Ds read-out transcription in transgenic tomato plants

Localization of Ds-transposon containing T-DNA inserts in the diploid transgenic potato: linkage to the R1 resistance gene against Phytophthora infestans (Mont.) de Bary

The Dissociation transposable element (Ds) of maize containing NPTII was introduced into the diploid potato (Solanum tuberosum) clone J91-6400-A16 through Agrobacterium tumefaciens mediated transformation. Genomic DNA sequences flanking the T-DNAs from 312 transformants were obtained with inverse polymerase chain reaction or plasmid rescue techniques and used as probes for RFLP linkage analysis. The RFLP map location of 60 T-DNAs carrying Ds-NPTII was determined. The T-DNA distribution per chromosome and the relative distance between them appeared to be random. All 12 chromosomes have been covered with Ds-containing T-DNAs, potentially enabling tagging of any gene in the potato genome. The T-DNA insertions of two transformants, BET92-Ds-A16-259 and BET92-Ds-A16-416, were linked in repulsion to the position of the resistance gene R1 against Phytophthora infestans. After crossing BET92-Ds-A16-416 with a susceptible parent, 4 desired recombinants (Ds carrying T-DNA linked in coupling phase with the R1 gene) were discovered. These will be used for tagging the R1 gene. The efficiency of the pathway from the introduction to localization of T-DNAs is discussed. Key words : Solanum tuberosum, Phytophthora infestans, Ds element, transposon tagging, R genes, euchromatin.

The activator/dissociation transposable elements comprise a two-component gene regulatory switch that controls endogenous gene expression in maize

The maize Activator/Dissociation (Ac/Ds) elements are able to replicate and transpose throughout the maize genome. Both elements preferentially insert into gene-rich regions altering the maize genome by creating unstable insertion alleles, stable derivative or excision alleles, or by altering the spatial or temporal regulation of gene expression. Here, we characterize an Ac insertion in the 5'-UTR of the Pink Scutellum1 (Ps1) gene and five Ds derivatives generated through abortive transposition events. Characterization of Ps1 transcription initiation sites in this allelic series revealed several that began within the terminus of the Ac and Ds elements. Transcripts originating within Ds or Ac accumulated to lower levels than the wild-type Ps1 allele, but were often sufficient to rescue the seedling lethal phenotype associated with severe loss-of-function alleles. Transcription initiation sites were similar in Ac and Ds derivatives, suggesting that Ac transposase does not influence transcript initiation site selection. However, we show that Ac transposase can negatively regulate Ps1 transcript accumulation in a subset of Ds-insertion alleles resulting in a severe mutant phenotype. The role of maize transposons in gene evolution is discussed.

An orange ripening mutant links plastid NAD(P)H dehydrogenase complex activity to central and specialized metabolism during tomato fruit maturation

In higher plants, the plastidial NADH dehydrogenase (Ndh) complex supports nonphotochemical electron fluxes from stromal electron donors to plastoquinones. Ndh functions in chloroplasts are not clearly established; however, its activity was linked to the prevention of the overreduction of stroma, especially under stress conditions. Here, we show by the characterization of Orr(Ds), a dominant transposon-tagged tomato (Solanum lycopersicum) mutant deficient in the NDH-M subunit, that this complex is also essential for the fruit ripening process. Alteration to the NDH complex in fruit changed the climacteric, ripening-associated metabolites and transcripts as well as fruit shelf life. Metabolic processes in chromoplasts of ripening tomato fruit were affected in Orr(Ds), as mutant fruit were yellow-orange and accumulated substantially less total carotenoids, mainly beta-carotene and lutein. The changes in carotenoids were largely influenced by environmental conditions and accompanied by modifications in levels of other fruit antioxidants, namely, flavonoids and tocopherols. In contrast with the pigmentation phenotype in mature mutant fruit, Orr(Ds) leaves and green fruits did not display a visible phenotype but exhibited reduced Ndh complex quantity and activity. This study therefore paves the way for further studies on the role of electron transport and redox reactions in the regulation of fruit ripening and its associated metabolism.

