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
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Notch, Wnt, or Hedgehog encoded by the completely sequenced genome of Arabidopsis
thaliana, the small flowering plant. Similarly, plants do not seem to use
cyclic AMP for intracellular signaling. Nevertheless, the general strategies underlying
signaling are frequently very similar in plants and animals. Both, for example,
use enzyme-coupled cell-surface receptors, as we now discuss.
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Receptor Serine/Threonine Kinases Are the Largest Class
of Cell-Surface Receptors in Plants
Most cell-surface receptors in plants are enzyme-coupled. However, whereas
the largest class of enzyme-coupled receptors in animals is the receptor tyrosine
kinase (RTK) class, this type of receptor is extremely rare in plants. Instead,
plants rely largely on a great diversity of transmembrane receptor serine/threonine
kinases, which have a typical serine/threonine kinase cytoplasmic domain and an
extracellular ligand-binding domain. The most abundant types of these receptors
have a tandem array of extracellular leucine-rich repeat structures and are therefore
called leucine-rich repeat (LRR) receptor kinases.
There are about 175 LRR receptor kinases encoded by the Arabidopsis genome.
These include a protein called Bri1, which forms part of a cell-surface steroid
hormone receptor. Plants synthesize a class of steroids that are called brassinosteroids
because they were originally identified in the mustard family Brassicaceae,
which includes Arabidopsis. These signal molecules regulate the growth and
differentiation of plants throughout their life cycle. Binding of a brassinosteroid
to a Bri1 cell-surface receptor kinase initiates an intracellular signaling cascade
that uses a GSK3 protein kinase and a protein phosphatase to regulate the phosphorylation
and degradation of specific transcription regulatory proteins in the
nucleus, and thereby specific gene transcription. Mutant plants that are deficient
in the Bri1 receptor kinase are insensitive to brassinosteroids and are therefore
dwarfs.
The LRR receptor kinases are only one of many classes of transmembrane
receptor serine/threonine kinases in plants. There are at least six additional families,
each with its own characteristic set of extracellular domains. The lectin receptor
kinases, for example, have extracellular domains that bind carbohydrate signal
molecules. The Arabidopsis genome encodes over 300 receptor serine/threonine
kinases, which makes them the largest family of receptors known in plants. Many
are involved in defense responses against pathogens.
Ethylene Blocks the Degradation of Specific Transcription
Regulatory Proteins in the Nucleus
Various plant growth regulators (also called plant hormones) help to coordinate
plant development. They include ethylene, auxin, cytokinins, gibberellins, and
abscisic acid, as well as brassinosteroids. Growth regulators are all small molecules
made by most plant cells. They diffuse readily through cell walls and can
either act locally or be transported to influence cells further away. Each growth
regulator can have multiple effects. The specific effect depends on environmental
conditions, the nutritional state of the plant, the responsiveness of the target cells,
and which other growth regulators are acting.
Ethylene is an important example. This small gas molecule (Figure 15–69A)
can influence plant development in various ways; it can, for example, promote
fruit ripening, leaf abscission, and plant senescence. It also functions as a stress
signal in response to wounding, infection, flooding, and so on. When the shoot of
a germinating seedling, for instance, encounters an obstacle, ethylene promotes
a complex response that allows the seedling to safely bypass the obstacle (Figure
15–69B and C).
Plants have various ethylene receptors, which are located in the endoplasmic
reticulum and are all structurally related. They are dimeric, multipass transmembrane
proteins, with a copper-containing ethylene-binding domain and a domain
that interacts with a cytoplasmic protein called CTR1, which is closely related
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Figure 15–69 The ethylene-mediated
triple response that occurs when the
growing shoot of a germinating seedling
encounters an obstacle underground.
(A) The structure of ethylene. (B) In the
absence of obstacles, the shoot grows
upward and is long and thin. (C) If the
shoot encounters an obstacle, such as
a piece of gravel in the soil, the seedling
responds to the encounter in three ways.
First, it thickens its stem, which can then
exert more MBoC6 force m15.84/15.67
on the obstacle. Second,
it shields the tip of the shoot (at top) by
increasing the curvature of a specialized
hook structure. Third, it reduces the shoot’s
tendency to grow away from the direction
of gravity, so as to avoid the obstacle.
(Courtesy of Melanie Webb.)