Interesting find on red light and phytochrome's influence on root elongation.
http://pcp.oxfordjournals.org/content/46/2/317.full.pdf
The Roles of Phytochromes in Elongation and Gravitropism of Roots
"Gravitropic orientation and the elongation of etiolated
hypocotyls are both regulated by red light through the phy-
tochrome family of photoreceptors. The importance of phy-
tochromes A and B (phyA and phyB) in these red light
responses has been established through studies using
phy mutants. To identify the roles that phytochromes play in
gravitropism and elongation of roots, we studied the effects
of red light on root elongation and then compared the grav-
itropic curvature from roots of phytochrome mutants of
Arabidopsis (phyA ,phyB, phyD and phyAB) with wild type.
We found that red light inhibits root elongation approxi-
mately 35% in etiolated seedlings and that this
response is
controlled by phytochromes. Roots from dark- and light-
grown double mutants (phyAB) and light-grown phyB seed-
lings have reduced elongation rates compared with wild
type. In addition, roots from these seedlings (dark/light-
grown phyAB and light-grown phyB) have reduced rates of
gravitropic curvature compared with wild type.
These
results demonstrate roles for phytochromes in regulating
both the elongation and gravitropic curvature of roots."
