A natural graft between a birch (left) and an oak (right). Sexually incompatible species can exchange chloroplast genomes at graft sites. Image: MPI of Molecular Plant Physiology
scientists were confounded by the fact that the DNA extracted from the
plants’ green chloroplasts sometimes showed the greatest similarities
when related species grew in the same area. They tried to explain this
phenomenon, for which they coined the term “chloroplast capture” with
the assumption that every once in a while those normally sexually
incompatible species crossed and produced offspring with a new
combination of nuclear and chloroplast genomes. Now, scientists around
Ralph Bock from the Max Planck Institute of Molecular Plant Physiology
in Potsdam discovered that a transfer of entire chloroplasts, or at
least their genomes, can occur in contact zones between plants.
Inter-species crossing is not necessary. The new chloroplast genome can
even be handed down to the next generation and, thereby, give a plant
new traits. These findings are of great importance to the understanding
of evolution as well as the breeding of new plant varieties
wooden plants, especially fruit and rose trees, are deliberately
damaged by gardeners. They chop off branches or cut dents into the bark
in order to put parts of another plant into the slots. The reason behind
such gardening measures is to reproduce varieties with an especially
high yield. According to the classic Mendelian Laws, only parts of the
progeny show the same traits as their parents. The rest of the offspring
will most likely be less valuable. By putting one branch of a
successful apple variety onto a new stock, the desired apple tree is
easily cloned. But graft junctions do not always have to be man-made.
Plants that simply grow in close vicinity to each other can fuse.
those above mentioned contact zones, so-called horizontal gene
transfers, the transfer of genes without sexual reproduction, can occur.
For a long time, scientists believed that such a gene transfer was
restricted to prokaryotes, organisms without nuclei. It was universally
accepted that, for example, bacteria can exchange genes that are crucial
to their survival, like the ones that transmit a resistance to
antibiotics. Nowadays it is increasingly appreciated that this
phenomenon is in fact not restricted to such organisms. It can also be
observed at the contact zone between different animal tissues after an
organ transplantation or—as shown here—between two fusing plants. In
2009, Ralph Bock and Sandra Stegemann discovered that genetic
information stored in the green chloroplasts can be transferred to
another plant by means of horizontal gene transfer. Their results were,
at that time, restricted to the transfer of genes between plants of the
In their new experiments, they grafted the sexually incompatible tobacco species Nicotiana benthamiana, a herbaceous species, and Nicotiana glauca, a tree tobacco, onto the cultivated tobacco Nicotiana tabacum. They equipped the nuclei of the two wild species N. benthamiana and N. glauca
with genes that encoded a resistance to an antibiotic as well as the
yellow fluorescent protein The cultivated tobacco, on the other hand,
had chloroplasts that carried genes coding for a resistance to another
antibiotic and a green fluorescent protein. After the successful fusion
of the two plants the graft sites were excised and cultivated on a
growth medium that contained both antibiotics. The antibiotics hinder
cell division and lead to cell death; only those cells which contain
both resistance genes can survive and proliferate. Due to the nature of
the experiment, it would have to be cells from N. benthamiana or N. glauca that acquired chloroplasts, or the chloroplast genome, from N. tabacum.
Indeed, new plantlets grew out of half the excised graft sites and
under the microscope one could see the characteristic green and yellow
results from the DNA analyses were especially interesting,” says Sandra
Stegemann, first author of the paper. “We found a completely identical
version of the chloroplast genome from N. tabacum in the two other species.”
mitochondria, another cell organelle with an individual genome, are
transferred across species barriers, the result is often a mixture of
the donor and recipient DNA.
new chloroplasts had kept their entire genetic information and fully
ousted the old ones. They were even inherited by the next generation,”
Stegemann further explains.
scientists are trying to find the answer to the question of how exactly
the chloroplasts leave their homes and find a new place to live. Do
they migrate through the plasmodesmata, the narrow tunnels that connect
neighbouring plant cells? Or do enzymes locally remove the cell wall and
allow small amounts of cytoplasm and cell organelles to pass from one
cell to another?
of now, we do not know how chloroplasts manage to get from one cell to
the other,” says group leader Ralph Bock. “But the decisive point is
that it happens and the discovery of this process offers a new
explanation for important evolutionary processes and opens up new
possibilities for plant breeders.”
After all, the chloroplast DNA vitally contributes to the fitness of a plant and can provide crucial advantages.