Chapter 10 Translocation in the phloem

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Chapter 10 Translocation in the phloem

• Root anchor, absorb water and nutrients
  absorption
• Leave absorb light and exchange gas
  assimilation
• efficiently exchange via long-distance
  transport
• Xylem transport water and nutrients from roots
  to aerial portions
• Phloem
• mature leaves (sugars)? growth and storage
  portions
• redistributes water and various compounds
• transmits signaling molecules such as
  proteins and RNA (06)
• The association of Pi translocation and
  co-expression of miR399s and PHO2 encoded a
  ubiquitin-conjugating E2 enzyme in vascular
  tissues suggests their involvement in
  long-distance signaling. (Apr 29 May 1, 2008)
• angiosperms major gymnosperms minor

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The long-distance transport pathways xylem and
phloem
The most innermost layer of secondary phloem is
functional.
Sclerenchyma cells
trefoil
Ash tree
p. 364 cell wall primary and secondary
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Xylem dead at maturity, lack plasma
membrane, lignified 2 wall tracheary
elements tracheids, vessel elements Phloem
living cells, nonlignified walls sieve
element directly involved in translocation
sieve tube element (angiosperms) sieve cell
(gymnosperms) no companion cells companion
cells parenchyma cells which store and
release food molecules (fibers and sclereids)
protection and strengthening of the tissue
(laticifers) latex-containing cell
Tracheids/ vessel elements
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Sieve element
Lack nuclei tonoplast
microfilaments microtubules Golgi bodies
ribosomes Retained mitochondria
plastids smooth ER
perforation plate
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3600?
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P-protein (slime) rich in phloem in all
dicots and in many monocots, absent in
gymnosperms different forms tubular,
fibrillar, granular, crystalline along the
periphery of the sieve tube element, or evenly
distribute in the lumen P-protein body a
discrete bodies in the cytosol of immature cells
are synthesized
in companion cells two types PP1 the
phloem filament protein, similar to cysteine
proteinase inhibitor ? defense
against insect PP2 the phloem lectin,
serve to anchor PP1 in the sieve
element P-protein (body) The major
function is in sealing off damaged sieve
elements ? short-term solution is
reversible and controlled by calcium in the
legume family, it is not known whether other
plants ? Callose deposition ? long-term
solution
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Callose a ?-1,3-glucan, ? callus is
accumulated between plasma membrane and cell wall
wound callose, mechanical stimulation and
Temp. stress dormancy disappear when the
damage is recovery
Companion cells is associated with sieve tube
element and connected by plamodesmata
(branched) dense cytoplasm and abundant
mitochondria
Functions ? to supply energy ? to transport
the photosynthetic products ? take over some of
the critical metabolic functions, such as
protein synthesis
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Three types (a) ordinary companion cell
smooth inner surface, few plasmodesmata (b)
transfer cell fingerlike wall ingrowth, the
scarcity of connections (c) intermediary cell
numerous plasmodesmata connect to
surrounding cells
Sugars of mesophyll cells of mature
leaf apoplast ? ? symplast ordinary
company cells, intermediary cells
transfer cells apoplast ? ? symplast
Sieve element
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The translocation patterns of phloem

• is not exclusively either an upward or downward
  direction
• irrespective to gravity
• Source area of supply, an exporting organ
• mature leaves, storage root beet
  (Beta maritima)
• Sink area of metabolism or storage, a
  receiving organ
• nonphotosynthetic organs, root and
  shoot apices,
• young tuber, developing fruits,
  immature leaves
• Not all sources supply all sinks on a plant
• ? certain sources preferentially supply
  specific sinks

plasticity
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The features of source-to-sink pathways a.
proximity b. development vegetative or
reproductive stage c. vascular connections
orthostichy d. modification of translocation
pathways wounding, pruning
Epifluorescent microscopic
dahlia
Anastomosis/ callose (yellow)
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Materials translocated in the phloem
Water the most abundant substance
Sugar-rich sap
Carbohydrate Sucrose (0.3 to 0.9 M) Nitrogen
Asx (Asp, Asn) Glx (Glu, Gln)
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RNAs mRNA, pathogenic RNA, small regulatory RNA
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Phloem sap can be collected and analyzed the
stylet of an aphid, a natural syringe (WebTopic
10.3)