Gene Expression Mechanism
Location: Outside U.S.
Date: Spring 2011
Last year I got my students to perform an experiment where
the germinated radish seeds, one exposed to light, the other kept in
the dark, The first batch grew well and were green. The second batch
were healthy but were stark white. I used this to show how
environmental factors affect gene expression. (The second batch did
eventually go green when exposed to light.) Problem is, what is the
exact mechanism for this process at the gene level?
By withholding light from the seedlings, you demonstrated the process
of etiolation. Etiolation refers to the bleached, spindly stems your
class observed. The plant simply thinks its being shaded by a
neighbor, so it resolves to grow out of its neighbor's shadow. Under
normal lighted conditions, plants will produce the light harvesting
pigment chlorophyll, which accounts for the green color.
Despite being in the dark, the etiolated plants actually had "prefab"
chloroplast precursors known as etioplasts. When you exposed the
plants to light, they turned on their chlorophyll making switches
(genes) and re-purposed their etioplasts into chloroplasts. In effect,
they de-etiolated. This is a reversible process, as exposure to the
dark will result in a re-etiolation.
In the dark, etioplast production makes sense from an energy
conservation standpoint. It's counterproductive to invest energy in
chloroplast production, so it readies itself by going halfway
(etioplast) and invests the rest to grow long and spindly to search
Once a light stimulus is received by the photoreceptor pigment
phytochrome, a multistep process ensues. This complex process is
coordinated by proteins encoded by genes on the etioplast and the
nuclear genome. Chlorophyll synthesis is ultimately stimulated via
cytokinin (plant hormone) production.
This is a fine example of differential gene expression. Despite having
the same genetic template, wholly different structures can be produced
in response to the prevailing conditions.
Dr. Tim Durham
Instructor, Office of Curriculum and Instruction
Department of Biological Sciences
Florida Gulf Coast University
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Update: June 2012