https://www.cell.com/trends/plant-science/pdf/S1360-1385(02)02251-3.pdf

GATEWAY vectors for Agrobacterium-mediated plant transformation.

It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.

Somaclonal variation — a novel source of variability from cell cultures for plant improvement

During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism’s pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Bacillus thuringiensis and Its Pesticidal Crystal Proteins

The complete nucleotide sequence (155 844 bp) of tobacco (Nicotiana tabacum var. Bright Yellow 4) chloroplast DNA has been determined. It contains two copies of an identical 25 339 bp inverted repeat, which are separated by a 86 684 bp and a 18 482 bp single-copy region. The genes for 4 different rRNAs, 30 different tRNAs, 39 different proteins and 11 other predicted protein coding genes have been located. Among them, 15 genes contain introns. Blot hybridization revealed that all rRNA and tRNA genes and 27 protein genes so far analysed are transcribed in the chloroplast and that primary transcripts of the split genes hitherto examined are spliced. Five sequences coding for proteins homologous to components of the respiratory-chain NADH dehydrogenase from human mitochondria have been found. The 30 tRNAs predicted from their genes are sufficient to read all codons if the ;two out of three' and ;U:N wobble' mechanisms operate in the chloroplast. Two sequences which autonomously replicate in yeast have also been mapped. The sequence and expression analyses indicate both prokaryotic and eukaryotic features of the chloroplast genes.

The complete nucleotide sequence of the tobacco chloroplast genome: its gene organization and expression.

Rice (Oryza sativa), a major staple food, is usually milled to remove the oil-rich aleurone layer that turns rancid upon storage, especially in tropical areas. The remaining edible part of rice grains, the endosperm, lacks several essential nutrients, such as provitamin A. Thus, predominant rice consumption promotes vitamin A deficiency, a serious public health problem in at least 26 countries, including highly populated areas of Asia, Africa, and Latin America. Recombinant DNA technology was used to improve its nutritional value in this respect. A combination of transgenes enabled biosynthesis of provitamin A in the endosperm.

https://science.sciencemag.org/content/sci/287/5451/303.full.pdf

Engineering the Provitamin A (β-Carotene) Biosynthetic Pathway into (Carotenoid-Free) Rice Endosperm

In the absence of efficient transcription termination correct 3'-end processing is an essential step in the synthesis of stable chloroplast mRNAs in higher plants We show here that 3'-end processing in vitro involves endonucleolytic cleavage downstream from the mature terminus, followed by exonucleolytic processing to a stem-loop within the 3'-untranslated region These processing steps require a high molecular weight complex that contains both endoribonucleases and an exoribonuclease In the presence of ancillary RNA binding proteins the complex correctly processes the 3'-end of precursor RNA In the absence of these ancillary proteins 3'-end maturation is prevented and plastid mRNAs are degraded Based on these results we propose a novel mechanism for the regulation of mRNA 3'-end processing and stability in chloroplasts

Chloroplast mRNA 3'-end processing by a high molecular weight protein complex is regulated by nuclear encoded RNA binding proteins.

Novel compositions and methods useful for genetic engineering of plant cells to provide increased expression in the plastids of a plant or plant cell of the Bacillus thuringiensis insecticidal protein.

Expression of bacillus thuringiensis cry proteins in plant plastids

Plastids of higher plants are semi-autonomous cellular organelles that have their own genome and transcription-translation machinery. Examples of plastid functions are photosynthesis and biosynthesis of starch, amino acids, lipids and pigments. Plastid functions are encoded in approximately 120 plastid genes and approximately 3,000 nuclear genes. Although many embryo and seedling lethal nuclear genes are required for chloroplast biogenesis, until now deletion of plastid genes either had no phenotypic consequence (8 genes), or caused a mutant phenotype but did not affect viability (13 genes). Here we identify an essential plastid gene. By using the CRE-lox site-specific recombination system we have deleted clpP1 (caseinolytic protease P1), one of the three genes (clpP1, ycf1 and ycf2) whose disruption had previously only been possible in a fraction of the 1,000-10,000 plastid genome copies in a cell. Loss of the clpP1 gene product, the ClpP1 protease subunit, results in ablation of the shoot system of tobacco plants, suggesting that ClpP1-mediated protein degradation is essential for shoot development.

The plastid clpP1 protease gene is essential for plant development.

The plastid psbA gene encodes the 32 kDa D1 polypeptide of photosystem II. It has previously been shown that the initiation of psbA mRNA translation in tobacco is regulated by sequences outside of the coding region. To identify the cis-acting regulatory sequences involved in the translational control, a series of chimeric uidA genes, encoding the beta-glucuronidase (GUS) reporter enzyme, were introduced into the plastid genome. GUS accumulation in response to the light (135- to 200-fold), and the arrest of uidA mRNA translation in light-grown seedlings following transfer for 2 h to the dark, was observed only if the transgenes contained the psbA 5'-untranslated leader region (UTR). Changes in GUS accumulation were accompanied by little or no changes in the uidA mRNA levels. The data indicate that the initiation of D1 translation in tobacco plastids is controlled via the psbA 5'-UTR.

Translation of psbA mRNA is regulated by light via the 5′-untranslated region in tobacco plastids

Plastid engineering currently relies on DNA delivery by the biolistic process. We report here stable plastid transformation in tobacco by an alternate direct transformation protocol that is based on polyethylene glycol (PEG) treatment of leaf protoplasts in the presence of the transforming DNA. Clones with transformed plastid genomes were selected by spectinomycin resistance encoded by a mutant 16S ribosomal RNA gene. Incorporation of the transforming DNA into the plastid genome was confirmed by two unselected markers, streptomycin resistance and a novel PstI site that flank the spectinomycin resistance mutation in plasmid pZS148. Our simple and inexpensive protocol eliminates the dependence on the particle gun for chloroplast transformation and should facilitate applications of plastome engineering in crops.

Pal Maliga

Papers

Chloroplast mRNA 3'-end processing by a high molecular weight protein complex is regulated by nuclear encoded RNA binding proteins.

Expression of bacillus thuringiensis cry proteins in plant plastids

The plastid clpP1 protease gene is essential for plant development.

Translation of psbA mRNA is regulated by light via the 5′-untranslated region in tobacco plastids

Stable Plastid Transformation in PEG-treated Protoplasts of Nicotiana tabacum