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Plant Responses to Internal and External Signals (Plant hormones help…
Plant Responses to Internal and External Signals
Signal transduction pathways link signal reception to response
These morphological adaptations for growing in darkness, called etiolation, also occur in seedlings germinated in the dark and make sense for plants that sprout underground.
Once a shoot reaches the sunlight, its morphology and biochemistry undergo profound changes, collectively called de-etiolation or greening.
Weak signals are amplified by second messengers—small molecules and ions that amplify the signal and transfer it from the receptor to proteins that cause the response.
In transcriptional regulation, specific transcription factors bind to specific regions of DNA and control the transcription of specific genes.
Plant hormones help coordinate growth, development, and responses to stimuli
Hormones are signaling molecules produced in tiny amounts in one part of the body and transported to other parts of the body, where they bind to specific receptors and trigger responses in target cells and tissues.
Any growth response that results in curvature of whole plant organs toward or away from stimuli is called a tropism.
The growth of a shoot toward light is called positive phototropism; growth away from light is negative phototropism.
The term auxin is used for any chemical substance that promotes the elongation of coleoptiles, although auxins actually have multiple functions in angiosperms.
Lowering the pH activates expansin enzymes that break the cross-links between cellulose microfibrils and other cell wall constituents, loosening the wall.
These growth regulators were named cytokinins because they stimulate cytokinesis, or cell division.
The fungus caused hyperelongation of rice stems by secreting a chemical called gibberellin.
Brassinosteroids are steroids similar to cholesterol and the sex hormones of animals.
Abscisic acid (ABA) was discovered in the 1960s, when one research group studying bud dormancy and another investigating leaf abscission isolated ABA.
In 1901, it was demonstrated that the gas ethylene was the active factor that caused leaves to drop prematurely from trees near leaking gas mains.
During programmed cell death, or apoptosis, there is active expression of new genes that produce enzymes that break down many chemical components.
Responses to light are critical for plant success
The effects of light on plant morphology are called photomorphogenesis.
A graph called an action spectrum depicts the relative effectiveness of different wavelengths of radiation in driving a particular process.
Researchers identified two major classes of light receptors: blue-light photoreceptors and phytochromes that absorb mostly red light.
Physiological cycles with a frequency of about 24 hours that are not directly paced by any known environmental variable are called circadian rhythms.
A physiological response to the photoperiod, such as flowering, is called photoperiodism.
Maryland Mammoth is a short-day plant that requires a light period shorter than a critical length to flower.
Long-day plants flower only when the light period is longer than a critical number of hours.
Day-neutral plants flower when they reach a certain stage of maturity, regardless of the day length.
The flowering signal, called florigen appears to be such a macromolecule.
Plants respond to a wide variety of stimuli other than light
The roots and shoots of plants respond to gravity by gravitropism.
Plants detect gravity by the settling of statoliths, dense cytoplasmic components that settle under the influence of gravity to the lower portions of cells.
Plants change form with mechanical perturbations, a response called thigmomorphogenesis.
Directional growth in response to touch is thigmotropism, which allows a vine to take advantage of whatever mechanical support it comes across as it climbs upward toward a forest canopy.
One remarkable feature of rapid leaf movement is that signals are transmitted from leaflet to leaflet via action potentials.
Environmental stresses, both biotic and abiotic, are important in determining the geographic range of plants.
Plants respond to attacks by pathogens and herbivores
A plant’s first line of defense against infection is the physical barrier of the epidermis of the primary plant body and the periderm of the secondary plant body.
The resulting systemic acquired resistance (SAR), associated with the systemic expression of some defense genes, is nonspecific, providing protection against many pathogens for days.
Methylsalicylic acid is produced around the infection site and carried by phloem throughout the plant, where it is converted to salicylic acid in areas remote from the site of infection.
Plants can recognize certain invading pathogens. Successful pathogens cause disease because they are able to evade recognition or suppress host defense mechanisms.