the gravity signal is rapidly transduced to a broad range of response elements. Interest-
ingly, a similar proteomic analysis of the mechanoresponse does not show an extensive
overlap in the proteins regulated at the post-transcriptional level in the mechanical and
gravity response systems (see Chapter 2). Thus, one possibility is that post-transcriptional
modification represents one theme of how specificity and selectivity may be imposed on
the touch versus gravity response and possibly how these response systems may interact.
5.6 Interaction of touch and gravity signaling/response
Evidence for touch and gravity signaling cross-talk is readily apparent in the gross mor-
phological response of a root upon hitting an obstacle to growth. When encountering a
barrier, the root forms a step-like growth habit due to the curvature at two sites (Massa
and Gilroy 2003a) (Figure 5.6). The first is convex and initially occurs in the central elon-
gation zone (CEZ) of the root, whereas the second is concave and located at the distal
elongation zone (DEZ), causing the root cap to track along at a fixed angle to the barrier.
Thus, the root body extending behind the DEZ aligns parallel to the barrier as growth
110 PLANT TROPISMS
Figure 5.6. Response of a root as it grows into a barrier (cov-
erglass). Initial bending of the root within 20 min of contact with
the barrier occurs in the central elongation zone (CEZ). A sec-
ond bend forms in the distal elongation zone (DEZ) as the root
traverses the barrier, with the tip tracking along the surface at a
fixed angle (in wild-type Arabidopsis, this angle is 42 degrees).
Scale bar = 1 mm.