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 Defenses II.Responses to Light III. Circadian Rhythms IV.Responses to Gravity V.Responses to Touch VI.Plant Hormones VII.Preparation for next lecture

Plants are susceptible to physical stresses Examples? Other threats include: viruses, bacteria, fungi, animals, and other plants 3 Plant Defenses Alfalfa plant bug Why are nonnative invasive species especially problematic? Dermal tissue: 1 st line of defense –Dermis covered with cutin or suberin: substances to reinforce cell walls –Silica inclusions, trichomes, bark, and even thorns can also offer protection

Many plants produce toxins that kill herbivores, make them ill, or repel them with strong flavors or odors Some plants have antimicrobial peptides 4 Toxin Defenses Secondary metabolites –Plants make defense compounds via modified metabolism Ex. Alkaloids [Wild tobacco has elevated nicotine levels lethal to tobacco hornworms] –Tannins

5 Ricin: alkaloid produced by castor bean plant –6X more lethal than cyanide –A single seed can kill a small child –Binds ribosomes - inhibits translation Toxin Defenses

6 Immediate Plant Responses -Plants may produce protective compounds -Plants may summon “bodyguards” when attacked -Plants may warn other plants of attack -Some plants move rapidly

Some plants “recruit” animals in mutualism Acacia trees and ants –Small armies of ants protect Acacia trees from harmful herbivores –Plant provides ants with food and shelter 7 Animal “Body Guards” Foolish katydid

–As caterpillar chews away, a wound response in the plant leads to release of a volatile compound –Female parasitoid wasp is attracted –Lays fertilized eggs in caterpillar –Eggs hatch and larvae kill caterpillar 8 Animal “Body Guards”

9 Chemical Warnings Volatile chemicals released by plants boost defenses in neighbors Many virally-attacked plants produce salicylic acid –Activates an immune response Attacked plant converts salicylic acid to methyl salicylate (wintergreen)  diffuses to air –Absorbed by neighboring healthy plants and reconverted to salicylic acid (aspirin)

10 Tobacco plants produce salicylic acid to fight viral infections Virus Infected Plant Methyl salicylate  Salicylic acid production  Salicylic acid production  Salicylic acid production  Salicylic acid production Chemical Warnings

11 Leaves have sensory “hairs” on inside –Fly triggers hairs - generates signal Cells in outer leaf epidermis pump H + into cell walls Enzymes activated  cells absorb water Outer epidermal cells expand, close leaf Reopening leaves takes several hours Venus fly trap Touch Responses

12 Self-Check DefenseExamples Chemical compounds Recruited animals Volatile chemicals Movement

13 Two major classes of light receptors: Blue-light photoreceptors stomatal movements phototropism Phytochromes – red/far-red receptor shade avoidance response photoperiodism A phytochrome consists of two identical proteins joined Photoreceptor activity. Enzyme - kinase activity. 13 Plant Timekeeping/Light Detection

14 Plant Orientation

15 Plant Responses to Light 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

16 Plant Responses to Light Blue light receptor: Embedded in cell membrane When blue light detected, changes conformation, signal transduction  differential elongation

Phototropism is defined as… A.Plant movement toward light B.Plant flowering seasonally C.Plant detection of shade D.Plant response to dawn/dusk

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 Plant Timekeeping/Light Detection

19 Response to time of year (seasons) 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 phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod Photoperiodism Plant Timekeeping/Light Detection

20 Short-day (long night) plants: flower when nights longer than critical period Long-day (short night) plants: flower when nights shorter than critical period. Plant Timekeeping/Light Detection

Which of the following would trigger flowering in a short-day plant… A.Night shorter than critical length B.Night longer than critical length with a flash of light C.Night longer than critical length D.Night shorter than critical length with a flash of light

22 Leaves detect lengths of night/day –An internal biological clock –A light-detecting phytochrome Pigments found in leaves Active/inactive depending on light conditions Still-unidentified chemical (florigens)  travel from leaf to bud to either trigger or inhibit flowering Plant Timekeeping/Light Detection A phytochrome Photoreceptor activity. Enzyme - kinase activity.

