Isolation and characterization of a light-inducible, organ-specific gene from potato and analysis of its expression after tagging and transfer into tobacco and potato shoots

Genome wide inherited modifications of the tomato epigenome by trans-activated bacterial CG methyltransferase

BACKGROUND

Epigenetic variation is mediated by epigenetic marks such as DNA methylation occurring in all cytosine contexts in plants. CG methylation plays a critical role in silencing transposable elements and regulating gene expression. The establishment of CG methylation occurs via the RNA-directed DNA methylation pathway and CG methylation maintenance relies on METHYLTRANSFERASE1, the homologue of the mammalian DNMT1.

PURPOSE

Here, we examined the capacity to stably alter the tomato genome methylome by a bacterial CG-specific M.SssI methyltransferase expressed through the LhG4/pOP transactivation system.

RESULTS

Methylome analysis of M.SssI expressing plants revealed that their euchromatic genome regions are specifically hypermethylated in the CG context, and so are most of their genes. However, changes in gene expression were observed only with a set of genes exhibiting a greater susceptibility to CG hypermethylation near their transcription start site. Unlike gene rich genomic regions, our analysis revealed that heterochromatic regions are slightly hypomethylated at CGs only. Notably, some M.SssI-induced hypermethylation persisted even without the methylase or transgenes, indicating inheritable epigenetic modification.

CONCLUSION

Collectively our findings suggest that heterologous expression of M.SssI can create new inherited epigenetic variations and changes in the methylation profiles on a genome wide scale. This open avenues for the conception of epigenetic recombinant inbred line populations with the potential to unveil agriculturally valuable tomato epialleles.

Agrobacterium-mediated transfer of the Fusarium graminearum Tri6 gene into barley using mature seed-derived shoot tips as explants

KEY MESSAGE

We transferred the Tri6 gene into the elite barley GemCraft via new transformation method through shoot organogenesis and identified the rearrangements of transgenes and phenotypic variations in the transgenic plants. Despite its agronomic and economic importance, barley transformation is still very challenging for many elite varieties. In this study, we used direct shoot organogenesis to transform the elite barley cultivar GemCraft with the RNAi constructs containing Tri6 gene of Fusarium graminearum, which causes fusarium head blight (FHB). We isolated 4432 shoot tips and co-cultured these explants with Agrobacterium tumefaciens. A total of 25 independent T0 transgenic plants were generated including 15 events for which transgene-specific PCR amplicons were observed. To further determine the presence of transgenes, the T1 progenies of all 15 T0 plants were analyzed, and the expected PCR products were obtained in 10 T1 lines. Droplet digital (dd) PCR analysis revealed various copy numbers of transgenes in the transgenic plants. We determined the insertion site of transgenes using long-read sequencing data and observed the rearrangements of transgenes. We found phenotypic variations in both T1 and T2 generation plants. FHB disease was evaluated under growth chamber conditions, but no significant differences in disease severity or deoxynivalenol accumulation were observed between two Tri6 transgenic lines and the wildtype. Our results demonstrate the feasibility of the shoot tip transformation and may open the door for applying this system for genetic improvement and gene function research in other barley genotypes.

Impact of Intron and Retransformation on Transgene Expression in Leaf and Fruit Tissues of Field-Grown Pear Trees

Stable and high expression of introduced genes is a prerequisite for using transgenic trees. Transgene stacking enables combining several valuable traits, but repeated transformation increases the risk of unintended effects. This work studied the stability and intron-mediated enhancement of uidA gene expression in leaves and different anatomical parts of pear fruits during field trials over 14 years. The stability of reporter and herbicide resistance transgenes in retransformed pear plants, as well as possible unintended effects using high-throughput phenotyping tools, were also investigated. The activity of β-glucuronidase (GUS) varied depending on the year, but silencing did not occur. The uidA gene was expressed to a maximum in seeds, slightly less in the peel and peduncles, and much less in the pulp of pear fruits. The intron in the uidA gene stably increased expression in leaves and fruits by approximately twofold. Retransformants with the bar gene showed long-term herbicide resistance and exhibited no consistent changes in leaf size and shape. The transgenic pear was used as rootstock and scion, but grafted plants showed no transport of the GUS protein through the graft in the greenhouse and field. This longest field trial of transgenic fruit trees demonstrates stable expression under varying environmental conditions, the expression-enhancing effect of intron and the absence of unintended effects in single- and double-transformed woody plants.

