39.1

Organisms receive signals and respond to them in ways that enhance survival and reproductive success Organisms must have appropriate receptors to elicit a response –Receptors initiate signal transduction pathways Receptors are proteins whose shape changes in response to a stimulus Second messengers amplify signals and transfer them from receptors to other proteins to carry out a response Signal transduction pathways lead to regulation of cellular activities including increased activity of particular enzymes –Transcriptional Regulation increases or decreases synthesis of mRNA that codes for specific enzymes by controlling transcription –Post-Translational Modification activates existing enzyme molecules

Signal Transduction Pathways in Potatoes Potatoes undergo etiolation responses and de-etiolation responses –Etiolation response Morophological adaptations for growing in darkness Adapt to surroundings using receptors –De-Etiolation response Undoing etiolation because plant reacts to sunlight Plant cell’s reception of light transduced into response

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

39.2

Growth Responses are caused by Hormones Hormone- a signaling molecule that is produced in small amounts in an organism’s body and transported to other parts where it binds to a specific receptor and triggers responses in target cells and tissues –Hormones help coordinate growth, development, and responses to environmental stimuli Responses to hormones depend on relative concentration compared with other hormones rather than the amount

Discovery of Plant Hormones Phototropism- the growth of a shoot toward light or away from it –E:\bc_campbell_biology_8ap\ CW\inde x.html Video of phototropism

Discovery of Plant Hormones –Charles Dawin and Son Francis conducted some of the earliest experiments on phototropism In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. EXPERIMENT In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen- Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. CONCLUSION RESULTS ControlDarwin and Darwin (1880) Boysen-Jensen (1913) Light Shaded side of coleoptile Illuminated side of coleoptile Light Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Light Tip separated by gelatin block Tip separated by mica

Discovery of Plant Hormones –Frits Went extracted the chemical messenger for phototropism (auxin) by modifying the experiments of Boysen-Jensen Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance. He separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact, but allowing chemicals to pass. These seedlings were phototropic. Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Control Offset blocks cause curvature RESULTS CONCLUSION In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. EXPERIMENT

Overview of Plant Hormones

Auxin A term used for any chemical substance that promotes elongation of coleoptiles –Auxin is transported down the stem from the shoot apex too fast for diffusion so moves from tip to base Known as polar transport –Acid Growth Hypothesis: protons pumps play a major role in the growth response of cells to auxin Activates enzymes called expansins which loosen the wall’s fabric

Auxin Auxin is also involved in/affects: –Lateral and adventitious root formation –Herbiciddes –Secondary growth –Promotion of fruit growth

Cytokinins Enhance growth and development of plant cells by Stimulate cytokinesis (cell division) Modified forms of adenine Produced in actively growing tissues, embryos, and fruits –Particularly roots Work with auxin to stimulate cell division and influence the pathway of differentiation –Without auxin, grows larger but does not divide –Ratio of cytokinins to auxin controls cell differentiation Retard the aging of certain plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

Gibberellins Cause/affect: –Stem elongation Stimulate stem and leaf growth but have little effect on roots Hypothesis proposing that they activate enzymes that loosen cell walls thus facilitation entry of expansin proteins –Fruit growth In many plants, both auxin and gibberellins must be present in order for the fruit to develop –Germination After water is imbibed into the embryo of a seed, gibberellins are released from the embryo and signal the seed to break dormancy and germinate

Brassinosteroids Steroids and therefore chemically similar to cholesterol and sex hormones of animals Induce: –Cell elongation –Division in stem segments and seedlings Retard leaf abscission and promote xylem differentiation

Abcisic Acid Cause: –Seed Dormancy High levels of abcisic acid in maturing seeds inhibit germination and induce production of certain proteins that help seeds withstand extreme dehydration Many types of dormant seeds germinate when abcisic acid is removed of inactivated –Drought Tolerance Primary internal signaling molecule that enables plants to withstand drought –Accumulates in leaves and causes stomata to close rapidly when a plant begins to wilt »Reduces transpiration

Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection –Ethylene instigates a growth maneuver known as the triple response that enables plants to avoid obstacles by: Slowing of stem elongation, thickening of the stem, curvature that causes the stem to start growing horizontally –Senescence: programmed death of certain cells or organs or the entire plant –A change in the balance of auxin and ethylene controls leaf abscission