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Figure 3.2(also see Color Section). Expression of auxin-responsive reporters is elevated on the lower side
of gravity-stimulated wild-type roots consistent with redistribution of auxin at the root tip. A–C. GUS ex-
pression was visualized in 7-day-old seedlings homozygous for the DR5-GUS reporter construct (D) or for
a DR5-GFP reporter. Root tips are shown for (A), a vertically grown seedling on control media, and (B and
C) seedlings 6 hours after gravity stimulation on (B) control media, (C) 1 μM NPA, or (D) 3 hours after grav-
ity stimulation on control media. The scale bar is equal to 100 microns. A model showing the mechanism of
IAA redistribution and gradient formation at the root tip is shown in E and F. To simplify the model, both
PIN proteins, which are part of the IAA efflux protein complex, and AUX1, an auxin influx protein, have
been shown to be asymmetrically localized in the diagram, although the localization of AUX1 was reported
to be axial. For gradients in IAA to form across a horizontal root, there are likely to be mechanisms to in-
crease IAA transport to lower side of the root and decrease IAA transport to the upper side that act to change
the activity and abundance of IAA efflux proteins.
Panels A through C reprinted from Rashotte et al. (2001), with kind permission of the American Society
of Plant Biologists.