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Oscillations in Pollen Tube Growth & ROP Signaling Network

Oscillations in Pollen Tube Growth & ROP Signaling Network. XIA, Fan. Observation of oscillations in pollen tube: Spontaneous Positive correlation between oscillation and the rate of growth. Phase relationship Mechanism underlying these oscillations:

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Oscillations in Pollen Tube Growth & ROP Signaling Network

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  1. Oscillations in Pollen Tube Growth & ROP Signaling Network XIA, Fan

  2. 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. Introduction 0°: tip F-actin & ROP1 activity 90°: Growth rate 120°: Calcium concentration 210°: Calcium influx

  3. 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

  4. 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 atPM 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°.

  5. 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.

  6. 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.

  7. RIC3 downstream pathway • Similar to high [Ca2+]ex, 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 [Ca2+]ex influx.

  8. Counteract between the two pathways • RIC3OX and RIC4OX resulted in the recovery of growth.---antagonistic effect • Optimal ratio RIC3/RIC4 = 1:2 • 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.

  9. 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. • [Ca2+]cyc feedback to ROP1 activity • Possible down regulation above threshold. • Mechanism unknown. • Common interaction between Ca2+ and GEFs, GAPs and GDIs.

  10. Quantitative Modeling • Target: • Oscillation with proper phase relationship between ROP activity, F-actin and [Ca2+]cyc. • Mutant experiment (OX elimination of oscillation)

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