Renate Schmidt

Construction of an Intron-Containing Marker Gene - Splicing of the Intron in Transgenic Plants and Its Use in Monitoring Early Events in Agrobacterium-Mediated Plant Transformation

SummaryAgrobacterium tumefaciens is a commonly used tool for transforming dicotyledonous plants. The underlying mechanism of transformation however is not very well understood. One problem complicating the analysis of this mechanism is the fact that most indicator genes are already active in Agrobacterium, thereby preventing the precise determination of timing and localisation of T-DNA transfer to plant cells. In order to overcome this obstacle a modified prokaryotic indicator gene was constructed. The expression of this indicator gene and its use in analysing early events in Agrobacterium-mediated plant transformation are described. A portable intron, derived from a plant intron, was introduced into the β-glucuronidase (GUS) gene. In transgenic plants containing this chimaeric gene the intron is spliced efficiently, giving rise to GUS enzymatic activity. Mapping of the splice junction indicates the exact removal of the intron. No GUS activity is detected in agrobacteria containing this construct due to the lack of a eukaryotic splicing apparatus in prokaryotes. Early phases after transformation of Arabidopsis cotyledon explants were analysed using this GUS-intron chimaeric gene showing that as early as 36 h after Agrobacterium infection significant GUS activity is detected. In vivo GUS staining of transformed cells clearly shows that quickly proliferating calli expressing GUS activity are formed, mainly at the cut surface. Minor transformation events occur however throughout the whole cotyledon. These data indicate that Agrobacterium-mediated T-DNA transfer to plants is much more efficient than has been judged from experiments where selection is applied immediately. The intron-containing GUS gene can be used as an optimised marker gene in transient and stable transformation experiments.

FCA, a Gene Controlling Flowering Time in Arabidopsis, Encodes a Protein Containing RNA-Binding Domains

A strong promoter of the transition to flowering in Arabidopsis is encoded by FCA. FCA has been cloned and shown to encode a protein containing two RNA-binding domains and a WW protein interaction domain. This suggests that FCA functions in the posttranscriptional regulation of transcripts involved in the flowering process. The FCA transcript is alternatively spliced with only one form encoding the entire FCA protein. Plants carrying the FCA gene fused to the strong constitutive 35S promoter flowered earlier, and the ratio and abundance of the different FCA transcripts were altered. Thus, FCA appears to be a component of a posttranscriptional cascade involved in the control of flowering time.

A novel zinc finger protein is encoded by the Arabidopsis LSD1 gene and functions as a negative regulator of plant cell death

Arabidopsis Isd1 mutants are hyperresponsive to cell death initiators and fail to limit the extent of cell death. Superoxide is a necessary and sufficient signal for cell death propagation. Thus, LSD1 monitors a superoxide-dependent signal and negatively regulates a plant cell death pathway. We isolated LSD1 via its map position. The predicted LSD1 protein contains three zinc finger domains, defined by CxxCxRxxLMYxxGASxVxCxxC. These domains are present in three additional Arabidopsis genes, suggesting that LSD1 defines a zinc finger protein subclass. LSD1 is constitutively expressed, consistent with the mutant phenotype. Alternate splicing gives rise to a low abundance mRNA encoding an extra five amino-terminal amino acids. We propose that LSD1 regulates transcription, via either repression of a prodeath pathway or activation of an antideath pathway, in response to signals emanating from cells undergoing pathogen-induced hypersensitive cell death.

Expression of a yeast-derived invertase in the cell wall of tobacco and Arabidopsis plants leads to accumulation of carbohydrate and inhibition of photosynthesis and strongly influences growth and phenotype of transgenic tobacco plants.

Chimeric genes consisting of the coding sequence of the yeast invertase gene suc 2 and different N‐terminal portions of the potato‐derived vacuolar protein proteinase inhibitor II fused to the 35S CaMV promoter and the poly‐A site of the octopine synthase gene were transferred into tobacco and Arabidopsis thaliana plants using Agrobacterium based systems. Regenerated transgenic plants display a 50‐ to 500‐fold higher invertase activity compared to non‐transformed control plants. This invertase is N‐glycosylated and efficiently secreted from the plant cell leading to its apoplastic location. Whereas expression of the invertase does not lead to drastic changes in transgenic Arabidopsis thaliana plants, transgenic tobacco plants show dramatic changes with respect to development and phenotype. Expression of the invertase leads to stunted growth due to reduction of internodal distances, to development of bleached and/or necrotic regions in older leaves and to suppressed root formation. In mature leaves, high levels of soluble sugars and starch accumulate. These carbohydrates do not show a diurnal turnover. The accumulation of carbohydrate is accompanied by an inhibition of photosynthesis, and in tobacco, by an increase in the rate of respiration. Measurements in bleached versus green areas of the same leaf show that the bleached section contains high levels of carbohydrates and has lower photosynthesis and higher respiration than green sections. It is concluded that expression of invertase in the cell wall interrupts export and leads to an accumulation of carbohydrates and inhibition of photosynthesis.

