L. Andrew Staehelin - Professor, Retired

<p>We are interested in the functional organization and dynamic properties of the organelles and cytoskeletal arrays that produce new plant cell walls during cytokinesis and allow the cell walls to expand during cell growth. The structures involved include the ER, the Golgi apparatus, and three types of cytoskeletal arrays, the preprophase band, the interphase array of cortical microtubules, and the cell plate-forming structure known as phragmoplast. In addition, we are studying how the movement of statoliths in the gravisensing columella cells is transduced into a root growth response. One of the main objectives of our Golgi studies is to obtain a better understanding of the relationship between the structural diversity, the functional organization and the dynamic properties of the Golgi apparatus in different types of cells. By determining which elements are common to all Golgi, and which structures are unique, we will evaluate different hypotheses about the functional organization and dynamic behavior of the Golgi apparatus in different cell types. To this end we are combining studies of GFP/DsRed-labeled Golgi in living cells with high resolution (~7 nm) 3-D structural studies of Golgi preserved by cryofixation and analyzed with the help dual-axis electron tomography. The three systems we are investigating are yeast (Pichia pastoris and Saccharomyces cerevisiae), the scale-forming alga Scherffelia dubia, and different types of plant Golgi. All of our findings support the cisternal progression/maturation hypothesis of Golgi trafficking. In addition, we have shown that TGN cisternae are shed from Golgi stacks and can funtion as a semi-autonomous compartments prior to fragmentation. Our plant cell cytokinesis studies are focused on the structural characterization and functional identification of components of the cytokinetic apparatus of plant cells preserved by high pressure freezing and freeze-substitution techniques. This work has demonstrated that the process of cell plate formation involves a number of clearly defined stages and structural intermediates, and these insights are now forming the basis for predicting the sites of action of specific cell plate-associated molecules and for understanding the phenotypes of cytokinesis mutants. We have also shown that the mechanism of cell plate formation in syncytial endosperm cells differs from the mechanism described for somatic cells. Currently, we are in the process of determining the 3-D architecture of cell plates and associated cytoskeletal structures using dual-axis electron tomography methods. The resolution of this technique allows us to identify individual molecular complexes such clathrin triskelions, vesicle-associated kinesin molecules, dynamin rings and exocyst complexes.</p>