Development of a minigenome cassette for Lettuce necrotic yellows virus: A first step in rescuing a plant cytorhabdovirus

Rhabdoviruses are enveloped negative-sense RNA viruses that have numerous biotechnological applications. However, recovering plant rhabdoviruses from cDNA remains difficult due to technical difficulties such as the need for concurrent in planta expression of the viral genome together with the viral nucleoprotein (N), phosphoprotein (P) and RNA-dependent RNA polymerase (L) and viral genome instability in E. coli. Here, we developed a negative-sense minigenome cassette for Lettuce necrotic yellows virus (LNYV). We introduced introns into the unstable viral ORF and employed Agrobacterium tumefaciens to co-infiltrate Nicotiana with the genes for the N, P, and L proteins together with the minigenome cassette. The minigenome cassette included the Discosoma sp. red fluorescent protein gene (DsRed) cloned in the negative-sense between the viral trailer and leader sequences which were placed between hammerhead and hepatitis delta ribozymes. In planta DsRed expression was demonstrated by western blotting while the appropriate splicing of introduced introns was confirmed by sequencing of RT-PCR product.

Cloning and functional analysis of the promoter of a stress-inducible gene (Zmap) in maize

The anionic peroxidases play an important role in a variety of plant physiological processes. We characterized and isolated the Zmap promoter (PZmap) at the 5' flanking region in order to better understand the regulatory mechanisms of Zmap gene expression. A series of PZmap deletion derivatives, termed a1 -a6, at positions -1694, -1394, -1138, -784, -527 and -221 from the translation start site were blended to the β-glucuronidase reporter gene. Agrobacterium-mediated transformation method was used to study each deletion construct in tobaccos. Sequence analysis showed that several cis-acting elements (MYB binding site, Box-II, a TGACG-element, a CGTCA-element and a low temperature responsive element) were located within the promoter. Deletion analysis suggested the sequence between -1,694 and -1394bp may contain cis-elements associated with GUS up regulation. The MYB binding site (-757) might act as a negative drought-responsive element. There might be repressor elements located in the region (-1,694 to -1394bp) to repress Zmap expression under 4°C. The characterized promoter would be an ideal candidate for genetic engineering for improving the resistance of maize to different stressors.

Isolation and identification of a vegetative organ-specific promoter from maize

To avoid the unregulated overexpression of the exogenous genes, specific or inducible expression is necessary for some exogenous genes in transgenic plants. But little is known about organ- or tissue-specific promoters in maize. In the present study, the expression of a maize pentatricopeptide repeat (PPR) protein encoding gene, GRMZM2G129783, was analyzed by RNA-sequencing data and confirmed by quantitative real time PCR. The results showed that the PPR _{GRMZM2G129783} gene specifically expressed in vegetative organs. Consequently, a 1830 bp sequence upstream of the start codon of the promoter for GRMZM2G129783 gene was isolated from maize genome (P ₁₈₃₀ ). To validate whether the promoter possesses the vegetative organ-specificity, the full-length and three 5'-end deletion fragments of P ₁₈₃₀ of different length (1387, 437, and 146 bp) were fused with glucuronidase (GUS) gene to generate promoter::GUS constructs and transformed into tobacco. The transient expression and fluorometric GUS assay in transgenic tobacco showed that all promoter could drive the expression of the GUS gene, the - 437 to - 146 bp region possessed some crucial elements for root-specific expression, and the shortest and optimal sequence to maintain transcription activity was 146 and 437 bp in length, respectively. These results indicate that the promoter of the PPR _{GRMZM2G129783} gene is a vegetative organ-specific promoter and will be useful in transgenic modification of commercial crops for moderate specific expression after further evaluation in monocotyledons.

Expression of the Galanthus nivalis agglutinin (GNA) gene in transgenic potato plants confers resistance to aphids

Aphids, the largest group of sap-sucking pests, cause significant yield losses in agricultural crops worldwide every year. The massive use of pesticides to combat this pest causes severe damage to the environment, putting in risk the human health. In this study, transgenic potato plants expressing Galanthus nivalis agglutinin (GNA) gene were developed using CaMV 35S and ST-LS1 promoters generating six transgenic lines (35S1-35S3 and ST1-ST3 corresponding to the first and second promoter, respectively). Quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that the GNA gene was expressed in leaves, stems and roots of transgenic plants under the control of the CaMV 35S promoter, while it was only expressed in leaves and stems under the control of the ST-LS1 promoter. The levels of aphid mortality after 5 days of the inoculation in the assessed transgenic lines ranged from 20 to 53.3%. The range of the aphid population in transgenic plants 15 days after inoculation was between 17.0±1.43 (ST2) and 36.6±0.99 (35S3) aphids per plant, which corresponds to 24.9-53.5% of the aphid population in non-transformed plants. The results of our study suggest that GNA expressed in transgenic potato plants confers a potential tolerance to aphid attack, which appears to be an alternative against the use of pesticides in the future.

