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hormoneLec - Lecture 6 Phloem Transport Control of plant...

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Unformatted text preview: Lecture 6 Phloem Transport Control of plant growth and development Phytohormone Overview Gibberellins and Abscisic Acid Ethylene Auxin Cytokinins 1 Carbohydrate movement Site and direction of movement Driving force for movement = pressure Fow Pulse label new photosynthate by *CO 2 feeding of leaf for 1 hr Allow 6 hr for transport of newly synthesized sugar away from source tissue Autoradiograph entire plant Sites where radioactivity imported = sink tissue Transport is multidirectional Water enters both funnels by osmosis Internal pressure builds until no more water enters sink Internal pressure builds more -> water and solutes forced out at sink Dual-osmometer model Phloem loading/unloading maintains source/sink identities Sucrose loaded at source cells Water follows osmotically, recycled via xylem Sucrose unloaded at sink cells Source/sink identities of organs change as they mature Yellow = carbohydrates in phloem Red = chlorophyll http://www.dbbe.fcen.uba.ar/materias/botanica/Phloem%20Translocation.htm Phloem contains two major cell types Sieve tubes: major conducting cells, alive but have minimal organelles Companion cells: small, metabolically active, control activity of adjacent sieve tube member, element Phloem loading involves both symplastic and apoplastic transfer from mesophyll cells Most plants have few connections between phloem and adjacent mesophyll cells: forces apoplastic transport into companion or sieve tube cells Sucrose loading by sucrose-proton symport Uses potential energy of H+ gradient, created by H+ pump, to drive sucrose uptake Xylem sap is under tension (usually) Phloem sap is under pressure Comparison of plant and animal Fuid transport mechanisms Environmental signals: Light Temperature Gravity Water availability Salinity Pathogens Internal signals: Developmental stage Tissue/organ (position within body) Nutrient status Phytohormones Genetically programmed options Approaches to understanding hormone response: 1) physiological: spray and pray Catalog possible responses 2) genetic: wreck and check Identify genes required for response Identify responses requiring common signaling factor Analyze biochemical function following cloning 3) reverse genetics Create speciFc mutations to determine physiological function of gene product with known biochemical function Phytohormones = chemicals made by plant to regulate their own growth and development; active at very low concentrations Five classes of classical hormones Small molecules: weak acids or bases, 1 gas Can cross membrane in uncharged form Active in nM - M range Leaf senescence: ABA & ethylene (+) Cytokinin (-) Stomatal opening: ABA (-), auxin (+) Embryo and seed development: All hormones at different stages Seed germination: GA (+), ABA (-) Cell/stem elongation: Auxin & GA (+) ABA and ethylene (-) GA and cytokinins synthesized: move to shoots Auxin and GA synthesized in young...
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hormoneLec - Lecture 6 Phloem Transport Control of plant...

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