Gravity-regulated differential auxin transport from columella to lateral root cap cells

^*Institut für Biologie II, Universität Freiburg, D-79014 Freiburg, Germany Europe; ^‡Department of Plant Biology, Ohio State University, Columbus, OH 43210; and ^¶Department of Forest Genetics and Plant Physiology, Umeå Plant Science Center, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden Europe

Communicated by Josef S. Schell, Max Planck Institute for Plant Breeding Research, Cologne, Germany (received for review November 10, 2002)

Abstract

Gravity-induced root curvature has long been considered to be regulated by differential distribution of the plant hormone auxin. However, the cells establishing these gradients, and the transport mechanisms involved, remain to be identified. Here, we describe a GFP-based auxin biosensor to monitor auxin during Arabidopsis root gravitropism at cellular resolution. We identify elevated auxin levels at the root apex in columella cells, the site of gravity perception, and an asymmetric auxin flux from these cells to the lateral root cap (LRC) and toward the elongation zone after gravistimulation. We differentiate between an efflux-dependent lateral auxin transport from columella to LRC cells, and an efflux- and influx-dependent basipetal transport from the LRC to the elongation zone. We further demonstrate that endogenous gravitropic auxin gradients develop even in the presence of an exogenous source of auxin. Live-cell auxin imaging provides unprecedented insights into gravity-regulated auxin flux at cellular resolution, and strongly suggests that this flux is a prerequisite for root gravitropism.

gravitropic root curvature‖polar auxin transport‖auxin carrier proteins

Footnotes

↵ † I.O. and P.W. contributed equally to this work.
↵ § Present address: Department of Botany–Microbiology, Ohio Wesleyan University, Delaware, OH 43015.
↵ ‖ To whom correspondence should be addressed. E-mail: klaus.palme{at}biologie.uni-freiburg.de.
Abbreviations:

EZ,

elongation zone;

IAA,

indole-3-acetic acid;

CI,

columella initial;

QC,

quiescent center;

1-NAA,

1-naphthyl-acetic acid;

2,4-D,

2,4-dichloro-phenoxyacetic acid;

NPA,

1-naphthylphthalamic acid;

1-NOA,

1-naphthoxyacetic acid;

BFA,

brefeldin A;

LRC,

lateral root cap;

dLRC,

distal LRC;

pLRC,

proximal LRC