Plant Responses to Stimuli

Signal Transduction Environmental signals trigger specific responses in plants Etiolation- adaptations for darkness Potatoes sprouting in a dark pantry De-etiolation- “greening”; occurs when a shoot reaches sunlight Potato leaves expand and roots elongate when exposed to sunlight

From Reception to Response Receptor proteins change shape in response to stimulus (hormonal, chemical, or environmental) Transduction Signal is amplified by second messengers Specific protein kinases activated Response Regulation of cell activity Regulation of transcription or translation Modification of proteins (post-translation)

Tropisms Tropism- growth response that orients plant toward or away from a stimuli Growth of a shoot toward light = phototrophism Coleoptile senses light and transmits signal (Darwin and Darwin/ Boyen-Jensen)

Plant Hormones Growth caused by the concentration of horomones within plant cells Phototropism caused by a higher concentration of auxin (growth-promoting horomone) on the dark side Fritts Went

Auxin Indoleacetic Acid (IAA) Promotes elongation of coleoptiles Synthesized in apical meristem; moves down the shoot Activates expansins (break down cellulose microfibrils) Alters gene expression, causing cells to produce new proteins Used in vegetative propigation of roots and also as a herbicide

Cytokinins Promote cytokinesis (cell division) Work with auxin to promote growth (not effective alone) Used in the control of axillary bud growth; promotes apical dominance Anti-aging effects Inhibit protein breakdown, stimulate transcription and protain synthesis, and mobilize nutrients

Gibberellins Stimulate growth of roots and leaves Present in seed embryos, roots, and young leaves Little effect on roots (can cause plants to topple if the concentration is too high) Stimulates cell-wall loosening enzymes Causes grapes to grow larger because stems are elongated, giving fruit more room to grow

Brassinosteroids and Abscisic Acid Induce cell elongation and division in stem segments Retard loss of leaves (abscission) Promote xylem differentation Abscisic Acid (ABA) Slows growth Keeps seeds dormant/ prevents early sprouting Increases drought tolerance

Ethylene Response to mechanical stress Apoptosis Leaf abscission Occurs if an emerging shoot meets resistance (rock, etc.) Triple response- slows elongation, thickens stem, and causes it to curve Apoptosis Programmed cell death Shedding leaves/ death of xylem components, etc. Leaf abscission Separates leaf from tree at petiole Fruit ripening Triggers ripening of fruit; causes chain reaction that ripens all fruit on the plant

Photomorphogenesis Plants can detect the direction, intensity, and wavelength of light Action spectrum- different wavelengths are more effective at driving particular processes Blue-light photoreceptors Phototrophism, opening of stomata, slowing of hypocotyl elongation Phytochromes (red light) Seed germination, shade avoidance in trees

Circadian Rhythms Plants “cycle” their processes and responses daily Length of day determines growth of plants Must have darkness for a certain amount of time… critical dark period Short-day plants- flower when darkness is longer than CDP (late summer/fall/winter) Long-day plants -flower when darkness is shorter than CDP (late spring/summer) Day-neutral plants- unaffected by photoperiods

Touchy, touchy… Gravitropism Adjust growth according to gravity Detect gravity by settling of statoliths (dense starch grains) Roots- positive gravitropism Shoots- negative gravitropism Thigmomorphogenesis Change in form due to touch Inhibition of growth/ change in leaf conformation

Environmental Stress Drought Flood Salt Heat Cold Reduce transpiration, inhibit new growth Flood O2 deprivation stimulates ethylene production Salt Can cause desiccation Heat Denatures proteins Cold Alters solute transport Freezing can cause cell death due to dehydration

Plant Defenses Herbivores Pathogens Physical (thorns, etc.) Chemical (Distasteful compounds, "recruitment” of predatory animals) Pathogens Virulent (little specific defense) and avirulent (cause mild harm) Gene-for-gene recognition- resistance (R) genes Elicitors (oligosaccharins) Stimulate antimicrobial compounds (phytoalexins) and produce pathogenesis-related (PR) proteins Hypersensitive response- seals off infected region Systemic Acquired Resistance- protects entire plant from infection due to phytoalexin and PR protein production