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

DEVELOPMENtaL TIMING
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cells at the growing apex of the plant shoot—the apical meristem. During ordinary
vegetative growth, these cells behave as stem cells, generating a steady succession
of new leaves and new segments of stalk. In flowering, the meristem cells switch
to making the components of a flower, with its sepals and petals, its stamens carrying
pollen, and its ovary containing the female gametes.
To time the switch correctly, the plant has to take account of both past and
present conditions. One important cue, for many plants, is day length. To sense
this, the plant uses its circadian clock—an endogenous 24-hour rhythm of gene
expression—to generate a signal for flowering only when there is light for the
appropriate part of the day. The clock itself is influenced by light, and the plant
in effect uses the clock to compare past to present lighting conditions. Important
parts of the genetic circuitry underlying these phenomena have been identified,
including the phytochromes and cryptochromes that act as light receptors (discussed
in Chapter 15). The flowering signal that is carried from the leaves to the
stem cells via the vasculature depends on the product of Flowering locus T (Ft).
But this signal will trigger flowering only if the plant is in a receptive condition
from prior long-term cold exposure. Many plants need winter before they
will flower—a process called vernalization. Cold over a period of weeks or months
progressively reduces the level of expression of a remarkable gene called Flowering
locus C (Flc). Flc encodes a transcriptional repressor that suppresses expression
of the Ft flowering promoter.
How does vernalization shut down Flc so as to lift the block to flowering? The
effect involves a noncoding RNA called Coolair that overlaps with the Flc gene and
is produced when the temperature is low (Figure 21–44). Together with cold-induced
chromatin modifiers, including Polycomb-group proteins, Coolair coordinates
the switching of Flc chromatin to a silent state (discussed in Chapters 4 and
7). The degree of silencing depends on the length of cold exposure enabling the
plants to distinguish the odd chilly night from the whole of winter.
The effect on the chromatin is long lasting, persisting through many rounds
of cell division even as the weather grows warmer. Thus vernalization creates a
persistent block in production of Flc, enabling the Ft signal to be generated when
day length is sufficiently long.
Flc active
flowering genes
OFF
open chromatin
Flc gene
time
autumn
chromatin
modifiers
Coolair RNA
made at lower
temperature
Flc inactive
flowering genes
OFF
Flc inactive
flowering genes
ON
closed chromatin
FLOWERING
winter
spring
Figure 21–44 Temporal control of
flowering in Arabidopsis. The Flc gene
is active and blocks flowering when plants
have been grown without exposure to
winterlike temperatures. Exposure to a
prolonged period of cold leads to the
production of the noncoding RNA Coolair,
which overlaps with the Flc gene. Coolair
induces long-term chromatin changes that
turn off Flc. These changes persist after
the end of the cold period and allow the
plant to flower when other environmental
conditions are favorable for flowering.