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How does auxin turn on genes?
The plant hormone auxin causes cellular responses, such as cell growth and vascular differentiation, by activating the transcription of a specific set of auxin-responsive genes. Auxin-responsive reporter gene constructs allow me to monitor auxin effects on gene expression by color reactions or light emission in genetically engineered tobacco plants.
This leaf is from a transgenic tobacco plant. The plant has been engineered to express the bacterial Gus gene under an auxin-responsive promoter. The leaf was stained for Gus enzyme activity, which is visible as a blue stain. Chlorophyll was extracted with alcohol, so that the blue Gus stain is more visible than it would be in the presence of green chlorophyll. The pattern of staining demonstrates that this particular auxin-inducible promoter is most active in cells associated with the leaf vein as well as with the hairs on the leaf surface.
This image is a close-up of the hairs on the leaf-surface, showing that the Gus activity (blue staining) is most intense in the round heads of each hair. You can also see that each hair is composed of two or more cells.
Tobacco plants produce defense chemicals in the tips of these hairs that deter herbivores. We do not know yet why the hair-tip cells are particularly auxin-responsive -- does auxin perhaps regulate the synthesis of defense chemicals? Or could it be required for the round cell shape of the tip cell?
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