A transplastomic plant is a genetically modified plant

Genetically modified plant

Genetically modified plants are plants whose DNA is modified using genetic engineering techniques. In most cases the aim is to introduce a new trait to the plant which does not occur naturally in this species...

 in which the new gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s have not been inserted in the nuclear DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 but in the DNA of the chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s. The major advantage of this technology is that in many plant species plastid

Plastid

Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...

 DNA is not transmitted through pollen, which prevents gene flow

Gene flow

In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...

 from the genetically modified plant to other plants.

Transformation technology

The most common method to transform plastids is particle bombardment: Small gold or tungsten particles are coated with DNA and shot into young plant cells or plant embryos. Some genetic material will stay in the cells and transform them. The transformation

Transformation (genetics)

In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...

 efficiency is lower than in agrobacterial

Agrobacterium tumefaciens

Agrobacterium tumefaciens is the causal agent of crown gall disease in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium...

 mediated transformation, which is also common in plant genetic engineering, but particle bombardment is especially suitable for plastid transformation.

In order to persist and be stably maintained in the cell, a plasmid DNA molecule must contain an origin of replication

Origin of replication

The origin of replication is a particular sequence in a genome at which replication is initiated. This can either be DNA replication in living organisms such as prokaryotes and eukaryotes, or RNA replication in RNA viruses, such as double-stranded RNA viruses...

, which allows it to be replicated in the cell independently of the chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

. Because transformation usually produces a mixture of rare transformed cells and abundant non-transformed cells, a method is needed to identify the cells that have acquired the plasmid. Plasmids used in transformation experiments will usually also contain a gene giving resistance to an antibiotic that the intended recipient strain of bacteria is sensitive to. Selection for cells able to grow on media containing this antibiotic can then select the cells that have acquired the plasmid by transformation, as cells lacking the plasmid will be unable to grow.

Biological containment and agricultural coexistence

Genetically modified plants must be safe for the environment and suitable for coexistence with conventional and organic crops

Co-existence of genetically modified and conventional crops and derived food and feed

In the context of agriculture and food and feed production, co-existence means using cropping systems with and without genetically modified organisms in parallel. For co-existence to be assured, the separation and the identity of the respective food and feed products must be maintained at all...

. Towards such safety, a major hurdle is posed by the potential outcrossing

Outcrossing

Outcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities...

 of the transgene

Transgene

A transgene is a gene or genetic material that has been transferred naturally or by any of a number of genetic engineering techniques from one organism to another....

 via pollen movement. Plastid transformation, which yields transplastomic plants in which the pollen does not contain the transgene, not only increases biosafety, but also facilitates the coexistence of genetically modified, conventional and organic agriculture. Therefore, developing such crops is a major goal of research projects such as Co-Extra

Co-Extra

Co-Extra is an EU-funded research programme on co-existence and traceability of genetically modified crops and their edible derivatives. It has been granted €13.5 million under the Sixth Framework Programme of the European Union, and is conducted by more than 200 scientists in 52 organisations in...

 and Transcontainer.

Transplastomic tobacco

However, plastid transformation is suitable only for certain crop species, and the reliability of this method has only been proven for tobacco. Led by Ralph Bock from the Max Planck Institute of Molecular Plant Physiology in Germany, researchers studied genetically modified tobacco in which the transgene was integrated in chloroplasts. Since past literature reported contradicting figures on the reliability of this process, the Co-Extra researchers analysed more than two million seedlings and found that less than 20 in 1,000,000 inherited the transgene. In the pollen of adult plants, the rate was even lower, remaining below 3 in 1,000,000. This reduction is because some parts of the seedlings are lost during their development into mature plants.

Because tobacco has a strong tendency towards self-fertilisation, the reliability of transplastomic plants is assumed to be even higher under field conditions. Therefore, the researchers believe that only one in 100,000,000 GM tobacco plants actually would transmit the transgene via pollen. Such values are more than satisfactory to ensure coexistence. However, for GM crops used in the production of pharmaceuticals, or in other cases in which absolutely no outcrossing is permitted, the researchers recommend the combination of chloroplast transformation with other biological containment

Biological containment

Biological containment describes measures aimed at preventing genetically modified organisms and their transgenes from spreading into the environment ....

 methods, such as cytoplasmic male sterility

Cytoplasmic male sterility

Cytoplasmic male sterility is total or partial male sterility associated with plant biology as the result of specific nuclear and mitochondrial interactions...

or transgene mitigation strategies.

External links

• Co-Extra Research on the co-existence and traceability of genetically modified plants
• Transcontainer Developing biological containment systems for genetically modified plants