Horizontal Transfer of a Synthetic Metabolic Pathway between Plant Species.

Transgene expression from the plastid (chloroplast) genome provides unique advantages, including high levels of foreign protein accumulation, convenient transgene stacking in operons, and increased biosafety due to exclusion of plastids from pollen transmission [1, 2]. However, applications in biotechnology and synthetic biology are severely restricted by the very small number of plant species whose plastid genomes currently can be transformed [3, 4]. Here we report a simple method for the introduction of useful plastid transgenes into non-transformable species.

The plastid-encoded PsaI subunit stabilizes photosystem I during leaf senescence in tobacco.

PsaI is the only subunit of PSI whose precise physiological function has not yet been elucidated in higher plants. While PsaI is involved in PSI trimerization in cyanobacteria, trimerization was lost during the evolution of the eukaryotic PSI, and the entire PsaI side of PSI underwent major structural remodelling to allow for binding of light harvesting complex II antenna proteins during state transitions. Here, we have generated a tobacco (Nicotiana tabacum) knockout mutant of the plastid-encoded psaI gene.

Horizontal genome transfer as an asexual path to the formation of new species.

Allopolyploidization, the combination of the genomes from two different species, has been a major source of evolutionary innovation and a driver of speciation and environmental adaptation. In plants, it has also contributed greatly to crop domestication, as the superior properties of many modern crop plants were conferred by ancient allopolyploidization events. It is generally thought that allopolyploidization occurred through hybridization events between species, accompanied or followed by genome duplication.

Horizontal transfer of chloroplast genomes between plant species.

The genomes of DNA-containing cell organelles (mitochondria, chloroplasts) can be laterally transmitted between organisms, a process known as organelle capture. Organelle capture often occurs in the absence of detectable nuclear introgression, and the capture mechanism is unknown. Here, we have considered horizontal genome transfer across natural grafts as a mechanism underlying chloroplast capture in plants.

Exchange of genetic material between cells in plant tissue grafts.

Tissue grafting includes applications ranging from plant breeding to animal organ transplantation. Donor and recipient are generally believed to maintain their genetic integrity, in that the grafted tissues are joined but their genetic materials do not mix. We grafted tobacco plants from two transgenic lines carrying different marker and reporter genes in different cellular compartments, the nucleus and the plastid.

Insensitivity of chloroplast gene expression to DNA methylation.

Presence and possible functions of DNA methylation in plastid genomes of higher plants have been highly controversial. While a number of studies presented evidence for the occurrence of both cytosine and adenine methylation in plastid genomes and proposed a role of cytosine methylation in the transcriptional regulation of plastid genes, several recent studies suggested that at least cytosine methylation may be absent from higher plant plastid genomes. To test if either adenine or cytosine methylation can play a regulatory role in plastid gene expression, we have introduced cyanobacterial genes for adenine and cytosine DNA methyltransferases (methylases) into the tobacco plastid genome by chloroplast transformation.

The plastome-encoded PsaJ subunit is required for efficient Photosystem I excitation, but not for plastocyanin oxidation in tobacco.

The functions of several small subunits of the large photosynthetic multiprotein complex PSI (Photosystem I) are not yet understood. To elucidate the function of the small plastome-encoded PsaJ subunit, we have produced knockout mutants by chloroplast transformation in tobacco (Nicotiana tabacum). PsaJ binds two chlorophyll-a molecules and is localized at the periphery of PSI, close to both the Lhca2- and Lhca3-docking sites and the plastocyanin-binding site.

Experimental reconstruction of functional gene transfer from the tobacco plastid genome to the nucleus.

Eukaryotic cells arose through the uptake of free-living bacteria by endosymbiosis and their gradual conversion into organelles (plastids and mitochondria). Capture of the endosymbionts was followed by massive translocation of their genes to the genome of the host cell. How genes were transferred from the (prokaryotic) organellar genome to the (eukaryotic) nuclear genome and how the genes became functional in their new eukaryotic genetic environment is largely unknown.

Rapid evolution of RNA editing sites in a small non-essential plastid gene.

Chloroplast RNA editing proceeds by C-to-U transitions at highly specific sites. Here, we provide a phylogenetic analysis of RNA editing in a small plastid gene, petL, encoding subunit VI of the cytochrome b6f complex. Analyzing representatives from most major groups of seed plants, we find an unexpectedly high frequency and dynamics of RNA editing.

High-frequency gene transfer from the chloroplast genome to the nucleus.

Eukaryotic cells arose through endosymbiotic uptake of free-living bacteria followed by massive gene transfer from the genome of the endosymbiont to the host nuclear genome. Because this gene transfer took place over a time scale of hundreds of millions of years, direct observation and analysis of primary transfer events has remained difficult. Hence, very little is known about the evolutionary frequency of gene transfer events, the size of transferred genome fragments, the molecular mechanisms of the transfer process, or the environmental conditions favoring its occurrence.