Silencing in Arabidopsis T-DNA Transformants: The Predominant Role of a Gene-Specific RNA Sensing Mechanism versus Position Effects

Pronounced variability of transgene expression and transgene silencing are commonly observed among independent plant lines transformed with the same construct. Single-copy T-DNA lines harboring reporter genes of various kind and number under the control of a strong promoter were established in Arabidopsis thaliana for a comprehensive analysis of transgene expression. Characterization of 132 independent transgenic lines revealed no case of silencing as a result of site of T-DNA integration. Below a certain number of identical transgenes in the genome, gene copy number and expression were positively correlated. Expression was high, stable over all generations analyzed, and of a comparable level among independent lines harboring the same copy number of a particular transgene. Conversely, RNA silencing was triggered if the transcript level of a transgene surpassed a gene-specific threshold. Transcript level–mediated silencing effectively accounts for the pronounced transgene expression variability seen among transformants. It is proposed that the RNA sensing mechanism described is a genome surveillance system that eliminates RNA corresponding to excessively transcribed genes, including transgenes, and so plays an important role in genome defense.

Physical Map and Organization of Arabidopsis thaliana Chromosome 4

A physical map of Arabidopsis thaliana chromosome 4 was constructed in yeast artificial chromosome clones and used to analyze the organization of the chromosome. Mapping of the nucleolar organizing region and the centromere integrated the physical and cytogenetic maps. Detailed comparison of physical with genetic distances showed that the frequency of recombination varied substantially, with relative hot and cold spots occurring along the whole chromosome. Eight repeated DNA sequence families were found in a complex arrangement across the centromeric region and nowhere else on the chromosome.

MOR1/GEM1 has an essential role in the plant-specific cytokinetic phragmoplast

MOR1 is a member of the MAP215 family of microtubule-associated proteins and is required to establish interphase arrays of cortical microtubules in plant cells. Here we show that MOR1 binds microtubules in vivo, localizing to both cortical microtubules and to areas of overlapping microtubules in the phragmoplast. Genetic complementation of the cytokinesis-defective gemini pollen 1-1 (gem1-1) mutation with MOR1 shows that MOR1 (which is synonymous with the protein GEM1) is essential in cytokinesis. Phenotypic analysis of gem1-1 and gem1-2, which contains a T-DNA insertion, confirm that MOR1/GEM1 is essential for regular patterns of cytokinesis. Both the gem1-1 and gem1-2 mutations cause the truncation of the MOR1/GEM1 protein. In addition, the carboxy-terminal domain of the protein, which is absent in both mutants, binds microtubules in vitro. Our data show that MOR1/GEM1 has an essential role in the cytokinetic phragmoplast.

High efficiency Agrobacterium tumefaciens-mediated transformation of Arabidopsis thaliana leaf and cotyledon explants

A highly efficient and fast Agrobacterium-mediated leaf disc transformation system for the Arabidopsis thaliana L. genotype C24 was developed. This protocol is also amenable to other ecotypes - as could be shown for Landsberg erecta and Wassllewskija. Besides the hygromycin selection also the G418 and kanamycin selection were established. Furthermore the described procedure is appliable not only to leaf explants but also to expanded cotyledons which proved to be an excellent alternative as explant source for transformation experiments.

Chilling sensitivity of Arabidopsis thaliana with genetically engineered membrane lipids.

Upon transfer of a genetically engineered Escherichia coli gene for glycerol‐3‐phosphate acyltransferase (plsB) to Arabidopsis thaliana (L.) Heynh., the gene is transcribed and translated into an enzymatically active polypeptide. This leads to an alteration in fatty acid composition of membrane lipids. From these alterations it is evident that the enzyme is located mainly inside the plastids. The amount of saturated fatty acids in plastidial membrane lipids increased. In particular, the fraction of high‐temperature melting species of phosphatidylglycerol is elevated. These molecules are thought to play a crucial role in determining chilling sensitivity of plants. An increase in sensitivity could be observed in the transgenic plants during recultivation after chilling treatment. Implications for the hypothesis of phosphatidylglycerol‐determined chilling sensitivity are discussed.

Nucleotide sequence of proteinase inhibitor II encoding cDNA of potato (Solanum tuberosum) and its mode of expression

SummaryTwo cDNA clones containing the complete coding region of a developmentally controlled (tuber-specific) as well as environmentally inducible (wound-inducible) gene from potato (Solanum tuberosum) have been sequenced. The open reading frame codes for 154 amino acids. Its sequence is highly homologous to the proteinase inhibitor II from tomato, indicating that the cDNAs encode the corresponding proteinase inhibitor II of potato. In addition the putative potato proteinase inhibitor II contains a sequence which is completely homologous with that of another small peptide proteinase inhibitor from potato, called PCI-I. Evidence is presented that this small peptide is probably derived from the proteinase inhibitor II by posttranslational processing.Northern type experiments using RNA from wounded and nonwounded leaves demonstrate that RNA homologous to the putative proteinase inhibitor II cDNAs accumulates in leaves as a consequence of wounding, whereas normally the expression of this gene is under strict developmental control, since it is detected only in tubers of potato (Rosahl et al. 1986). In addition the induction of this gene in leaves can also be achieved by the addition of different polysaccharides such as poly galacturonic acid or chitosan. In contrast to the induction of its expression by wounding in leaves, wounding of tubers results in a disappearance of the proteinase II inhibitor m-RNA from these organs.