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. What kinds of molecules are the receptors?

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 only minute amounts are required produced by 1 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

Phototropism Went 1926 Growth towards light

Auxin Indolacetic acid (IAA) stimulates cell elongation near apical meristems enhances apical dominance classical explanation of phototropism asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side

Zones of meristem growth shoot root

Cytokinins Family of hormones Effects modified forms of adenine produced in roots, fruits & embryos Effects control of cell division & differentiation enhances apical dominance interaction of auxin & cytokinins

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

Abscisic acid (ABA) Effects slows growth seed dormancy high concentrations of ABA germination only after ABA is inactivated down or leeched out 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

Apoptosis & Leaf abscission What is the evolutionary advantage of loss of leaves in autumn? Balance of ethylene & auxin many events in plants involve pre-programmed cell death death of annual plant after flowering differentiation of xylem vessels loss of cytosol shedding of autumn leaves The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn. Photo: Abscission of a maple leaf. Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM).

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 breakdown of cell wall = softening conversion of starch to sugar = sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production

Applications Truth in folk wisdom! one bad apple spoils the whole bunch ripening apple releases ethylene to speed ripening of fruit nearby Ripen green bananas by bagging them with an apple Climate control storage of apples high CO2 storage = reduces ethylene production

Responses to light Photomorphogenesis Light detection effect of light on plant growth Light detection intensity direction wavelength blue-light receptors phytochromes (red-light receptors) Why does it make “biological sense” that red & blue light have greater effects on plants response that other wavelengths?

Phytochrome photoreceptors Molecular switch reaction to red light conversion of Pr  Pfr in sunlight stimulates germination, flowering, branching… conversion of Pfr  Pr in dark inhibits response, & stimulates other responses: growth in height Chromophore Photorecptor Light induced Kinase activity Phytochrome Phytochrome Response: Vertical growth

Practical Application Why do you plant lettuce seed by scattering them on the ground instead of burying seed? What is the evolutionary advantage to lettuce seeds?

Flowering Response Triggered by photoperiod relative lengths of day & night night length—“critical period”— is trigger Plant is sensitive to red light exposure What is the evolutionary advantage of photoperiodism? Synchronizes plant responses to season Short-day plants Long-day plants

Is there a flowering hormone? Plant on left is induced to flower & then grafted onto plant on right plant on right is triggered to flower What can you conclude?

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

Responses to gravity How does a sprouting shoot “know” to grow towards the surface from underground? environmental cues? roots = positive gravitropism shoots = negative gravitropism settling of statoliths (dense starch grains) may detect gravity

Responses to touch Thigmotropism Mimosa (Sensitive plant) closes leaves in response to touch Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells

Plant defenses Defenses against herbivores

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

Plant defenses Defenses against pathogens