Long Distance Transport of Assimilates

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Long Distance Transport of Assimilates
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Transport of Assimilates
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Overview

• Phloem allows mass flow of carbohydrates from
  source to sink (translocation)
• Phloem loading photosynthates from mesophyll to
  sieve tubes
• Two pathways
• Symplasmic
• Apoplasmic
• Correspond to ecological distribution

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The phloem-loading pathway

• Production compartment
• Mesophyll cells
• Bundle sheath cells
• Collection compartment Phloem
• Companion cells
• Sieve elements
• Export compartment Phloem
• Sieve tubes
• Sieve elements

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The phloem-loading pathway
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Major transport compounds

• Why not glucose?
• Sucrose
• Linking sucrose with 1,2,or 3 galactose
• ? Oligosaccharides
• Raffinose
• Stachyose
• Verbascose

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Chemical structure of major sugars
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Minor vein anatomy

• Large differences in plasmodesmatal connectivity
  between mesophyll and phloem, range from
• Entirely symplasmic (Type 1)
• 60 plasmodesmata mm-2
• Evergreen trees
• Entirely apoplasmic (Type 2)
• 0.03 plasmodesmata mm-2
• Annual herbs
• Decrease in plasmodesmatal connectivity between
  minor vein phloem and mesophyll during evolution
  from evergreen trees to annual herbs

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Two phloem-loading pathways
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Taxonomic distribution of minor vein types
(Gamalei 1989)
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Evolution of minor vein anatomy
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Transport against a concentration gradient

• Symplastmic pathway
• Sucrose metabolized to make galactose in
  intermediary cells
• Sucrose linked to make oligosaccharides in an
  ATP-requiring process
• Too large to diffuse back to mesophyll
• polymerization trap mechanism
• Apoplasmic pathway
• Companion cells between mesophyll and sieve
  elements take up sucrose
• Sucrose-proton-co-transport carrier in plasma
  membrane (requires ATP)
• transfer cells

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Lythrum salicaria (Purple loosestrife)
Senecio vulgaris (Common groundsel)
Transfer cells (T) Sieve tube (S) Xylem
(X) Mesophyll cells (MC)
Intermediary cells (I) Sieve tube (S) Xylem (X)
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Variation in Transport Capacity

1. Species with symplastic phloem-loading pathway
   have lower capacity to export
2. Accumulate more non-structural carbohydrates
3. Why?
4. Limited by oligosaccharide production
5. Raffinose solubility at low temperatures

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Raffinose SolubilityAs temp ? other sugar
solutes utilized
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Phloem Loading and Ecological Distribution

1. Apoplasmic pathway dominates in temperate and
   arid zones
2. Symplasmic pathway typical in tropics
3. Raffinose common in tropical trees vines
4. Ancient familes show symplasmic configuration
   progressive evolution toward apoplasmic phloem
   loading pathway

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Phloem Loading and Ecological Distribution
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Phloem Loading and Ecological Distribution

• Environmental Factors
• Extreme temperatures
• lt10C plasmodesmata may not function well
• lt10C oligosaccharide production ?
• Viscosity of oligosaccharides ? with decreasing
  temp
• Water stress
• Consistent types found in hot deserts
• Bundle sheath cells of C4 plants

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Transport in climbing plants
Trees
Lianas
Lianas
Trees
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Phloem Loading and Plant Distribution Summary

1. Symplasmic pathway with its trapping mechanism
   sufficient in tropics
2. As plants evolved into cooler habitats,
   symplasmic pathway became limiting
3. Apoplasmic pathway important trait for species to
   occupy cool habitats
4. Inhibition of photosynthesis at cool temperatures
   may be a secondary effect of feedback inhibition
5. Does the proton-pump of transfer cells require
   more ATP than the polymerization in intermediary
   cells?
6. How does this affect the costs and benefits of
   each pathway?