The pea rbcS-3A enhancer-like element directs cell-specific expression in transgenic tobacco

Regulated genes in transgenic plants

Transgenic plants are an effective system for the study of regulated gene expression. Developmental control of expression can be monitored by assaying different tissues or by assaying a plant at different developmental stages. Analysis of the petunia 5-enolpyruvylshikimate-3-phosphate synthase gene, which is highly expressed in flowers, allowed identification of an upstream region that confers tissue-specific and developmentally regulated expression. The cell specificity of expression in floral tissues has been defined by histochemical localization. This expression is contrasted to that of the 35S promoter of cauliflower mosaic virus, a nominally constitutive promoter that shows a definite specificity of expression in floral tissues. Moreover, this expression differs in transgenic hosts of different species.

Tissue-specific and light-responsive regulation of the promoter region of the Arabidopsis thaliana chloroplast omega-3 fatty acid desaturase gene (FAD7)

The Arabidopsis FAD7 gene encodes a chloroplast omega-3 fatty acid desaturase that catalyzes the desaturation of lipid-linked dienoic fatty acids (18:2 and 16:2). An 825 bp FAD7 promoter fragment upstream from the transcriptional start point contained several short sequences which were homologous to the cis-elements (box II, G-box, etc.) conserved in many light-responsive genes. We introduced the FAD7 promoter fused to the beta-glucuronidase (GUS) or the luciferase (LUC) reporter gene into tobacco plants. The -825 promoter sequence conferred tissue-specific and light-responsive expression to both these reporter genes in transgenic tobacco, indicating that these expressions of the FAD7 gene were regulated mainly at the transcriptional level. Histochemical GUS staining showed that the activity of the FAD7 promoter is restricted to the tissues with chloroplast-containing cells although the staining was noticeably absent in the chloroplast-containing cells associated with vascular systems. The 5' deletion experiments of the promoter revealed that the -362/-166 region, containing two putative box II sequences, was responsible for the tissue-specific and light-responsive expression of the FAD7 gene.

The promoter from the rice nuclear gene encoding chloroplast aldolase confers mesophyll-specific and light-regulated expression in transgenic tobacco

The rice genome contains at least four separate loci that encode aldolase isozymes. Among these, the aldolase P (AldP) gene, a nuclear gene coding for chloroplast aldolase, is expressed predominantly in the leaf blade mesophyll cells in rice. To dissect promoter elements that regulate such tissue- or cell type-specific expression, we constructed various AldP promoter-beta-glucuronidase (GUS) fusion genes and transferred them into Nicotiana tabacum (tobacco) plants. Analysis of GUS activities in the transgenic tobacco revealed the presence of at least two elements within 2.0 kb AldP promoter region. One is located within the segment from position -2.0 kb to -1.2 kb and acts as a negative element. The other is a positive element located between -1.2 kb and -0.31 kb that confers developmentally regulated, mesophyll cell-specific expression. In addition, the 1.2 kb rice promoter segment flanking the transcription start site contains an element(s) that serves as target for light induction in tobacco. The results suggest that the AldP gene promoter of rice, a monocot promoter, can function in an essentially physiological manner in the dicot tobacco plant.

Electroporation-mediated DNA delivery to seedling tissues ofPhaseolus vulgaris L. (common bean)

DNA was delivered to intact embryonic axes of the legumePhaseolus vulgaris L. through electroporation. Expression of the ß-glucuronidase reporter gene was observed in hypocotyl and epicotyl tissue in a spot-like manner. Transgene expression was high when a single pulse of 260 ms at a field strength of 225 V·cm(-1) was applied but could be achieved within a wide range of electrical conditions. Linearization of plasmid DNA greatly enhanced transient expression levels. The procedure was successful for embryonic axes of all testedP. vulgaris cultivars, for similar explants of several large-seeded leguminous species, as well as for some other tissues ofP. vulgaris.

