Plant Responses to Internal and External Signals 31 Plant Responses to Internal and External Signals

Outline of topics Phototropism Role of auxin/plant experiments Gravitropism, chemotropism, thigmatropism Plant Hormones Role of gibberelins/ABA in seed germination Ethylene Circadian Rhythms Photoperiodism Short day vs long day plants

Do now: Hypothesize the direction of the light source in the picture below. A tropism is a response towards or away from a stimulus. Identify the type of tropism displayed here and explain the evolutionary benefit of this process.

Overview: The Race to Live Young seedlings must outcompete their neighbors in the race for resources in order to survive Unlike animals, which respond through movement, plants must respond to environmental challenges by altering their growth and development 4

Concept 31.1: Plant hormones help coordinate growth, development, and responses to stimuli Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ Most aspects of plant growth and development are under hormonal control 5

The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present 6

They postulated that a signal was transmitted from the tip to the elongating region Video: Phototropism 7

Results Shaded side Control Light Illuminated side Boysen-Jensen Light Figure 31.2 Results Shaded side Control Light Illuminated side Boysen-Jensen Light Darwin and Darwin Gelatin (permeable) Mica (impermeable) Figure 31.2 Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? Light Opaque shield over curvature Trans- parent cap Tip removed Opaque cap 8

Quick Check: Why does removing the tip of the coleoptile result in a lack of positive phototropism? Identify the specific signal and response parts of phototropism in the signal transduction pathway

In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance 10

In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments 11

Results Excised tip on agar cube Growth-promoting chemical diffuses Figure 31.3 Results Excised tip on agar cube Growth-promoting chemical diffuses into agar cube Control (agar cube lacking chemical) Offset cubes Control Figure 31.3 Inquiry: Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light? 12

A Survey of Plant Hormones The major classes of plant hormones include Auxin* Cytokinins Gibberellins* Brassinosteroids Abscisic acid* Ethylene* * know these hormones 13

Table 31.1 Table 31.1 Overview of plant hormones 14

Auxin The term auxin refers to any chemical that promotes elongation of coleoptiles Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA Auxin is produced in shoot tips and is transported down the stem Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell 15

Results Cell 1 100 m Cell 2 Epidermis Cortex Phloem 25 m Xylem Figure 31.4 Results Cell 1 100 m Cell 2 Epidermis Cortex Phloem Figure 31.4 Inquiry: What causes polar movement of auxin from shoot tip to base? Xylem 25 m Basal end of cell Pith 16

Gibberellins Gibberellins (GAs) have a variety of effects, such as stem elongation, fruit growth, and seed germination 17

Gibberellins are produced in young roots and leaves Stem elongation: Gibberellins stimulate stem and leaf growth by enhancing cell elongation and cell division Gibberellins are produced in young roots and leaves They can induce bolting, rabid growth of the floral stalk 18

Figure 31.6 Figure 31.6 Effects of gibberellins on stem elongation and fruit growth 19

Germination: After water is imbibed, release of gibberellins from the embryo signals seeds to germinate 20

Aleurone Endosperm Water Radicle Scutellum (cotyledon) GA GA -amylase Figure 31.7 Aleurone Endosperm -amylase Sugar GA GA Water Figure 31.7 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Radicle Scutellum (cotyledon) 21

Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA include Seed dormancy Drought tolerance 22

Seed dormancy ensures that the seed will germinate only in optimal conditions In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Precocious (early) germination can be caused by inactive or low levels of ABA 23

ABA accumulation causes stomata to close rapidly Drought tolerance: ABA is the primary internal signal that enables plants to withstand drought ABA accumulation causes stomata to close rapidly 24

Quick check: What 3 effects do gibberellins have on a plant? ABA is water soluble while gibberellins are not. Why is it necessary to water a plant once you have planted seeds?

Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening 26

Explain the phrase “one rotten apple spoils the bunch” Fruit ripening: A burst of ______ production in a fruit triggers the ripening process Ethylene triggers ripening, and ripening triggers release of more ethylene Which kind of feedback is this? Explain. Explain the phrase “one rotten apple spoils the bunch” 27

Quick Check: Grocery stores need ripe fruit for customers to buy, but it is easier to transport in an unripened state. How could ethylene solve this dilemma? An enzyme PG softens fruit by enzymatic action of the cell wall to break it down. What would occur if PG was released but the enzyme was not in ideal conditions? Explain. Why might putting fruits in the refrigerator delay fruit ripening?

