9.3 - Chemical control in plants

What are three groups of plant growth factors?

• Auxins

• Cytokinins

• Gibberellins

What are the functions of auxins?

• Involved in trophic responses (e.g. IAA)

• Control cell elongation

• Suppress lateral buds to maintain apical dominance

• Promote root growth (e.g. in rooting powders)

How do auxins cause cell elongation?

Acid growth hypothesis:

1. IAA causes active transport of H⁺ ions into cell wall

2. Disruption to hydrogen bonds between cellulose molecules & action of expansins make cell more permeable to water

3. Cells with higher turgor pressure elongate faster

What are the functions of gibberellins?

Stimulate:

• germination

• elongation at cell internodes

• fruit growth

• rapid growth/flowering

How is germination stimulated?

1. Seeds absorb water, activating embryo to secrete gibberellins

2. Gibberellins diffuse to aleurone layer, which produces amylase

3. Amylase diffuses to endosperm layer to hydrolyse starch

4. Hexose sugars act as respiratory substrate to produce ATP as ‘energy currency’

What are the functions of cytokinins?

• Stimulate the development of lateral buds by promoting cell division at apical meristems

• Promote leaf abscission synergetically with ethene

What are the two ways that plant growth hormones interact?

• Synergetically to achieve same effect (e.g. auxins & gibberellins)

• Antagonistically with inverse effects (e.g. auxins (suppress lateral buds) & cytokinins (stimulate lateral buds)

What is phytochrome?

Plant photoreceptor with bilin chromophore group. Converts between two forms:

• biologically inactive Pr absorbs red light

• biologically active Pfr absorbs far-red light

What are the times of day when each form of phytochrome is most abundant?

• In darkness: Pr abundant

• In sunlight: Pfr abundant

(ratio of Pr : Pfr enables plants to detect how long days are)

How does phytochrome control flowering?

Pr absorbs red light & converts to Pfr, which stimulates flowering → signifies that light intensity is high enough for photosynthesis

What are long & short day plants?

• Long day plants: flower when sunlight hours exceed a critical value

• Short day plants: flower when darkness hours exceed a critical value

What is photomorphogenesis?

Pattern of plant growth & development determined by light intensity

How does phytochrome control photomorphogenesis?

• Transition from Pr to Pfr controls localisation of proteins within cells, transcription of certain genes & phosphorylation of proteins

• Therefore affects germination, circadian rhythms & flowering