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

I.Plant Signal Transduction II.Auxins (and Cytokinins) III.Phytochromes! IV. Ethylene and senescence V. Gibberellins & germination VI. Abscisic acid & dormancy VII. Summary Lecture 11 Outline (Ch. 39)

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 Signals of focus in this lecture: light and hormones!

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 Example hormone, there are others Plant responses to hormones

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 (IAA) - 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

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

On the left is a potato left in the dark; on the right, a potato left in the light. De-etiolation (“greening”) Etiolation = morphological changes for growing in dark

De-etiolation response in potato

Phytochromes – red light/far-red light receptors Lettuce seeds germinate if exposed to red light. But if this is followed by far- red light, they will not germinate. When light pulses were alternated between red and far-red, the results are shown at left.

Phytochromes – red light/far-red light receptors The light wavelengths induce conformational changes. P fr is the active form for generating cell responses. The P r /P fr ratio is crucial! Phytochromes are receptors for red/far-red light.

17 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 Phytochromes also control detection of night length = photoperiod

18 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

Really it is red light detection that interrupts the dark period. Photoperiodism Again, depending on the order of red/far-red detection, responses are altered!

If you were working your way through college for a large florist, and in December…. You realize that you left your study materials in the greenhouse so you turn on the lights in the middle of the night. The greenhouse is growing several hundred poinsettias which are short day plants. Will your unexpected midnight visit cause a problem, why or why not?

21 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

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

23 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

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

25 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+ 25

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