical and fluorometric assays. Transcript expression of TaMIOX::UidA was significantly up-regulated by heat (five fold), cold (seven fold), and drought (five fold) stresses as compared to transgenic plants grown without stress-induced conditions. The CaMV35S::UidA plants showed very high GUS activity even in normal conditions. In contrast, the TaMIOX::UidA plants showed prominent GUS activity only in stress treatments (cold, heat, and drought), which suggests the inducible behavior of the TaMIOX promoter. The substrate myo-inositol feeding assay of TaMIOX::UidA plants showed lesser GUS activity as compared to plants treated in abiotic stress conditions. Results support that the TaMIOX promoter could be used as a potential candidate for conditional expression of the transgene in abiotic stress conditions.

Functional characterization of an inducible bidirectional promoter from Fusarium oxysporum f. sp. cubense

Bidirectional promoters (BDPs) are regulatory DNA sequences (~1000 bp long) intervening two genes arranged on opposite strands with their 5' ends in close proximity. These genes are mostly co-expressed; but, instances of anti-correlation and independent transcription have been observed. In fungal systems, BDPs have shown to provide an improved genetic circuit by assembling and regulating transcription of different genes of a common metabolic pathway. We have identified an intergenic region (1063 bp) from the genome of Fusarium oxysporum f. sp. cubense (Foc), a banana root pathogen. This intergenic region regulates the expression of a gene pair required for the breakdown of hemicellulose. For characterization, it was cloned into pCSN44 vector backbone between two reporter genes, namely β-glucuronidase (GUS) and enhanced green fluorescent protein (EGFP). The newly formed vector was transformed into Foc and tested for its bidirectional expression activity. Using histochemical staining and fluorescence microscopy, the kinetics for both, GUS and EGFP expression were tested under different growth conditions respectively. The activity was differentially regulated by inducers such as xylan, arabinogalactan and pectin. This is the first report on the isolation of the intergenic region with inducible bidirectional promoter activity from Fusarium. Characterization of such BDPs will find applications in genetic engineering, metabolic engineering and synthetic biology using fungal systems.

Plant Defenses Against Pests Driven by a Bidirectional Promoter

The plant defense responses to pests results in the synchronized change of a complex network of interconnected genes and signaling pathways. An essential part of this process is mediated by the binding of transcription factors to the specific responsive cis-elements within in the promoters of phytophagous-responsive genes. In this work, it is reported the identification and characterization of a bidirectional promoter that simultaneously co-regulate two divergent genes, At5g10300 and At5g10290, upon arthropod feeding. Computational analysis identified the presence of cis-elements within the intergenic region between two loci, mainly from the DOF but also from the AP2/ERF, Golden 2-like and bHLH families. The function of the bidirectional promoter was analyzed using two enhanced variants of the GFP and CherryFP reporter genes, in both orientations, in transient tobacco and stably transformed Arabidopsis plants. Promoter activity was tested in response to feeding of Tetranychus urticae and Pieris brassicae, as well as wounding, flagellin and chitin treatments. Using RT-qPCR assays and confocal microscopy, it was shown that all treatments resulted in the induction of both reporter genes. Furthermore, our findings revealed the asymmetric character of the promoter with stronger activity in the forward than in the reverse orientation. This study provides an example of a bidirectional promoter with a strong potential to be used in plant biotechnology in pest control that requires stacking of the defense genes.