Targeted and Untargeted Approaches Unravel Novel Candidate Genes and Diagnostic SNPs for Quantitative Resistance of the Potato (Solanum tuberosum L.) to Phytophthora infestans Causing the Late Blight Disease

The oomycete Phytophthora infestans causes late blight of potato, which can completely destroy the crop. Therefore, for the past 160 years, late blight has been the most important potato disease worldwide. The identification of cultivars with high and durable field resistance to P. infestans is an objective of most potato breeding programs. This type of resistance is polygenic and therefore quantitative. Its evaluation requires multi-year and location trials. Furthermore, quantitative resistance to late blight correlates with late plant maturity, a negative agricultural trait. Knowledge of the molecular genetic basis of quantitative resistance to late blight not compromised by late maturity is very limited. It is however essential for developing diagnostic DNA markers that facilitate the efficient combination of superior resistance alleles in improved cultivars. We used association genetics in a population of 184 tetraploid potato cultivars in order to identify single nucleotide polymorphisms (SNPs) that are associated with maturity corrected resistance (MCR) to late blight. The population was genotyped for almost 9000 SNPs from three different sources. The first source was candidate genes specifically selected for their function in the jasmonate pathway. The second source was novel candidate genes selected based on comparative transcript profiling (RNA-Seq) of groups of genotypes with contrasting levels of quantitative resistance to P. infestans. The third source was the first generation 8.3k SolCAP SNP genotyping array available in potato for genome wide association studies (GWAS). Twenty seven SNPs from all three sources showed robust association with MCR. Some of those were located in genes that are strong candidates for directly controlling quantitative resistance, based on functional annotation. Most important were: a lipoxygenase (jasmonate pathway), a 3-hydroxy-3-methylglutaryl coenzyme A reductase (mevalonate pathway), a P450 protein (terpene biosynthesis), a transcription factor and a homolog of a major gene for resistance to P. infestans from the wild potato species Solanum venturii. The candidate gene approach and GWAS complemented each other as they identified different genes. The results of this study provide new insight in the molecular genetic basis of quantitative resistance in potato and a toolbox of diagnostic SNP markers for breeding applications.

Transgenic potato plants expressing cry3A gene confer resistance to Colorado potato beetle

The Colorado potato beetle (Leptinotarsa decemlineata Say, CPB) is a fatal pest, which is a quarantine pest in China. The CPB has now invaded the Xinjiang Uygur Autonomous Region and is constantly spreading eastward in China. In this study, we developed transgenic potato plants expressing cry3A gene. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that the cry3A gene expressed in leaves, stems and roots of the transgenic plants under the control of CaMV 35S promoter, while they expressed only in leaves and stems under the control of potato leaf and stem-specific promoter ST-LS1. The mortality of the larvae was higher (28% and 36%) on the transgenic plant line 35S1 on the 3rd and 4th days, and on ST3 (48%) on the 5th day after inoculation with instar larvae. Insect biomass accumulation on the foliage of the transgenic plant lines 35S1, 35S2 and ST3 was significantly lower (0.42%, 0.43% and 0.42%). Foliage consumption was lowest on transgenic lines 35S8 and ST2 among all plant foliage (7.47 mg/larvae/day and 12.46 mg/larvae/day). The different transgenic plant foliages had varied inhibition to larval growth. The survivors on the transgenic lines obviously were smaller than their original size and extremely weak. The transgenic potato plants with CPB resistance could be used to develop germplasms or varieties for controlling CPB damage and halting its spread in China.

