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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