Minisymposium 19: Membrane Transport

Abs # 33001: The multidrug-resistance associated protein ZmMRP1 is involved in anthocyanin sequestration in Zea mays

Presenter:	Goodman, Christopher D., cdgood@stanford.edu
Authors	Goodman, Christopher D. (A)   Walbot, Virginia  (A)
Affiliations:	(A): Stanford University

Anthocyanin biosynthesis is one of the most well-studied enzymatic pathways in biology but little remains known about the molecular mechanisms of its final stage; the transport of the anthocyanin pigment into the vacuole. Genetic, biochemical and expression studies suggest that anthocyanin sequestration requires the activity of a specific anthocyanin transporter, with the most likely candidate being a member of the ATPase-binding cassette (ABC) protein family. We have identified an ABC protein - the multidrug-resistance associated protein ZmMRP1 - that is involved in anthocyanin transport. ZmMRP1 expression is controlled by the regulators of anthocyanin biosynthesis and mirrors the expression of other anthocyanin structural genes. Expression of a GFP-ZmMRP1 fusion construct in vivo shows that the ZmMRP1 protein is localized to the tonoplast, the site at which anthocyanin transport occurs. Mutants generated using antisense constructs have a distinct beige pigmentation phenotype in the adult plant that results from a significant reduction in anthocyanin levels but no alteration in the chemical structure of the anthocyanin species produced. This indicates that the anthocyanin biosynthetic pathway is intact and suggests that the pigmentation phenotype is the result of a defect in the vacuolar sequestration of anthocyanin. This conclusion is supported by the similarity in the phentoypes seen in ZmMRP1 antisense mutants and maize mutants lacking the glutathione S-transferase Bronze2. The bronze2 mutation prevents vacuolar sequestration of anthocyanin and results in the oxidation of cytoplasmically localized anthocyanin to a bronze-colored compound. Surprisingly, no aleurone phenotype was observed in the ZmMRP1 antisense mutant plants. This appears to result from the presence of second, highly homologous gene - ZmMRP2 - that is also co-regulated with the anthocyanin pathway but is expressed exclusively in the aleurone. The expression of ZmMRP2 is unaffected by the antisense transgene. The identification of ZmMRP1 as an anthocyanin transporter is the first description of a plant MRP with a known endogenous substrate. As such, ZmMRP1 provides a new model for examining the biological and biochemical mechanisms involved in the MRP-mediated transport of plant secondary metabolites from the site of synthesis to their final site of deposition in the cell. Research supported by NSF Grant IBN0083221. CDG was supported in part by a NSERC PGS-B fellowship.