Composite potato plants with transgenic roots on non-transgenic shoots: a model system for studying gene silencing in roots

Composite potato plants offer an extremely fast, effective and reliable system for studies on gene functions in roots using antisense or inverted-repeat but not sense constructs for gene inactivation. Composite plants, with transgenic roots on a non-transgenic shoot, can be obtained by shoot explant transformation with Agrobacterium rhizogenes. The aim of this study was to generate composite potato plants (Solanum tuberosum) to be used as a model system in future studies on root-pathogen interactions and gene silencing in the roots. The proportion of transgenic roots among the roots induced was high (80-100%) in the four potato cultivars tested (Albatros, Desirée, Sabina and Saturna). No wild-type adventitious roots were formed at mock inoculation site. All strains of A. rhizogenes tested induced phenotypically normal roots which, however, showed a reduced response to cytokinin as compared with non-transgenic roots. Nevertheless, both types of roots were infected to a similar high rate with the zoospores of Spongospora subterranea, a soilborne potato pathogen. The transgenic roots of composite potato plants expressed significantly higher amounts of β-glucuronidase (GUS) than the roots of a GUS-transgenic potato line event. Silencing of the uidA transgene (GUS) was tested by inducing roots on the GUS-transgenic cv. Albatros event with strains of A. rhizogenes over-expressing either the uidA sense or antisense transcripts, or inverted-repeat or hairpin uidA RNA. The three last mentioned constructs caused 2.5-4.0 fold reduction in the uidA mRNA expression. In contrast, over-expression of uidA resulted in over 3-fold increase in the uidA mRNA and GUS expression, indicating that sense-mediated silencing (co-suppression) was not functional in roots. The results suggest that composite plants offer a useful experimental system for potato research, which has gained little previous attention.

Genetic approaches for studying transgene inheritance and genetic recombination in three successive generations of transformed tobacco

Transgene integration into plant genomes is a complex process accompanied by molecular rearrangements. Classic methods that are normally used to study transgenic population genetics are generally inadequate for assessing such integration. Two major characteristics of transgenic populations are that a transgenic genome may harbor many copies of the transgene and that molecular rearrangements can create an unstable transgenic locus. In this work, we examined the segregation of T1, T2 and T3 transgenic tobacco progenies. Since transfer DNA (T-DNA) contains the NptII selectable marker gene that confers resistance to kanamycin, we used this characteristic in developing a method to estimate the number of functional inserts integrated into the genome. This approach was based on calculation of the theoretical segregation ratios in successive generations. Mendelian ratios of 3:1, 15:1 and 63:1 were confirmed for five transformation events whereas six transformation events yielded non-segregating progenies, a finding that raised questions about causal factors. A second approach based on a maximum likelihood method was performed to estimate recombination frequencies between linked inserts. Recombination estimates varied among transformation events and over generations. Some transgenic loci were unstable and evolved continuously to segregate independently in the T3 generation. Recombination and amplification of the transgene and filler DNA yielded additional transformed genotypes.

Agroinoculation of Citrus tristeza virus causes systemic infection and symptoms in the presumed nonhost Nicotiana benthamiana

Citrus tristeza virus (CTV) naturally infects only some citrus species and relatives and within these it only invades phloem tissues. Failure to agroinfect citrus plants and the lack of an experimental herbaceous host hindered development of a workable genetic system. A full-genome cDNA of CTV isolate T36 was cloned in binary plasmids and was used to agroinfiltrate Nicotiana benthamiana leaves, with or without coinfiltration with plasmids expressing different silencing-suppressor proteins. A time course analysis in agroinfiltrated leaves indicated that CTV accumulates and moves cell-to-cell for at least three weeks postinoculation (wpi), and then, it moves systemically and infects the upper leaves with symptom expression. Silencing suppressors expedited systemic infection and often increased infectivity. In systemically infected Nicotiana benthamiana plants, CTV invaded first the phloem, but after 7 wpi, it was also found in other tissues and reached a high viral titer in upper leaves, thus allowing efficient transmission to citrus by stem-slash inoculation. Infected citrus plants showed the symptoms, virion morphology, and phloem restriction characteristic of the wild T36 isolate. Therefore, agroinfiltration of Nicotiana benthamiana provided the first experimental herbaceous host for CTV and an easy and efficient genetic system for this closterovirus.

Transgenic plants as tools to study the molecular organization of plant genes

Transgenic plants are generated in nature by Agrobacterium tumefaciens, a pathogen that produces disease through the transfer of some of its own DNA into susceptible plants. The genes are carried on a plasmid. Much has been learned about how the plasmid is transferred, how the plasmid-borne genes are organized, regulated, and expressed, and how the bacteria's pathogenic effects are produced. The A. tumefaciens plasmid has been manipulated for use as a general vector for the transfer of specific segments of foreign DNA of interest (from plants and other sources) into plants; the activities of various genes and their regulation by enhancer and silencer sequences have been assessed. Future uses of the vector (or others like it that have different host ranges) by the agriculture industry are expected to aid in moving into vulnerable plants specific genes that will protect them from such killers as nonselective herbicides, insects, and viruses.

