Although the phototropins are the primary photoreceptors responsible for initiating
phototropic responses, another family of blue-light-absorbing photoreceptors in plants,
the cryptochromes, is also involved in regulating the response (Figure 4.1). The principal
roles of cryptochromes are in photomorphogenesis and the entrainment of circadian
rhythms (Cashmore 2003), but they have also been found, in mediating blue-light-
induced stomatal opening, to act synergistically with phototropins (Mao et al. 2005).
Cryptochromes are not necessary for the induction of phototropic responses (Lasceve et
al. 1999; Sakai et al. 2000). However, at low fluence rates, cryptochrome-deficient mu-
tants have reduced phototropic responses (Ahmad et al. 1998; Lasceve et al. 1999; Whippo
and Hangarter 2003). Cryptochromes are also important in regulating light-induced
growth inhibition in hypocotyls, together with the phototropins (Parks et al. 2001).
Because phototropism is caused by complex changes in growth rate, alterations in growth
inhibition by cryptochromes is likely to intersect phototropic signaling. Indeed, Whippo
and Hangarter (2003) have shown that at high light intensities, the cryptochromes, along
with the phototropins, inhibit phototropism. Thus, the cryptochromes may interact with
phototropins to modulate the phototropic pathway (Figure 4.1 and Color Section) and also
control the inhibition of growth of hypocotyls in a fluence-rate-dependent manner, which
can limit the potential phototropic response.
CHAPTER 4 PHOTOTROPISM AND ITS RELATIONSHIP TO GRAVITROPISM 81
Figure 4.1 (also see Color Section). Light quality and phototropic responses. Several photoreceptors are
important in phototropic responses, sensing light in the blue, red, and far-red ranges of the visible spectrum.
The signaling pathways from these photoreceptors interact to regulate the blue-light-based phototropic re-
sponses. The phytochromes separately also regulate some other phototropism-related responses, including
red-light root phototropism and alteration of stem gravitropism, which can enhance phototropic responses.
Some ferns and algae have a chimeric photoreceptor, neochrome, which can mediate phototropism. BL, blue
light; Dk, darkness; RL, nondirectional red light; WL, white light.