Lecture 12 Sensing the Environment and Controlling Growth

How do plants sense the environment and develop in response?

Hormones and photoreceptors

Hormones

Chemical signals that act at very low concentrations at sites often far from where they are produced

Photoreceptors

Pigments associated with proteins that absorb light

Plant hormones

Auxin and ethylene

Auxin

Controls the direction and arrangement of apical growth, gravitropism, and apical dominance

Ethylene

Gaseous compound controls fruit ripening, leaf drop, germination, and the development of apical hook

Auxin functions

Orchestrates most plant growth, stimulates cell elongation

Auxin was discovered in the context of ___.

phototropism

Phototropism

response to light → stems bend toward light source

What is phototropism caused by?

Cell elongation stimulated by auxin

Where is the stimulus for phototropism initiated?

The tip of the growing stem; signal then travels down stem to cause bend

Conclusion of Darwin’s phototropism experiment

The signal moves down the shaded side of the shoot and causes it to bend

What happens when the mica barrier is inserted on the shaded side?

No bend

What happens when the mica barrier is inserted on the light side?

Bend

Auxin molecules congregate on what side of the plant?

The shaded side, causing elongation there → bend towards the sun

The biphasic effect of auxin

In roots, auxin inhibits cell elongation. In stem, auxin promotes growth.

Auxin contributes to ___.

gravitropism

Negative gravitropism

Upward gravitropic response of shoots

Positive gravitropism

Downward gravitropic response of roots

What explains the biphasic effect of auxin?

The acid growth theory of auxin-induced cell elongation

Auxin leads to ___.

apical dominance

Apical dominance

Apical buds inhibit the growth of axillary buds. Terminal bud produces auxin, prevents lateral buds from growing.

Examples of strong vs. weak apical dominance

Ethylene gas

Promotes leaf abscission and senescence, speeds ripening

Ethylene causes…

an increase in its own production.

Climacteric fruit

continue ripening after being harvested

Phytochromes

Photoreceptor; exists in two interconvertible isoforms (Pr and Pfr)

How do phytochromes interconvert?

Exposure to red light and exposure to far-red light

Active phytochrome

Pfr

Inactive phytochrome

Pr

Ratio of Pr and Pfr determines

Seed germination, flowering, shoot development

During daylight, ___ predominates

Pfr

In shade, ___ predominates

Pr

Phytochrome affects the plant by ___.

stimulating gene transcription.

Flowering time is synchronized by…

temperature and photoperiod (length of night)

Fruit ripening

Light-regulated chloroplast reorientation