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Figure 2.1(also see Color Section). Histological differences between roots and inflorescence stems trans-
late into gravitropic signaling specificities in Arabidopsis thaliana. A. Root from a 4-day-old light-grown
seedling (left) showing the cap, distal and central elongation zones (DEZ and CEZ), and the mature zone
(MZ). The inset (right) represents a confocal image of a propidium iodide-stained root tip showing the root
cap (with its L1, L2, and L3 layers of columella cells, and lateral cap cells, LRC), the promeristem (with its
quiescent center cells, QC, surrounded by initials). The root proper is composed of several cell layers, includ-
ing the epidermis (Ep); the cortex (C); the endodermis (En); and the stele (St). The primary site of gravity
perception in roots is the root cap, with the statocytes located in the first two layers of columella cells,
whereas the curvature response initiates in the DEZ. Therefore, a gravitropic signal has to be transmitted
basipetally upon gravistimulation. B. Top of an inflorescence stem showing the stem ended by a shoot apical
meristem (SAM), flowers, and siliques. The entire stem region located below the SAM in this picture is part
of the elongation zone (EZ). The middle drawing corresponds to a region of shoot stem. Microscopical image
of a longitudinal section of this region (indicated by the rectangle in the drawing) is represented on the right
showing the epidermis (Ep), three layers of cortical cells (C), one layer of endodermis (En), and the stele (St).
Sedimenting amyloplasts, located in the endodermal statocytes, are indicated by black arrowheads. Hence,
the sites of gravity sensing and curvature response overlap in stems, and the gravitropic signal has to be trans-
mitted outward, rather than basipetally, in order for a curvature response to develop.
The American Society of Plant Biologists is the copyright holder for this figure, which first appeared in
Masson et al. (2002).