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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. By grafting sexually incompatible species, we show that complete chloroplast genomes can travel across the graft junction from one species into another. We demonstrate that, consistent with reported phylogenetic evidence, replacement of the resident plastid genome by the alien genome occurs in the absence of intergenomic recombination. Our results provide a plausible mechanism for organelle capture in plants and suggest natural grafting as a path for horizontal gene and genome transfer between sexually incompatible species.

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. Analysis of the graft sites revealed the frequent occurrence of cells harboring both antibiotic resistances and both fluorescent reporters. Our data demonstrate that plant grafting can result in the exchange of genetic information via either large DNA pieces or entire plastid genomes. This observation of novel combinations of genetic material has implications for grafting techniques and also provides a possible path for horizontal gene transfer.

The formation of a new species can occur by an asexual mechanism by transfer of entire nuclear genomes between plant cells as shown by the creation of a new allopolyploid plant from parental herbaceous and woody plant species, this mechanism is a potential new tool for crop improvement. Speciation by growing together Grafting is a common occurrence in nature and is familiar as a means of manipulating plants and trees for use in agriculture and horticulture. Here Ralph Bock and colleagues demonstrate that entire nuclear genomes can be transferred across the graft junction from plant to plant. In grafting experiments between the tree tobacco Nicotiana glauca and the cigarette tobacco N. tabacum (woody and herbaceous species, respectively), horizontal transfer of nuclear genomes can result in the formation of a new polyploid species — Nicotiana tabauca . This is an example of allopolyploidization, the combination of the genomes from two different species that has contributed to evolutionary innovation, adaptation, speciation and domestication. It is thought to occur through hybridization events between species, accompanied or followed by genome duplication, but this work shows that it can also occur through an asexual mechanism that is readily available as a potential tool for crop improvement. 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 1 , 2 , 3 , 4 . 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 5 , 6 . It is generally thought that allopolyploidization occurred through hybridization events between species, accompanied or followed by genome duplication 6 , 7 . Although many allopolyploids arose from closely related species (congeners), there are also allopolyploid species that were formed from more distantly related progenitor species belonging to different genera or even different tribes 8 . Here we have examined the possibility that allopolyploidization can also occur by asexual mechanisms. We show that upon grafting—a mechanism of plant–plant interaction that is widespread in nature—entire nuclear genomes can be transferred between plant cells. We provide direct evidence for this process resulting in speciation by creating a new allopolyploid plant species from a herbaceous species and a woody species in the nightshade family. The new species is fertile and produces fertile progeny. Our data highlight natural grafting as a potential asexual mechanism of speciation and also provide a method for the generation of novel allopolyploid crop species.

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. Here, we report the successful experimental reconstruction of functional gene transfer between an organelle and the nucleus, a process that normally occurs only on large evolutionary timescales. In consecutive genetic screens, we first transferred a chloroplast genome segment to the nucleus and then selected for gene activation in the nuclear genome. We show that DNA-mediated gene transfer can give rise to functional nuclear genes if followed by suitable rearrangements in the nuclear genome. Acquisition of gene function involves (1) transcriptional activation by capture of the promoter of an upstream nuclear gene and (2) utilization of AT-rich noncoding sequences downstream of the plastid gene as RNA cleavage and polyadenylation sites. Our results reveal the molecular mechanisms of how organellar DNA transferred to the nucleus gives rise to functional genes and reproduce in the laboratory a key process in the evolution of eukaryotic cells.

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. We show that PsaI is not required for the redox reactions of PSI. Neither plastocyanin oxidation nor the processes at the PSI acceptor side are impaired in the mutant, and both linear and cyclic electron flux rates are unaltered. The PSI antenna cross section is unaffected, state transitions function normally, and binding of other PSI subunits to the reaction centre is not compromised. Under a wide range of growth conditions, the mutants are phenotypically and physiologically indistinguishable from wild-type tobacco. However, in response to high-light and chilling stress, and especially during leaf senescence, PSI content is reduced in the mutants, indicating that the I-subunit plays a role in stabilizing PSI complexes.

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. We describe here a genetic system based on transgenic chloroplasts carrying a nuclear selectable marker gene that allows the efficient selection of plants with a nuclear genome that carries pieces transferred from the chloroplast genome. We can select such gene transfer events from a surprisingly small population of plant cells, indicating that the escape of genetic material from the chloroplast to the nuclear genome occurs much more frequently than generally believed and thus may contribute significantly to intraspecific and intraorganismic genetic variation.

The formation of a new species can occur by an asexual mechanism by transfer of entire nuclear genomes between plant cells as shown by the creation of a new allopolyploid plant from parental herbaceous and woody plant species, this mechanism is a potential new tool for crop improvement.

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. Using DNA cleavage with methylation-sensitive and methylation-dependent restriction endonucleases, we show that the plastid genomes in the transplastomic plants are efficiently methylated. All transplastomic lines are phenotypically indistinguishable from wild-type plants and, moreover, show no alterations in plastid gene expression. Our data indicate that the expression of plastid genes is not sensitive to DNA methylation and, hence, suggest that DNA methylation is unlikely to be involved in the transcriptional regulation of plastid gene expression.

Eukaryoti 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. Here, we report the successful experimental reconstruction of functional gene transfer between an organelle and the nucleus, a process that normally occurs only on large evolutionary timescales. In consecutive genetic screens, we first transferred a chloroplast genome segment to the nucleus and then selected for gene activation in the nuclear genome. We show that DNA-mediated gene transfer can give rise to functional nuclear genes if followed by suitable rearrangements in the nuclear genome. Acquisition of gene function involves (1) transcriptional activation by capture of the promoter of an upstream nuclear gene and (2) utilization of AT-rich noncoding sequences downstream of the plastid gene as RNA cleavage and polyadenylation sites. Our results reveal the molecular mechanisms of how organellar DNA transferred to the nucleus gives rise to functional genes and reproduce in the laboratory a key process in the evolution of eukaryotic cells.

Background Coronavirus disease 2019 has become a health problem spreading worldwide with pandemic characteristics since March 2020. Post coronavirus disease 2019 symptoms are more frequent than initially expected, with fatigue as an often-mentioned issue. Case presentations We describe a 32-year-old white male and a 55-year-old white female who suffered from post coronavirus disease 2019 fatigue syndrome. On polysomnography, rapid eye movement associated sleep apnea with an increased hypopnea index during rapid eye movement phases of 36.8 and 19.5 events per hour was found. Based on the patients’ burdensome fatigue symptoms, we initiated automatic positive airway pressure therapy, which diminished sleep apnea (rapid eye movement index: 0.0 in both patients) and, consequently, also the fatigue symptoms. Conclusions Since sleep apnea and coronavirus disease 2019 are both associated with fatigue, a screening for sleep apnea might be considered in coronavirus disease 2019 patients with fatigue syndrome.