Symposium I: Tropisms

Abs # 10002: Auxin transport

Presenter:	Palme, Klaus , klaus.palme@biologie.uni-freiburg.de
Authors	Palme, Klaus  (A)   Ottenschläger, Iris  (A)   Wolff, Patricia  (A)   Santos, Filipa  (A)   Wanke, Dierk  (B)   Tietz, Olaf  (A)
Affiliations:	(A): Institut fuer Biologie, Universitaet Freiburg, Schaenzlestr. 1, D-79104 Freiburg, Germany (B): Botanisches Institut, Universitaet Koeln, Gyrhofstr.15, D-50931 Koeln

Auxin is a major regulator in plant development and regulated auxin transport is thought to be important to mediate its effects. We have studied a family of genes encoding auxin efflux carrier proteins. In Arabidopsis all eight members encode membrane proteins with up to 12 putative transmembrane segments and similarities with the major facilitator superfamily. These proteins are important tools for the analysis of auxin transport, cell polarity and developmental processes. In this presentation we will focus on the analysis of gravity signalling. 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. A GFP-based auxin biosensor was developed to monitor auxin during Arabidopsis root gravitropism at cellular resolution. Our data suggest that elevated auxin levels occur at the root apex in columella cells. This was also confirmed by mass spectrometric analysis. From the site of gravity perception an asymmetric auxin flux occurs to the lateral root cap (LRC) and towards the elongation zone (EZ), upon gravistimulation. Using this auxin biosensor we were able to 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 EZ. It was also possible to demonstrate that endogenous gravitropic auxin gradients develop even in the presence of an exogenous source of auxin. Live cell auxin imaging provides insights into gravity regulated auxin flux at cellular resolution, and strongly suggests that this flux is a prerequisite for root gravitropism. To study the function of PIN proteins we have expressed them in several expression systems and measured auxin efflux to proof its suggested function. Overexpression of AtPIN1in Arabidopsis revealed phenotypes indicating altered auxin homeostasis/transport in roots. Measurement of auxin transport in Arabidopsis stems suggests a regulatory function for AtPIN1 in auxin transport. We are now aiming to identify and characterize its partners using yeast based screening systems suitable to detect membrane protein interactions.