Plant Responses AP Biology Chapter 39

Plants respond by signal transduction pathways just like we do! Plants have cellular receptors that detect changes in their environment For a stimulus to elicit a response, certain cells must have an appropriate receptor Stimulation of the receptor initiates a specific signal transduction pathway

A potato’s response to light is an example of cell-signal processing Fig. 39-2 A potato’s response to light is an example of cell-signal processing Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes (a) Before exposure to light (b) After a week’s exposure to natural daylight

Ca2+ channel opened Ca2+ Fig. 39-4-3 Reception Transduction Response 1 Reception 2 Transduction 3 Response Transcription factor 1 CYTOPLASM NUCLEUS NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP P Second messenger produced Transcription factor 2 Phytochrome activated by light P Cell wall Specific protein kinase 2 activated Transcription Light Translation Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response De-etiolation (greening) response proteins Ca2+ channel opened Ca2+

Signaling pathways due to Auxin

A signal transduction pathway leads to regulation of one or more cellular activities In most cases, these responses to stimulation involve increased activity of enzymes involved in photosynthesis and chlorophyll production They may also lead to changes in gene expression.

The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism Tropisms are often caused by hormones

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 They postulated that a signal was transmitted from the tip to the elongating region

F. Went concluded that the chemical was auxin and that it migrated to the shady side of the plant and caused cell growth in that area. .

Boysen-Jensen demonstrated that the substance was mobile and could move through a block of gelatin.

But, maybe the light stimulates a GROWTH INHIBITOR on the lighted side!

RESULTS Shaded side of coleoptile Control Light Illuminated side of Fig. 39-5a RESULTS Shaded side of coleoptile Control Light Illuminated side of coleoptile Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted?

A Survey of Plant Hormones In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ

How does auxin work in stimulating cell elongation in phototropism?

AUXIN The term auxin refers to any chemical that promotes elongation of coleoptiles. The Role of Auxin in Cell Elongation According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric With the cellulose loosened, the cell can elongate

3 4 2 1 5 Expansins separate microfibrils from cross- Fig. 39-8 Expansins separate microfibrils from cross- linking polysaccharides. 3 Cell wall–loosening enzymes Cross-linking polysaccharides Expansin CELL WALL Cleaving allows microfibrils to slide. 4 Cellulose microfibril H2O Cell wall Cell wall becomes more acidic. 2 Plasma membrane Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis Auxin increases proton pump activity. 1 Nucleus Cytoplasm Plasma membrane Vacuole CYTOPLASM 5 Cell can elongate.

Uses of auxin Cell elongation in phototropism and gravitropism root formation and branching affects secondary growth by stimulating cambium growth. An overdose of synthetic auxins can kill eudicots ! weedkillers

Plant growth involves interaction between metabolites such as sugars, phytohormones and their action on gene expression. Auxin as a signaling molecule has various effects depending upon the tissue where it acts.

CYTOKININS Cytokinins are so named because they stimulate cytokinesis (cell division). Cytokinins retard the aging of some plant organs

Control of Apical Dominance Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds If the terminal bud is removed, plants become bushier

(a) Apical bud intact (not shown in photo) Fig. 39-9 Lateral branches “Stump” after removal of apical bud (b) Apical bud removed Figure 39.9 Apical dominance Axillary buds (a) Apical bud intact (not shown in photo) (c) Auxin added to decapitated stem

Gibberellins Gibberellins or gibberellic acid (GA) have a variety of effects, such as stem elongation, fruit growth, and seed germination

Seed Germination Gibberellins (GA) send signal to aleurone. Fig. 39-11 Gibberellins (GA) send signal to aleurone. 1 Aleurone secretes -amylase and other enzymes. 2 Sugars and other nutrients are consumed. 3 Aleurone Endosperm -amylase Sugar GA GA Water Figure 39.11 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Radicle Scutellum (cotyledon)

Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA: Seed dormancy In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Drought tolerance ABA is the primary internal signal that enables plants to withstand drought

Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection Also induces leaf fall (abscision) and fruit ripening.