A genome-wide transcriptional activity survey of rice transposable element-related genes

A genome-wide survey of the transcriptional activity of TE-related genes that were associated with fifteen developmental stages and stress conditions revealed clear, albeit low, general transcription of TE-related genes.

Background

Transposable element (TE)-related genes comprise a significant portion of the gene catalog of grasses, although their functions are insufficiently characterized. The recent availability of TE-related gene annotation from the complete genome sequence of rice (Oryza sativa) has created an opportunity to conduct a comprehensive evaluation of the transcriptional activities of these potentially mobile elements and their related genes.

Results

We conducted a genome-wide survey of the transcriptional activity of TE-related genes associated with 15 developmental stages and stress conditions. This dataset was obtained using a microarray encompassing 2,191 unique TE-related rice genes, which were represented by oligonucleotide probes that were free from cross-hybridization. We found that TE-related genes exhibit much lower transcriptional activities than do non-TE-related genes, although representative transcripts were detected from all superfamilies of both type I and II TE-related genes. The strongest transcriptional activities were detected in TE-related genes from among the MULE and CACTA superfamilies. Phylogenetic analyses suggest that domesticated TE-related genes tend to form clades with active transcription. In addition, chromatin-level regulations through histone and DNA modifications, as well as enrichment of certain cis elements in the promoters, appear to contribute to the transcriptional activation of representative TE-related genes.

Conclusion

Our findings reveal clear, albeit low, general transcription of TE-related genes. In combination with phylogenetic analysis, transcriptional analysis has the potential to lead to the identification of domesticated TEs with adapted host functions.

Dissociation (Ds) constructs, mapped Ds launch pads and a transiently-expressed transposase system suitable for localized insertional mutagenesis in rice

We have developed a transiently-expressed transposase (TET)-mediated Dissociation (Ds) insertional mutagenesis system for generating stable insertion lines in rice which will allow localized mutagenesis of a chromosomal region. In this system, a Ds containing T-DNA construct was used to produce Ds launch pad lines. Callus tissues, from single-copy Ds/T-DNA lines, were then transiently infected with Agrobacterium harbouring an immobile Ac (iAc) construct, also containing a green fluorescent protein gene (sgfpS65T) as the visual marker. We have regenerated stable Ds insertion lines at a frequency of 9-13% using selection for Ds excision and GFP counter selection against iAc and nearly half of them were unique insertion lines. Double transformants (iAc/Ds) were also obtained and their progeny yielded approximately 10% stable insertion lines following excision and visual marker screening with 50% redundancy. In general, more than 50% of the Ds reinsertions were within 1 cM of the launch pad. We have produced a large number of single-copy Ds/T-DNA launch pads distributed over the rice chromosomes and have further refined the Ds/T-DNA construct to enrich for "clean" single-copy T-DNA insertions. The availability of single copy "clean" Ds/T-DNA launch pads will facilitate chromosomal region-directed insertion mutagenesis. This system provides an opportunity for distribution of gene tagging tasks among collaborating laboratories on the basis of chromosomal locations.

Specific expression of the Fusarium transposon Fot1 and effects on target gene transcription

The Fot1 transposon is active in some strains of the plant pathogenic fungus Fusarium oxysporum. In a high-copy-number strain that contains autonomous elements, we have detected a transcript of 1.7 kb hybridizing to Fot1 in very low amounts. Mapping the 3' and 5' termini of this transcript confirms that it corresponds to a Fot1-specific transcript. In this strain, five independent mutants of the transgene (niaD) encoding nitrate reductase have arisen by insertion of Fot1 into the third intron. The analysis of the effect of Fot1 insertion in these mutants shows that, depending on the orientation of Fot1 relative to niaD, different truncated chimeric niaD-Fot1 transcripts are produced. Mapping the 5' and 3' ends of these transcripts reveals (i) premature polyadenylation at sites present in the 5' and 3' untranslated regions of Fot1, and (ii) initiation of some transcripts in the 3' part of the niaD gene at sites located immediately downstream of the Fot1 insertion. Thus, a novel promoter, associated with the end of Fot1, directs transcriptional activity outwards from the element into the coding sequence of the niaD gene. These effects demonstrate that Fot1 insertion provides an additional general mechanism controlling fungal gene expression.

