Lecture 10 Outline (Ch. 39, 36) I. Plant Hormones II. Plant Orientation/Shape III. Plant Timekeeping (?!) Senescence & Dormancy “Fast” Responses Plant transport A. Water pressure B. Xylem C. Phloem Lecture Concepts

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

Plant Hormone Overview Plants respond to stimuli and lead a stationary life Plants, being rooted to the ground Must respond to whatever environmental change comes their way Figure 38.9a Two common types of seed germination

Plant Hormones Five major classes of plant hormones (table 44-1 summary) Hormone effects depend on - target cell - developmental stage of the plant - amount of hormone - presence of other hormones

Plant Hormones 1. Auxins: Elongation of cells Root elongation stimulate (low concentrations) inhibit (high concentrations) Vascular tissues and fruit development Responses to light (phototropism), gravity (gravitropism), and touch (thigmotropism)

Cell elongation in response to auxin 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. Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2O 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. 1 Auxin increases the activity of proton pumps. 5 With the cellulose loosened, the cell can elongate. Figure 39.8

Other Auxin Stimulated Responses: Lateral / branching root formation Promote fruit growth (tomato sprays) As herbicide, overdose kills eudicots Auxin is produced: At the shoot apex, seeds, other actively growing tissues. In a variety of molecular structures.

Plant Hormones 2. Gibberellins: Stem elongation, flowering, and fruit development Seed germination and bud sprouting

Gibberellins stimulate germination After water is imbibed, the release of gibberellins from the embryo Signals the seeds to break dormancy and germinate 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. Figure 39.11 Embryo

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

Control of Apical Dominance 11 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 “Stump” after removal of apical bud Lateral branches Axillary buds Figure 39.9

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

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

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

Abscisic Acid 15 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 K+ Mutation that prevents the synthesis of ABA Why is that one kernel (seed) germinating prematurely? 15

Sprouts know where to go Plant Orientation 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

Plant Orientation Opaque cap over tip. FIGURE E44-1 Tip doesn't bend in dark

Plant Orientation Clear cap over tip. Opaque sleeve over bending region. FIGURE E44-2 Tip perceives direction of light

Plant Orientation Cells elongate rapidly. Cells elongate slowly. FIGURE E44-3 Cells elongate on shaded side

Plant Orientation Shoot Elongation Root Growth In shoot, light and gravity cause auxin movement to the lower side Auxin stimulates elongation of stem cells Stem bends away from gravity & toward light Root Growth Due to gravity, auxin builds up on the lower side of the root Auxin retards elongation of root cells, and the root bends toward gravity

How Do Plants Detect Gravity? Plant Orientation How Do Plants Detect Gravity? Starch-filled plastids In specialized stem cells and root caps Orient within cells toward gravity Changing plastid orientation may trigger high levels of auxin plastids cell in root cap root 21

Plant Timekeeping/Light Detection 22

Plant Timekeeping/Light Detection 23 Plant Timekeeping/Light Detection 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. Figure 39.18

Plant Timekeeping/Light Detection Circadian Rhythms Cyclical responses to environmental stimuli approximately 24 hours long entrained to external clues of the day/night cycle Phytochrome conversion marks sunrise and sunset Providing the biological clock with environmental cues Many legumes Lower their leaves in the evening and raise them in the morning Pulvinus: tip of petiole/base of blade Noon Midnight Figure 39.20 24

Plant Timekeeping/Light Detection Photoperiodism 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 that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod »

Plant Timekeeping/Light Detection 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 26

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

Senescence Lack of nutrients Lack of sugars Cut flowers undergo senescence due to: Reduced water uptake Lack of nutrients Lack of sugars

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

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

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

Immediate Plant Responses 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) Barley and willow, alder, and birch trees warn members of their own species Sagebrush and lima bean plants warn members of differing species (wild tobacco and cucumber plants, respectively) 32

Immediate Plant Responses Some plants respond to attack by releasing volatile chemicals Chemicals attract parasitic wasps and predaceous mites that feed on plant predators

Immediate Plant Responses Touch generates an electrical signal Increases permeability to ions (K+) of “motor cells” at bases of leaflets and petiole K+ flow out of motor cells; water follows Motor cells shrink  leaflets and petiole droop Sensitive plant (mimosa)

Immediate Plant Responses 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

Self-Check Hormone Name Functions Auxin Gibberellin Cytokinin Ethylene Abscisic Acid

Transport in vascular plants occurs on three scales Transport in Plants Physical forces drive the transport of materials in plants over a range of distances Transport in vascular plants occurs on three scales Transport of water and solutes by individual cells, such as root hairs Short-distance transport of substances from cell to cell at the levels of tissues and organs Long-distance transport within xylem and phloem at the level of the whole plant 37

