Oscillations in Pollen Tube Growth

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Oscillations in Pollen Tube Growth ROP
Signaling Network

• XIA, Fan

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Introduction

• Observation of oscillations in pollen tube
• Spontaneous
• Positive correlation between oscillation and the
  rate of growth.
• Phase relationship
• Mechanism underlying these oscillations
• The oscillations with a period of minutes or
  shorter can are unlikely to involve periodic
  changes in gene expression.
• ROP1 signaling network plays a key role.

0 tip F-actin ROP1 activity
90 Growth rate
120 Calcium concentration
210 Calcium influx
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ROP1 Signaling Network

• ROP1
• Location of the active ROP1 establish the tip
  growth domain.
• Recruitment A tip-localized recruitment factor
  and a tip-localized Rop sequestering factor
  (AtGDI1).
• Maintenance active ROP1 promotes the recruitment
  to the apical PM region.

Jae-Ung Hwang et al., 2005
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• Oscillation of ROP1 activity

Result

• Experiment
• RIC4 localization to the PM is the direct result
  of its binding to the active form of ROP1.
• RIC4?C loses the effort function of RIC4 in the
  promotion of F-actin assembly, but faithfully
  reports its interaction with active ROP1.
• Intensity of RIC4?C at PM reflects the activity
  of ROP1.

RIC4 localization to the apical cap, i.e., the
apparent tip-localized ROP1 activity, is
oscillatory and leads growth bursts by 90.
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• RIC3 and RIC4
• RICs ROP-interactive CRIB-containing proteins.
• In pollen tubes, only RIC3 and RIC4 OX induced
  depolarized growth.
• Structurally different
• Two different pathways
• Interaction with ROP1
• Only interact with active form
• Recruit from the cytoplasm to the PM
• While RIC4 is tightly associated with PM domain,
  RIC3 is primarily cytoplasmic. RIC3 interacts
  with ROP1 in transient and dynamic manner, as
  AtGDI1.

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• RIC4 downstream pathway
• Similar to ROP1OX, RIC4OX induced dense F-actin
  network and depolarization.
• LatB could recover the oscillation of tip F-actin
  in RIC4OX tubes.
• RIC4 promotes the actin assembly.

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• RIC3 downstream pathway
• Similar to high Ca2ex, RIC3OX induced loss of
  F-actin and protrusion of the axial actin cables
  toward the extreme apex.
• EGTA and LaCl3 could recover RIC3OX induced
  growth inhibition and actin reorganization.
• RIC3 affects Ca2ex influx.

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• Counteract between the two pathways
• RIC3OX and RIC4OX resulted in the recovery of
  growth.---antagonistic effect
• Optimal ratio RIC3/RIC4 12
• Not due to the competition to bind to ROP1 but
  due to downstream targets.
• Different location of action
• RIC3 acts through Ca2 to promote the disassembly
  of RIC4-dependent F-actin.
• RIC4 acts through F-actin to counteract the RIC3
  pathway.

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• F-actin feedback to ROP1 activity
• LatB treatments recovered the polarized growth in
  both ROP1OX and RIC4OX.
• LatB treatments significantly reduced the peak of
  GFP-RIC4?C localization/ROP1 activity.
• Mechanism unknown.
• Analog in other system.
• Ca2cyc feedback to ROP1 activity
• Possible down regulation above threshold.
• Mechanism unknown.
• Common interaction between Ca2 and GEFs, GAPs
  and GDIs.

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• Quantitative Modeling
• Target
• Oscillation with proper phase relationship
  between ROP activity, F-actin and Ca2cyc.
• Mutant experiment (OX elimination of
  oscillation)