Touch-Responsive Behaviors and Gene Expression in Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 252

(indole-3-acetic acid) have been implicated as mediators of the differential
growth effecting coiling (Jaffe and Galston 1968; Jaffe 1985; Weiler et al.
1993, 1994; Stelmach et al. 1998; Blechert et al. 1999).
Root tips are also highly sensitive to touch stimulation; perception of
touch is thought to enable roots to avoid obstacles as they penetrate soils.
Darwin (1880) and more recently Massa and Gilroy (2003) have observed
that when root tips encounter an impenetrable surface such as a glass
plate, the root tip flattens and turns 90 ◦ Ctogrowoverthesurfaceuntil
it is once again allowed to display positive gravitropism and turn to grow
downward. Remarkably, perception of touch appears to delay the gravity-
induced downward movement of columella cell starch granules and in this
way may interfere with root gravitropism (Massa and Gilroy 2003).
17.3
Thigmomorphogenesis - Plasticity of Shoot Growth
Notonlydotherootsofnonspecializedplantssenseandrespondtotouch,
but the shoots are also mechanosensitive and mechanoresponsive. Mechan-
ical perturbations, like touch or wind, generate gradual morphogenetic
alterations in most, if not all, plants (Jaffe 1973). Plants subjected to repet-
itive mechanical stimuli develop with shorter and often stockier pheno-
types (Fig. 17.1f). These changes occur slowly over time and therefore
are not often recognized, but they can result in quite dramatic morpho-
genetic alterations. Jaffe, who conducted much of the pioneering work on
this phenomenon, called the touch-induced changes in growth “thigmo-
morphogenesis” (Jaffe 1973). Thigmomorphogenesis likely evolved as an
adaptive response to environmental stresses like wind and often results in
increased rigidity or increased flexibility, depending upon the species, and
may therefore improve resistance to further mechanical perturbation (Jaffe
et al. 1984; Biddington 1986; Telewski and Jaffe 1986; Depege et al. 1997;
Coutand et al. 2000).
Mechanical perturbation, like wind or touch, is likely perceived through
the resulting longitudinal strain experienced by the shoot tissue. The ex-
tent of thigmomorphogenetic changes correlates strongly with the degree
of longitudinal strain (Coutand et al. 2000). Furthermore, because thig-
momorphogenetic alterations can affect subsequent strain, there are likely
direct feedback pathways at play. The degree of thigmomorphogenesis
wouldthusbetailoredappropriatelytothedegreerequiredforacclimation
to the condition. For example, tobacco plants engineered to have weakened
xylem composition would be predicted to have reduced tensile stiffness
and should experience greater strain than wild-type plants. However, these
transgenic tobacco are nearly indistinguishable from the wild-type control