Arabidopsis LHT1 is a high-affinity transporter for cellular amino acid uptake in both root epidermis and leaf mesophyll

Amino acid transport in plants is mediated by at least two large families of plasma membrane transporters. Arabidopsis thaliana, a nonmycorrhizal species, is able to grow on media containing amino acids as the sole nitrogen source. Arabidopsis amino acid permease (AAP) subfamily genes are preferentially expressed in the vascular tissue, suggesting roles in long-distance transport between organs. We show that the broad-specificity, high-affinity amino acid transporter LYSINE HISTIDINE TRANSPORTER1 (LHT1), an AAP homolog, is expressed in both the rhizodermis and mesophyll of Arabidopsis. Seedlings deficient in LHT1 cannot use Glu or Asp as sole nitrogen sources because of the severe inhibition of amino acid uptake from the medium, and uptake of amino acids into mesophyll protoplasts is inhibited. Interestingly, lht1 mutants, which show growth defects on fertilized soil, can be rescued when LHT1 is reexpressed in green tissue. These findings are consistent with two major LHT1 functions: uptake in roots and supply of leaf mesophyll with xylem-derived amino acids. The capacity for amino acid uptake, and thus nitrogen use efficiency under limited inorganic N supply, is increased severalfold by LHT1 overexpression. These results suggest that LHT1 overexpression may improve the N efficiency of plant growth under limiting nitrogen, and the mutant analyses may enhance our understanding of N cycling in plants.

Enhancement of virus-induced gene silencing through viral-based production of inverted-repeats

Plant virus-based vectors carrying sequences homologous to endogenous genes trigger silencing through a homology-dependent RNA degradation mechanism. This phenomenon, called virus-induced gene silencing (VIGS), has potential as a powerful reverse-genetics tool in functional genomic programmes through transient, loss-of-function screens. Here, we describe a method to enhance the robustness of the VIGS phenotype by increasing the level of dsRNA molecule production, a critical step in the VIGS response. Incorporation of 40-60 base direct inverted-repeats into a plant viral vector generates RNA molecules that form dsRNA hairpins. A tobacco mosaic virus (TMV)-based vector carrying such inverted-repeats, homologous to a green fluorescent protein (gfp) transgene or an endogenous phytoene desaturase (pds) gene, generated a stronger and more pervasive VIGS phenotype than constructs carrying corresponding cDNA fragments in sense or antisense orientation. Real-time RT-PCR indicated that there was up to a threefold reduction in target mRNA accumulation in the tissues where VIGS was triggered by constructs carrying inverted-repeats compared to those where it was triggered by sense or antisense constructs. Moreover, an enhanced VIGS pds phenotype was observed using a different vector, based on barley stripe mosaic virus, in the monocotyledonous host barley. This demonstrates that VIGS can be significantly improved through the inclusion of small inverted-repeats in plant virus-based vectors, generating a more robust loss-of-function phenotype. This suggests that dsRNA formation can be a limiting factor in the VIGS phenomenon.

Direct measurement of the transfer rate of chloroplast DNA into the nucleus

Gene transfer from the chloroplast to the nucleus has occurred over evolutionary time. Functional gene establishment in the nucleus is rare, but DNA transfer without functionality is presumably more frequent. Here, we measured directly the transfer rate of chloroplast DNA (cpDNA) into the nucleus of tobacco plants (Nicotiana tabacum). To visualize this process, a nucleus-specific neomycin phosphotransferase gene (neoSTLS2) was integrated into the chloroplast genome, and the transfer of cpDNA to the nucleus was detected by screening for kanamycin-resistant seedlings in progeny. A screen for kanamycin-resistant seedlings was conducted with about 250,000 progeny produced by fertilization of wild-type females with pollen from plants containing cp-neoSTLS2. Sixteen plants of independent origin were identified and their progenies showed stable inheritance of neoSTLS2, characteristic of nuclear genes. Thus, we provide a quantitative estimate of one transposition event in about 16,000 pollen grains for the frequency of transfer of cpDNA to the nucleus. In addition to its evident role in organellar evolution, transposition of cpDNA to the nucleus in tobacco occurs at a rate that must have significant consequences for existing nuclear genes.

