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 ist 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?