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

SIGNALING IN PLANTS
883
(A)
(B) ABSENCE OF AUXIN
CH 2 COOH
auxin-response factor (ARF)
N
H
transcriptional repressor (Aux/IAA)
AUXIN TARGET GENES OFF
(C) PRESENCE OF AUXIN
Figure 15–71 The auxin signaling
pathway. (A) The structure of the auxin
indole-3-acetic acid. (B) In the absence of
auxin, a transcriptional repressor protein
(called Aux/IAA) binds and suppresses
a transcription regulatory protein (called
auxin-response factor, ARF), which is
required for the transcription of auxinresponsive
genes. (C) The auxin receptor
proteins are mainly located in the
nucleus and form part of ubiquitin ligase
complexes (not shown). When activated
by auxin binding, the receptor–auxin
complexes recruit the ubiquitin ligase
complexes, which ubiquitylate the Aux/IAA
proteins, marking them for degradation in
proteasomes. ARF is now free to activate
the transcription of auxin-responsive genes.
There are many ARFs, Aux/IAA proteins,
and auxin receptors that work as illustrated.
auxin
active
ARF
Aux/IAA
auxin receptor protein
ubiquitin ligase
complex
ubiquitylation
and degradation of
Aux/IAA protein
TRANSCRIPTION OF AUXIN TARGET GENES
cells via the circulatory system, auxin has its own transport system. Specific plasma-membrane-bound
influx transporter proteins and efflux transporter proteins
move auxin into and out of plant cells, respectively. The efflux transporters can be
distributed asymmetrically in the plasma membrane to make the efflux of auxin
directional. A row of cells with their auxin efflux transporters confined to the basal
plasma membrane, for example, will transport auxin from the top of the plant to
the bottom.
In some regions of the plant, the localization of the auxin transporters, and
therefore the direction of auxin flow, is highly MBoC6 dynamic m15.86/15.69
and regulated. A cell can
rapidly redistribute transporters by controlling the traffic of vesicles containing
them. The auxin efflux transporters, for example, normally recycle between intracellular
vesicles and the plasma membrane. A cell can redistribute these transporters
on its surface by inhibiting their endocytosis in one domain of the plasma
membrane, causing the transporters to accumulate there. One example occurs in
the root, where gravity influences the direction of growth. The auxin efflux transporters
are normally distributed symmetrically in the cap cells of the root. Within
minutes of a change in the direction of the gravity vector, however, the efflux transporters
redistribute to one side of the cells, so that auxin is pumped out toward the
side of the root pointing downward. Because auxin inhibits root-cell elongation,
this redirection of auxin transport causes the root tip to reorient, so that it grows
downward again (Figure 15–72).
Phytochromes Detect Red Light, and Cryptochromes Detect Blue
Light
Plant development is greatly influenced by environmental conditions. Unlike
animals, plants cannot move when conditions become unfavorable; they have
to adapt or they die. The most important environmental influence on plants is