Activity of the promoter of the Lhca3.St.1 gene, encoding the potato apoprotein 2 of the light-harvesting complex of Photosystem I, in transgenic potato and tobacco plants

We have isolated cDNA and genomic clones for the potato (Solanum tuberosum) apoprotein 2 of the light harvesting complex of Photosystem I, designated Lhca3.St.1. The protein shows all characteristics of the family of chlorophyll a/b-binding proteins. Potato Lhca3.1 gene expression occurs predominantly in leaves, and is transcriptionally regulated by light. One gene copy is present per haploid genome. The sequence of the 5' upstream region was determined. Most boxes identified in the promoter sequences of genes whose expression is light-regulated recur in the Lhca3.St.1 sequence. Functional analyses of the Lhca3.St.1 promoter and two deletion derivatives in transgenic potato transformed with a promoter-GUS fusion show high promoter activity in leaves and other green parts of the plant, which depends on light. Activity is absent in roots and potato tubers. The 500 bp promoter fragment is as active as the full 2.0 kb sequence, showing that all regulatory elements are present on the smallest deletion derivative. In transgenic tobacco (Nicotiana tabacum) plants carrying the largest promoter derivative a similar distribution of activity is found. Promoter activity is not restricted to the phloem, but also prominent in the xylem of the young stem, which contrasts with promoters of other photosynthesis-associated genes.

Light-regulated expression of the Arabidopsis thaliana ferredoxin A gene involves both transcriptional and post-transcriptional processes

Ferredoxin is part of the photosynthetic apparatus of the chloroplast and is encoded in the nucleus. In Arabidopsis thaliana expression of the ferredoxin A gene is influenced by both the presence of chloroplasts and light. Tobacco plants transformed with a ferredoxin promoter-GUS fusion gene showed a tissue-specific and light-dependent expression pattern. The effect of light on the expression of the fusion gene in transgenic seedlings was only two- to fourfold, which is less pronounced than the 20-fold effect in Arabidopsis itself. Run-on transcription assays with nuclei isolated from Arabidopsis revealed a twofold modulation of transcriptional activity of the ferredoxin A gene under the influence of light. These results suggest the involvement of post-transcriptional processes in light-regulated gene expression. A ferredoxin promoter deletion series ranging from -1205 to -143 was studied. All but the smallest deletion construct (at position -143 relative to the translation start site) showed comparable expression levels in mature leaves, suggesting the presence of a positive regulating element between -269 and -143. The same pattern of tissue specificity was found in all promoter deletions studied. Expression of the fusion genes is high in all chloroplast-containing cells: mesophyll, chlorenchyma, paravascular tissue, epidermal and stomatal guard cells and trichomes. Transgenic seedlings treated with norflurazon, which blocks the development of green chloroplasts, showed a two- to fourfold reduction in GUS expression for all constructs. In Arabidopsis seedlings the effect of norflurazon on the expression of the ferredoxin A was eightfold. This again can be explained by the need for post-transcriptional processes of the regulated gene expression of Arabidopsis ferredoxin A.

The pea plastocyanin promoter directs cell-specific but not full light-regulated expression in transgenic tobacco plants

A series of 5' deletions of the pea plastocyanin gene (petE) promoter fused to the beta-glucuronidase (GUS) reporter gene has been examined for expression in transgenic tobacco plants. Strong positive and negative cis-elements which modulate quantitative expression of the transgene in the light and the dark have been detected within the petE promoter. Disruption of a negative regulatory element at -784 bp produced the strongest photosynthesis-gene promoter so far described. Histochemical analysis demonstrated that all petE-GUS constructs directed expression in chloroplast-containing cells, and that a region from -176 bp to +4 bp from the translation start site was sufficient for such cell-specific expression. The petE-promoter fusions were expressed at high levels in etiolated transgenic tobacco seedlings but there was no marked induction of GUS activity in the light. The endogenous tobacco plastocyanin genes and the complete pea plastocyanin gene in transgenic tobacco plants were also expressed in the dark, but showed a three- to sevenfold increase in the light. This indicates a requirement for sequences 3' to the promoter for the full light response of the petE gene.

Characterization of the gene encoding the plastid-located glutamine synthetase of Phaseolus vulgaris: regulation of beta-glucuronidase gene fusions in transgenic tobacco

The gln-delta gene, which encodes the plastid-located glutamine synthetase of Phaseolus vulgaris, was cloned and its promoter region was sequenced. Primer extension analysis was used to map the two major transcription initiation sites which are about 90 nucleotides apart. A fusion of 2.3 kb of the upstream region of the gln-delta gene to the reporter gene uidA encoding beta-glucuronidase was shown to be expressed in the chlorophyllous cell types of leaves and stems and in the root meristem region of transgenic tobacco. Analysis of a series of three 5' promoter deletion fusions revealed the presence of a region essential for promoter activity between -786 and -327 and regions involved in tissue-specific regulation and light regulation between -786 and +43.

