els, as expression of PHOT1 (Sakamoto and Briggs 2002; Kong et al. 2006) and several
phytochromes (Sharrock and Clack 2002) is down-regulated by light. In the case of grav-
itropic adaptation, however, it is unclear where in the signaling pathway the modulation
occurs. Part of the reduction in curvature may also be due to autotropic straightening, a
reversal of curvature which can occur following tropistic responses (Orbovic and Poff
1991; Stankovic et al. 1998). There can also be differences in the spatial distribution of
the responses, adding complexity to the overall growth pattern. In roots, phototropic cur-
vature develops farther from the root tip than gravitropic curvature (Mullen et al. 2002;
Kiss et al. 2003), whereas in shoots, there can be differences in the extent of the region
of curvature between gravitropic and phototropic responses (Tarui and Iino 1999).
Nondirectional light, more generally, may modulate gravitropic responses apart from
the integration of gravitropic and phototropic growth responses. There have been observa-
tions of red light sensitizing hypocotyls to gravitropic stimulation (Britz and Galston 1982;
Woitzik and Mohr 1988). However, it appears more common for red light to inhibit grav-
itropism. In hypocotyls of Arabidopsis, red light acting via phytochrome appears to inhibit
gravitropism, causing a randomization of shoot orientation (Figure 4.1; Liscum and
Hangarter 1993; Poppe et al. 1996; Robson and Smith 1996). This inhibition of a counter-
ing gravitropic response may explain how red light enhances blue-light phototropism
(Hangarter 1997; Parks et al. 1996). Red-light attenuation of shoot gravitropism has also
been observed in pea and tobacco (McArthur and Briggs 1979; Hangarter 1997). And in
mosses, at least, red light does not act to inhibit gravitropism at the level of perception, as
red-light treatments did not repress amyloplast sedimentation (Kern and Sack 1999). This
signaling pathway may also be acting during blue-light phototropism, as phytochrome ab-
sorption of blue light can also cause randomization of shoot orientation (Lariguet and
Fankhauser 2004). Light inhibition of gravitropism has also been observed in leaves
(Mano et al. 2006), although whether it is also mediated by phytochrome is unclear.
4.5 Importance to plant form and function
At a whole-plant level, the positioning of branches and leaves through phototropic and
gravitropic responses will play an important role in the overall functioning of the organ-
ism. Yet tests of the roles of specific phototropic responses at specific stages of the life
cycle of a plant remain limited, although it appears that a key function of phototropism is
to help the plant maximize photosynthesis. Experiments with Arabidopsis photmutants
suggest that PHOT1 and PHOT2 may be important at different developmental stages,
consistent with their different sensitivities to light intensity (Galen et al. 2004; Galen et
al. 2007). In young seedlings, one of the important roles of phototropism may be orient-
ing the root system away from the surface to aid in dealing with dry conditions (Galen et
al. 2007).
As plants develop, positioning of leaves becomes increasingly important, allowing
them to exploit light gaps or orient at the correct angle for light capture from the sun. For
solar-tracking plants, phototropism allows for increased photosynthesis in the morning
and evening. This appears to be particularly important for plants found in habitats with
short growing seasons (Ehleringer and Forseth 1980). However, because solar tracking