THE QUESTION OF DIFFERENTIATION --- JAY PENNING
Scientists have uncovered detail after detail about plant
development until it seems that we must have solved all the big problems in
plant biology. One question still prowls our minds: "how do plant cells
differentiate?"
Botanists have provided valuable information using the ideal plant
subjects for studying cell growth and proliferation under controlled
conditions. For one thing, some of them can regenerate from bits and pieces
of plant. Many plants can regenerate roots from stem cuttings, stems from
bits of root, and even entire plants from leaves. This knowledge has helped
the area of agriculture, as it supplies clues to the question of
differentiation. It supports the idea that plants are totipotent, that is
the cell can undergo development and differentiation along any of the lines
inherently possible for the species. If this is true the question then
arises: "can individual plant cells duplicate the regenerative that occurs
in the cuttings previously stated?"
In the 1930's and 1940's researcher, Johannes van Overbeek,
discovered that a suitable medium was coconut milk. Besides containing some
nutrients, it also contains some critical hormones needed in plant growth.
Van Overbeek was able to grow individual cells that he had separated out
from young carrot embryos. He cultured them in the coconut milk and then
planted them in soil. After some deliberation they produced normal adult
carrot plants. These reproducible results indicated that cells in the
embryo are indeed totipotent.
In the late 1950's mature tissue was cultured in a simmilar manner
by F. C. Steward of Cornell University. The mature carrot cells were
removed from their parent plant and grown in a nutrient medium. These
mature cells grew into rootlike structures that, upon planting, became
entire carrot plants. This added to the studies done by van Overbeek
showing totipotency in plant cells at a variety of life cycle postions.
We see that many cells retain the capability necessary to produce
the entire plant. Also, when removed from their cell associations, cells
can regress to an undifferentiated state from which they can then proceed
through normal embryonic development. How does this answer the question
about differentiation? It tells us that genetic information is not lost as
development proceeds and that it is not irreversibly repressed.
References: "Science" magazine and "Biology, the science of life"
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