Translocation in the Phloem

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• Lecture 19
• Translocation in the Phloem
• Assimilate allocation and
• partitioning

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Summary apoplastic and symplastic loading

• Species with apoplastic loading
• Mainly sucrose transport
• Herbs, temperate or arid climate plants
• Species with symplastic loading
• Mostly intermediary companion cells
• Transport raffinose and stachyose
• Trees, shrubs, tropical and subtropical plants
• Species with both types of loading
• Coleus

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Summary phloem unloading

• Symplastic unloading
• E.g. growing leaves, roots and storage sinks, in
  which carbon is stored as starch or protein
• Passive.no membranes crossed
• Apoplastic unloading
• Activesugar molecules cross at least two
  membranes (PM of exporting cell and PM of sink
  cell)
• Presence of energy requiring transporters

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How do metabolites move in the phloem?
Pressure-driven bulk flow (Ernst Muench, 1930)
Source to sink High phloem transport velocities
( 100 cm h-1) So diffusion is not involved
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Pressure-flow model of translocation in the phloem
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Allocation and partitioning
Allocation the regulation of the distribution
of fixed carbon into various metabolic
pathways (i.e. the fate of fixed carbon)
- includes storage (starch), utilization
(metabolic energy, synthesis of other
com- pounds), and synthesis of transport
sugars Partitioning the differential
distribution of photo- synthates within the
plant - various sinks partition sugars -
distribution must be balanced - Many cultivars
are economically important because they
partition to edible plant parts (fruits,
grains)
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Source leaves regulate allocation
Increase in photosynthesis rate in a source leaf
results in increased translocation rate from the
source. Control points for allocation - starch
synthesis - sucrose synthesis (including
distribution of sucrose between transport
and temporary storage) - regeneration of
intermediates in the reduction cycle of the
Calvin Cycle
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Source leaves regulate allocation
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Sink tissues compete for available
translocatedphotosynthates

• Example young leaves compete with roots for
  photosynthates
• In sugar beet and bean
• Over short term rates of photosynthesis and
  export from single source do not change young
  leaves receive relatively more sugar than roots
• Shading decreases partitioning in roots
• Young leaves can deplete sugar content of sieve
  elements more rapidly, thus increasing the
  pressure gradient and translocation toward
  themselves

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Sink Strength depends on sink size and activity

• Sink strength sink size x sink activity
• Sink size total biomass of the sink tissue
• Sink activity rate of uptake of photosynthates
  per unit biomass of sink tissue
• Plant types and storage strategies
• Annuals, plants that usually germinate, flower
  and die in one year (pea)
• Biennials, plants that take between twelve and
  twenty-four months to complete their lifecycle
  (carrot, celery, parsley)
• Perennials, plants that live for more than two
  years (trees, shrubs)

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Environmental influences on partitioning

• Water stress
• More root growth
• Nitrogen
• High N supplyless root growth
• Low N supplymore root growth

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Transport of signal molecules

• Growth regulators (plant hormones, e.g. abscisic
  acid, gibberellins, cytokinin)
• Affect partitioning by controlling sink growth,
  leaf senescence etc.
• Abscisic acid enhances, auxin inhibits sucrose
  uptake by some sink tissue
• Proteins (P-proteins, H/sucrose symporter)
• Turgor pressure may control activities of sources
  and sinks
• E.g. phloem unloading is rapid due to rapid sugar
  utilization at sink, ?P ? in sieve elements this
  reduction in pressure is transmitted to source,
  i.e.
• loading increases

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Phloem transport - Summary
1. Metabolites are transported from source to
sink. SOURCE Tissue/organ that makes or stores
food reserves. A net exporter. E.g. Seed
endosperm, Green Photosynthetic leaf. SINK
Organ/tissue or cell that requires metabolites
for energy and for biosynthesis. A net importer.
E.g. Shoot meristem, roots, developing seeds. 2.
Phloem contains 3 types of cells. 3. PHLOEM
LOADING in source tissue is by energy-dependent
transport. Sucrose is actively loaded into
companion cell or phloem parenchyma by H-
coupled symport. PHLOEM UNLOADING via the
apoplast and symplast. 4. LONG DISTANCE
MOVEMENT in phloem is driven by a
pressure gradient. 1. At source, accumulation of
sugars in sieve elements ? high solute
concentration, i.e. ??S ? (more negative) ? Water
moves in ? ??P? 2. At sink, unloading causes
decrease in solute concentration, ??S? (more
positive) ? H2O leaves ? ??P? 3. Solution moves
by mass flow under pressure gradient from source
to sink.
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Phloem transport - Summary
5. Allocation and Partioning 6. Transport of
signal molecules physical ? turgor
pressure chemical ? plant hormones, proteins