Use of a vector based on Potato virus X in a whole plant assay to demonstrate nuclear targeting of Potato spindle tuber viroid

Potato spindle tuber viroid (PSTVd) is a covalently closed circular RNA molecule of 359 nucleotides that replicates within the nucleus of host cells. To determine how this small, highly structured RNA enters the nucleus, we have developed a virus-based, whole plant in vivo assay that uses green fluorescent protein (GFP) as the reporter molecule. The coding region of GFP was interrupted by insertion of an intron derived from the intervening sequence 2 of the potato ST-LS1 gene. A cDNA copy of the complete PSTVd genome was, in turn, embedded within the intron, and this construct was delivered into Nicotiana benthamiana plants via a vector based on Potato virus X. The intron-containing GFP subgenomic RNA synthesized during virus infection cannot produce a functional GFP unless the RNA is imported into the nucleus, where the intron can be removed and the spliced RNA returned to the cytoplasm. The appearance of green fluorescence in leaf tissues inoculated with constructs containing a full-length PSTVd molecule embedded in the intron indicates that nuclear import and RNA splicing events did occur.

Intron-tagged epitope: a tool for facile detection and purification of proteins expressed in Agrobacterium-transformed plant cells

Epitope tagging provides a useful tool for immunological detection and cellular localization of proteins in vivo. Using T-DNA-mediated transformation, the detection of epitope-tagged proteins in planta is currently feasible only in transgenic plants, because an artificial expression of cDNA and gene constructs driven by plant promoters in bacteria obscures an early detection of epitope-tagged proteins in Agrobacterium-infected plant cells. We have developed a method for labelling plant coding sequences with intron-tagged epitope-coding domains that are not processed in Agrobacterium. Here we show that the expression of HA-epitope-tagged constructs encoding beta-glucuronidase and S-phase kinase-associated (AtSKP1/ASK1) proteins can be specifically and exclusively detected in cultured Arabidopsis cells as early as five days after Agrobacterium infection. This epitope-tagging approach offers an unlimited source of transformed material for purification and localization of proteins expressed individually or simultaneously in Agrobacterium-transformed plant cells.

A 200 bp region of the pea ENOD12 promoter is sufficient for nodule-specific and nod factor induced expression

ENOD12 is one of the first nodulin genes expressed upon inoculation with Rhizobium and also purified Nod factors are able to induce ENOD12 expression. The ENOD12 gene family in pea (Pisum sativum) has two members. A cDNA clone representing PsENOD12A [26] and a PsENOD12B genomic clone [7] have been previously described. The isolation and characterization of a PsENOD12A genomic clone is presented in this paper. By using a Vicia hirsuta-Agrobacterium rhizogenes transformation system it is shown that both genes have a similar expression pattern in transgenic V. hirsuta root nodules. Promoter analyses of both PsENOD12 promoters showed that the 200 bp immediately upstream of the transcription start are sufficient to direct nodule-specific and Nod factor-induced gene expression.

Air pollutant stress changes the steady-state transcript levels of three photosynthesis genes

Six-week-old tobacco plants (Nicotiana tabacum L., cv. SR-1) were fumigated with different, but defined and realistic combinations of NO(2), automobile exhaust and ozone for two days. To determine the effect of an additional environmental stress during exposure, plants were either wounded mechanically or treated with salicylic acid to simulate pathogen attack. The steady-state mRNA levels of genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS), chlorophyll a/b-binding protein (cab) and a 10 kDa protein of the water-evolving complex of photosystem II (ST-LS1) were probed in Northern analysis. Our results clearly show a decline of the mRNA levels for all three photosynthesis genes under automobile exhaust and/or ozone fumigation. Wounding and also elicitor treatment enhance this effect. In contrast, exposure to NO(2) either increased the transcription level of all three genes, or counteracted the negative effect of automobile exhaust and ozone on their expression. At this time no major changes in the concentrations of the corresponding proteins could be detected by Western blot analyses. These results are discussed in the present paper.

Expression of the alpha-thionin gene from barley in tobacco confers enhanced resistance to bacterial pathogens

Thionins are cysteine-rich, 5 kDa polypeptides which are toxic to plant pathogens in vitro. Expression of the gene encoding alpha-thionin from barley endosperm, under the 35S promoter from cauliflower mosaic virus, conferred to transgenic tobacco enhanced resistance to the bacterial plant pathogens Pseudomonas syringae pv. tabaci 153 and P. syringae pv. syringae. The barley alpha-thionin gene, which has two introns, was correctly spliced in tobacco. The alpha-thionin in transgenic plants had the expected mobility in the gradient, when separated by high-performance liquid chromatography, reacted with monospecific antibodies and showed the expected antibiotic properties in vitro.

