Skotomorphogenesis Seed germination Genes, enzymes and signal transduction Embryo and Seed...
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Transcript of Skotomorphogenesis Seed germination Genes, enzymes and signal transduction Embryo and Seed...
Skotomorphogenesis
Seed germination
Genes, enzymes and signal transduction
Embryo and Seed development
PhotomorphogenesisPhotoreceptors
Phytochrome
Cells and cell growth
Phytochrome: regulation oflight responses
Photosynthesis: light reaction
Photosynthesis: carbon fixationPhotorespiration
Nitrogen fixation Plant nutrition
Respiration
Sucrose, starch, cellulose
Phloem translocation
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Carbohydrates: where are they coming from, where are they going???
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Phloem Translocation, and Photosynthate Allocation and PartitioningHORT 301 – Plant Physiology
October 29, 2010Taiz and Zeiger, Chapter 10
Phloem translocation - bidirectional transport of photosynthate, assimilated nutrients, signaling molecules and redistribution of water and molecules
Sources to sinks
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Lecture topics:Phloem translocationPhloem loading and unloadingPhotosynthate allocation and partitioning
Taiz & Zeiger (2010) Web Topic 10.1.A
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Photosynthate moves in the phloem
10.1 Transverse section of a vascular bundle of trefoil, a clover (Trifolium)
Phloem tissue is adjacent to the xylemProximity of the xylem and phloem is critical for translocation
10.2 Transverse section of a 3-year-old stem of an ash (Fraxinus excelsior) tree
Leaf
Stem (perennial)
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Phloem - sieve (tube) elements and companion cells
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Sieve elements – protoplasmic w/modified plastids and ERCompanion cells – sieve element metabolic functioning, phloem loading and unloading
Sieve plates – pores predominantly at the ends of sieve elementsSieve elements are interconnected cells
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Callose plugs seal sieve pores
Companion cells facilitate phloem loading and unloading Interconnected with sieve elements
Ordinary companion cell Transfer cell
Intermediary cell
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OCC and TC – minimal interconnection w/adjacent cells – apoplastic to symplastic uptake
IC – interconnection to adjacent cells, symplastic uptake-
Source to sink translocation
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Source – photosynthate production (availability) is greater than metabolic need, exportSink – requires photosynthate, import
Taiz & Zeiger (2010) Web Topic 10.1.C
Change in photosynthate distribution in sink leaves by removal of sources
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Taiz & Zeiger (2010) Web Topic 10.3.A
Phloem sap sampling for analysis
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Sugars are a primary component of sap
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Sugars are translocated in non-reducing form – sucrose is the predominant photosynthate
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Pressure flow model for phloem translocation from source to sink
Sink – phloem unloading decreases ψp
Ψp gradient between source and sink drives solution movementΨw gradient between sieve elements and xylem vessel elements recirculates the solution
Source – phloem loading increases ψp
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Phloem translocation requires minimal energy – passive transport
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Phloem loading via symplastic or apoplastic pathwaysSymplastic loading is passive transportApoplastic loading is active transport
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Sucrose-H+ symporter involved in phloem loading via an apoplastic pathway – active transportSymporter is driven by the H+ gradient generated by the plasma membrane H+-ATPase
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Polymer trapping model for symplastic phloem loading – passive uptakeSucrose diffuses from the bundle sheath/mesophyll cells into intermediary cellsSucrose is synthesized into raffinose, concentration decreases in intermediary cellsMore sucrose enters the intermediary cellsRaffinose cannot move to bundle sheath/mesophyll cells because plasmodesmata are too small
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Phloem unloading by symplastic and apoplastic pathways into sinksDeveloping leaves, roots, tubers and reproductive organsUnloading, short-distance transport and sink loading
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Leaf transition from sink to source14C import (dark) from a source leaf
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Sucrose-H+ symporter gene expression in source and sink leavesAtSUC2 promoter::GUS reporter fusion – phloem loading
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Photosynthate allocation and partitioningAllocation – regulation of carbon assimilate distribution into various metabolic pathways or sink cells
Partitioning – distribution of photosynthates, assimilation products, ions, hormones, signaling molecules (proteins and perhaps mRNAs)
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Allocation is regulated by sinks and sources
Partitioning of triose-phosphatesSink strength
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