Regulation of Plant Morphogenesis During Growth & Development
Plant forms have long delighted artists and naturalists with their variety and beauty. These forms arise through morphogenesis in a process that depends on growth. Cells specify their growth rates in each spatial dimension and these rates are usually different from one another, that is, the growth of plant cells is anisotropic. To build an organ with a defined shape, the plant must control precisely the direction of maximal expansion and the magnitude of expansion anisotropy. Understanding the mechanisms whereby plant cells govern growth anisotropy is the crux of my research.
To unearth these mechanisms I am digging in three types of terrain. The first is to understand how cell division and expansion are regulated coordinately. Cell division supplies the plant with building blocks whereas cell expansion determines the shape of the blocks and hence of the whole structure. These processes must be coordinated precisely for morphogenesis to succeed, but those interested in division have typically ignored expansion, and vice versa. My laboratory is quantifying the spatial profiles of cell expansion and division at high spatio-temporal resolution and studying how these change in different environments or in different genetic backgrounds. As part of this effort, I collaborated with a computer scientist to develop a novel image processing routine allowing growth profiles to be measured algorithmically.
The second terrain is the role of the cytoskeleton in regulating anisotropic expansion. For years, the cytoskeleton has been known to be important for morphogenesis by virtue of the aberrant morphology that results when the cytoskeleton is disrupted by chemical inhibitors. But how does the cytoskeleton act? This question requires more than inhibitors to answer. My laboratory has isolated mutants of arabidopsis in which root morphology is aberrant and we are using those to identify proteins that make up the pathway for the control of organ shape. Additionally, we have designed a novel in vitro assay specifically for cortical microtubules, where there behavior can be studied readily and the function of putative players tested directly.
The third terrain is the cell wall, the ultimate regulator of cell and organ shape. Cells can expand anisotropically only when the cell wall is mechanically anisotropic. The mechanical anisotropy is provided by cellulose microfibrils, long polymers of glucose crystallized into microfibrils with the tensile strength of steel; however, it is not known how cellulose alignment is controlled. In addition to the mutational approach mentioned above, my laboratory uses several approaches to study the ultrastructure of the cell wall, including quantitative polarized-light microscopy, field-emission scanning electron microscopy, and atomic force microscopy. The overall goal here is to uncover how anisotropic wall yielding is conditioned by the structural elements of the cell wall.