Subdomains of the octopine synthase upstream activating element direct cell-specific expression in transgenic tobacco plants

Development of a salicylic acid inducible minimal sub-genomic transcript promoter from Figwort mosaic virus with enhanced root- and leaf-activity using TGACG motif rearrangement

In Figwort mosaic virus sub-genomic transcript promoter (F-Sgt), function of the TGACG-regulatory motif, was investigated in the background of artificially designed promoter sequences. The 131bp (FS, -100 to +31) long F-Sgt promoter sequence containing one TGACG motif [FS-(TGACG)] was engineered to generate a set of three modified promoter constructs: [FS-(TGACG)(2), containing one additional TGACG motif at 7 nucleotides upstream of the original one], [FS-(TGACG)(3), containing two additional TGACG motifs at 7 nucleotides upstream and two nucleotides downstream of the original one] and [FS-(TGCTG)(mu), having a mutated TGACG motif]. EMSA and foot-printing analysis confirmed binding of tobacco nuclear factors with modified TGACG motif/s. The transcription-activation of the GUS gene by the TGACG motif/s in above promoter constructs was examined in transgenic tobacco and Arabidopsis plants and observed that the transcription activation was affected by the spacing/s and number/s of the TGACG motif/s. The FS-(TGACG)(2) promoter showed strongest root-activity compared to other modified and CaMV35S promoters. Also under salicylic acid (SA) stress, the leaf-activity of the said promoter was further enhanced. All above findings were confirmed by real-time and semi-qRT PCR analysis. Taken together, these results clearly demonstrated that the TGACG motif plays an important role in inducing the root-specific expression of the F-Sgt promoter. This study advocates the importance of genetic manipulation of functional cis-motif for amending the tissue specificity of a plant promoter. SA inducible FS-(TGACG)(2) promoter with enhanced activity could be a useful candidate promoter for developing plants with enhanced crop productivity.

Enhancer-promoter interference and its prevention in transgenic plants

Biotechnology has several advantages over conventional breeding for the precise engineering of gene function and provides a powerful tool for the genetic improvement of agronomically important traits in crops. In particular, it has been exploited for the improvement of multiple traits through the simultaneous introduction or stacking of several genes driven by distinct tissue-specific promoters. Since transcriptional enhancer elements have been shown to override the specificity of nearby promoters in a position- and orientation-independent manner, the co-existence of multiple enhancers/promoters within a single transgenic construct could be problematic as it has the potential to cause the mis-expression of transgene product(s). In order to develop strategies with, which to prevent such interference, a clear understanding of the mechanisms underlying enhancer-mediated activation of target promoters, as well as the identification of DNA sequences that function to block these interactions in plants, will be necessary. To date, little is known concerning enhancer function in plants and only a very limited number of enhancer-blocking insulators that operate in plant species have been identified. In this review, we discuss the current knowledge surrounding enhancer-promoter interactions, as well as possible means of minimizing such interference during plant transformation experiments.

Strategies to mitigate transgene-promoter interactions

The expression pattern of tissue-specific promoters in transgenes can be influenced by promoter/enhancer elements employed for the expression of selectable marker genes or elements found in DNA flanking the insertion site. We have developed an analytical system in Arabidopsis thaliana to investigate strategies useful in blocking or reducing nonspecific interactions. These experiments confirm that the DNA configuration and the insertion of spacer DNA aid in the appropriate expression of tissue-specific promoters. It is also demonstrated that the novel tobacco cryptic promoter (tCUP), when used to replace the cauliflower mosaic virus (CaMV) 35S promoter/enhancer, does not show nonspecific interactions. Furthermore, it is shown that insulators isolated from yeast and animals may have potential application in plants. Our results may allow the design of strategies that, individually or in combination, can be used to minimize nonspecific interactions and to design vectors for individual tissue-specific promoters.

