Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

884 Chapter 15: Cell Signaling
(A)
(B)
(C)
vascular
tissue
epidermis
root
cap
ROOT DISPLACED
THROUGH 90°
(D)
cells in center
of root cap
respond to
gravity
inhibition of
epidermal cell
elongation in
this region restores
downward growth
GRAVITY
auxin inhibits
epidermal cell
elongation on
downward side
of root
reorientation of auxin
efflux transporters
in center of root cap
directs auxin to
downward side of root
Figure 15–72 Auxin transport and root gravitropism. (A–C) Roots respond to a 90° change
in the gravity vector and adjust their direction of growth so that they grow downward again. The
cells that respond to gravity are in the center of the root cap, while it is the epidermal cells further
back (on the lower side) that decrease their rate of elongation to restore downward growth. (D) The
gravity-responsive cells in the root cap redistribute their auxin efflux transporters in response to
the displacement of the root. This redirects the auxin flux mainly to the lower part of the displaced
root, where it inhibits the elongation of the epidermal cells. The resulting asymmetrical distribution
of auxin in the Arabidopsis root tip shown here is assessed indirectly, using an auxin-responsive
reporter gene that encodes a protein fused to green fluorescent protein (GFP); the epidermal cells
on the downward side of the root are green, whereas those on the upper side are not, reflecting
the asymmetrical distribution of auxin. The distribution of auxin efflux transporters in the plasma
membrane of cells in different regions MBoC6 of the m15.87/15.70
root (shown as gray rectangles) is indicated in red, and
the direction of auxin efflux is indicated by a green arrow. (The fluorescence photograph in D is from
T. Paciorek et al., Nature 435:1251–1256, 2005. With permission from Macmillan Publishers Ltd.)
light, which is their energy source and has a major role throughout their entire life
cycle—from germination, through seedling development, to flowering and senescence.
Plants have thus evolved a large set of light-sensitive proteins to monitor
the quantity, quality, direction, and duration of light. These are usually referred
to as photoreceptors. However, because the term photoreceptor is also used for
light-sensitive cells in the animal retina (see Figure 15–38), we shall use the term
photoprotein instead.
All photoproteins sense light by means of a covalently attached light-absorbing
chromophore, which changes its shape in response to light and then induces a
change in the protein’s conformation. The best-known plant photoproteins are the
phytochromes, which are present in all plants and in some algae but are absent in
animals. These are dimeric, cytoplasmic serine/threonine kinases, which respond
differentially and reversibly to red and far-red light: whereas red light usually activates
the kinase activity of the phytochrome, far-red light inactivates it. When activated
by red light, the phytochrome is thought to phosphorylate itself and then
to phosphorylate one or more other proteins in the cell. In some light responses,
the activated phytochrome translocates into the nucleus, where it activates transcription
regulators to alter gene transcription (Figure 15–73). In other cases, the
activated phytochrome activates a latent transcription regulator in the cytoplasm,
which then translocates into the nucleus to regulate gene transcription. In still
other cases, the photoprotein triggers signaling pathways in the cytosol that alter
the cell’s behavior without involving the nucleus.