Lin, Y. and E. Grotewold

Department of Plant Biology and Plant Biotechnology Center, The Ohio State University, Columbus Ohio.

We have used flavonoid biosynthesis as a model system to investigate plant metabolic pathways and the mechanisms by which plants control gene expression. In maize, two branches of flavonoid biosynthesis are independently regulated by the Myb-domain proteins P and C1, providing one of the best understood regulatory networks in plants. The ectopic expression of the P and C1 regulators in cultured maize Black Mexican Sweet (BMS) cells provides an attractive alternative to engineering plant secondary metabolite production. The analysis of the compounds induced by the ectopic expression of these Myb factors indicated that they play novel regulatory functions, not obvious from previous studies. Autofluorescent compounds induced by P provide unique markers to investigate trafficking and subcellular localization of secondary metabolites. P induces the accumulation of two fluorescent compounds targeted to distinct subcellular structures. A yellow fluorescent compound accumulates in discrete bodies within the vacuolar compartment. Bodies filled with a green fluorescent compound accumulate, upon P induction, in the cytoplasm of BMS cells. These bodies move to the cell surface, fuse with the cell membrane, and release the green fluorescent compound to the cell wall. Golgi-disturbing agents don’t inhibit the transport of these bodies to the cell membrane. Rather, the number of cells with green fluorescent bodies and green fluorescence in the cell wall increases. In contrast, the transport of the yellow fluorescent compound to the vacuole is not affected by these treatments. Together, our findings suggest that the transport of plant secondary metabolites involves novel trafficking pathways. Unique tools to dissect these pathways have now become available.