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Pollen Tube
-- the model system for the study of cell polarity
control and tip growth
Speaker: Chunhui Cai
Introduction
• Following germination on the surface of
the stigma, pollen tubes are directed
through the stigma surface, grow within
the transmitting track, emerge from the
transmitting tract and finally are targeted
toward the micropyle to deliver sperms to
the ovule.
Introduction
• Pollen tubes provide a simple model
system for studying cell growth and cell
polarity control.
• Pollen tubes extend exclusively at the cell
apex via an extreme form of polar growth,
known as tip growth, producing uniformly
shaped cylindrical cells.
Introduction
• Pollen tube growth requires
spatiotemporal regulation of many cellular
functions,
-- ion fluxes, organization and dynamics of
cytoskeletal elements, vesicular trafficking,
exocytosis, endocytosis, and wall
synthesis, assembly and remodelling
Introduction
• An important issue: tip growth domain (PM
domain) which leads to localized extension
of the plasma membrane and cell walls.
• Both internal signals and external
guidance signals may regulate the tip
growth.
Introduction
• Questions of interest:
– How the tip growth domain is established in
pollen tubes;
– How internal tip growth signals and external
growth guidance signals spatially regulate the
tip growth domain;
– How the tip growth domain signals to the
machinery for exocytosis and vesicle targeting
Rop GTPase
• Rop is a plant-specific subfamily of the Rho
family of G protein that include Cdc42, Rac and
Rho subfamilies from animals and fungi.
(Rho GTPase signaling has been shown to
regulate a wide variety of cellular processes,
including reorganization and dynamics of F-actin,
microtubule organization, gene transcription,
RNA processing, cell cycle progression, and
activation of specific enzymes such as NADPH
oxidase and glucan synthase)
• A global study of Rop signaling network in
Arabidopsis
• Arabidopsis: 11 ROPs, divided into four groups
based on their amino acid sequence similarities.
• Members within the same groups appear to be
functionally redundant; different groups are
distinct.
• ROP1 regulates pollen germination and pollen
tube growth.
• The existence of a tip-localized ROP1dependent signaling pathway activates the
pollen germination and pollen tube growth and
the active ROP1 signaling complex defines a PM
domain for growth.
• ROP1 localization and/or its localized activation
indeed define the tip growth domain
• The active ROP1 promoted ROP1 recruitment to
the apical PM region, forming a ROP
activation/recruitment positive feedback loop,
which provides a mechanism for the rapid
reproduction of tip growth site. The positive
feedback loop is activated locally, amplified
laterally and inhibited globally.
Rop1 signaling network
• Rop1 activates two downstream pathways
through direct targets: RIC3 and RIC4.
– RIC4 promotes F-actin assembly, whereas
RIC3 activate Ca2+ signaling that leads to Factin disassembly. This Rop-dependent
dynamics of tip F-actin is important for polar
growth.
RIC3 and RIC4
• RIC3 and RIC4 are two distinct ROP1 target proteins
• RIC4 promotes the assembly of apical F-actin
• RIC3 regulates and promotes the tip-localized influx of
intracellular Ca2+, which subsequently modulates for the
formation of the tip-focused Ca2+ gradient.
• RIC3 and RIC4 counteract to control actin dynamics and
apical growth
• A balance between RIC3 and RIC4 is critical for efficient
tip growth
• RIC3 acts through Ca2+ to promote the disassembly of
RIC4-dependent F-actin
• RIC4 acts through F-actin to counteract the RIC3
pathway.
• The mechanism by which RIC4 promotes
actin assembly and the structural nature of
RIC4-mediated F-actin remain to be
investigated.
• RIC4-actin pathway counteract the RICCa2+ pathway; RIC4-mediated actin
assembly appears to inhibit the
accumulation of Ca2+ at the tip by
inhibiting a Ca2+ permeable PM channel.
Phases
• RIC3 in the cytosol and RIC4 in the PM.
Because RIC3 and RIC4 both contain a CRIB
motif and are expected to bind the same effector
domain on ROP1, it is important to have distinct
kinetics during their interaction with ROP1 so
that they would not compete with each other in
binding to ROP1, but would co-ordinate with
each other to transmit ROP1 signals.
• These two ROP signaling targets coordinately
oscillate with the same period but in the different
phases during the growth of pollen tubes