Plant Cell Growth. As the world population increases and natural resources become more limited, it is critical that we identify, understand, and improve valuable traits in plants, such as robust and faster growth. This is critical because plants provide the basis for all our food, either directly or indirectly, it therefore becomes absolutely crucial that we understand the organizing principles of plant growth at the cellular level. Although a great deal is known about plant cell growth, we are only starting to understand how self-organizing intracellular structures, such as the cytoskeleton, participate in this process. Polarized cell growth is a specialized type of cell growth where expansion is restricted to the tip of elongated cells. This type of cell growth has been studied in great detail because it is fundamental for plant sexual reproduction, water and nutrient uptake, and for the propagation of ferns and mosses. Polarized cell growth is highly dependent on exocytosis and is regulated by the actin cytoskeleton, but how these two systems are coupled during tip growth is not well understood. It is thought that members of the myosin XI family of motors function as the link between the vesicle trafficking machinery and the actin cytoskeleton 
The central focus of my work at WPI has been investigating the function of myosin XI in polarized cell growth. Our detailed quantitative analysis of myosin XI and F-actin localization challenged the view that myosin XI simply transports secretory vesicles and suggested a more active function of myosin in actin organization. This unexpected observation resulted from our implementation of fluorescence fluctuation cross-correlation analysis of time series data. This allowed us to determine that myosin XI accumulation at the tip anticipates F-actin’s. At the same time, we identified ectopic foci of myosin-associated endomembrane structures that recruit actin filaments; these filaments propel them inside of the cell. These myosin-associated structures had not bee observed before in plants, and they only emerge when cells are recovering from treatment with reagents that cause actin de-polymerization. Together, our results support a model where myosin XI plays an active role in F-actin organization. To further explore this model, we have generated a moss line expressing a temperature sensitive allele of myosin XI. 


Chloroplast Photorelocation. Chloroplast photorelocation is an essential process in plants because it is required to maximize energy harvesting and to avoid photodamage. Several components relevant for light perception and actin-based motility have been identified, mainly in Arabidopsis thaliana, but chloroplast motility in mosses is more complicated. In addition to having the conserved actin-dependent pathway, mosses have a microtubule-dependent system that can work independently of the actin cytoskeleton. This allows for experimental separation of the two systems and to independently study the mechanisms that drive motility. One of our initial goals has been to identify the microtubule dependent motors responsible for chloroplast motility. With this end in view, we completed a comprehensive phylogenetic analysis of all the kinesin genes present in the Physcomitrella genome. Out of this phylogeny, several subfamilies emerged as possible candidates that we are currently testing for a function in chloroplast motility.