Ch. 39: Plant Responses to Internal And External Signals

I.Signal transduction pathways link internal and environmental signals to cellular responses. A.Example: Plants response to light Once a shoot reaches the sunlight, greening takes place.

Greening is an example of a signal transduction pathway.

II.Plant responses to hormones: A. Discovery: The Early Experiments by Charles Darwin and his son, Francis, and by Peter Boysen-Jensen Gelatin: permeable barrier Mica: solid barrier

1.Phototropism: growth of a shoot towards light.  The cells on the darker side elongate faster than the cells on the brighter side.

B. The Went Experiment: Auxin: Hormone that causes cell elongation, which causes a curvature.

C.Plant hormones: 1.Auxins: stimulates stem elongation (only low concentrations), root growth, cell differentiation, and branching, regulates development of fruit, enhances apical dominance, functions in phototropism and gravitropism. Cell elongation: The acid growth hypothesis

2.Cytokinins: -Source = roots -Affect root growth and differentiation, stimulate cell division and growth (must be with auxin), stimulate germination, delay senescence (anti-aging), signal axillary buds to grow (counteract auxin).

3.Gibberellins (over 100 kinds): -Source = meristem of apical buds and roots, young leaves, embryo -Promote seed and bud germination, stem elongation, and leaf growth, stimulate flowering (bolting) and developing of fruit, affect root growth and differentiation.

4.Abscisic Acid: -Source = Leaves, stems, roots, green fruit -Inhibits growth, closes stomata during water stress, counteracts breaking of dormancy  Many seeds will start to germinate once abscisic acid is removed or inactivated.

5.Ethylene: gaseous hormone -Source = tissues of ripening fruit, nodes of stems, aging leaves and flowers -Promotes fruit ripening, opposes some auxin effects, promotes or inhibits growth and development of roots, leaves, and flowers, depending on species; also, there is a burst of ethylene during apoptosis. -Responsible for the “triple response” in seedlings: a. Horizontal growth of seedling if there is an obstacle (rock) on top of it.

-Ethylene causes cell to produce enzymes that digest the cellulose of cells walls for leaf abscission. When an autumn leaf falls, the breaking point is at the abscission layer.

6.Brassinosteroids: -Source = seeds, fruits, shoots, leaves, and floral buds -Inhibits root growth, retards leaf abscission, promotes xylem differentiation

III.Plant response to light A. Photomorphogenesis: the effects of light on plant morphology. B. Blue-light detection due to 3 pigments: cryptochromes, phototropin, zeaxanthin

C.Circadian rhythms: physiological cycles with a period of 24 hours. 1. Light influences circadian rhythms.

2.Photoperiodism synchronizes plant responses to changes of season. -Short day plants only flower if the light period is less than a critical length -Long day plants only flower if the light period is more than a critical period In 1920, Garner and Allard termed:

3.In the 1940’s scientists discovered that it is the length of night that controls flowering. Short day plants only flower when they have continuous dark periods that exceed the critical period. Long day plants only flower when they have continuous dark periods that do not exceed the critical period.

A.Statoliths: specialized plastids (starch grains) in root cells. Corn roots IV.Plant response to other environmental stimuli: A. Gravitropism

Gravity pulls the statoliths groundward. This redistributes calcium, which triggers an accumulation of auxin. High conc. of auxin prevents cell elongation  Curvature

B.Mechanical Stimuli: 1.Thigmomorphogenesis: mechanical stress due to touch. TouchedUntouched

2.Thigmotropism: directional growth in response to touch (Ex. Vines and tendrils).

3.Rapid leaf movements in response to stimulation: Ex. Mimosa plant A signal travels 1 cm per second by an electrical impulse (action potential); creates turgid and flaccid cells.

V.Plant defense in response to herbivores and pathogens: A.Herbivores: 1.Chemical defense: a.Canavanine: resembles amino acid, arginine. When eaten by an insect, canavanine is incorporated into the insect’s protein in place of arginine.  Kills insect because canavanine is different enough from arginine to allow for incorrect function of protein.

2.Leaf destroyed by an herbivore releases parasitoid wasp attractants.

3.Lima bean plants infested with spider mites will send out chemical signals to neighboring plants to activate their own defense genes. B.Plant defense against pathogens: 1.Epidermis of plant body 2.Gene-for-gene resistance: avirulent pathogens and plants have a compromise.

R gene = codes for receptor that signals the plant’s defenses so it can survive.  This compromise allows for the pathogen and plant to survive.

3.Hypersensitive response and the Systemic acquired resistance: HR: -Sealing off of infected area -Self destruction SAR: -Signal (salicylic acid) received from infected cells