In most cases, red light does not induce phototropism in flowering plants. Red and far-
red light are sensed by the phytochromes (Figure 4.1)—photoreceptors which are involved
in numerous aspects of the plant life cycle, including seed germination, flowering, and cir-
cadian rhythms (Schepens et al. 2004). The phytochrome gene family has five members
(PHYA–E) in Arabidopsis. Although unidirectional red light does not induce phototropism
inArabidopsishypocotyls (Liscum and Briggs 1996), red-light pretreatments are known to
greatly enhance blue-light phototropism, in a process mediated by phytochromes (Janoudi
et al. 1992; Liu and Iino 1996; Hangarter 1997). Phytochromes can also absorb blue light
and they modulate blue-light phototropic responses, even in the absence of a red-light pre-
treatment (Correll et al. 2003). At low fluence rates of unidirectional blue light, phy-
tochromes (particularly PHYA and PHYB) reduce the latent period and enhance the mag-
nitude of the phototropic response (Janoudi et al. 1997; Hangarter 1997). However, at
higher fluence rates of blue light, phytochrome (primarily PHYA) causes an attenuation in
the response (Whippo and Hangarter 2004). Phytochromes can interact with the cryp-
tochromes (Ahmad et al. 1998; Mas et al. 2000); and like the cryptochromes, phytochromes
also regulate light-induced growth inhibition in hypocotyls (Folta and Spalding 2001).
Therefore, in addition to a possible role in directly modulating the phototropism signaling
pathway, phytochromes also appear to act in coordination with the cryptochromes and pho-
totropins (Figure 4.1) to regulate shoot growth rate more generally in response to light stim-
uli, a process that also modulates the overall phototropic response observed.
Some algae, mosses, and ferns engage in red-light phototropism, mediated by phy-
tochrome (Wada and Sei 1994; Esch et al. 1999), in addition to blue-light phototropism.
The green alga Mougeotiaand a group of ferns have independently evolved a chimeric
photoreceptor that is a hybrid between phytochrome and phototropin, termed neochrome
(Figure 4.1), which controls the red-light phototropism in these plants (Kawai et al. 2003;
Suetsugu et al. 2005). However, neochromes have not been found in mosses or flowering
plants. This chimeric photoreceptor broadens the response by allowing strong absorption
of both red light and blue light. The signaling of neochrome for the two wavelengths is
synergistic, so that the photoreceptor has increased sensitivity to weak white light; this
allows plants with these pigments (e.g., polypodiaceous ferns) to sense and respond to
low-light signals in their naturally shaded light environment (Kanegae et al. 2006).
There have been a few reports of phototropism mediated by normal phytochromes in
flowering plants. In roots of Arabidopsis, there is a positive red-light phototropic re-
sponse controlled by phytochromes A and B, in addition to the negative blue-light re-
sponse (Kiss et al. 2003). In shoots, there are reports of phytochrome-regulated phototro-
pism in mesocotyls of maize (Iino et al. 1984) and negative far-red phototropism in
cucumber (Ballare et al. 1992, 1995), as well as positive far-red phototropism in the par-
asitic plant Cuscuta planiflora(Orr et al. 1996).
4.3 Signal transduction and growth response
Although there has been considerable progress in understanding the cell and molecular
biology of the primary photoreceptors including phototropin, cryptochrome, and phy-
tochrome, relatively little is known about the downstream signaling events (i.e., signal