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Name: Paul
Status: educator
Grade: other
Location: Outside U.S.
Country: Australia
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 for light.

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 University Colloquium Department of Biological Sciences Florida Gulf Coast University

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