Our research is broadly focused on deciphering the molecular mechanisms underlying growth and development of flowering plants. Characterization of sterol biosynthesis mutants of Arabidopsis led to the emerging view that sterols in addition to brassinosteroids play critical roles in patterning during development. What is the connection between sterol biosynthesis and signaling networks in plants? Of key importance will be the identification of sterol signals and their binding sites.
Our long-term goals are to elucidate the signaling roles of sterols in plant growth and development. We are using Arabidopsis and Mendel's pea as model systems to study the mechanisms underlying the functions of sterols in embryogenesis. Sterol biosynthesis enzymes are differentially expressed in the embryo, and sterol contents appear to vary temporally and spatially in development. Sterol production may reflect the metabolic state of cells, which in turn can be exploited to regulate developmental processes such as cell-type differentiation. Such a mechanism would ensure a proper sequence of events in controlling cell fate decisions in the developing organism.
START lipid/sterol binding domains in plants
Candidate sterol-binding proteins include plant-specific transcription factors of the homeodomain class termed HD-Zip-START, also known as Class III and IV HD-Zip. Proteins of this family contain a DNA-binding domain, the homeodomain (HD) associated with a leucine zipper dimerization domain (Zip or ZLZ), and a putative lipid/sterol binding domain (START). START (for StAR-related lipid transfer) domains were first discovered in mammalian steroidogenic acute regulatory protein orthologs (StARs). In humans, misexpression of several START domain proteins is associated with cancer. It is hypothesized that START domains are critical in linking lipid metabolism to cell division.
The presence of START domains in transcription factors reveals a potential mechanism by which lipid/sterol ligands regulate gene transcription in plants. HD-Zip-START transcription factors are implicated in cell differentiation during development. Several correspond to striking mutant phenotypes in Arabidopsis and typically have layer-specific expression patterns. One postulate is that ligand binding to the START domain controls transcription factor activity. We are currently investigating the role of START domains in this family of HD transcription factors using genetic, molecular, and biochemical approaches in Arabidopsis.
Sterols and cellulose synthesis
Our discovery that sterol biosynthesis is crucial for cellulose production suggests an underlying mechanism by which sterols influence the cellulose synthase machinery. One proposed model suggests that steryl glucosides act as primers in cellulose synthesis, by supplying glucose residues to growing cellodextrin polymers. We are characterizing sterol glucosyltransferase mutants of Arabidopsis to address this hypothesis. If specific steryl glucosides and/or sterols are critical for cellulose synthesis, their modulation may yield improved cellulose production in biomass crops.
Khosla, A., Paper, J.M., Boehler, A.P., Bradley, A.M., Neumann, T.R., and Schrick, K. 2014. HD-Zip proteins GL2 and HDG11 have redundant functions in Arabidopsis trichomes and Gl2 activates a positive feedback loop via MYB23. Plant Cell 26:2184-2200.
Schrick, K., DeBolt, S., and Bulone, V. 2012. Deciphering the molecular functions of sterols in cellulose biosynthesis. Front. Plant Sci. 3:84.
Schrick, K., Shiva, S., Arpin, J.C., Delimont, N., Isaac, G., Tamura, P. and Welti, R. 2012. Steryl glucoside and acyl steryl glucoside analysis of Arabidopsis seeds by electrospray ionization tandem mass spectrometry. Lipids 47:185-193.
Schrick, K., Cordova, C., Li, G., Murray, L. and Fujioka, S. 2011. A dynamic role for sterols in embryogenesis of Pisum sativum. Phytochemistry 72: 465-475.
DeBolt, S., Scheible, W.-R., Schrick, K., Auer, M., Beisson, F., Bischoff, V., Bouvier-Navé, P., Carroll, A., Hematy, K., Li, Y., Milne, J., Nair, M., Schaller, H., Zemla, M. and Somerville, C. 2009. Mutations in UDP-glucose:sterol-glucosyltransferase in Arabidopsis cause transparent testa phenotype and suberization defects in seeds. Plant Physiology 151: 78-87.
Venkata, B.P. and Schrick, K. 2006. START domains in lipid/sterol transfer and signaling in plants. In C. Benning and J. Ohlrogge (eds.): Current Advances in Biochemistry and Cell Biology of Plant Lipids: Proceedings of the 17th International Symposium on Plant Lipids, Michigan State University Press, East Lansing, Michigan. ISPL2006, pp. 57-61.
Schrick, K., Nguyen, D., Karlowski, W.M. and Mayer, K.F.X. 2004. START lipid/sterol binding domains are amplified in plants and are predominately associated with homeodomain transcription factors. Genome Biology 5: R41.
Schrick, K., Fujioka, S., Takatsuto, S., Stierhof, Y.-D., Stransky, H., Yoshida, S. and Jürgens, G. 2004. A link between sterol biosynthesis, the cell wall and cellulose in Arabidopsis. Plant J. 38: 227-243.
Schrick, K., Mayer, U., Martin, G., Bellini, C., Kuhnt, C., Schmidt, J. and Jürgens, G. 2002. Interactions between sterol biosynthesis genes in embryonic development of Arabidopsis. Plant J. 31: 61-73. (article featured in cover illustration)
Schrick, K. and Laux, T. 2001. Zygotic Embryogenesis: The formation of an embryo from a fertilized egg. In S.S. Bhojwani & W.Y. Soh (eds.) Current Trends in the Embryology of Angiosperms, Kluwer Academic Publishers, Dordrecht, pp. 249-277.
Schrick, K. 2000. Perspective: Plant developmental biologists show their colors: Toward a virtual understanding of green
development. Science´s STKE (Dec. 5).
Schrick, K., Mayer, U., Horrichs, A., Kuhnt, C., Bellini, C., Dangl, J., Schmidt, J. and Jürgens, G. 2000. FACKEL is a sterol C-14 reductase required for organized cell division and expansion in Arabidopsis embryogenesis. Genes Dev. 14: 1471-1485. (article featured in cover illustration)
Schrick, K., Garvik, B. and Hartwell, L.H. 1997. Mating in Saccharomyces cerevisiae: The role of the pheromone signal transduction pathway in the chemotropic response to pheromone. Genetics 147: 19-32.
Dorer, R., Pryciak, P., Schrick, K. and Hartwell, L.H. 1994. The induction of cell polarity by pheromone in Saccharomyces cerevisiae. Harvey Lect. 90: 95-104.
Bennetzen, J. L., Schrick, K., Springer, P.S., Brown, W.E. and SanMiguel, P. 1994. Active maize genes are unmodified and flanked by diverse classes of modified, highly repetitive DNA. Genome 37: 565-576.