4.2 Light perception
The light environment in which plants naturally live and grow can be a very heteroge-
neous one, with both diffuse and directional light varying throughout the landscape.
Spatial differences in the light intensity impinging upon a plant manifest as a gradient in
light intensity across the particular plant organ, due to scattering and absorption by the
plant tissue (Vogelmann et al. 1996). This leads to a gradient in photoreceptor activation
in the plant organ, providing directional information regarding the light stimulus. The
presence of screening pigments, such as carotenoids, in the plant can increase the light
gradient and have been found to increase the magnitude of phototropic responses
(Piening and Poff 1988). In leaves of solar-tracking plants, however, gradients in light in-
tensity across the organs do not play an important role in the phototropic response. In
these plants, the leaf surface is perpendicular to the direction of the light beam, and pho-
totropic leaf reorientation occurs when the direction of the light beam becomes oblique
(Schwartz and Koller 1978). This vectorial phototropic response is sensed by cells above
the veins of the leaves, and the ability to sense light direction is related to the angle be-
tween the light beam and the directions of the major axes of the veins. Sensing of the di-
rection of light, in this case, has been postulated to be due to localization of photorecep-
tors to the end walls of the cells along the leaf veins (Koller et al. 1990).
Plants are generally bathed in a full spectrum of light, but nevertheless they have dif-
ferent photoreceptor pigments to sense specific qualities of light (Figure 4.1). The action
spectrum for phototropism in flowering plants clearly shows that blue light is the most
effective in this process. Although this observation was known for more than a century
(reviewed in Whippo and Hangarter 2006), it was only in the 1990s that the specific blue-
light-absorbing photoreceptor was identified by the use of nph (non-phototropic
hypocotyl) mutants of Arabidopsis. This family of photoreceptors, renamed phototropins,
has two members found in Arabidopsis, PHOT1 and PHOT2 (Christie et al. 1999; Sakai
et al. 2001). The phot1 and phot2 holoproteins have different sensitivities to light inten-
sity, with phot2 being functionally active only at the relatively higher light intensities of
greater than approximately 10 μmol m–2s–1(Sakai et al. 2001). The phototropins are au-
tophosphorylating serine/threonine kinases with two LOV (light-, oxygen-, or voltage-
sensitive) domains, a motif that is present in a wide diversity of organisms including
plants, fungi, and bacteria. The LOV domains bind flavin mononucleotide as chro-
mophores for the photoreceptor (Christie et al. 1999). However, although both LOV do-
mains are evolutionarily conserved, only the second domain (LOV2) appears to be func-
tionally important for photoperception (Christie et al. 2002; Cho et al. 2007). The
phototropins are localized to the plasma membrane, though following stimulation with
blue light a fraction dissociate from the membrane (Sakamoto and Briggs 2002; Kong et
al. 2006). In parenchyma cells near the vascular tissue, phot1 is asymmetrically distrib-
uted, with most of the photoreceptor found along the end walls normal to the major axis
of the veins (Sakamoto and Briggs 2002). This localization is consistent with the pro-
posed mechanism for vectorial light sensing (Koller et al. 1990). Despite their name, pho-
totropins are not specific to the phototropic response. They can also function in other re-
sponses such as chloroplast movement in leaves and stomatal opening (Briggs and
Christie 2002; Kimura and Kagawa 2006).