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

E Balugka et al.: Mitogen-activated protein kinase SIMK in alfalfa 267
the nucleus, some late-G2 cells showed association of
SIMK with preprophase bands. This could be brought
about by translocation of some of the nuclear SIMK
protein. Alternatively, because cell fractionation of
suspension-cultured cells had shown that SIMK is
present in both the nucleus and the cytoplasm
(Munnik et al. 1999), SIMK association with preprophase
bands may occur by recruitment of the
kinase from a diffuse cytoplasmic pool. Whatever the
mechanism, most investigations have so far revealed
that MAPKs are extremely dynamic molecules, and
green-fluorescent-protein fusion technology may be
necessary to follow changes in MAPK localization in
real time. Besides affecting the subcellular localization
of SIMK in dividing cells, the major effect of salt stress
was found in increased SIMK protein levels in both
dividing cells of the root apex and nondividing cells of
the transition and elongation zone. It remains to be
analyzed whether the increased amounts of SIMK in
these cells after salt stress are due tO increased gene
expression, mRNA, or protein stability.
Overall, our data show that both expression as well
as localization of SIMK in root tips is regulated in a
highly complex manner. Besides revealing a cell cycle
phase dependency, subcellular localization of SIMK
was also sensitive to salt stress. In addition to sal t
stress, SIMK may also be involved in sensing of other
environmental factors like temperature and mechanical
stress (unpubl. data). Future experimental work
will be necessary to reveal the physiological significance
of environmental-stress-induced changes in
amounts and intracellular localization of SIMK.
Although our present knowledge is scarce, the SIMK
pathway appears to be well suited to study the intracellular
dynamics of stress signaling in plants.
Acknowledgments
The work was supported grants from the Austrian Science Foundation
(P12188-GEN and P11729-GEN) and from the TMR program
of the European Union. This work was also supported by Grant
Agency VEGA, grant nr. 3009 from the Slovak Academy of Sciences.
Financial support to AGRAVIS by the Deutsche Agentur fiir
Raumfahrtangelegenheiten (Bonn, Federal Republic of Germany)
and the Ministerium fiir Wissenschaft und Forschung (D~sseldorf,
Federal Republic of Germany) is gratefully acknowledged.
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