1Photomorphogenesis1. Most significant environmental factor affecting plant development:Light qualityLight quantity2. Low-level light-triggered responses are called photomorphogenic responses18-4 Buchanan Light regulates plant development17-17. Taiz. Induction of mRNA for Light-harvestingcomplex (LHC) after transfer of seedling to lightPhotomorphogenesis: OUTLINEEffect of light on Growth and Development-Light quality and quantity are the most significant environmental factors affecting plant development. -Light induces dramatic changes in morphology and biochemical (protein) composition. How does light induce such changes? e.g. increase in rubisco, LHCA Simple Model of Signal-induced Responses1. Signal perception by a receptor2. Signal transductiona) Communicate signal to other cell partsb) Amplify the signalc) Network and cross talk3. Primary responsee.g. Increase or decrease in gene expressione.g . Change from inactive protein ---> active protein 4. Cellular and Physiological responsesPlants have 3 types of Photoreceptors1. Phytochrome 660 nm2. Blue light receptor ~400-500 nm3. UV-B Receptors ~300 nm1. Phytochrome: a red light receptoracts as a light-activated switchTwo types of phytochrome: Phy A & PhyB-PhyEMode of action2. Blue-light induced responsesMultiple receptorsMode of actionSignal transduction1. Each cell is programmed to respond to specific combination of signals.2. Different cells respond differently to the same signal.a. response depends on receptors in the cellb. response depends on the intracellular machinery coupled to the receptor2Model of signal perception and transduction1. Signal perception by a receptor2. Signal transductiona) Communicate signal to other cell partsb) Amplify the signalc) Network and cross talk3. Primary responsee.g. Increase or decrease in gene expressione.g . Change from inactive protein ---> active protein or an Ion channel opens4. Cellular and Physiological responsese.g. light stimulated guard cell movement, Light stimulated greening of etiolated seedling17-7 Taiz: How do plants respond to light?Different intensities of light induce different responses.Examples of physiological responsesLight quantity matters.Inhibit stem elongationsynthesis of anthocyanin[not photoreversible]>10,000 umole/m210 mmole/m2HIRHigh irradiancePromote lettuce seed germination[Photoreversible]1-1000 umole/m2LFR Low fluenceInduce gene expression LHCBArabidopsis germination[not photoreversible]1-100 nmol /m2VLFRvery low fluenceHow can plants respond to light quality & quantity?Ans. Plant has several different photoreceptors.Examples of responses mediated by phytochrome light receptora. Lettuce seed germination (VLFR)17-13. Taiz. Circadian rhythm of Albizzia leaf movement is controlled by phytochrome.Samanea317-13 Taiz. Inhibition of stem elongation by light. High IRPhyBRatio of Pfr/Pr is important.17-8 Taiz. LFR action spectra for Arabidopsis seed germinationWhat wavelength is important? Evidence for a role of phytochrome (Phy A)17-9. Taiz. Inhibition of hypocotyl elongation in dark grown lettuce seedlingsHIRPhyB & UV-A,Blue17-4. Structures of Pr and PfrWhat is phytochrome?1streceptor discovered in plants. Purified from dark-grown seedling. A protein with a chromophore.Absorbs R and FR light17-3 Taiz Purified phytochrome absorbs Red and FR lightPhotoreversiblePr PfrLight induces conformational changes in the phytochrome molecule417-8 Taiz Phytochrome level is high in apical meristemsMultiple types of Phytochrome: Two types:I.PhyA, and II. Phy B-PhyEMultiple phytochrome genesI. Phy A, Abundant, active in dark grown seedlingUnstable in light, degradedPr ----RED---> Pfr<---FR----<---dark--II. Phy B: stable form in green seedlingsPhy CPhy DPhy E-----------------------------How can plants sense light of different quantity and quality & respond ?How do you know if phytochrome is acting in a light-induced response?1. The wavelength needed for the response (action spectrum)2. Photoreversibility (Low fluence response)3. Response is affected by ratio of Pfr/Pr17-7. PhyA is labile. PhyB-E are more stable.Arabidopsis has 5 genes encoding phytochrome. phyA, B, C, D, EDark grown seedlingGreen seedling/plant[continued in