2Plant Reactions Stimuli & a Stationary Life animals respond to stimuli by changing behaviormove toward positive stimulimove away from negative stimuliplants respond to stimuli by adjusting growth & development
3What mechanism causes this response? grown in dark1 week exposure to light
4Signal Transduction Pathway model signal triggers receptorreceptor triggers internal cellular messengers & then cellular responsereceptorsignal pathway (2° messengers)responseGeneral 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?
5Signal Transduction Pathway example 1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathwaysSignal 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 pathwaysOne 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.
6Plant hormonesChemical signals that coordinate different parts of an organismonly minute amounts are requiredproduced by 1 part of bodytransported to another partbinds to specific receptortriggers response in target cells & tissues
9Auxin Indolacetic acid (IAA) stimulates cell elongation near apical meristemsenhances apical dominanceclassical explanation of phototropismasymmetrical distribution of auxincells on darker side elongate faster than cells on brighter side
11Polarity of auxin transport Auxin picks up H+ between cells & is neutralizedNeutral auxin passes through membraneCellular pH 7 causes auxin to ionize & is trapped in cellAuxin stimulates proton pumpAuxin leaves through carriers
12Auxin response Acid growth hypothesis According to a model called the acid growth hypothesis, proton pumps play a major role in the growth response of cells to auxin. In a shoot’s region of elongation, auxin stimulates the plasma membrane’s proton pumps. This pumping of H+ increases the voltage across the membrane (membrane potential) and lowers the pH in the cell wall within minutesAcidification of the wall activates enzymes called expansins that break the cross–links (hydrogen bonds) between cellulose microfibrils and other cell wall constituents, loosening the wall’s fabric. (Expansins can even weaken the integrity of filter paper made of pure cellulose.) Increasing the membrane potential enhances ion uptake into the cell, which causes osmotic uptake of water and increased turgor. Increased turgor and increased cell wall plasticity enable the cell to elongate.
13Cytokinins Family of hormones Effects modified forms of adenine produced in roots, fruits & embryosEffectscontrol of cell division & differentiationenhances apical dominanceinteraction of auxin & cytokinins
14Gibberellins Family of hormones Effects over 100 different gibberellins identifiedEffectsstem elongationfruit growthseed germination
15Brassinosteroids Steroids Effects similar to auxins cell elongation & division in shoots & seedlings
16Abscisic acid (ABA) Effects slows growth seed dormancy high concentrations of ABAgermination only after ABA is inactivated down or leeched outsurvival value: seed will germinate only under optimal conditionslight, temperature, moisturedrought tolerancerapid stomate closing
17Ethylene Ethylene is a gas released by plant cells Multiple effects response to mechanical stresstriple responseslow stem elongationthickening of stemcurvature to horizontal growthapoptosisleaf abscissionfruit ripening
18Apoptosis & Leaf abscission What is the evolutionary advantage of loss of leaves in autumn?Balance of ethylene & auxinmany events in plants involve pre-programmed cell deathdeath of annual plant after floweringdifferentiation of xylem vesselsloss of cytosolshedding of autumn leavesThe 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).
19Fruit ripeningHard, tart fruit protects developing seed from herbivoresRipe, sweet, soft fruit attracts animals to disperse seedburst of ethylene triggers ripening processbreakdown of cell wall = softeningconversion of starch to sugar = sweeteningpositive feedback systemethylene triggers ripeningripening stimulates more ethylene production
20Applications Truth in folk wisdom! one bad apple spoils the whole bunchripening apple releases ethylene to speed ripening of fruit nearbyRipen green bananas by bagging them with an appleClimate control storage of appleshigh CO2 storage = reduces ethylene production
21Responses to light Photomorphogenesis Light detection effect of light on plant growthLight detectionintensitydirectionwavelengthblue-light receptorsphytochromes (red-light receptors)Why does it make “biological sense” that red & blue light have greater effects on plants response that other wavelengths?
22Phytochrome photoreceptors Molecular switch reaction to red lightconversion of Pr Pfr in sunlight stimulates germination, flowering, branching…conversion of Pfr Pr in dark inhibits response, & stimulates other responses: growth in heightChromophorePhotorecptorLight inducedKinase activityPhytochromePhytochromeResponse:Vertical growth
23Practical 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?
24Flowering Response Triggered by photoperiod relative lengths of day & nightnight length—“critical period”— is triggerPlant is sensitive to red light exposureWhat is the evolutionary advantage of photoperiodism?Synchronizes plant responses to seasonShort-day plantsLong-day plants
25Is there a flowering hormone? Plant on left is induced to flower & then grafted onto plant on rightplant on right is triggered to flowerWhat can you conclude?
27Responses to gravityHow does a sprouting shoot “know” to grow towards the surface from underground?environmental cues?roots = positive gravitropismshoots = negative gravitropismsettling of statoliths (dense starch grains) may detect gravity
28Responses to touch Thigmotropism Mimosa (Sensitive plant) closes leaves in response to touchCaused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells