Touch-Responsive Behaviors and Gene Expression in Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 255

repetitive mechanical stimulation can lead to enhanced disease resistance
(Biddington 1986); however, the basis of this potential resistance is not
well understood. Notably a number of the disease-resistance-related genes
upregulated in expression by touch are members of the nucleotide bind-
ing site (NBS) leucine-rich repeat (LRR) gene family. This family contains
the majority of plant disease-resistance genes (R genes) identified to date
(Cannon et al. 2002). The NBS domain is thought to contribute in signal
transduction, and the LRR domain may be responsible chiefly for elici-
tor recognition (Cannon et al. 2002). The upregulation of expression of
these genes suggests a potentiation of defense in touch-stimulated plants.
Whether this potentiation results in an enhanced resistance response by
plants is still to be determined.
It is also intriguing that many of the touch-inducible genes have been
associated with the jasmonic acid (JA) pathway. In addition to genes encod-
ing enzymes involved in JA biosynthesis, other touch-inducible genes are
thosethathavebeenusedasJA-dependentresistancemarkers,including
those encoding proteinase inhibitors that block digestive enzymes of some
insect herbivores. Recent literature indicates that plant defense responses
against insect herbivores and some microbial pathogens are coordinated
by JA signaling pathways (Howe 2001). JA-mediated defenses are typically
preceded by accumulation of JA in response to biotic stress (Wasternack
and Hause 2002). Whether touch results in the accumulation of JA and/or
the establishment of an enhanced disease resistance state awaits further
study.
Touch-inducible genes provide important tools for investigating how
plants perceive mechanical stimuli; however, to date, most genes found
to be touch-inducible in expression are also upregulated in expression by
other types of stimuli. The original TCH genes, encoding CaM2, CML12,
CML24, and XTH22, are not only upregulated in expression by various
forms of stimuli that have mechanical properties, such as touch, wind and
wounding, but also by stimuli such as cold, heat, and darkness that, at
least superficially, do not appear to be mechanical in nature (Braam 1992,
2000; Sistrunk et al. 1994; Xu et al. 1995; Polisensky and Braam 1996). To
address the question whether all touch-inducible genes share this property
of being upregulated by diverse stimuli, microarray experiments were done
using darkness as a stimulus (Lee et al. 2005). More than half of all touch-
inducible genes were also upregulated in expression by darkness, and the
coregulation was most apparent in those genes with the greatest fold touch
inducibility (Lee et al. 2005). Indeed, among the top 60 touch-inducible
genes with at least tenfold upregulation, only four were not at least twofold
upregulated in plants treated with darkness (Lee et al. 2005). All but three
of the 68 genes that are most strongly upregulated by darkness are also
touch-inducible (Lee et al. 2005). These findings are consistent with an