Because plant cells are surrounded by rigid cell walls that prevent cell migration or movement, organization of plant tissues and organs during growth and development is achieved by directed cell expansion and cell division.  During cell division, cells can theoretically divide along any number of possible division planes.  My research focuses on how cells execute a chosen division plane by controlling the cytoskeletal structures that form and breakdown sequentially throughout the cell cycle.  In plant cells, the plane of the division is specified prior to mitosis by the location of a cortical band of microtubules and microfilaments called the preprophase band (PPB).  The PPB encircles the mother cell at the future plane of division and breaks down prior to the start of mitosis.  A variety of experiments support the hypothesis that the PPB “marks” the mother cell cortex establishing the cortical division site.  During cytokinesis, another plant-specific cytoskeletal structure called the phragmoplast arises from the remnants of the mitotic spindle.  The phragmoplast deposits the new cell wall as it expands towards the cortical division site previously established by the PPB (for review of the above processes, see Muller et al., 2009; Wright and Smith 2007). 

           

            To gain insight into the forces that modulate cell division orientation in plants by regulating these cytoskeletal structures, we have been working with Zea mays (maize) mutants that have disrupted division plane orientations in the leaf epidermis. 

All of the mutants we study have a similar phenotype that is manifested by abnormally shaped subsidiary cells in the leaf epidermis.  Subsidiary cells are a great model for studies of division plane orientation because the mother cells that give rise to them have an invariant division plane making it simple to identify cells that have divided incorrectly.  One mutant, dcd1, has been cloned and encodes a regulatory subunit of the conserved PP2A phosphatase (Wright et al., 2009).  DCD1, and a related protein, ADD1, are required for PPB formation in maize.  These proteins are thought to direct the PP2A phosphatase complex to a particular sub-cellular location.  We are in the process of cloning two additional mutants, dcd2 and dcd3, both of which have phenotypes reminiscent of dcd1.  Positional cloning of dcd2 has narrowed the gene down to a ~2.1 MB interval which contains approximately 100 genes.  The phenotype of dcd3 is caused by two independent mutations, both of which have been localized to different chromosomes.  The molecular identity of these genes will suggest future avenues of research.