Efficient gene introduction into rice by electroporation and analysis of transgenic plants: use of electroporation buffer lacking chloride ions

We have developed a method for reproducibly obtaining transgenic rice at a high frequency (10(-4)): electroporation with a buffer in which chloride ions are replaced with organic acids. Co-transformation frequencies of the β-glucuronidase (GUS) and hygromycin phosphotransferase (HPT) genes located on two separate plasmids were higher than 50%. Transgenic rice plants contained multiple copies of introduced genes integrated into their genomes in a complex manner. GUS enzyme activity was not proportional to gene copy number. Introduced HPT genes were detected and expressed in the progeny of transformants.

Analysis of the promotors of the single-copy genes for plastocyanin and subunit delta of the chloroplast ATP synthase from spinach

The promotors of the single-copy genes for subunit delta of the chloroplast ATP synthase (atpD) and plastocyanin (PC) from spinach have been sequenced, dissected and analysed in transgenic F0 and F1 tobacco plants using the bacterial GUS gene as a reporter for promotor activity. The transcription of these genes is photo-controlled. The results have been compared with those obtained for the spinach rbcS-1 gene, one of the light-regulated genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, and for the cauliflower mosaic virus (CaMV) 35S RNA promotor. We find that the 5' upstream regions of about 1200 nucleotides contain all the sequences required for light regulation, organ-, tissue- and development-specific expression, and that they are structurally diverse. Their cis-acting elements are functionally defined. The proximal regions of the spinach promotors contain potential TATA, CAAT and T-cyt boxes at appropriate positions, but only sequence elements with low similarity to published light-responsive elements. Positive light-stimulated regions, regions with constitutive, light-independent enhancing effects and with 'silencer'-like activity in complete darkness are found in proximal and far upstream promotor segments. Highest activity of these promotors is correlated with the presence of chloroplasts but is not confined to photosynthetic tissue. Surprisingly, expression is observed in the phloem regions of transgenic leaves, leaf and floral stems, in the vascular area of anthers and in pollen. No histochemical staining has been detected in roots. The distal region of atpD located between -1137 and -590 contains elements for expression in the outer phloem, the region from -590 to -185 for activity in the inner phloem of floral stems. Similar tissue-specific patterns are observed with a fusion between the caufliflower mosaic virus 35S RNA promotor and the GUS gene.

GT-1 binding site confers light responsive expression in transgenic tobacco

Light-dependent expression of rbcS, the gene encoding the small subunit of ribulose-1,5-bisphosphate carboxylase, which is the key enzyme involved in carbon fixation in higher plants, is regulated at the transcriptional level. Sequence analysis of the gene has uncovered a conserved GT motif in the -150 to -100 region of many rbcS promoters. This motif serves as the binding site of a nuclear factor, designated GT-1. Analysis of site-specific mutants of pea rbcS-3A promoter demonstrated that GT-1 binding in vitro is correlated with light-responsive expression of the rbcS promoter in transgenic plants. However, it is not known whether factors other than GT-1 might also be required for activation of transcription by light. A synthetic tetramer of box II (TGTGTGGTTAATATG), the GT-1 binding site located between -152 to -138 of the rbcS-3A promoter, inserted upstream of a truncated cauliflower mosaic virus 35S promoter is sufficient to confer expression in leaves of transgenic tobacco. This expression occurs principally in chloroplast-containing cells, is induced by light, and is correlated with the ability of box II to bind GT-1 in vitro. The data show that the binding site for GT-1 is likely to be a part of the molecular light switch for rbcS activation.

Regulation of nuclear genes encoding chloroplast proteins in transgenic plants

Transgenic plants have been particularly useful in studying nuclear genes encoding for photosynthetic functions. The expression of these genes and their chimeric constructs in transgenic plants faithfully mimics their natural counterparts. The use of sensitive chimeric reporter genes has enabled localizing the activity of genes encoding photosynthetic proteins to individual cells. Cab and rbcS transgenes have been shown to retain sensitivity to light quality, which is modulated by phytochrome. Conditional light activation under the influence of a circadian rhythm has been shown for Cab transgenes. Transgenic plants containing truncated promoters have helped delineate cis-regulatory positive and negative elements involved in light-mediated transcriptional induction and tissue specificity.