Translocation: Distribution of Assimilates

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Translocation Distribution of Assimilates

• Chapter 8

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Transport

• Photosynthetic products are transported primarily
  as sucrose in phloem
• Sieve elements (primary phloem conducting cells)
  are living cells stacked end-to-end
• Phloem transport is up or down, but always from
  source to sink

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Bidirectional Flow of Phloem Contents
How can this occur? Apparently, sieve tubes
adjacent to each other can flow in opposite
directions. If this occurs in same cell then
the mechanisms used to describe phloem transport
are suspect.
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Source vs. Sink

• Source a net exporter of assimilate
• Sink a net importer of assimilate

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Source vs. Sink

• Source mostly mature leaves
• Sink apical meristems
• lateral meristem
• fruit
• seed
• developing leaves
• etc.

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Trivia Pesticides

• Systemic pesticides (fungicides, insecticides,
  herbicides) are either phloem mobile or xylem
  mobile
• Phloem mobile pesticides must be applied to
  leaves
• Xylem mobile pesticides must be soil applied

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Sieve Tube Structure

• Living Cells
• Thin walled with sieve plates at the end of each
  cell
• No nucleus
• No vacuole
• Have plasmalemma

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Sieve Plate
Sieve tube members
Phloem Parenchyma cells
Phloem Parenchyma cells
Sieve Tube Plastids
Companion Cell
Parenchyma plastids
From Hopkins, 1999
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Sieve Tube Transport

• Sugars concentrated (up to 25 or 600 mM
  sucrose) for transport
• Transported up to 250 cm/hr
• Movement of sugars in phloem is understood in
  theory only
• pH is relatively high (8)

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Companion Cell Characteristics

• It is a Parenchyma Cell
• Large Nuclei
• Many Ribosomes
• Active Mitochondria and Endoplasmic Reticulum
• Provide support for sieve tube member
• Facilitate sucrose transport into sieve tube

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Phloem Parenchyma Cells

• Difficult to distinguish from companion cells.
• In herbaceous dicots, they may share a large
  surface area with companion cells and serve as
  transfer cells.

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P-protein

• Phloem Protein begins as discrete bodies
• Range in mass from 15 to 220 kD
• Its function is not totally clear but it may bind
  carbohydrates and participate in transport. It
  is located along inner wall of sieve element and
  does not block sieve plate.

From Hopkins, 1999
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Callose

• Callose is a 1 3-glucan related to starch.
• Located on surface of the sieve plate or near
  pores between elements.
• May help seal off sieve tube if injured to
  preserve plant integrity.

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Phloem TransportMass Flow Theory

• In leaves, sucrose moves from mesophyll to phloem
  (sieve element - companion cell complex) via
  plasmodesmata the symplastic pathway or is pumped
  into apoplast and then to phloem.
• Sucrose is pumped into phloem (uses ATP, called
  phloem loading). This can be inhibited by
  apoplastic PCMBS

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Phloem TransportMass Flow Theory

• Phloem loading occurs against a concentration
  gradient.
• Water enters phloem by osmosis
• Pressure builds - Münch pressure-flow hypothesis

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From Hopkins, 1999
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Phloem TransportMass Flow Theory

• Solution is forced to move through phloem
• More sugar continues to be loaded
• Phloem is under positive pressure
• An active sink is required for transport to
  continue
• May move at 50 to 250 cm h-1

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From Galston et al., 1980
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From Galston et al., 1980
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Recent Findings About Phloem Loading
Some species may exhibit predominantly apoplastic
loading, others may exhibit symplastic loading.
Some species that load symplastically tend to
transport stachyose rather than sucrose.
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Xylem and Phloem Contents
mg liter-1
From Galston et al., 1980
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Why is sucrose used for transport rather than
glucose or fructose?
Sucrose is a nonreducing sugar and is therefore
less reactive.
The bond linking glucose and fructose (the
?-fructoside linkage) has a relatively high
negative free energy of hydrolysis (just below
ATP).
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What happens at the sink?
Phloem unloading and sink loading. Option 1
(Symplastic only) At the sink, sucrose flows to
the sink via plasmodesmata and is hydrolyzed to
glucose and fructose. This helps maintain the
osmotic gradient from source to sink.
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What happens at the sink?
Option 2 Sucrose released into apoplast where it
is converted to glucose and fructose by acid
invertase. The seed tissue then takes up these
monomers. This pathway is prominent in maize,
sorghum, and pearl millet.
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What happens at the sink?
Option 3 Sucrose actively pumped into apoplast
by an energy-dependent transport. Sucrose is
then taken up by seed tissue.
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Assimilate Distribution
Distribution or Allocation consists of 1. Leaf
Metabolism and Biomass 2. Storage 3. Export from
Leaf
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Leaf Metabolism and Biomass
1. Maintenance (e.g., respiration) 2. Cellulose,
Protein, and Lipid Synthesis
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Assimilate Storage
1. Sucrose and starch are common storage
forms but many grass species store
fructans. 2. In leaves, vacuolar sucrose
appears to be transported out at night prior
to transport of cytoplasmic sucrose.
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Export
1. In young leaves that retain a large portion of
their photosynthate, acid invertase and sucrose
synthase are active. 2. In older leaves, sucrose
phosphate synthase is more active.
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Export
3. Levels of starch are often low in the morning
and high at dusk. Sucrose is less variable
although it can build up. Starch is synthesized
in chloroplasts and sucrose in cytosol. Many
researchers believe leaves are programmed to
deliver carbon evenly throughout the day.
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Photosynthate Partitioning
On a whole plant basis, partitioning is called
dry matter distribution. It is generally the
result of sink strength. Sink Strength sink
size (g) ? sink activity Sink activity refers to
the uptake rate. Temperature, rate of phloem
unloading, turgor, and concentrations of hormones
dictate sink activity.
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Photosynthate Partitioning
The delivery of photosynthate to sinks is often
buffered by storage carbohydrates, at least on a
short-term basis. Researchers have used drought,
shade, and nutrient stress to quantify effects of
stress on partitioning to sinks.
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Role of Hormones in Photosynthate Partitioning
Abscisic acid may stimulate growth rate of fruit,
translocation of sugar to bean and sugarbeet
roots, and unloading of sucrose into apoplast
from soybean seed coat.
From Hopkins, 1999
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Xenobiotics
Xenobiotics are biologically active substances
that are foreign to a given species. The most
practical examples are pesticides. For many
pesticides to be effective, they must enter the
phloem. Foliar-applied substances may diffuse
into leaf tissue through cuticle or enter
stomata. Movement through the plasmalemma is
dependent upon polarity/hydrophobicity. This is
why pesticide formulations are so important.
From Hopkins, 1999