Randall A. Kerstetter

Much of this variation in leaf shape reflects differing patterns of leaf development that are under genetic control.  Leaves formed by an individual plant can also display remarkable diversity, both in shape, and in the expression of a wide range of other traits.  My research interests center on elucidating the genetic and developmental basis for variation in leaf form and uncovering the underlying mechanisms that guide leaf morphogenesis.

The specification of leaf sidedness

The leaves of most plants exhibit striking structural and functional differences between their upper (adaxial or dorsal) and lower (abaxial or ventral) surfaces.  This sidedness in leaves, otherwise known as adaxial-abaxial polarity, is apparent in the size and shape of cells in different tissue layers, and in the distribution of differentiated cell types, such as xylem and phloem, stomata and trichomes (leaf hairs). In Arabidopsis, this polarity difference is particularly obvious in the first two rosette leaves that possess trichomes on their adaxial surface but not on their abaxial surface.

Loss-of-function mutations in the KANADI gene result in a partial loss of abaxial character in Arabidopsis leaves.  KANADI encodes a plant-specific transcription factor that is expressed on the abaxial side of developing leaves and floral organs and in the peripheral cells of the early embryo.

Over-expression of KANADI dramatically disrupts leaf polarity and normal embryonic patterning.

Specifically, misexpression of KANADI abaxializes embryonic leaves (cotyledons) and completely suppresses the formation of the embryonic shoot apical meristem and vascular tissue of the embryonic stem (hypocotyl). These observations indicate that KANADI is critical for specifying both abaxial identity in leaves and peripheral identity in the developing embryonic shoot.

Current research projects address the genetic mechanisms by which adaxial-abaxial polarity is established and maintained during leaf development.  The KANADI gene provides an important tool with which to identify other genes and determinants of this process.  We are working on finding upstream regulators of KANADI expression, genetic enhancers and suppressors of the kanadi mutant phenotype, downstream targets of KANADI, and gene products that interact with KANADI that will yield other key players in this developmental pathway.

A complementary approach to identify determinants of adaxial-abaxial polarity in Arabidopsis takes advantage of a set of transgenic lines that express the yeast transcriptional activator GAL4-VP16 in tissue- or cell-specific patterns.  Similar lines have been widely used for regulating gene expression in Drosophila and, more recently, in mice.  This approach involves fusing the coding region of a gene-of-interest (GOI) to a GAL4 upstream-activating-sequence (UAS), introducing this gene into plants by transformation, and then crossing this UAS:GOI line to a line that expresses GAL4-VP16 in a desired pattern.

  Jim Haseloff designed an "enhancer trap" vector containing both a promoterless GAL4-VP16 and the reporter gene UAS:GFP that has been used to generate lines with stable patterns of GFP expression in the root, shoot, and inflorescence (see http://enhancertraps.bio.upenn.edu/). Several of these lines display differential GFP expression in adaxial or abaxial tissues.

I have produced a set of vectors that will facilitate the use of this system for manipulating gene expression, mosaic analysis, genetic ablation studies, and tissue-specific activation tagging mutagenesis. Currently, my lab is identifying genes involved in leaf morphogenesis by characterizing gain-of-function mutants caused by the misexpression of genes under the control of an enhancer trap GAL4 that is expressed in abaxial leaf tissue.