THE TOMATO-CLADOSPORIUM FULVUM INTERACTION: A Versatile Experimental System to Study Plant-Pathogen Interactions

Over the past 20 years, the interaction between the biotrophic fungal pathogen Cladosporium fulvum and tomato has developed into a versatile experimental system for molecular plant pathology and resistance breeding. This interaction provided the resources for cloning of fungal avirulence genes for the first time and interesting clues on recognition of their extracellular products by tomato, as well as mechanisms employed by the fungus to circumvent this recognition. A wealth of information has become available on the structure and genomic organization of Cf resistance genes. The occurrence of many clustered Cf homologues allows the generation of new genes with additional recognitional specificities by reshuffling. It is anticipated that potentially all proteins secreted by C. fulvum are recognized by one or more individuals in a population of tomato genotypes, a hypothesis that has been experimentally confirmed. The future challenge will be to elucidate the mechanisms of perception of avirulence factors and the subsequent signaling eventually leading to activation of host defense responses.

Identification and Ds-tagged isolation of a new gene at the Cf-4 locus of tomato involved in disease resistance to Cladosporium fulvum race 5

Leaf mould disease in tomato is caused by the biotrophic fungus Cladosporium fulvum. An Ac/Ds targeted transposon tagging strategy was used to isolate the gene conferring resistance to race 5 of C. fulvum, a strain expressing the avirulence gene Avr4. An infection assay of 2-week-old seedlings yielded five susceptible mutants, of which two had a Ds element integrated in the same gene at different positions. This gene, member of a gene family, showed high sequence homology to the C. fulvum resistance genes Cf-9 and Cf-2. The gene is predicted to encode an extracellular transmembrane protein containing a divided domain of 25 leucine-rich repeats. Three mutants exhibited a genomic deletion covering most of the Lycopersicon hirsutum introgressed segment, including the Cf-4 locus. Southern blot analysis revealed that this deletion includes the tagged gene and five homologous sequences. To test whether the tagged gene confers resistance to C. fulvum via Avr4 recognition, the Avr4 gene was expressed in planta. Surprisingly, expression of the Avr4 gene still triggered a specific necrotic response in the transposon-tagged plants, indicating that the tagged resistance gene is not, or is not the only gene, involved in Avr4 recognition. Mutants harbouring the genomic deletion did not show this Avr4-specific response. The deleted segment apparently contains, in addition to the tagged gene, one or more other genes, which play a role in the Avr4 responses. The tagged gene is present at the Cf-4 locus, but it does not necessarily recognize Avr4 and is therefore designated Cf-4A.

A promoter identified in the 3' end of the Ac transposon can be activated by cis-acting elements in transgenic Arabidopsis lines

In experiments directed to develop a promoter trap strategy in Arabidopsis, using a Ds chimaeric element containing a promoterless beta-glucuronidase (GUS) gene, we identified a promoter in the 3' end region of the Ac transposable element. The promoter initiates most of the transcripts at coordinate 4250 in the Ac sequence and is oriented towards the internal part of the element. When fused to a promoterless GUS gene, the promoter allows transient expression in Arabidopsis leaves. After stable integration into the Arabidpsis genome, no GUS activity was observed in most of the transformed lines analysed. Only two of them exhibited different tissue-specific GUS expression. When a CaMV 35S promoter was introduced into the transformation vector, downstream to the reporter gene, a high level of GUS activity was observed in all the transformants. These results strongly suggest that the promoter is not normally expressed at a significant level in Arabidopsis transformed lines except when activated by neighbouring cis-acting enhancer elements. This opens an interesting possibility for using this promoter to develop 'enhancer trap' strategies in Arabidopsis. Since only one Ac transcript, initiating in the 5' end region of the element has been reported to date in maize, the putative biological function of the promoter remains an open question.