Functional tagging of regulatory elements in the plant genome

In comparison with animals, relatively few plant genes have been identified that have been shown to be under organ-, tissue- or cell-type-specific regulation. In this paper, we describe how the beta-glucuronidase (GUS) reporter gene (gusA or uidA), fused to a weak promoter (a truncated (-90 bp) CaMV35S promoter), can be used to identify tissue-specific markers in transgenic tobacco plants. The rationale was that the expression of gusA would be determined primarily by position effect. Quantitative analysis revealed that, of 184 -90-gus transgenic plants, 73% exhibited gusA gene activation in leaf tissue, and the level of GUS enzyme activity varied over a 300-fold range within the population. In comparison, transformation with a promoterless gusA gene resulted in GUS expression in 78% of all plants analyzed (in leaf and/or root) and expression levels were three-fold or more lower. Qualitative GUS analysis of single locus -90-gus transformants revealed differential expression in diverse tissues. The spatial pattern of GUS activity was unique to individual transformants, was a reflection of differential gusA gene transcription, and was stably transmissible to progeny. Evidence for preferential expression in roots not only of the -90-gus, but also the promoterless gusA gene is presented. The value of the -90 bp promoter-gusA sequence, which is termed an 'interposon', as a tool both to identify native enhancer sequences in situ and to investigate position effects in plants, is discussed.

The effect of T-DNA copy number, position and methylation on reporter gene expression in tobacco transformants

Inter-transformant variability in the expression of introduced genes was studied in the R1 and R2 generations of 10 tobacco transformants, produced by Agrobacterium-mediated transformation. In replicated and physiologically equivalent material, tranformants showed considerable variability in the expression of the reporter gene uidA as shown by transcript levels and beta-glucuronidase (GUS) activity. However, homozygous R2 material could be investigated for seven of the transformants and among these, and in one line in which two inserts could segregate independently, this inter-transformant variability was reduced to simple bimodal expression. The two levels of expression for GUS activity in leaves were high or low (approximately 2.5 or 0.3 nmol cm-2 min-1 respectively), with no continuous variation. Transformants in the high group had single T-DNA insertions, while those in the low group had multiple T-DNA insertions, at the same or different loci. Within each group, although T-DNA was apparently integrated at different sites in the plant genome, there was no evidence of position effects. GUS activity levels of the transformants were very similar in the field and in environmentally controlled conditions under high or low light. Plants with multiple insertions and low expression also tended to have increased methylation of the integrated T-DNA.

Comparison of the expression of two highly homologous members of the soybean ribulose-1,5-bisphosphate carboxylase small subunit gene family

Two soybean ribulose-1,5-bisphosphate carboxylase small subunit (SSU) genes, SRS1 and SRS4, are highly homologous over a region that includes 4 kb of 5' and 1 kb of 3' flanking sequences. The expression of these genes was compared using synthetic oligonucleotide probes. Analysis of a soybean leaf cDNA library indicates that SRS1 and SRS4 are the most highly expressed members of the soybean SSU gene family. Similar changes were observed in the RNA levels for these genes in response to white light, far-red light and darkness, although SRS1 was expressed at a four-fold higher level in total RNA than SRS4 under all conditions. However, nuclear run-on assays indicate that SRS1 is transcribed at a lower rate than SRS4, which suggests that SRS1 RNA is more stable. S1 nuclease analysis and oligonucleotide directed RNase H cleavage indicate that transcripts from both genes are polyadenylated within two principle regions separated by 35 nt. Sequence analysis of 16 independent cDNA clones identified seven different polyadenylation sites, and six of these sites lie within these two regions. Although SRS1 RNA was poorly recovered during poly(A)+ fractionation, RNase H cleavage experiments showed that transcripts from SRS1 and SRS4 had similar poly (A) tail lengths ranging from 0 to 220 nt. In addition, and despite differences in the untranslated leader sequences, SRS1 and SRS4 RNAs are assembled into polysomes with equal efficiencies. The overall similarity in expression patterns for these two genes further illustrates the coordinate evolution of individual members of a SSU gene family and is consistent with the proposal that gene conversion homogenizes both the coding and regulatory regions of these genes.

