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RESEARCH INTERESTS
Cell polarity; plant embryogenesis and development
Silvetia and Arabidopsis
cytoskeleton
PUBLICATIONS
We are studying development in the brown alga Silvetia compressa and in the angiosperm Arabidopsis thaliana. Our primary interest in the algal project is in the mechanisms by which an embryonic axis is established and expressed during the first cell cycle. This polarization is of fundamental importance because the spatial control of all subsequent development processes depends on a properly oriented axis. The goal is to understand the molecular, cellular and physiological basis of embryonic polarity. Because polarization is likely to involve many interrelated cellular processes, we take a multidisciplinary approach in our investigations.
At present, much of our work concerns the cytoskeleton. Cellular components that are uniformly distributed about the egg must become localized to one region of the cytoplasm during polarization, and cytoskeletal filaments mediate this reorganization. We are therefore investigating the dynamic reorganizations of microtubules and microfilaments during early development, and assessing their roles in embryogenesis. The emerging picture is that a branching network of dynamic F-actin, nucleated by an Arp2/3 complex, defines one pole of the axis. This Arp2/actin 'patch' serves as a target site for secretion, creating a specialized cortical domain at this pole. Microtubules emanating from centrosomes are captured by this cortical domain and exert force, which rotates the nucleus and aligns the spindle with the embryonic axis. Cytokinesis occurs centrifugally (as in plants) and bisects the spindle resulting in an asymmetric division invariantly oriented transverse to the axis.
These studies have inspired an investigation of microtubule search and capture proteins, specifically the recently discovered plus-end tracking proteins (+ TIPs). + TIPs preferentially bind microtubule plus ends, increasing microtubule dynamics and facilitating 'searching'. Localized (kinetochore, cell cortex, etc.) capture proteins bind directly or indirectly to + TIPs, thereby anchoring and stabilizing microtubules. We are studying the EB1 family of + TIPs in Silvetia and in Arabidopsis. Arabidopsis has three EB1 genes that are co-expressed in several organs and we are pursuing reverse genetic and cell biological approaches to understand their function in plant development. Since plants have unique microtubule arrays that control growth and division, we expect that the EB1 proteins may have evolved distinct functions in plants.
Selected publications
Hable, Whitney E. and Darryl L. Kropf (2004) Studies of the Arp2 protein and actin nucleation in fucoid zygotes. Submitted.
Bisgrove, Sherryl R., Henderson, David C. and Darryl L. Kropf (2003) Asymmetric division in fucoid zygotes is positioned by telophase nuclei. Plant Cell 15, 854-862.
Bisgrove, Sherryl R. and Darryl L. Kropf. (2001) Asymmetric cell division in fucoid algae; a role for the cell cortex in alignment of the mitotic apparatus. J. Cell Sci. 114, 4319-4328.
Hable, Whitney E. and Darryl L. Kropf (2000) Sperm entry induces polarity in fucoid zygotes. Development 127, 493-501.
Alessa, Lilian and Darryl L. Kropf (1999) F-actin marks the rhizoid pole of Pelvetia zygotes. Development 126, 201-209.
Kropf, D. L., Sherryl R. Bisgrove and Whitney E. Hable (1999) Establishing a growth axis in fucoid algae. Trends Plant Sci. 4: 490-494.
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