Plant Responses to Internal and External Signals

Hormones: chemical messengers that coordinate different parts of a multicellular organism Important plant hormones: Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide) Cytokinins – cell division (cytokinesis) & differentiation Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

The effects of gibberellin on stem elongation and fruit growth

Ethylene Gas: Fruit Ripening Canister of ethylene gas to ripen bananas in shipping container Untreated tomatoes vs. Ethylene treatment

Plant Movement Tropisms: growth responses  SLOW Phototropism – light (auxin) Gravitropism – gravity (auxin) Thigmotropism – touch Turgor movement: allow plant to make relatively rapid & reversible responses Venus fly trap, mimosa leaves, “sleep” movement

Experiments with Light Phototropism Experiments with Light Conclusion: Tip of stem senses light  some signal was sent from tip to elongating region of coleoptile

Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Offset blocks cause curvature Control (agar block lacking chemical) has no effect Further Experiments: Cells on darker side elongate faster than cells on brighter side AUXIN = chemical messenger that stimulates cell elongation

Gravitropism

Positive gravitropism in roots: the statolith hypothesis.

Thigmotropism: rapid turgor movements by Mimosa plant  action potentials

Plant Responses to Light Plants can detect direction, intensity, & wavelenth of light Phytochromes: light receptors, absorbs mostly red light Regulate seed germination, shade avoidance

Biological Clocks Circadian rhythm: biological clocks Persist w/o environmental cues Frequency = 24 hours Phytochrome system + Biological clock = plant can determine time of year based on amount of light/darkness

Night length is a critical factor! Photoperiodism: physiological response to the relative length of night & day (i.e. flowering) *more predictable than air temperature changes* Short-day plants: flower when nights are long (mums, poinsettia) Long-day plant: flower when nights are short (spinach, iris, veggies) Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions) Night length is a critical factor!

How does interrupting the dark period with a brief exposure to light affect flowering?

Plant responses to stress

Flooding (O2 deprivation): Drought (H2O deficit): close stoma release abscisic acid to keep stoma closed Inhibit growth roll leaves  reduce SA & transpiration deeper roots Flooding (O2 deprivation): release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

Excess Salt: Heat: Cold: cell membrane – impede salt uptake produce solutes to ↓ψ - retain H2O Heat: evap. cooling via transpiration heat shock proteins – prevent denaturation Cold: alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity) increase cytoplasmic solutes antifreeze proteins

Herbivores: Pathogens: physical (thorns) chemicals (garlic, mint) recruit predatory animals (parasitoid wasps) Pathogens: 1st line of defense = epidermis 2nd line = pathogen recognition, host-specific