Chapter 39. Plant Response

Plant Reactions Stimuli & a Stationary Life animals respond to stimuli by changing behavior move toward positive stimuli move away from negative stimuli plants respond to stimuli by adjusting growth & development

What mechanism causes this response? grown in dark 1 week exposure to light

Signal Transduction Pathway model signal triggers receptor receptor triggers internal cellular messengers & then cellular response receptor signal pathway (2° messengers) response General model for signal transduction pathways: A hormone or another signal binding to a specific receptor stimulates the cell to produce relay molecules, such as second messengers. These relay molecules trigger the cell’s various responses to the original signal. In this diagram, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside.

Signal Transduction Pathway example 1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways Signal Transduction Pathway controlling greening (de-etiolation) of plant cells, like in a sprouting potato tuber. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways One pathway uses cGMP as a 2nd messenger to activate a protein kinase.The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase. Both pathways lead to expression of genes for proteins that function in greening response of plant. 2. One pathway uses cGMP as a 2nd messenger to activate a protein kinase. The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase. 3. Both pathways lead to expression of genes for proteins that function in greening response of plant.

Plant hormones Chemical signals that coordinate different parts of an organism minute amounts required produced by one part of body transported to another part binds to specific receptor triggers response in target cells & tissues

Plant hormones auxins cytokinins gibberellins brassinosteroids abscisic acid ethylene

Auxin Indolacetic acid (IAA) stimulates cell elongation enhances apical dominance classical explanation of phototropism

Phototropism Growth towards light

Cytokinins Family of hormones Effects produced in roots, fruits & embryos Effects control of cell division & differentiation Enhance axillary growth interaction of auxin & cytokinins

Gibberellins Family of hormones Effects over 100 different gibberellins Effects stem elongation fruit growth seed germination

Brassinosteroids Steroids Effects Slow leaf loss (abscission) cell elongation & division in shoots & seedlings

Abscisic acid (ABA) Effects slows growth seed dormancy survival value: seed will germinate only under optimal conditions light, temperature, moisture drought tolerance rapid stomate closing

Ethylene Ethylene is a gas released by plant cells Multiple effects response to mechanical stress triple response slow stem elongation thickening of stem curvature to horizontal growth apoptosis leaf abscission fruit ripening

Fruit ripening Hard, tart fruit protects developing seed from herbivores Ripe, sweet, soft fruit attracts animals to disperse seed burst of ethylene triggers ripening process conversion of starch to sugar = sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production

Responses to light Photomorphogenesis Light detection effect of light on plant growth Light detection intensity direction wavelength

Flowering Response Triggered by photoperiod relative lengths of day & night night length—“critical period”— is trigger Long-day plants flower if dark shorter than critical period Short-day plants flower if dark longer than critical period Short-day plants Long-day plants

Circadian rhythms Internal (endogenous) 24-hour cycles 4 O’clock Noon Midnight Morning glory

Responses to gravity roots = positive gravitropism shoots = negative gravitropism settling of statoliths detects gravity

Responses to touch Thigmotropism

Plant defenses Defenses against herbivores

Plant defenses Defenses against herbivores Parasitoid wasp larvae emerging from a caterpillar