1 Thought Question Plants can’t fight or hide or run away, so how do they adapt to a changing environment?

2 Lecture 8 Outline (Ch. 39) I.Plant Signaling & Hormones II.Auxins (and Cytokinins) A. Responses to Light B. Responses to Gravity C. Other responses III.Ethylene and senescence IV.Gibberellins and germination V.Abscisic acid and dormancy VI. Responses to Touch IV.Preparation for next lecture

3 Signal transduction pathways link signal reception to response – Plants have cellular receptors - detect important changes in environment For stimulus to elicit response - cells need specific receptor Plant Response - Overview Reception Transduction Response Receptor Relay molecules Signaling molecule cellular response Atomic model of the plant steroid receptor BRI1

4 (Plant) Hormone: Chemicals made in one location and transported to other locations for action Plant Hormones Growth Reproduction Movement Water balance Life cycles Usually produced in small amounts Dormancy, germination, primary growth, secondary growth, production and maturation of reproductive structures, phototropism, geotropism, leaf abscission, stomatal regulation, cell differentiation, cell death, branching structure, etc. Example hormone, there are others

5 Plant Orientation Phototropism – growth in response to directional light due to: cell ELONGATOIN / DIVISION on the SHADDED versus SUNNY side of a plant stem The growth of a plant part toward or away from light (from Greek tropos, meaning “turn”)

6 Plants not only detect the presence of light, - also direction, intensity, and wavelength (color) Blue light receptor: Directional growth responses Connect environmental signal with cellular perception of the signal, transduction into biochemical pathways, and ultimately an altered growth response Phototropism – Light Detection

7 Two major classes of light receptors (we’ll discuss the other later in this lecture): Blue-light photoreceptors stomatal movements phototropism 7 Phototropism – Light Detection Blue light receptor: Embedded in cell membrane When blue light detected, changes conformation, signal transduction  differential elongation

8 Plant Responses to Light First hormone discovered in phototropism: Auxin - hormone that promotes cell elongation - Auxin exits basal end of one cell and enters apical end of adjacent cell

9 Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ ATP Plasma membrane Cytoplasm Cell elongation in response to auxin Auxin increases activity of proton pumps. Enzymatic cleavage of polysaccharides allows microfibrils to slide.Cell wall can extend. Turgor causes the cell to expand. Cell wall becomes acidic. Expansins (active at low pH) cleave cellulose microfibrils from polysaccharides. Exposed polysaccharides now accessible to enzymes.

10 Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2OH2O Cell elongation in response to auxin With the cellulose loosened, the cell can elongate.

11 Other Auxin Stimulated Responses: Lateral / branching root formation Promote fruit growth (tomato sprays) As herbicide, overdose kills dicots Auxin is produced: At the shoot apex, seeds, other actively growing tissues. Other responses to auxin

12 Control of Apical Dominance Cytokinins (another plant hormone) and auxins interact in the control of apical dominance –The ability of a terminal bud to suppress development of axillary buds If the terminal bud is removed –Plants become bushier Axillary buds “ Stump” after removal of apical bud Lateral branches 12

Response of a plant to the gravitational field of the Earth Shoots exhibit negative gravitropism Roots have a positive gravitropic response 13 Response to Gravity

14 Gravity Response How Do Plants Detect Gravity? Starch-filled plastids –In specialized stem cells and root caps –Orient within cells toward gravity Changing plastid orientation triggers elongation –  involves auxin! plastids cell in root cap root

15 Other major class of light receptors: Phytochromes – red/far-red receptor shade avoidance response photoperiodism A phytochrome consists of two identical proteins joined Photoreceptor activity. Enzyme - kinase activity. 15 Photoperiodism Photoperiodism: is the response to time of year (seasons)

16 Photoperiod - relative lengths of night and day Triggers many developmental processes –Bud break –Flowering –Leaf drop in deciduous trees Are actually controlled by night length, not day length Photoperiodism A phytochrome phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod

17 Short-day (long night) plants: flower when nights longer than critical period Long-day (short night) plants: flower when nights shorter than critical period. Photoperiodism

18 Many legumes –Lower their leaves in the evening and raise them in the morning Noon Midnight Circadian Rhythms Cyclical responses to stimuli –about 24 hours long –entrained to external clues of the day/night cycle Phytochrome changes mark sunrise and sunset –Providing the biological clock with environmental cues Photoperiodism

19 Ethylene – only hormone that is a gas! Rapid increase in ethylene triggers: apoptosis fruit ripening leaf abscission Senescence Abscission Ethylene stimulates production of enzyme that digests cell walls at base of petiole Leaf falls when cells are sufficiently weakened leaf petiole bud abscission layer

20 Why will these ripe bananas help the green avocados ripen faster? 20 Self-Check

21 After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate Gibberellins stimulate germination Responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. Aleurone Endosperm Water cotyledon GA amylase Sugar embryo releases gibberellin as a signal Nutrients absorbed from the endosperm by the cotyledon are consumed during growth of the embryo into a seedling. Embryo

22 Abscisic Acid and plant stress Abscisic Acid: Initiates closing stomata in water-stressed plants Induces and maintains dormancy in buds and seeds –(inhibits gibberellins)

23 Two of the many effects of abscisic acid (ABA) are Seed dormancy –Ensures seeds germinate only when conditions are optimal Drought tolerance –Closes stomata, decreases shoot growth Coleoptile Abscisic Acid Why is that one kernel (seed) germinating prematurely? K+K+ K+K+ K+K+ 23

24 Self-Check Hormone NameFunctions Auxin Gibberellin Cytokinin Ethylene Abscisic Acid

Thigmotropism is directional growth of a plant or plant part in response to contact Examples of touch responses: Venus flytrap leaves Tendrils around objects Mimosa tree leaves 25 Thigmotropism Often due to differential elongation or manipulated water/turgor pressure

Things To Do After Lecture 8… Reading and Preparation: 1.Re-read today’s lecture, highlight all vocabulary you do not understand, and look up terms. 2.Ch. 39 Self-Quiz: #1, 2, 6, 7 (correct answers in back of book) 3.Read chapter 39, focus on material covered in lecture (terms, concepts, and figures!) 4.Skim next lecture. “HOMEWORK” (NOT COLLECTED – but things to think about for studying): 1.Describe in detail how auxin regulates stem directional growth. 2.Explain the difference between phototropism and photoperiodism – what wavelengths (colors) of light control each? 3.Diagram the direction of growth in relation to positive vs. negative phototropism, gravitropism, and thigmotropism 4.List the five main plant hormones and give a once sentence description of the main function/job of each one.