Plant Responses to Internal and External Signals

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There are two major classes of light receptors: blue-light photoreceptors and phytochromes.

Blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism.

Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life.

These responses include seed germination and shade avoidance.

Slide 46

Structure of a phytochrome

Two identical subunits

Chromophore

Photoreceptor activity

Kinase activity

Slide 47

Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses.

Red light

Far-red light

Pr

Pfr

Slide 48

Phytochrome: a molecular switching mechanism

Synthesis

Pr

Far-red

light

Slow conversion

in darkness

(some plants)

Enzymatic

destruction

Responses:

seed germination,

control of

flowering, etc.

Pfr

Red light

Slide 49

Biological Clocks and Circadian Rhyths

Many plant processes oscillate during the day.

Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions.

Slide 50

Sleep movements of a bean plant

Noon

Midnight

Slide 51

Circadian rhyths are cycles that are about 24 hours long and are governed by an internal “clock.”

Circadian rhyths can be entrained to exactly 24 hours by the day/night cycle.

The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes.

Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues.

Slide 52

Photoperiodism and Responses to Seasons

Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year.

Photoperiodism is a physiological response to photoperiod.

Slide 53

Photoperiodism and Control of Flowering

Some processes, including flowering in many species, require a certain photoperiod.

Plants that flower when a light period is shorter than a critical length are called short-day plants.

Plants that flower when a light period is longer than a certain number of hours are called long-day plants.