ABCB11 functions with B1 and B19 to regulate rootward auxin transport

Reemmer, Jesica Elyse M.S., Purdue University, August 2014. ABCB11 functions with B1 and B19 to regulate rootward auxin transport. Major Professor: Angus Murphy. Auxin transport is essential for the architecture and development of erect plants. In a network of transporters directing auxin flows, ATP-Binding Cassette (ABC) transporters are a ubiquitous family of proteins that actively transport important substrates, including auxins, across the plasma membrane. ABCB1 and ABCB19 have been shown to account for the majority of rootward auxin transport, but residual fluxes to the root tip in Arabidopsis b1b19 double mutants implies the involvement of at least one additional auxin transporter in this process. Of specific interest, the severe dwarfism seen in abcb1abcb19 is strikingly reminiscent of that seen in mutants defective in the FK506-binding protein 42 (FKBP42), known as TWISTED DWARF1 (TWD1). FKBPs function in the maturation and stabilization of proteins, and biochemical evidence indicates that TWD1 functions in ABCB protein maturation and activation in particular. However, although b1b19 largely phenocopies twd1, the relative severity of the twd1 phenotype further suggests TWD1 activity may regulate the missing rootward auxin transporter. A broad screen including 12 ABCBs now reveals that ABCB11 acts in concert with ABCB1 and ABCB19 in long-distance transport, with an additional role in basipetal auxin transport in leaf tissues. Support for this conclusion comes from analyses of ABCB11 expression, ABCB11 protein localization and interaction, growth phenotypes of b11 single and abcb1b19b11 triple mutants, and auxin transport and accumulation mediated by ABCB11. The comparative analysis of the ABCB knock-out lines with twd1-3 now provides the means to deconvolute the relationship between auxin, ABCB transporters and FKBP42, as well as the mechanisms leading to the phenotypes seen in the abcb and twd1 mutants. My work thus concludes both that B11 mediates long-distance rootward auxin efflux and that such function by ABCB transporters is crucial to describing the twd phenotype. Future uses of this work include the possibility of customizing plant architecture through the manipulation of substrate specificity and transport directionality of ABCB transporters.