PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Abscisic Acid: Seed Maturation and Anti-stress hormoneChapter 23Produced in the chloroplast via the terpenoid pathwayXanthophylls are precursors to ABA synthesisABA is a sesquiterpene (C15), three 5C isoprene unitsHow does ABA1 contribute to the observed phenotype?ABA BiosynthesisTissue concentrations can be quite high- µM rangeConcentration is highly variable – influenced by developmental and environmental conditionsLeaves-increases 50 fold in response to drought stressDeveloping seeds-100 fold more than mature seedsABA concentration and activity regulated through:Synthesis/degradationCompartmentationInactivation-oxidation/conjugationABA RegulationTransported via the vascular tissue- more abundant in phloemDrought -ABA transported from roots to leaves via xylemAnion trap: under well water conditions apoplastic pH is 6.3 which keeps ABA in its associated form (ABAH). Membrane permeable ABAH diffuses into the mesophyll cells and becomestrapped after dissociating (ABA-) under higher cytosolic pH. This keeps apoplastic ABA levels lowUnder drought conditions the apoplast becomes more alkaline (pH 7.2) which converts ABA intoits dissociated form increasing itsconcentration in the apoplastABA regulation: transport and compartmentationRegulator of seed maturation, promotes seed dormancyMaintains bud dormancy- not well understoodInvolved in leaf senescence- stimulates ethylene productionMajor hormone involved in plant response to drought:Increases root:shoot ratioPromotes stomatal closuresPhysiological effects of ABASeed dormancy and germinationDuring seed development, the embryo enters a quiescent phase in response to desiccationDormancy: the condition in which a living embryo within a seed drastically slows its metabolism and growth Seed germination: the resumption of embryo growth of the mature seedFor seeds to germinateWater and oxygen must be availableThe temperature must be suitableThere must be no inhibitory substances presentSeed dormancyTwo types of seed dormancyEmbryo dormancy: controlled by hormones Inhibitors: ABA high concentration Germination promoters (GA)- low concentrationSeed coat-imposed dormancyPrevents germination under unfavorable conditionsSeed bankGreater seed dispersalWhy seed dormancy?Gas exchange limitationsWater uptake preventionWaxy cuticles, suberized layers, and lignified scleridsMechanical constraintInhibitor production- ABARetention of inhibitorsLeaching of inhibitors by precipitation often triggers germination- common in desert environmentsFive mechanisms of seed coat induced dormancyScarificationPhysical damage to the seed coatDecompositionAnimal involvementStratification (chilling)Many seeds require a period of cold (0 to 10°C) while in a fully hydrated state in order to germinateAfter-ripeningMoisture content reduced to certain level by drying (deserts)LightMany seeds have a light requirement for germination (phytochrome)Light activates enzymatic weakening of the seed coat triggering germinationFire/smoke Why use fire/smoke as a germination cue?Environmental cues influence germination timingNicotiana attenuataABA maintains seed dormancyVivipary: precocious seed germinationGenetic germination analysis-double mutants Koornneef GA mutants- revertant screen Turned out to be ABA biosynthesis mutants Highlighted the fact that the ABA:GA ratio controlled germinationABA inhibits α-amylase transcription Seed dormancy is regulated by the ABA:GA ratioMaarten KoornneefCanna compacta seeds can live for at least 600 yearsViable Canna seeds were obtained from inside a tomb in ArgentinaCassia multijuga (false sicklepod) seeds, collected in 1776, were still viable in 1934Seeds under retreating glaciersWhy are some seeds able to survive in a dormant state for hundreds of years, whereas others lose viability in less than 5 years? Almost nothing is known about the mechanisms that determine the longevity of seeds…environmental conditions…species…Seed longevityABA increases the root to shoot ratio under drought stressHow does increasing the root:shoot ratio help plants cope with drought stress?1) More roots increase soil moisture capture 2) Decreased shoots result in less water demand and transpirational water lossABA triggers stomatal closure under drought stress Blue light triggered stomatal opening is dependent on secondary active transport ABA inhibits plasma membrane proton transporters which slows the guard cell uptake capacity of K+ and Cl-Starting pH was the same displaced for easy viewingModel of ABA control of stomatal closureABA binds to receptor which activates two separate signaling pathways that increase cytosolic Ca2+ concentrations by activating vacuole and plasma membrane Ca2+ transportersHigh cytosolic Ca2+ Inhibits proton pumping which slows K+ & Cl- uptake into guard cell Activates K+ and Cl- export transportersChapter 23 Homework QuestionsIn our Arabidopsis mutant experiment the ABA1 mutant is significantly smaller than wild-type. Give two explanations for the observed size difference. What are two additional phenotypes you would expect to see in ABA1 based on your understanding of the effect of ABA on plants?Outline the two types of seed dormancy and describe how and what Maarten Koorneef discovered about the role of hormones in seed dormancy; give three reasons why there is a period of seed dormancy. What are the five major environmental triggers of seed germination we discussed in class and explain how each one can contribute to the survival of the developing embryo.Explain how ABA concentrations increase in leaves in response to drought, and then describe how ABA triggers stomatal closure at the cellular