Plant Response to Signals Ch 39

Plant Response 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

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.

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.

Plants do not have brains Or nervous systems for that matter, So how do they communicate with itself and coordinate beneficial responses?

Plant hormones Chemical signals that coordinate different parts of an organism only tiny 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 abscisic acid ethylene 2005-2006 Plant hormones auxins cytokinins gibberellins abscisic acid ethylene

Hormones in review: AUXINS __ Promote cellular elongation __ by softening of cell walls __ Involved in phototropism __ Involved in geotropism __ Involved in apical dominance CYTOKININS __ Promotes lateral growth_ growth in size of leaf cells __ Stimulate cell division (hence name) __ Release buds from apical dominance GIBERELLINS __ seasonal growth___ Stimulate cell elongation __ Produce bolting in biennials __ Stimulate production of starch digestion enzymes in some seeds ABSCISIC ACID __ opposite of giberellins__causes slow down or “cut off” of growth__Promotes stomatal closure __ __ Promotes seed and bud dormancy _ ETHYLENE __ Promotes ripening of fruit

Response to light: Phototropism Growth towards light Hormone: Auxin asymmetrical distribution of auxin, moves away from sunny side of stem (-ve phototropism, -ve gravitropism) cells on darker side elongate faster than cells on brighter side

Apical dominance Controls cell division & differentiation 2005-2006 Controls cell division & differentiation axillary buds do no grow while apical bud exerts control shoot root

Cell elongation in response to auxin: the acid growth hypothesis Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis Cross-linking polysaccharides Cell wall–loosening enzymes Expansin CELL WALL Cellulose microfibril H2O H Plasma membrane H H Cell wall H H H H H Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis. Nucleus Plasma membrane Cytoplasm ATP H Vacuole CYTOPLASM

Gibberellins Family of hormones Effects over 100 different gibberellins identified Effects fruit growth seed germination plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

Abscisic acid (ABA) Effects slows growth seed dormancy high concentrations of Abscisic acid 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 hormone gas released by plant cells Multiple effects response to mechanical stress triple response slow stem elongation thickening of stem curvature to stem growth leaf drop (like in Fall) apoptosis fruit ripening

Apoptosis & Leaf drop: combination of hormones What is the evolutionary advantage of loss of leaves in autumn? Ethylene & auxin many events in plants involve apoptosis (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 Adaptation Ethylene hard, tart fruit protects developing seed from herbivores ripe, sweet, soft fruit attracts animals to disperse seed 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….. 2005-2006 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

Plant stimuli

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

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

2005-2006 Response 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

Response to touch Thigmotropism 2005-2006 Response 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 2005-2006 Defenses against herbivores

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

Any Questions??