We are happy to offer some of our favorite plants we propagated using tissue culture. I wanted to provide some background to answer questions about the process people might have:
What is tissue culture?
A method to propagate plants.
How did you learn Plant Tissue Culture?
I have a background in genetics and plant biology and previous work experience with plant TC. I was lucky to have as mentors two people who are pioneers in the field of plant TC.
How does it work?
Plant tissue containing merisitems (for more information about meristems see my previous blog) is sterilized to kill bacteria and fungi. With haworthia we use young flower stems.
This tissue is cut up and placed on special media containing nutrients, sugars and plant hormones (auxins and cytokinins). The plant tissue will grow and transform into a "Callus" tissue. Callus tissue is embryonic and strange looking.
The callus tissue is transferred to media containing specific hormone mixtures that encourage the callus to grow into shoots. These shoots can eventually be transferred to soil. Though the process sounds simple, monitoring and nurturing the development of different stages is an art. Getting high quality plants from tissue culture is also an art.
Are tissue cultured plants the same as regular cuttings?
Yes and no. Plants can mutate and change as they grow and are propagated and these kinds of variant shoots are sometimes called "sports". For example there are pink lemons that originated as offshoots from a normal lemon tree. When people propagate plants through normal cuttings, what they obtain can sometimes be different than the mother plant. For reasons not fully understood plants in tissue culture have a higher rate of this kind of variation. This phenomenon is called "Somoclonal Variation". Wikipedia has more information about this. Somoclonal variation probably is influenced by the embryonic nature of callus tissue and epigenetcs. Epigenetics has to do with how DNA is packaged in cells. In general as cells develop from un-differentiated (embryonic) to differentiated (leaves, shoots etc.) changes occur in how DNA is packaged in cells. These changes may influence gene expression and mutation. Because callus tissue is in an embryonic state it may have an increased rate of someclonal variation. If overly high concentrations of hormones are used when encouraging shoot formation from callus tissue, the epigenetic state of the tissue can be perturbed and this can greatly effect the vigor of the resulting plants.
Another factor that influences health of the resulting plants is how long the callus cultures are grown. Organisms evolve to grow successfully in their environment. If plants are maintained too long in tissue culture they will gradually become too adapted to the in-vitro growth environment. They can evolve to carry mutations that benefit growth in tissue culture but are detrimental to growth outside of tissue culture. In our tissue culture methods the tissue is maintained as callus and in TC only long enough to produce sufficient plantlets. Additionally, we use the lowest concentrations of hormones necessary.
Even though we are careful some somoclonal variation is likely to occur.
What about polyploidy?
Polyploidy means extra sets of chromosomes. Most organisms are diploid. This means they have 2 copies of each chromosome. Organisms with 4 copies of each chromosome are referred to as tetraploid. Animals are not very tolerant of extra chromosomes. For example an extra chromosome 21 leads to Down's syndrome in humans. Plants on the other hand are very tolerant of extra chromosomes and extra sets of chromosomes. About 1/3 of ferns found in nature are polyploid. Many crops such as wheat and canola are polyploid. Sugarcane is probably the most polyploid organism on earth with most varieties having more than 100 sets of chromosomes!
Plants in tissue culture may spontaneously become polyploid. This is not a bad thing since in Haworthia polyploidy results in fatter leaves and increase in size. But these plants will not be fertile when crossed with normal plants.