Osmosis : movement of water Transport in Plants To survive Plants must balance water uptake and loss Osmosis : movement of water Water potential : measure water movement due to solute concentration & pressure designated as psi (ψ) Water flows from regions of high water potential to regions of low water potential

Transport in Plants The solute potential (ψs) of a solution Pressure potential (ψp) Is the physical pressure on a solution Therefore, the water potential equation is: Ψ = ψs + ψp psi is measured in megapascals (MPa) 0 MPa = the water potential of pure water in a container open to the atmosphere »

The addition of solutes Transport in Plants The addition of solutes Reduces water potential (Figure 36.8)» 0.1 M solution H2O Pure water P = 0 S = 0.23  = 0.23 MPa  = 0 MPa (a) Figure 36.8a

Application of physical pressure Transport in Plants Application of physical pressure Increases water potential » H2O P = 0.23 S = 0.23  = 0 MPa  = 0 MPa (b) H2O P = 0.30 S = 0.23  = 0.07 MPa  = 0 MPa (c) Figure 36.8b, c

Transport in Plants Water potential Affects uptake and loss of water by plant cells If a flaccid cell is placed in an environment with a higher solute concentration The cell will lose water and become plasmolyzed (Figure 36.9)» Figure 36.9a 0.4 M sucrose solution: Initial flaccid cell: Plasmolyzed cell at osmotic equilibrium with its surroundings P = 0 S = 0.7 S = 0.9  = 0.9 MPa  = 0.7 MPa

Transport in Plants If the same flaccid cell is placed in a solution with a lower solute concentration The cell will gain water and become turgid » Distilled water: Initial flaccid cell: Turgid cell at osmotic equilibrium with its surroundings P = 0 S = 0.7 S = 0 P = 0.7 Figure 36.9b  = 0.7 MPa  = 0 MPa  = 0 MPa These process occur in cells that can affect tissues, which can in turn affect the entire plant (e.g., wilting)

44 Uses of turgor pressure: Inexpensive cell growth Hydrostatic skeleton Phloem transport

Transport in Plants 45 Water molecules are attracted to: Each other (cohesion) Solid surfaces (adhesion)

cytoplasmic continuum apoplast Transport in Plants Most plant tissues cell walls and cytosol are continuous from cell to cell cytoplasmic continuum called the symplast apoplast continuum of cell walls Plus extracellular spaces »

47 Transport in Plants How do water and minerals get from the soil to the vascular tissue? Apoplastic Symplastic transmembrane

Transport in Plants 48 Symplast Apoplast Endodermis  Xylem What happens to psi between soil and endodermis? Where is osmosis occurring?

49 Transport in Plants Transpiration = loss of water from the shoot system to the surrounding environment. What drives water loss?

50 Transport in Plants Tension in water in mesophyll cell walls

51 Bulk Flow = movement of fluid due to pressure gradient Transpiration drive bulk flow of xylem sap. Water is PULLED up a plant. Ring/spiral wall thickening protects against vessel collapse

Xylem Sap Ascent by Bulk Flow: A Review The movement of xylem sap against gravity Is maintained by the transpiration-cohesion-tension mechanism Stomata help regulate the rate of transpiration Leaves generally have broad surface areas And high surface-to-volume ratios Both of these characteristics Increase photosynthesis Increase water loss through stomata

53 H+ pumped out K+ flow in H2O flow in stomata open Stomata Control Why? Why? K-channels, aquaporins and radially oriented cellulose fibers play important roles.

54 Transport in Plants What happens if rate of transpiration nears zero? Guttation

Transport in Plants 55 Direction is source to sink Near source to near sink Adjacent sieve tubes can flow in different directions. Are tubers and bulbs sources or sinks? Phloem tissue

Transport in Plants 56 Phloem sap composition: Sugar (mainly sucrose) amino acids hormones minerals enzymes Phloem sap under positive pressure

In studying angiosperms Pressure Flow Vessel (xylem) H2O Sieve tube (phloem) Source cell (leaf) Sucrose Sink cell (storage root) 1 Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis. 2 4 3 This uptake of water generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink. In the case of leaf-to-root translocation, xylem recycles water from sink to source. Transpiration stream Pressure flow Figure 36.20 In studying angiosperms Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure

Lecture 10 concepts Name the five major plant hormones & list two roles for each one. Explain how plants get their roots to grow down and their shoots to grow up. Define: thigmotropism, phototropism, gravitropism What happens (hormonally) if you cut the growing top off of a plant? What shape does the plant take? How does day length relate to flowering? Define senescence. What happens (hormones) to cause leaf senescence? Give two examples (be specific) of how plants can “quickly” respond to their environment. Describe fluid movement in plants: local, bulk transport. How is water balance regulated in plants? How is water transported? From where to where? How are sugars transported? From where to where?