Do now: Explain why the following hormones are evolutionarily favorable Auxin gibberellins/ABA ethylene

Do now: Identify the stimuli for the following tropisms: Gravitropism Thigmotropism Phototropism

Identify the following pictures as positive (towards) or negative (away) and the tropism

Identify the following pictures as positive (towards) or negative (away) and the tropism

Identify the following pictures as positive (towards) or negative (away) and the tropism

Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” Which plant is at Noon and which is at Night? Do humans have a circadian rhythm? 34

Plants respond to light There are two major classes of light receptors: blue-light photoreceptors phytochromes*, photoreceptors that absorb mostly red light How does it function ? (refer to bozeman video questions) 35

The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues 36

Photoperiodism and Responses to Seasons Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year Photoperiodism is a physiological response to photoperiod 37

Photoperiodism and Control of Flowering Some processes, including flowering in many species, require a certain photoperiod Plants that flower when a light period is shorter than a critical length are called ________ Plants that flower when a light period is longer than a certain number of hours are called __________ Flowering in day-neutral plants is controlled by plant maturity, not photoperiod 38

If this is a short day plant, describe for each scenario what the status of the plant would be (flowered vs Non flowered) Definition: Plants that flower when a light period is shorter than a critical length

If this is a long day plant, describe For each scenario what the status Of the plant would be (flowered vs Non flowered) Definition: plants that flower when a light period is longer than a certain number of hours

Critical night length: In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length 43

Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness 44

Quick check: Why is it an error to call plants “short day” and “long day” plants? Use evidence from the observations shown

24 hours (a) Short-day (long-night) plant Light Flash of light Figure 31.16 24 hours (a) Short-day (long-night) plant Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Figure 31.16 Photoperiodic control of flowering Flash of light 46

Red light can interrupt the nighttime portion of the photoperiod A flash of red light followed by a flash of far-red light does not disrupt night length Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light 47

Short-day (long-night) plant Long-day (short-night) plant Figure 31.17 24 hours R R F R Figure 31.17 Reversible effects of red and far-red light on photoperiodic response R F R R R F R R F R Short-day (long-night) plant Long-day (short-night) plant Critical dark period 50

Summary Quick Check If you were a florist, and you had short day plants, in which seasons would they flower? What can you do to induce flowering in the other seasons? (so you don’t go out of business?)

Exit Slip A student makes the claim that plants behave according to environmental cues. Support or refute this claim with some examples.

Other plants need several successive days of the required photoperiod Some plants flower after only a single exposure to the required photoperiod Other plants need several successive days of the required photoperiod Still others need an environmental stimulus in addition to the required photoperiod For example, vernalization is a pretreatment with cold to induce flowering 53

Concept 31.3: Plants respond to a wide variety of stimuli other than light Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms 54

Gravity Response to gravity is known as gravitropism Roots show positive gravitropism by growing downward; shoots show negative gravitropism by growing upward Plants may detect gravity by the settling of statoliths, dense cytoplasmic components Video: Gravitropism 55

(a) Primary root of maize bending gravitropically (LMs) Figure 31.19 Statoliths 20 m Figure 31.19 Positive gravitropism in roots: the statolith hypothesis (a) Primary root of maize bending gravitropically (LMs) (b) Statoliths settling to the lowest sides of root cap cells (LMs) 56

Mechanical Stimuli The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls 57

Figure 31.20 Figure 31.20 Altering gene expression by touch in Arabidopsis 58

Thigmotropism is growth in response to touch It occurs in vines and other climbing plants Another example of a touch specialist is the sensitive plant, Mimosa pudica, which folds its leaflets and collapses in response to touch Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials Video: Mimosa Leaf 59

(a) Unstimulated state (leaflets spread apart) Figure 31.21 Figure 31.21 Rapid turgor movements by the sensitive plant (Mimosa pudica) (a) Unstimulated state (leaflets spread apart) (b) Stimulated state (leaflets folded) 60