Overproduction of alfalfa glutamine synthetase in transgenic tobacco plants

We have obtained transgenic tobacco plants overexpressing the enzyme glutamine synthetase (GS) by fusing an alfalfa GS gene to the cauliflower mosaic virus 35S promotor and integrating it into Nicotiana tabacum var. W38 plants by Agrobacterium tumefaciens mediated gene transfer. The amount of RNA specific to alfalfa GS was about 10 times higher in transgenic tobacco plants than in alfalfa. The alfalfa GS produced by these transgenic plants was identified by Western blotting and represented 5% of total soluble protein in the transformed plants, amounting to a 5-fold increase in specific GS activity and in a 20-fold increase in resistance to the GS inhibitor L-phosphinothricin in vitro. Tissue from GS overproducing plants showed a sevenfold lower amount of free NH3. The amino acid composition of the plant tissue was not altered significantly by GS overproduction. GS overproducing plants were fertile and grew normally. These data show that a high level of expression of a key metabolic enzyme such as glutamine synthetase does not interfere with growth and fertility of plants.

A 10 kDa polypeptide associated with the oxygen-evolving complex of photosystem II has a putative C-terminal non-cleavable thylakoid transfer domain

The N-terminal amino acid sequence of the 10 kDa polypeptide associated with the oxygen-evolving complex of wheat photosystem II has been determined and shown to be homologous to the amino acid sequence of the product of the ST-LS1 gene from potato. The N-terminal sequence of the mature protein indicates that the polypeptide is synthesized with a 39 amino acid N-terminal presequence which is similar to chloroplast import sequences but which lacks a hydrophobic domain for transfer of the protein across the thylakoid membrane. The mature polypeptide has a C-terminal hydrophobic region which shows homology to the hydrophobic thylakoid transfer domain of other lumenal proteins and this hydrophobic region of the 10 kDa polypeptide is suggested to facilitate transfer of the protein across the thylakoid membrane.

Primary structures and catalytic properties of isoenzymes encoded by the two 4-coumarate: CoA ligase genes in parsley

We have determined the primary structures of two 4-coumarate: CoA ligase (4CL) isoenzymes in parsley (Petroselinum crispum) by sequencing near full-length cDNAs corresponding to the two 4CL genes, Pc4CL-1 and Pc4CL-2, present in this plant. Comparison of the cDNA and genomic nucleotide sequences showed that each 4CL gene is organized in five exons separated by introns of varying lengths. The positions of introns are the same in both genes and 97-99% of the corresponding nucleotide sequences are identical. The two isoenzymes, which are nearly identical in their primary structures, were separated by ion-exchange chromatography, and were found to be indistinguishable with regard to substrate specificity. Assignment to Pc4CL-1 and Pc4CL-2 was achieved by comparison with catalytically active 4CL proteins, isolated from Escherichia coli cells which had been transformed with plasmids harboring the corresponding cDNAs.

Introduction of foreign genes into potato cultivars Bintje and Désirée using an Agrobacterium tumefaciens binary vector

Tuber discs of Solanum tuberosum cv Bintje and Désirée were cocultivated with an Agrobacterium tumefaciens binary vector, carrying both the neomycine phosphotransferase and the E. coli β-glucuronidase gene fused to resp. the nopaline synthase and Cauliflower mosaic virus 35S promotor.Inoculated tuber discs produce transgenic shoots in selective media containing kanamycin. The transgenic plants are phenotypically normal and contain the euploid number of chromosomes. Both the neomycin phosphotransferase as well as the β-glucuronidase gene are expressed conferring resp. kanamycin resistance and β-glucuronidase activity to the plants.

The 5' flanking DNA of a patatin gene directs tuber specific expression of a chimaeric gene in potato

A member of the patatin multigene family which encodes the major soluble tuber protein was isolated from potato cultivar Désirée. Analysis by strategic nucleotide sequencing demonstrated close homology to analogous regions of previously isolated patatin genomic clones from different cultivars. A 3.8-kb fragment containing the promoter and 5' flanking DNA of the patatin gene was used to construct a transcriptional fusion gene with the coding DNA of the bacterial chloramphenicol acetyltransferase (CAT) gene and the polyadenylation/termination sequences of the nopaline synthase gene (nos). The chimaeric gene was reintroduced into potato cultivar Désirée by agrobacterial transformation of tissue slices. Regenerated transformed plants showed expression of the chimaeric gene (as determined by CAT activity) in tubers, but not in leaves, stems or roots of in vitro grown plants. Independent transformants did not show substantial variation in the level of induced tuber-specific CAT activity. Thus, information contained within 3.8 kb of the 5' flanking DNA of the patatin gene analysed is sufficient to direct tuber-specific expression, largely independent of position effects.