37.1 Information Processing in Plants

• Signal receptor proteins change shape in response to an environmental stimulus, such as being struck by a particular wavelength of light, having pressure applied, or binding to a particular type of molecule.

• If the receptor is located in the plasma membrane, the subsequent process is called signal transduction; the verb “transduce means convert from one form to another.”

• Phosphorylation cascades are triggered when the change in the receptor protein’s shape leads to the transfer of a phosphate group from ATP to the receptor or an associated protein.

  • Phosphorylation activates proteins involved in signal transduction cascades, causing them to phosphorylate and activate a different set of proteins, which in turn catalyze the phosphorylation and activation of still other proteins.

• Second messengers are produced when receptor proteins trigger the production of intracellular signals or their release from storage areas.

  • Calcium ions stored in the vacuole, ER, or cell wall are one of several ions or molecules that function as the second messenger in plants.

37.2 Blue Light: The Phototropic Response

• A coleoptile is a modified leaf that forms a sheath protecting the emerging shoots of young grasses.

• Directed movement in response to light is called phototropism.

• Any directional plant movement that occurs in response to a directional stimulus is considered a tropism.

• Although biologists knew that the blue-light receptor must be a pigment-a molecule that absorbs certain wavelengths of light it took decades to find it.

• Collectively, photoreceptors that detect blue light and initiate phototropic responses are known as phototropin.

• When proton pumping lowers the pH of the wall to 4.5, cell wall proteins called expansins are activated.

  • Expansins “unzip” the hydrogen bonds at form between cellulose microfibrils and other polymers in the cell wall, loosening the structure.

37.3 Red and Far-Red Light: Germination, Stem Elongation, and Flowering

• The idea was that switching behavior, or photoreversibility, occurs because light absorption makes the photoreceptor pigment change shape, like a light switch moving up or down in response to touch.

• Etiolation describes the phenomenon in which plants have a pale yellow color and lengthen their stems and grow long, thin, and spindly when they are shaded or kept in dark conditions.

• Photoperiodism is any response by an organism that is based on photoperiod-the relative lengths of day and night.

• The length of time in darkness at must be exceeded to initiate flowering in short-day plants, or not exceeded to initiate flowering in long da plants, is called the critical night length.

37.4 Gravity: The Gravitropic Response

• In 1881 Charles and Francis Darwin published one of the first experimental results on gravitropism (“gravity-turn”)-the ability to move in response to gravity.

• Amyloplasts are organelles that contain starch granules; the starch is denser than water.

  • These organelles are pulled to the bottom of root cap cells by the force of gravity.

  • The position of the amyloplasts activates pressure receptor proteins located in the plasma membrane.

  • These receptors initiate the gravitropic response.

• The grains of sand are called statoliths (“place-stones”).

37.5 How Do Plants Respond to Wind and Touch?

• Rapid, non-directional movements such as these are referred to as thigmotactic movements.

  • These types of movements occur when a touch-receptor cell transduces the mechanical signal to an electrical signal.

• The separation of charges creates a membrane voltage or membrane potential.

• To travel from a sensory cell to a response cell, electrical signals are propagated in a characteristic form called an action potential.

37.6 Youth, Maturity, and Aging: The Growth Responses

• When apical dominance occurs, growth is restricted to the main stems; the lateral buds in the axils of each leaf remain dormant.

• Auxin transport is polar, or unidirectional.

  • Auxin is the only plant hormone known to be transported through individual cells in one direction only.

• Falling auxin concentrations are involved in the abscission, or shedding, of leaves and fruits, which is associated with the genetically programmed aging process called senescence.

• Dormancy is a temporary state of reduced metabolic activity.

• Gibberellins, a large family of closely related compounds, stimulate growth in plants.

• Abscisic acid, commonly abbreviated ABA, inhibits growth.

37.7 Pathogens and Herbivores: The Defense Responses

• Disease-causing agents are termed pathogens.

  • If plants were not able to sense attacks by pathogens and respond to them quickly and effectively, the landscape would be littered with dead and dying vegetation.

• In addition, many plants lace their tissues with secondary metabolites-molecules that are closely related to compounds in key synthetic pathways but are not found in all plants.

• Responses to attacks and infections in both plants and animals are called inducible defenses because they are triggered by the presence of a threat.

• When contact is made, these “surveillance” proteins signal to the plant cell that a pathogen is present, which triggers a cascade of signals that lead to:

  • Stomatal closure prevents more pathogen cells from entering the plant.

  • Production of toxin molecules targeted to the pathogen.

  • Reinforcement of the neighboring plant cell walls to limit movement of the pathogen.

  • Rapid suicide of cells in the infected region.

• These events take place within hours of the start of an infection and are termed the hypersensitive response (HR).

  • The HR results in small brown spots in leaves-those cells dead by suicide that accumulate over the growing season.

• When biologists set out to locate the hormone responsible for SAR, they found that the level of methyl salicylate (MeSA)-a molecule derived from salicylic acid-increases dramatically after tissues are infected with a pathogen.

• When researchers started studying why some plant tissues are more palatable and digestible than others, biochemists discovered that many seeds and some storage organs, such as potato tubers, contain proteins called proteinase inhibitors.

• An organism that is free living as an adult but parasitic as a larva, such as these wasps, is called a parasitoid.