Hormone distribution and transport

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Hormone distribution and transport
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Contents

• Concepts and definitions
• Auxins distribution and transport
• Auxin distribution old views and new
  developments
• Auxin biosynthesis not restricted to the shoot
  anymore
• Auxin transport
• Mass-flow-dependent distribution of auxin
• Polar auxin transport
• Conclusion a joint effort required for auxin
  transport?

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Concepts and definitions

• ? Distribution of phytohormones
• ? depends on a concentration gradient
  established in the plant body
• ? mediated by free diffusion (gaseous compound
  ethylene)
• ? polar transport of auxin, strictly
  unidirection, a complex machinery of carrier
  protein, mediating cellular uptake and efflux
• ? 3 paradigms for explain phytohormone
  distribution
• Auxin, GA, BR

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Auxins distribution and transport

• Auxin distribution old views and new
  developments
• Auxtin mediates multiple aspects of plant
  growth and development (flowering, vascular
  development, tropism) and division, enlargement
  and differentiation of individual plant cell by
  temporal and spatial control of auxin levels
  (homoeostasis).
• ? Old views auxin is primarily synthesized in
  the apical parts of the shoot and subsequently
  delivered to other parts of the plant body where
  it exerts its activity in the regulation of a
  variety of growth responses.
• ? New development in the last few years, sites
  of auxin biosynthesis and distribution during
  plant development specifically in Arabidopsis
  thaliana.

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• Auxin biosynthesis not restricted to the shoot
  anymore

? Auxin biosynthesis ? Auxin metabolism ?
Transport into or out of tissue concerned ?
Active auxin/inactive auxin
Auxin (IAA, IBA, 4-Cl-IAA) steady-state level
? Analyses in Arabidopsis confirmed the old view
as auxin concentration is highest in the youngest
leaves formed by the shoot apical meristem (SAM).
?Essentially, all vegetative organs are capable
of synthesizing IAA de novo, in general, rates of
auxin biosynthesis were found to be highest in
meristematic tissue, correlating with the
important role in control of cell proliferation.
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Figure 9.1 Total IAA pool sizes in different
parts of a 10-day-old wild-type Arabidopsis
seedling grown under LD. The circles show the
absolute pool sizes of IAA in different organs of
the seedling. Black parts of circles show the
amount of IAA that had been synthesized de novo
in different tissues after 24 h of incubation in
30 deuterated water and 40 mM NPA.
? Auxin found in the roots of very young
Arabidopsis plantlets is most likely derived form
shoot sources. ? Primary root meristems as well
as emerged lateral roots appear to represent
important sources for root-synthesized auxin. ?
Auxin, drived from the root as well as from the
shoot, exerts a combinatorial effect on the
regulation of auxin-mediated responses in the
control of root growth.
The Plant Journal (2001) 28(4), 465-474
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Auxin transport

• Newly synthesized auxin, or auxin
  enzymatically liberated from storage forms,
  travels through a combination of two processes
• (i) mass flow, a rapid (510 cm/h) nondirectional
  transport occurs in the mature phloem
• (ii) polar auxin transport (PAT), slow
  (7-15mm/h)(510 mm/h), establishing
  physiologically relevant auxin gradients
• Polar auxin transport, which is important in the
  early stage of seedling growth, appears to be
  superseded subsequently by phloem-based
  transport.

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Mass-flow-dependent distribution of auxin

• ? Site In phloem cells
• ? Characteristics The direction and velocity of
  auxin translocation depends on spatial
  variations in auxin concentrations
• ? The role of auxin in phloem sap in
  auxin-mediated growth responses?

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Polar auxin transport
Figure 9.2 sites and direction of PAT in higher
plants(arrows).
BioEssays 2712461255,
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Figure 9.3 Chemiosmotic model for PAT.
BioEssays 2712461255,
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Auxin transporters

• Most of the research to date has been on
  Arabidopsis
• thaliana, in which the following set of putative
  auxin
• carriers have so far been identified
• The AUX1/LAX influx carriers comprising four
  members of amino acid permease-type proteins.
• The PIN efflux carriers comprising eight members
  resembling putative bacterial transporters.
• The MDR/PGP efflux carriers comprising many
  members homologous to multidrug
  resistance/P-glycoproteins transporters

The role of transporters has been implied from
two features ? There polar localisation in
cells which supports the chemiosmotic
hypothesis. ?Their homology to other
permeases/transporters
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• Difficulties in proving auxin transporter
  activity for
• putative carriers.
• What is being measured is not the activity of a
  specific carrier but the combined effect of all
  influx and efflux activities, including passive
  diffusion.
• Evidence of auxin transport activity can also
  mean that the carrier are required as part of a
  complex that transports auxin.
• Putative transporters have to show specificity
  for auxin.

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Regulation of the carriers
amount of protein (e.g. by increasing
transcription or reducing degradation of
protein) actual activity of the protein (e.g. by
covalent modification or allosteric
activation) rate of cycling polarity of carrier
distribution
PAT can be regulated by
Gravitropic and phototropic stimulation Auxin
treatment BFA treatment TIBA and NPA
treatment Various mutation pid (PINOID kinase
mutant) tt4
(flavonoid-deficient mutant)
smt/orc (sterol methyltransferase mutant)
Carrier membrane localization can be affected by
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(1) Various protein kinase and phosphatase
inhibitors in general inhibit auxin efflux or
basipetal PAT (2) Acidification of the
intracellular pH of cells reduces active auxin
influx and efflux, while alkalinisation of the
extracellular pH reduces influx. (3) NPA,
inhibitor of general auxin efflux and PAT. (4)
Flavonoids have a negative effect on PAT and can
inhibit auxin efflux/leakage from roots.
Factors affecting PAT with uncertain mechanism
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AUX1 PIN AtPGP1/19 AXR4 PINOID kinase TWD
immunophilin like protein AtPGP4 uncharged Auxin
diffusion trafficking of AUX1 to the plasma
membrane PIN polarity controlled by PINOID and an
unconfirmed phosphatase
Function and regulation of auxin-transport
proteins
Biochem. J. (2007) 401, 613622 (Printed in Great
Britain)
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Conclusion a joint effort required for auxin
transport

• We mentioned earlier that measurements of
  PAT reflect the sum of all influx and efflux
  paths of auxin in cells or tissues, and that it
  was thus not possible to measure activities of
  the individual carriers. However, the fact that
  PIN localisation can change has given researchers
  a good tool to observe direct effects on these
  carriers in response to a certain treatment.
  Following up this with PAT measurements ( or
  observing secondary effects of PAT) allows
  further postulations on the putative role of the
  PINs in mediating PAT.
• The technical inability to assign any
  observed changes in PAT to specific carriers is a
  disadvantage when analyzing the role of each
  carrier, but it also simply reflects the true
  situation, that is the carriers act together. And
  no doubt future research will also discover other
  proteins involved in the carrier machinery.