Fate of exogenous DNA in Arabidopsis thaliana. Translocation and integration

Beyond the fringe: when science moves from innovative to nonsense

Microbiology has experienced examples of highly productive researchers who have gone beyond just interpreting their experimental results with hypotheses and published nonsense that was readily recognized as such by readers. Although the most discussed cases of this pathology come from physics, studies of single-celled microorganisms, virology, and immunology have provided many examples. Five cases are described here along with some generalizations. These are the Lamarckian inheritance of acquired characteristics reported by distinguished and experienced researchers, vectorless DNA transfer and incorporation of bacterial DNA into chromosomes of plants years before vector construction of genetically modified plants was invented, water with memory of immunoglobulin IgE, a new electromagnetic radiation method for identifying bacterial and viral pathogens by the discoverer of human immunodeficiency virus, and the claim of isolation of a new bacterial isolate with arsenic replacing phosphorus in DNA. These examples represent very dissimilar areas, and the only common factor is hubris on the part of experienced researchers. Secondarily, failure of peer review sometimes happens, and journal editors do not step in, sometimes even when alerted before publication. These failures of the publishing process teach us that unnecessary mistakes occur and should warn us all to watch our own enthusiasms.

Prospects and perspectives in mutation breeding

Induction of mutations, primarily a method of generating variation, can contribute to plant improvement when combined with selection, or recombination and selection, or with other methods of manipulating genetic variation. As a source of variability, induced mutations supplement naturally occurring variation. When specific mutants are selected following mutagenic treatments it is highly likely that a number of mutational changes will have occurred in the selected genotype. Hence, although most of the mutant varieties released so far have resulted from mutation and direct selection, the future trend will be for increasing use of mutants in association with recombination. Whereas induced mutations are generally regarded as random events, there are suggestions of some mutational specificity in response to different mutagenic agents and treatments. The best immediate prospects for increasing specificity lie in the manipulation of the selection environment. Biochemical selection applied to large number of plant cells in culture to locate mutations in specific biosynthetic pathways and the subsequent regeneration of whole plants offers great prospect for reducing the cost of breeding programs and altering the amount or composition of a desired end or intermediate product. Mutations in conbination with other techniques of genetic engineering will constitute the tools of the plant breeders of the future. Their present role in plant breeding has been established. They have advantages in certain situations, disadvantages in others. Greater understanding will lead to their more widespread use.

Biophysical and genetic evidence for transformation in plants

In thiamine mutants of Arabiodopsis, genetic corrections have been obtained by treatment with DNA bearing a thiamine information. When correction is attempted under selective conditions, about 0.7% of the treated plants grow and set fruit. Their progeny and the following ones, obtained by selfing, behave as homozygotes. Segregation of characters is found only when correction is attempted under nonselective conditions or when the correcting genes were of plasmidian origin. The correction is hereditary; results of backcrosses and test crosses indicate that it is dominant, nuclear, and strongly bound to the genome. The corrective factor appears to be added to the mutated genome and not substituted for the mutation, as it can be suppressed by outcrossing with the wild type or with a plant corrected by another DNA.

Studies on the fate of homologous DNA applied to seedlings of Matthiola incana

Seedlings of Matthiola incana (crucifer) are able to take up exogenous homologous DNA by the roots. DNA homogenously labelled with [3H]adenine and 5-bromodeoxyuridine is incorporated into the plants in a macromolecular form. Intact donor DNA and a fraction with a buoyant density intermediate between that of the donor and the recipient DNA can be recovered. Analysis of this intermediate fraction by ultrasonication and alkali treatment allows the suggestion that homologous DNA is integrated as a double-stranded DNA which becomes covalently linked to the recipient DNA. Control experiments in which seedlings were incubated in a mixture simulating donor DNA degradation products in the presence and absence of unlabelled competitors suggest that these results are not due to the breakdown of donor DNA and reincorporation of the products during DNA synthesis in the recipient plants. When ultrasonicated or thermally denatured DNA is applied to the plants it may be degraded and reused for recipient DNA synthesis but it is not recovered in a macromolecular form. The possibility that the intermediate DNA fraction arises by bacterial contamination of the plants can be excluded by several arguments. Autoradiographic studies show that at least part of the radioactivity of the donor DNA taken up by the plants is associated with the cell nucleus.

Uptake of bacterial DNA by Chlamydomonas reinhardi

Escherichia coli [3H]DNA supplied to vegetative cultures of wild-type (mt+) and CW15 (mt+;mutant lacking the cell wall) Chlamydomonas reinhardi could bind to the cell wall of the wild-type and to the cell membrane of CW15 mutant cells. The extent of this binding decreased with time and was to a large degree (over 90%) DNA-ase-sensitive. Nevertheless, about 0.01% of the bacterial DNA remained irreversibly associated with the cells when they reached stationary phase. The irreversible binding of the donor bacterial DNA to Chlamydomonas cells could be increased by treatment of the cultures with polycations such as DEAE-dextran, poly-L-lysine and poly-L-ornithine. Although the CW15 cells rapidly degraded bacterial DNA in the culture medium wild-type cells showed only a small effect on the molecular weight of the donor DNA. The acid-insoluble radioactivity irreversibly bound to WT (+) cells consisted mainly of oligonucleotides with a small proportion present as less depolymerized donor DNA. No radioactivity, however, was found to be associated with the recipient high molecular weight Chlamydomonas DNA. No labeled donor DNA could be recognized in the cells given bacterial [3H]DNA in early stationary phase. Instead, radioactivity found in Chlamydomonas DNA corresponded to reutilization of [3H]thymine derivatives released as a result of [3H]DNA degradation. No evidence for the integration of detectable amounts of donor DNA sequences into the host cell DNA was obtained.

On the question of the integration of exogenous bacterial DNA into plant DNA

Extensive studies with pea, tomato, and barley failed to confirm the evidence presented by previous investigators for integration or replication of exogenously applied bacterial DNA in these plants. Labeled DNA of buoyant density in CsCl intermediate between that of high density donor bacterial DNA and of plant DNA was never observed with axenic plants. Intermediate peaks, similar to those used as evidence for recombination by earlier investigators, were observed only when the plants were contaminated with bacteria. Plant DNA prepared by a published procedure [Ledoux, L. & Huart, R. (1969) J. Mol. Biol. 43, 243-262] was found to be contaminated with unidentified impurities. Such DNA was partially protected from the action of DNase and produced aberrant banding patterns in CsCl after shearing. Much of the published evidence for integration of foreign DNA in plants is based upon experiments with plant DNA prepared by this procedure. We conclude that contamination is the likely explanation for what has been interpreted as evidence for integration.

The fate of a bacterial plasmid in mammalian cells

When hamster cells are infected with the bacterial plasmid colicinogenic factor E1 (ColE1), as much as 5-8% of the input plasmid radioactivity is found in the recipient cell, mainly in the nuclear fraction. Density shift experiments with bromodeoxyuridine labeled ColE1 DNA indicate that part of the input DNA may be replicated in the nucleus. ColE1 specific RNA but no colicin E1, can be detected during the first two generations after the uptake of ColE1 DNA. However, extrachromosomal ColE1 DNA is unstable in the mammalian cells and is degraded to acid soluble fragments after a few generations.