Plant Responses to Signals IV Photomorphogenesis Circadian Rhythms Gravitropism http://sunflower.bio.indiana.edu/~rhangart/plantsinmotion.html Germination Action Spectra • Action Spectrum, – graph of the magnitude of a biological response to light, – as a function of wavelength. Stem elongation (inhibition) Molecular Switch …looking for a photoreceptor, hypothesis Germination …look for a photo-reversible pigment. Absorption vs. Action Spectra …looking for a photoreceptor, Germination Phytochrome photoreceptor molecule red light dimer FR light Phytochrome photoreceptor molecule Quantity, Time, Quality. Phytochrome Location Phtyochrome is a cytosolic protein. Phytochrome …has multiple functions, • Seed Germination, • Flowering time (photoperiodism), • Entraining (setting) the biological clock, • End of day, • Stem elongation, • Leaf Expansion, • Pigment synthesis. Photoperiodism …flowering times, Long-day plants, Short-day plants, …night breaks induce flowering. …night breaks inhibit flowering. Phytochrome …photoperiodism, …use photoreversibility to establish phytochrome function. Germination Phytochrome Stem elongation (inhibition) absorbtion spectra, …the wavelengths absorbed by specific pigments. not phytochrome Cryptochromes blue light photoreceptors (I), …evolved from a light dependent DNA repair enzyme, ...across phylogeny, these proteins have been used for many functions, • ranging from blue-lightdependent development in plants, • blue-light-mediated phase shifting of the circadian clock in insects, • to a core circadian clock component in mammals. Stem elongation (inhibition) Phototropins …mediate phototropism, blue light photoreceptors II …contribute to stem, root and leaf movements in response to directional information, • to maximize light gathering capacity, • and to minimize light damage at high irradiances. Phototropism action spectrum Concept Map Functions Photoreversible Phytochrome Cryptochrome Signal transduction Photoperiodism Photomorphogenesis Phototropin Action Spectra, Absorption spectra Circadian Rhythms • Relating to, or exhibiting approximately 24-hour periodicity, – circa around + dies day. • Internal Biochemical Oscillators, – found in all eukaryotes, – eubacteria as well. sleep movements Circadian Rhythms amplitude …of response, Circadian processes continue even if light (or dark) is continuous... Entrainment …amplitude and period entrainment is continuous, <- Period (24h) -> - allows fine control. ~ 480 of 8,000 (tested) genes are under circadian control, • ~1,500 (estimated) Arabidopsis genes, or ~6% follow circadian cycles of expression. Photosynthesis genes... Secondary metabolism (wood, defense). shoots, …are negatively gravitropic. Gravitropism … the gravity directed growth processes that direct root and shoot orientation during a plants life-cycle, – about 1.7%, or roughly 500 genes, are transcribed in Arabidopsis when it is re-oriented 90o. roots, …are positively gravitropic. Gravitropic Set Point 180o 90o 0o Plant organs orient themselves to the gravity vector. http://www.biosci.ohio-state.edu/~plantbio/Sacklab/timelapse.htmlMoss Starch Statolith Hypothesis Re-orientation of heavy starch grains signals gravity vector.