1Photomorphogenesis: OUTLINEEffect of light on Growth and Development-Light quality and quantity are the most significant environmental factorsaffecting 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 transduction a) Communicate signal to other cell parts b) Amplify the signal c) Network and cross talk3. Primary response e.g. Increase or decrease in gene expression e.g . Change from inactive protein ---> active protein4. Cellular and Physiological responsesPlants have 3 types of Photoreceptors1. Phytochrome 660 nm2. Blue light receptor ~400-500 nm3. UV-B Receptors ~300 nmPhotomorphogenesis and SkotomorphogenesisExamples of responses mediated by phytochrome lightreceptorFigure 17.2 Lettuce seed germination (LFR)HIRPhyB& UV-A,BlueFigure 17.5 Inhibition of hypocotyl elongation in dark grownlettuce seedlingsFigure 17.7Structures of Prand PfrWhat is phytochrome?1st receptor discovered in plants.Purified from dark-grown seedling.A protein with a chromophore.Absorbs R and FR lightFigure 17.6 Purified phytochrome absorbs Red and FR light PhotoreversiblePr FR RPfrPhotostationary state Pfr: 3-85%2Light Quantity and Plant Development Light Quantity and Plant Development (Figure 17.4)(Figure 17.4)Light quantity mattersInhibit stem elongationsynthesis of anthocyanin[not photoreversible]>10,000 umole/m2 10 mmole/m2HIRHighirradiancePromote lettuce seedgermination[Photoreversible]1-1000 umole/m2LFRLow fluenceInduce gene expression LHCBArabidopsis germination[not photoreversible]1-100 nmol /m2VLFRvery lowfluenceLight induces conformational changes inthe phytochrome molecule (Figure 17.8)Phytochrome FamilyMultiple phytochrome encoding genesI. Light-liable form - PHYA Pfr,Abundant in dark grown seedlingUnstable in light, degradedII. Light-stable form - PHYB, stable form in greengreenseedlingsPHYC, hypocotyl elongationPHYD, petiole elongationPHYE, seed germination in VLFR and petioleelongation in LFRFigure 17.13 Difference in phytochrome gene familystructure3Figure 17.15 COP1 represses expression of light-regulatedgenes by targeting protein for degradation DegradationDegradationCOP1 = E3 Ubiquitin Ligase that targets proteins ( HY5) for degradationHY5 is a TF that binds to light-inducible promoters.In the dark, COP1 targets HY5 for degradation.With light, COP1 leaves the nucleus. HY5 binds and activates light-regulated genes.PHYB directly regulates gene expression in nucleusPIF3 is amaster TF= [masterswitch]Figure 17.18 Circadian rhythm of Albizia leaf movement iscontrolled by phytochrome.17.19 Leaf movement results from turgor changes in ventraland dorsal cells. Light alters ion fluxes.Figure 17.20 Inhibition of stem elongation by light. High IRPhyBRatio ofPfr/Pr isimportant.4Summary- mode of actionSummary- mode of action1. Plant growth and developments is regulated by light.2. Phytochromes are important photoreceptors.PHYA: Pr -->PfrPHYB is stable; senses Pfr/Ptotal3. Mode of actiona. Fast responses- changes in ion fluxesno gene expression;activated by protein modificationb. Changes in gene expressionEarly response genes: 1-5 min after stimulimostly TFlate response genes: seen later 3-10 minQ3:What is the ecological significance ofecological significance ofcircadian rhythm?circadian rhythm?1. Sleep movements of leaves2. Shade perception and avoidance3. Small seed