BIO 206 Plants Sensing the Enviroment

signal transduction

the process of converting an external signal into an internal signal

• can result in either a phosphorylation cascade or second messengers

Steps for Sensing

1. Sensory cells perceive external stimuli and transduce (change) into intercellular signal 

2. Cell-cell signal is sent by sensory cells to target cells in body that can respond 

3. Target cells receive this signal and change their active to produce appreciate response

hormone

organic compound produced in small amounts in one part of a plant (or in glands in animals) that is transported to target cells, where it causes a physiological response

Phototropism

differential cell elongation exhibited by a plant organ in response to directional blue light

• grows toward light to light to maximize photosynthesis

• auxin accumulates on the shaded side of the plant, causing extra growth

Tropism

Any directional plant movement that occurs in response to a directional stimulus 

nastic movements

Other plant movements in which the direction of the response is not dependent on the direction of the stimulus (closing venus flytrap) 

Pigment

molecule that absorbs certain wave lengths of light

Photoreceptors

Phototropins

• blue-light receptors mediate effects of blue light

Phytochrome

• red-light and far-red-light receptors mediate the effects of red light

Types of receptors

• Photoreceptor: light

• Mechanoreceptor: physical

• Chemoreceptor: chemical 

Signal transduction

• Step 1: signal arrives 

• Step 2: receptor protein conformation change in response to signal 

• Step 3: receptor or associated protein catalyzed phosphorylation 

• Step 4: phosphorylation triggers either

  • Phosphorylation

  • Second messenger 

• Step 5: Phosphorylated proteins or 2nd messenger initiate response

• Step 6: multiple possibilties

  • active/repress transcription

  • activate/repress translation

  • Change ion flow 

Three Types of Signals

1. Light: photoperiod, quality, quantity

2. Physical: touch, air/fluid movement, gravity

3. Chemical: specific molecules, gradients

Light quality

the wavelengths of light that can be absorbed by molecules in the plant

Light Quantity

the intensity and duration of light exposure

Phototropins

photoreceptor that changes conformation, exposing active site for protein kinase, initiates signal transduction cascade, stimulates cell elongation with hormone auxin

Acid growth hypothesis

• Protons are pumped from cytoplast into the cell wall

• pH in the cell wall is lowered

• Cell wall proteins, expansins, are activated

• Cross-link fiber (aka hemicellulose) H+ bonds interrupted

Steps for sensing

1. Sensory cells perceive external stimuli and transduce (change) into intercellular signal 

2. Cell-cell signal is sent by sensory cells to target cells in body that can respond 

3. Target cells receive this signal and change their activity to produce response 

Coleoptile

modified leaf that forms sheath protecting emerging shoots of young plant

auxin

hormone that maintains the long axis of the plant body, playing a key role in phototropism and gravitropism

Photoreversibility

pigment in two shapes/confirmations, one shape absorbs red light and one shape absorbs far red light

critical night length

length of time in darkness that must be exceeded to initiate flowering (short) or not exceeded (long day) 

Statolith hypothesis

1.  Amyloplasts are organelles that contain starch granules, starch is denser than water so sink to bottom of root cells by the force of gravity 

2. Position of amyplast actives pressure receptor that initiate gravimetric response 

Ethylene

gaseous hormone that triggers senescence (cell aging) and fruit ripening, and it plays a role in leaf abscission.

1. Synthesized from methionine and helps flowers fading, and leaf abscission 

Vegetative growth to flowering

Apical meristem to leaves, then apical meristem becomes floral/inflorescent meristem, controlled by photoperiodism

Types of flowering

• Short day plants (SDPs) flower when the day is shorter than a critical maximum; flower in plate summer or fall, need long nights

• Long-day plants (LDPs) flower when day is longer than a critical maximum; flower in mid summer