title: SAUR Inhibition of PP2C-D Phosphatases Activates Plasma Membrane H+-ATPases to Promote Cell Expansion in Arabidopsis.
abstract: The plant hormone auxin promotes cell expansion. Forty years ago, the acid growth theory was proposed, whereby auxin promotes proton efflux to acidify the apoplast and facilitate the uptake of solutes and water to drive plant cell expansion. However, the underlying molecular and genetic bases of this process remain unclear. We have previously shown that the SAUR19-24 subfamily of auxin-induced SMALL AUXIN UP-RNA (SAUR) genes promotes cell expansion. Here, we demonstrate that SAUR proteins provide a mechanistic link between auxin and plasma membrane H+-ATPases (PM H+-ATPases) in Arabidopsis thaliana. Plants overexpressing stabilized SAUR19 fusion proteins exhibit increased PM H+-ATPase activity, and the increased growth phenotypes conferred by SAUR19 overexpression are dependent upon normal PM H+-ATPase function. We find that SAUR19 stimulates PM H+-ATPase activity by promoting phosphorylation of the C-terminal autoinhibitory domain. Additionally, we identify a regulatory mechanism by which SAUR19 modulates PM H+-ATPase phosphorylation status. SAUR19 as well as additional SAUR proteins interact with the PP2C-D subfamily of type 2C protein phosphatases. We demonstrate that these phosphatases are inhibited upon SAUR binding, act antagonistically to SAURs in vivo, can physically interact with PM H+-ATPases, and negatively regulate PM H+-ATPase activity. Our findings provide a molecular framework for elucidating auxin-mediated control of plant cell expansion.
authors.affiliation: Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108.
authors.forename: Angela K
authors.initials: AK
authors.lastname: Spartz
authors.affiliation: Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108.
authors.forename: Hong
authors.initials: H
authors.lastname: Ren
authors.affiliation: Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108.
authors.forename: Mee Yeon
authors.initials: MY
authors.lastname: Park
authors.affiliation: Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108.
authors.forename: Kristin N
authors.initials: KN
authors.lastname: Grandt
authors.affiliation: Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108.
authors.forename: Sang Ho
authors.initials: SH
authors.lastname: Lee
authors.affiliation: Department of Plant Science and Landscape Architecture, University of Maryland, College Park, Maryland 20742.
authors.forename: Angus S
authors.initials: AS
authors.lastname: Murphy
authors.affiliation: Biotechnology Center and Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Sussman
authors.affiliation: Department of Biology, Macalester College, St. Paul, Minnesota 55105.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Overvoorde
authors.affiliation: Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108 grayx051@umn.edu.
authors.forename: William M
authors.initials: WM
authors.lastname: Gray