The dosage effect of ethylene on impatiens The dosage effect of ethylene on impatiens. Plants not exposed to ethylene (A). Plants exposed to 2 ppm ethylene for one day (B), two days (C), and three days (D). Initially only open flowers abscised, then buds began to abscise. After three days of exposure, all flowers and buds had been shed

Light Cues in Plants Effects of light on plant morphology are called photomorphogenesis

(b) Coleoptile response to light colors Fig. 39-16b Light Time = 0 min Effects of light on plant morphology are called photomorphogenesis Time = 90 min Figure 39.16 Action spectrum for blue-light-stimulated phototropism in maize coleoptiles (b) Coleoptile response to light colors

Phytochromes as Photoreceptors Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life These responses include seed germination and shade avoidance Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses

Fig. 39-19 Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Figure 39.19 Phytochrome: a molecular switching mechanism Absorption of red light causes the Pr to change to Pfr. Far-red light reverses the conversion. Mostly, it is the Pfr that switches on physiological and developmental responses.

Fig. 39-17 How does the order of red and far-red illumination affect seed germination? RESULTS red-light ? Far-red ? Determing factor? Are the effects reversible? Simulates Inhibits Final-light exposure yes Dark (control) Red Dark Red Far-red Dark Figure 39.17 How does the order of red and far-red illumination affect seed germination? Red Far-red Red Dark Red Far-red Red Far-red

Biological Clocks and Circadian Rhythms Many plant processes oscillate during the day Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions

Fig. 39-20 Figure 39.20 Sleep movements of a bean plant (Phaseolus vulgaris) Noon Midnight

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 Some processes, including flowering in many species, require a certain photoperiod

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

What does this experiment indicate? Fig. 39-21 24 hours (a) Short-day (long-night) plant What does this experiment indicate? Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Figure 39.21 Photoperiodic control of flowering Red light (received by phytochromes) can interrupt the nighttime portion of the photoperiod Flash of light

A flash of far-red can reverse the effect though. Fig. 39-22 24 hours A flash of far-red can reverse the effect though. R RFR Figure 39.22 Reversible effects of red and far-red light on photoperiodic response RFRR RFRRFR Short-day (long-night) plant Long-day (short-night) plant Critical dark period

Other Responses: Gravity Response to gravity is known as gravitropism Roots show positive gravitropism; shoots show negative gravitropism Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains

(a) Root gravitropic bending (b) Statoliths settling Fig. 39-24 Statoliths 20 µm Figure 39.24 Positive gravitropism in roots: the statolith hypothesis (a) Root gravitropic bending (b) Statoliths settling

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

Fig. 39-25 Figure 39.25 Altering gene expression by touch in Arabidopsis

Thigmotropism is growth in response to touch It occurs in vines and other climbing plants Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials

Fig. 39-26 (a) Unstimulated state (b) Stimulated state Side of pulvinus with flaccid cells Leaflets after stimulation Side of pulvinus with turgid cells Figure 39.26 Rapid turgor movements by the sensitive plant (Mimosa pudica) Pulvinus (motor organ) Vein 0.5 µm (c) Cross section of a leaflet pair in the stimulated state (LM)

How plants react to environmental stresses Drought: close stomata, slow leaf growth, reduce exposed surface, deep roots Heat stress – heat shock proteins protect them Cold – alter lipids in cell membrane Salt – increased solute conc in cells Flooding – make air spaces in root cortex

How plants resist herbivores and pathogens Physical and chemical defenses Recruit predatory animals Immune system – gene for gene recognition, hypersensitive response, system acquired response, salicylic acid* *In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments.

Beware! Chemical Defenses

Physical Defenses

Recruiting predatory animals Ants and acacia tree

4 3 1 1 2 Recruitment of parasitoid wasps that lay their eggs Fig. 39-28 4 Recruitment of parasitoid wasps that lay their eggs within caterpillars 3 Synthesis and release of volatile attractants Figure 39.28 A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 1 Wounding 1 Chemical in saliva 2 Signal transduction pathway

Recognizing plant pathogens Fig. 39-29 Recognizing plant pathogens Signal Hypersensitive response Signal transduction pathway Signal transduction pathway Acquired resistance Figure 39.29 Defense responses against an avirulent pathogen Avirulent pathogen R-Avr recognition and hypersensitive response Systemic acquired resistance