Plant Responses

Basic Signal Transduction Pathway: 1. Reception: chemicals (hormones) bind to receptor proteins and cause a conformational change 2. Transduction: conformational change stimulates an amplification pathway by causing the activation of secondary messengers 3. Response: secondary messengers trigger cellular changes that activate transcription of mRNA (act as transcription factors or enhancers – page 365 and 366) or directly activating existing enzymes - activated enzymes leads to cellular activity

CELL WALL CYTOPLASM   1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus Plasma membrane

1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 Transcription factor 1 NUCLEUS P Translation De-etiolation (greening) response proteins Ca2+ Ca2+ channel opened kinase 2 factor 2

Plant Hormones Hormone Major Functions Auxins Stem elongation, root growth, fruit development Cytokinins Root growth, stimulates germination Gibberellins Seed and Bud Germination, Stem elongation, fruit development Brassinosteroids Inhibit root growth & leaf abscission Abscisic Acid Inhibits growth, promotes seed dormancy Ethylene Fruit ripening, abscission of leaves

Plant Response to Auxin Acid growth hypothesis - auxin binds to cells and activates proton pumps - H+ pumped into cell wall – cell wall becomes more acidic - acid activates “expansin” proteins – separate the cellulose fibers in the wall – wall becomes less rigid - turgor pressure expands the wall

Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2O

Auxin and Phototropism (growth toward light) - light hits a stem and blocks auxin production on lit side - shaded side produces auxin – growth happens on shaded side causing the stem to bend toward the light - page 792 Auxins and Seedless Tomatoes – seeds produce auxins which promotes fruit development – spraying developing fruits stimulates fruit without seeds

Plant Responses to Cytokinins Stimulate cell differentiation when in combination with auxins - in undifferentiated tissue: – high cytokinin and low auxin = shoots - low cytokinin and high auxin = roots - in developed tissues: - levels of leads to the regulation of apical dominance in roots and stems - high auxin in shoots (produced by the apical bud) migrates down the stem and promotes apical growth, but suppresses lateral growth - removal of apical bud allows for the production of lateral buds because cytokinin levels become higher - high levels of auxins in roots promotes lateral root growth which is inhibited by the high levels of cytokinins which promotes apical root growth

Anti-Aging: cytokinins stimulate protein synthesis and inhibit protein breakdown which prolongs the life of plant organs Florists spray cytokinin solutions on cut flowers to help them last longer

Plant Responses to Gibberellins Stem elongation – stimulate expansins without acidification - extra gibberellins act on inhibited stems – Mendel’s pea plants Fruit Growth – increase fruit size Seed Germination – released when water is absorbed – water can remove abscisic acid (keeps seed dormant)

After water is imbibed, the release of gibberellins from the embryo Signals the seeds to break dormancy and germinate 2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling. 2 Figure 39.11

Plant Response to Brassinosteroids - similar to auxin

Plant Responses to Abscisic Acid WHAT IT DOES NOT DO – cause leaf abscission (loss of leaves) What it does: - slows growth - causes seed dormancy – ABA removed when water is absorbed or inactivated by light or cold - produced in dry conditions  closes stomata

Plant Response to Ethylene - stress response – drought, flooding, mechanical pressure, injury and infection -Triple Response to Mechanical Pressure: 1. Slowing of stem growth 2. Thickening of stem 3. Curving of stem - allows germinating seeds to grow around rocks

Ethylene concentration (parts per million) Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response. CONCLUSION Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the triple response. Response was greater with increased concentration. RESULTS 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million)

Leaf abscission: loss of leaves in deciduous trees prevents desiccation during winter As a leaf ages it produces less auxin and makes it more susceptible to ethylene Ethylene causes the production of enzymes that break down the tissues at the abscission layer of the leaf (base of petiole) Weakened tissue eventually breaks after a protective layer of cork forms

0.5 mm Protective layer Abscission layer Stem Petiole

Fruit ripening – release ethylene – causes break down of cells – loss of chlorophyll – break down of starch – fruit becomes colored and sweet - stimulates the production of more ethylene - one bad apple …….

Effects of Light on Plants - Photomorphogenesis – changes due to light - based on Blue pigment photoreceptors called Phytochromes - composed of two identical proteins with a blue pigment that absorbs red light (660 nm) or absorbs far red light (730 nm) depending on the isomer form - two forms – Pr and Pfr - Pr form absorbs red light and is converted to Pfr - in the presence of far red light or in the absence of light, Pfr converts back to Pr - Pr/Pfr ratio determines plant morphological changes

Synthesis Far-red light Red light Slow conversion in darkness (some plants) Responses: seed germination, control of flowering, etc. Enzymatic destruction Pfr Pr

- Seeds that need light to germinate – get hit with light – Pr  Pfr - Apical dominance and Branching - Pr = apical growth - Pfr = branching - in light – Pr  Pfr = branching growth - in shade – Pfr  Pr = apical growth (shade avoidance)

Photoperiodism: - based on the amount of light and dark Ex: Flowering – based on critical night length – interruption of dark period prevents response - short “day” plants – (long night plants) – need long period of dark to produce correct levels of signals for flowering - long “day” plants – (short night plants) – need short period of dark to produce the correct signals for flowering - day neutral plants – flower when plant body is mature

(a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

Florigen – flowering hormone Figure 39.24 To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced. EXPERIMENT RESULTS CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant. Plant subjected to photoperiod that induces flowering that does not induce flowering Graft Time (several weeks)

TROPISMS: plant responses to other signals Gravitropism – statoliths (starch granules) and dense organelles – migrate to the lower portion of root cells and stimulate the growth of the root in that direction

Gravitropism Statoliths 20 m (a) (b)

Thigmotropism/Thigmomorphogenesis– response to touch (a) Unstimulated (b) Stimulated Side of pulvinus with flaccid cells turgid cells Vein 0.5 m (c) Motor organs Leaflets after stimulation Pulvinus (motor organ)

Thigmotropism

Phototropism – light

Environmental Stresses Drought – ABA – close stomata Flooding – aerial roots - ethylene gas and apoptosis creates air spaces in roots

Vascular cylinder Air tubes Epidermis 100 m (a) Control root (aerated) (b) Experimental root (nonaerated)

Salt stress – pump extra solutes into cell and some have salt glands (modified hydathodes) Heat stress – heat shock proteins – chaperonins – maintain protein shape Cold stress – modifications of phospholipids bilayer Herbivores – thorns, hairs (trichomes), poisons, hormonal signals Pathogens – secrete PR proteins – pathogen resistant

Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants 1 Chemical in saliva Wounding 2 Signal transduction pathway

4 Before they die, infected cells release a chemical signal, probably salicylic acid. 3 In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf. Signal 5 The signal is distributed to the rest of the plant. 4 5 Hypersensitive response Signal transduction pathway 6 3 6 In cells remote from the infection site, the chemical initiates a signal transduction pathway. Signal transduction pathway 2 Acquired resistance 7 2 This identification step triggers a signal transduction pathway. 1 7 Systemic acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days. Avirulent pathogen 1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells. R-Avr recognition and hypersensitive response Systemic acquired resistance Figure 39.31