Response of a plant to the gravitational field of the Earth Shoots exhibit negative gravitropism Roots have a positive gravitropic response 23 Response to Gravity

24 Response to Gravity Four general steps lead to a gravitropic response: 1.Gravity is perceived by the cell 2.Mechanical signal transduced into physiological signal 3.Physiological signal transduced inside cell & to other cells 4.Differential cell elongation occurs in the “up” and “down” sides of root and shoot

25 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 plastids cell in root cap root

Plant shoots exhibit… A.Negative phototropism and negative gravitropism B.Negative phototropism and positive gravitropism C.Positive phototropism and positive gravitropism D.Positive phototropism and negative gravitropism

Thigmotropism is directional growth of a plant or plant part in response to contact Thigmonastic responses occur in same direction independent of the stimulus Examples of touch responses: Venus flytrap leaves Tendrils around objects 27 Thigmotropism Often due to differential elongation or manipulated water/turgor pressure

Responses to Mechanical Stimuli Mimosa leaves have swollen structures called pulvini at base of leaflets –Stimulation triggers electrical signal –Triggers ions to outer side of pulvini –Water follows by osmosis –Decreased interior turgor pressure causes the leaf to fold 28

Responses to Mechanical Stimuli Bean leaves –Pulvini rigid during the day –Lose turgor at night –Reduce transpiration during the night –Maximize photosynthetic surface area during the day 29

30 (Plant) Hormone: Chemicals made in one location and transported to other locations for action Plant Hormones Growth Reproduction Movement Water balance Dormancy

31 Plant Hormones Five major classes of plant hormones Auxin Gibberellins Cytokinin Ethylene Abscisic Acid

32 Plant Hormones 1. Auxins: Elongation of cells Root elongation Vascular tissues and fruit development Responses to light (phototropism), gravity (gravitropism), touch (thigmotropism) Sprouts know where to go Auxin controls direction of sprouting seedling Distribution of auxin within shoot and root cells is influenced by gravity and light

33 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 Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2OH2O Cell elongation in response to auxin 1 Auxin increases the activity of proton pumps. 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 5 With the cellulose loosened, the cell can elongate. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes.

34 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.

35 Plant Hormones 2. Gibberellins: Stem elongation, flowering, and fruit development (enhanced if auxin also present) Seed germination and bud sprouting

36 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

37 Plant Hormones 3. Cytokinins: Anti-aging effects. Inhibit protein breakdown Stimulate RNA and protein synthesis Mobilize nutrients from surrounding tissues 58 day old cutting: Genetically engineered to express more cytokinin on right (florist sprays) Stimulate cell division and differentiation (enhanced by auxin) Produced in actively growing tissues such as roots, embryos, and fruits

38 Control of Apical Dominance Cytokinins 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 38

39 Plant Hormones 4. Ethylene: Gas at room temperature Promotes abscission (falling off) of fruits, flowers, and leaves Required (with auxin) for fruit development

40 Why will these ripe bananas help the green avocados ripen faster? 40 Self-Check

41 Plant Hormones 5. Abscisic Acid: Initiates closing stomata in water-stressed plants Induces and maintains dormancy in buds and seeds –(inhibits gibberellins)

42 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+ 42

Which plant hormone has a dominant role in stem and root elongation? A.auxin B.gibberellin C.cytokinin D.ethylene E.abscisic acid

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

45 Senescence Process by which leaves, fruits, and flowers age rapidly –Promoted by changes in hormone levels Cytokinin and auxin production decreases Ethylene production increases

46 Proteins, starches, and chlorophyll broken down –Products stored in roots and other permanent tissues Senescence Abscission Ethylene stimulates production of enzyme that digests cell walls at base of petiole Leaf falls when cells are sufficiently weakened

47 leaf petiole bud abscission layer Senescence

48 Period of reduced metabolic activity in which the plant does not grow and develop Dormancy Maintained by abscisic acid Dormancy broken by: increased temperature, longer day length  occur in the spring

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 at least three ways plants avoid predation/infection 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.