In vivo characterization of plant promoter element interaction using synthetic promoters

Short directly-repeated (DR) DNA enhancer elements of plant viral origin were analyzed for their ability, both individually and in combination, to influence in vivo transcription when inserted upstream from a minimal CaMV35S promoter. Synthetic promoters containing multiple copies and/or combinations of DR cassettes were tested for their effect upon reporter gene (luciferase) expression using an Agrobacteria-based leaf-infiltration transient assay and within stably transformed plants (Nicotiana tabacum). Transgenic plants harboring constructs containing different numbers or combinations of DR cassettes were further tested to look for tissue-specific expression patterns and potential promoter response to the infiltration process employed during transient expression. Multimerization of DR elements produced enhancer activity that was in general additive, increasing reporter activity in direct proportion to the number of DR cassettes within the test promoter. In contrast, combinations of different DR cassettes often functioned synergistically, producing reporter enhancement markedly greater then the sum of the combined DR activities. Several of the DR constructs responded to Agrobacteria (lacking T-DNA) infiltration of transgenic leaves by an induction (2 elements) or reduction (1 element) in reporter activity. Combinations of DR cassettes producing the strongest enhancement of reporter activity were used to create two synthetic promoters (SynPro3 and SynPro5) that drive leaf reporter activities at levels comparable to the CaMV35S promoter. Characterization of these synthetic promoters in transformed tobacco showed strong reporter expression at all stages of development and in most tissues. The arrangement of DR elements within SynPro3 and SynPro5 appears to play a role in defining tissue-specificity of expression and/or Agrobacteria-infusion responsiveness.

A 42 bp fragment of the pmas1' promoter containing an ocs-like element confers a developmental, wound- and chemically inducible expression pattern

Synthesis of mannopine in plant tissues infected with Agrobacterium tumefaciens is controlled by a divergent promoter (pmas2' and pmas1') that in 479 bp contains all the cis-acting elements necessary to direct tissue-specific and wound-inducible expression. In this report, using transgenic tobacco plants harboring a pmas1'-beta-glucuronidase (GUS) gene fusion, we investigated the developmental expression pattern directed by pmas1' in the early stages of development and the responses of pmas1' to different chemical inducers. It was found that this promoter can respond to auxins, cytokinins, methyl jasmonate (MJ), salicylic acid (SA) and its analogue 2,6-dichloroisonicotinic acid (iNA). Treatment with chemical inducers also showed that the effects of iNA are organ-dependent, that wound-induction is a complex response mediated by at least two different chemical signals, and that MJ stimulates changes in the tissue-specific and developmental expression pattern directed by the ptmas1' promoter. Using chimeric promoters we demonstrate that an ocs-like element (ocs+1) directs MJ responses in an orientation-dependent manner and that sequences around the ocs+1 are important to maintain the inducible and developmental properties of this cis-regulatory element.

5'-regulatory region of Agrobacterium tumefaciens T-DNA gene 6b directs organ-specific, wound-inducible and auxin-inducible expression in transgenic tobacco

The regulatory activity of a 826 bp DNA fragment located upstream of the pTiBo542 TL-DNA gene 6b coding region was analyzed in transgenic tobacco, using beta-glucuronidase (gus) as a reporter gene. The region was shown to drive organ-specific, wound- and auxin-inducible expression of the reporter, the effect being dependent on the type and concentration of auxin.

Site-directed mutagenesis of the enhancer region of the 780 gene promoter of T-DNA

Potential regulatory sequences within the enhancer-like region of the 780 gene promoter (Agrobacterium tumefaciens T-DNA) were identified by site-directed mutagenesis. Transcriptional activity of the mutated promoter was analyzed by S1 nuclease mapping of RNA from crown gall tumors of sunflower incited using a T-DNA-based vector. Variability in expression levels were minimized by the use of an internal reference gene and the pooling of at least 200 tumors per construct tested. This approach identified numerous sequences that influence transcriptional activity in either a positive or negative manner. Eight regions of positive influence and three of negative were identified from analysis of those mutations that exhibited low variability in expression (P < 0.005) and affected activity by at least 20%.

