Post on 22-Dec-2015
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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
paul.m.hasegawa.1@purdue.edu
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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