Chapter 17 Tissue Culture: Micropropagation 437
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Medium Composition
The sterile multiplication medium is typically composed of a gel
containing vitamins, sugars (basal medium), and a cytokinin growth
regulator. Cytokinins are a class of growth regulators used to encourage
shoot production. Using a high cytokinin to auxin ratio in the multiplication
stage encourages axillary or adventitious shoot formation. A high number
of adventitious shoots that do not elongate may result if the medium has too
high a concentration of cytokinin.
Subculturing
The plantlets developing in the multiplication medium remain in
stage 2 for about 5–7 weeks (depending on the plants’ particular needs
and growth) before reculturing. Plantlets at this stage may be divided,
detached, and cultured again in another stage 1 or 2 medium, depending on
the technician’s motive. When plantlets are recultured in stage 2 medium,
the process is referred to as subculturing (establishing a new culture by
moving some cells from an existing culture). There is a limit to how many
times plantlets can be subcultured. Undesirable or unintentional changes
may occur if a plantlet is subcultured too many times. This occurrence is
called somaclonal variation. Somaclonal variations can be intentional, too.
Breeding specialists often welcome unique changes in genetic material that
could possibly yield a new cultivar.
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STEM STEM Connection Connection
Cincinnati Zoo’s CREW
CryoBioBank®
The Carl H. Lindner Jr. Family Center for
Conservation and Research of Endangered Wildlife
(CREW) is a research facility dedicated to saving
endangered plants and animals from extinction. The
CryoBioBank, a genome database, is a division of
this facility. The CryoBioBank stores thousands of
plant and animal tissue samples through a process
known as cryopreservation. Cryopreservation
uses liquid nitrogen to preserve the samples at
subzero temperatures. Tissues can be stored and
remain viable for decades under these unique
environmental conditions.
Scientists and horticulturists work together at
this facility to preserve various species, especially
those in danger of extinction. Plants such as the
four-petal pawpaw (Asimina tetramera) have been
given a new lease on life through the CryoBioBank.
This plant, native to and found only in southeastern
Florida, has been nearly driven to extinction by loss
of habitat and the difficulty of propagating by seed.
The scientists at CREW developed a means of
propagation by tissue culturing the meristems and
preserving these tips in liquid nitrogen. Additionally,
the four-petal pawpaw has been micropropagated
and prepared for transplant to its native Florida.
Corner Question
What is the actual size
of a meristem?