A comparative structural analysis of direct and indirect shoot ...

J. Samaj et al. / Cell Biology International 27 (2003) 257–259 259
actin stabilization with jasplakinolide, an inducer of
actin polymerization, SIMK co-localized with thick
actin cables in the cytoplasm. Importantly, latrunculin B
and jasplakinolide were both found to activate SIMK in
a root-derived cell culture (Samaj et al., 2002). Loss of
tip-focused SIMK and actin was induced by the MAPK
kinase inhibitor UO 126 and resulted in aberrant root
hairs. UO 126 inhibits polarized root hair growth by
changing polar vesicle trafficking and root hair cytoarchitecture.
After UO 126 treatment, arrest of hair
growth and vacuolation of root hair tips were observed
by video enhanced microscopy. Tubular small vacuoles
within the tip inflated to large roundish vacuoles that
oscillated in random motion, and the shape of the apical
dome became swollen. SIMK was distributed evenly in
cytoplasm and nuclei of UO 126 treated hairs without
any preferable accumulation in subcellular compartments
(Samaj et al., 2002). Thus, UO 126 inhibits root
hair tip growth by modifying the cytoarchitecture and
actin-dependent polar vesicle targeting and trafficking.
In contrast to SIMK inhibition, overexpression of gainof-function
SIMK induced rapid tip-growth of root
hairs and could bypass growth inhibition by UO 126.
3. Conclusions
An intact actin cytoskeleton and vesicle trafficking
are necessary for the proper localization of SIMK within
root hair tips. Depolymerization and stabilization of
F-actin activate SIMK, indicating that the MAPK is not
only associated with the actin cytoskeleton but its
activity is also directly affected by altering the dynamics
of F-actin. Inhibition of MAPK activity caused remodeling
and changes in the subcellular distribution of
actin and SIMK, resulting in tip-growth inhibition and
aberrant root hair morphology. On the other hand,
overexpression of gain-of-function SIMK in transgenic
plants resulted in increased root hair formation and
growth. Taken together, our data suggest that SIMK
plays an important role in root hair tip growth linking
MAPK signaling to actin cytoskeleton.
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