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

880 Chapter 15: Cell Signaling
the transcription of its target genes. NFκB also activates the transcription of the gene
that encodes IκBα, creating a negative feedback loop, which can produce prolonged
oscillations in NFκB activity with sustained extracellular signaling.
Some small, hydrophobic signal molecules, including steroid and thyroid hormones,
diffuse across the plasma membrane of the target cell and activate intracellular
receptor proteins that directly regulate the transcription of specific genes.
In many cell types, gene expression is governed by circadian clocks, in which
delayed negative feedback produces 24-hour oscillations in the activities of transcription
regulators, anticipating the cell's changing needs during the day and night.
SIGNALING IN PLANTS
In plants, as in animals, cells are in constant communication with one another.
Plant cells communicate to coordinate their activities in response to the changing
conditions of light, dark, and temperature, which guide the plant’s cycle of growth,
flowering, and fruiting. Plant cells also communicate to coordinate activities in
their roots, stems, and leaves. In this final section, we consider how plant cells signal
to one another and how they respond to light. Less is known about the receptors
and intracellular signaling mechanisms involved in cell communication in
plants than is known in animals, and we will concentrate mainly on how the receptors
and intracellular signaling mechanisms differ from those used by animals.
Multicellularity and Cell Communication Evolved Independently in
Plants and Animals
Although plants and animals are both eukaryotes, they have evolved separately
for more than a billion years. Their last common ancestor is thought to have been
a unicellular eukaryote that had mitochondria but no chloroplasts; the plant lineage
acquired chloroplasts after plants and animals diverged. The earliest fossils
of multicellular animals and plants date from almost 600 million years ago. Thus,
it seems that plants and animals evolved multicellularity independently, each
starting from a different unicellular eukaryote, some time between 1.6 and 0.6 billion
years ago (Figure 15–68).
If multicellularity evolved independently in plants and animals, the molecules
and mechanisms used for cell communication will have evolved separately and
would be expected to be different. There should be some degree of resemblance,
however, because the genes in both plants and animals diverged from those contained
by their last common unicellular ancestor. Thus, whereas both plants and
animals use nitric oxide, cyclic GMP, Ca 2+ , and Rho family GTPases for signaling,
there are no homologs of the nuclear receptor family, Ras, JAK, STAT, TGFβ,
Figure 15–68 The proposed divergence
of plant and animal lineages from a
common unicellular eukaryotic ancestor.
The plant lineage acquired chloroplasts
after the two lineages diverged. Both
lineages independently gave rise to
multicellular organisms—plants and
animals. (Paintings courtesy of John Innes
Foundation.)
mitochondrion
nucleus
eukaryotic
common
ancestor cell
DIVERGENCE
OF ANIMAL
AND PLANT
LINEAGES
unicellular ancestor
of animals
ACQUISITION OF
MULTICELLULARITY
ACQUISITION OF
MULTICELLULARITY
unicellular
ancestor of plants
ACQUISITION OF
CHLOROPLASTS
chloroplast