transduction) following light perception. Nevertheless, in recent years, numerous mutants
in light-signaling intermediates, particularly in the phytochrome pathways, have been iso-
lated and used to investigate light signaling in plants (Møller et al. 2002).
In terms of signaling intermediates that are downstream from the phototropins, RPT2
(root phototropism) and NPH3 (non-phototropic hypocotyl) have been shown to bind to
PHOT1 and to function very early in some blue-light-based signaling pathways (Sakai et
al. 2000; Inada et al. 2004). For instance, RPT2 is involved in phototropism and stomatal
opening but not in chloroplast movements, and although NPH3 is involved in phototro-
pism, it is not needed for stomatal opening or chloroplast relocation. RPT2 and NPH3 are
considered to be in the same biochemical family, but the exact structure and precise phys-
iological functions of these proteins are not known. Although NPH3-deficient mutants of
Arabidopsisshowed no phototropism in shoots or roots (Sakai et al. 2000), the cpt1mu-
tant of rice, orthologous to NPH3, retained some root phototropism, suggesting that other
members of the gene family may also play a role in this plant (Haga et al. 2005).
The growth response of phototropism involves differential elongation on opposite
sides of a plant organ, which eventually leads to phototropic curvature. It is well-
established that the plant hormone auxin plays an integral role in the differential growth
that results in curvature. In fact, the discovery of auxin and the classical Cholodny-Went
model for auxin transport have been tied to research in tropisms, especially phototropism.
TheNPH4locus, which is important for both phototropism and gravitropism, encodes an
auxin response factor involved in auxin-sensitive transcriptional regulation (Harper et al.
2000). Much recent focus in auxin research, especially in conjunction with understand-
ing the role of auxins in tropisms, has centered on the auxin transport proteins in the PIN
(termed such because of the pin-shaped inflorescence stem in mutants) family (Blakeslee
et al. 2005; Paponov et al. 2005; see also Chapter 3 of this book). Understanding the link
among auxin transport proteins, NPH3/RPT2, and the actin cytoskeleton will be impor-
tant in determining the precise molecular role of these molecules in linking light percep-
tion in phototropism to the differential growth effects mediated by auxin (Maisch and
Nick 2007).
4.4 Interactions with gravitropism
Once a phototropic stimulus causes curvature in a plant, the orientation of the particular
plant organ will begin to change in relation to the gravity vector. This generally will lead
to a countering gravitropic response following the initial phototropic curvature. The two
signaling pathways thus need to be integrated into an overall growth response. Experi-
ments examining simultaneous gravitropic and phototropic stimulation have found that
the equilibrium growth response can be more complex than simple additivity (Nick and
Schäfer 1988; Galland 2002). Interpretation of these experiments is made somewhat dif-
ficult by differences in the kinetics of gravitropic and phototropic responses. For exam-
ple, the responses can have different latent periods, and there may be adaptation to the
stimulus during the responses, depending on stimulus strength, such that the magnitude
of curvature decreases with time (Iino 1988; Mullen et al. 2002; Kiss et al. 2003). In the
case of phototropism, part of the adaptation is likely due to changes in photoreceptor lev-