Expression of Agrobacterium rhizogenes auxin biosynthesis genes in transgenic tobacco plants

Plant oncogenes aux1 and aux2 carried by the TR-DNA of Agrobacterium rhizogenes strain A4 encode two enzymes involved in the auxin biosynthesis pathway in transformed plant cells. The short divergent promoter region between the two aux-coding sequences contains the main regulatory elements. This region was fused to the uidA reporter gene and introduced into Nicotiana tabacum in order to investigate the regulation and the tissue specificity of these genes. Neither wound nor hormone induction could be detected on transgenic leaf discs. However, phytohormone concentration and auxin/cytokinin balance controlled the expression of the chimaeric genes in transgenic protoplasts. The expression was localised in apical meristems, root tip meristems, lateral root primordia, in cells derived from transgenic protoplasts and in transgenic calli. Histological analysis showed that the expression was located in cells reactivated by in vitro culture. Experiments using cell-cycle inhibitors such as hydroxyurea or aphidicolin on transgenic protoplast cultures highly decreased the beta-glucuronidase activity of the chimaeric genes. These results as well as the histological approach suggest a correlation between expression of the aux1 and aux2 genes and cell division.

ocs element promoter sequences are activated by auxin and salicylic acid in Arabidopsis

ocs elements are a group of promoter elements that have been exploited by two distinct groups of plant pathogens, Agrobacterium and certain viruses, to express genes in plants. We examined the activity of single and multiple ocs elements linked to a minimal plant promoter and the uidA reporter gene in transgenic Arabidopsis. beta-Glucuronidase activity was detected only in root tips and in callus tissue after auxin treatment. A more sensitive assay revealed that auxin treatment also increased ocs element activity in aerial parts of the plant, although the absolute levels of ocs element activity were greater in roots. The response of ocs elements to exogenous auxin began within 1 h. Salicylic acid, a disease-resistance signal in plants, also increased ocs element activity in both roots and aerial parts of the plant. The question of whether the induction in ocs element activity is mediated through auxin and/or salicylic acid signal transduction pathways or is part of a more general stress response is discussed.

Isolation of a maize bZIP protein subfamily: candidates for the ocs-element transcription factor

Ocs-elements, a family of 20 bp DNA sequences, are components of a number of promoters active in plants. In the maize BMS cell line the dominant ocs-element binding activity is the ocs-element transcription factor complex called OTF. The isolation of cDNA clones from a BMS cDNA expression library for two bZIP (basic region-leucine zipper) proteins that bind the ocs-element sequence and are good candidates for forming at least part of OTF is described. The two ocs-element binding proteins, called OBF3.1 and OBF3.2, are closely related, with the OBF3.1 protein sharing 95.8% amino acid homology with part of the OBF3.2 protein although there were significant differences in the 3' untranslated regions. Genomic Southern blot analysis revealed a small gene family with a minimum of two OBF3 loci mapping to chromosomes 3L105 and 8L075. The OBF3.1 protein shared considerable homology with the wheat HBP1b protein (80% amino acid identity) and to a lesser extent with the tobacco TGA1aa protein. OBF3.1 like HBP1b was able to bind well to the Hex sequence but poorly to G-box/ABRE sequences. Interestingly, OBF3.1 bound eightfold more efficiently to an ocs-element sequence than TGA1a, raising the possibility that OBF3.1 and TGA1a may be distinct members of an OBF3/TGA subfamily.

Levels and location of expression of the Agrobacterium tumefaciens pTiA6 ipt gene promoter in transgenic tobacco

The location of gene expression of the Agrobacterium tumefaciens ipt gene promoter in transgenic tobacco plants was examined using the beta-glucuronidase (GUS) reporter gene. Expression of GUS was detected in every organ and most cell types examined. The highest levels of GUS activity were found in roots. To further examine the transcriptional basis of this broad expression pattern, deletions in the 5' non-coding region of the gene were translationally fused to two promoterless reporter genes, encoding the enzymes chloramphenicol acetyl transferase (CAT) and beta-glucuronidase (GUS). Reporter enzyme assays revealed the existence of an upstream segment required for maximal promoter function, the 5' end of which is between -442 and -408 of the Pipt ATG codon. This upstream segment is required for maximal levels of GUS expression in roots, but not in other organs, and a tobacco suspension-cultured cell line. The implications of broad ipt expression on the process of crown gall tumorigenesis are discussed.