75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

387 Molecular Functions of ARL2 and ARG1 in Arabidopsis Gravitropism
Benjamin Harrison, Changhui Guan, Patrick Masson
University of Wisconsin - Madison
Root gravitropism is an important model for studying signal transduction as well as fundamental aspects of plant growth and
behavior. Root gravitropism involves the sensing of the gravity vector within the columella cells of the root cap, transduction
of that information into a biochemical signal, transmission of that signal to the cells in the elongation zone, and a differential
growth response.
ARG1 and its paralog ARL2 are mediators of early gravity signaling in Arabidopsis roots and hypocotyls. Mutations in
either gene specifically result in reduced kinetics of gravitropic bending in both organs. Analysis of arg1 mutants indicates
that ARG1 is active in gravity-sensing cells, where it is required for the fast and transient alkalinization of the cytoplasm upon
gravity-stimulation (1). In addition ARG1 and ARL2 are required for the development of a lateral gradient of auxin across the
root cap of gravity-stimulated roots (1 and herein). The development of a lateral auxin gradient following gravity stimulation
appears to be essential for gravitropism and is thought to be mediated by the relocalization of PIN3 within gravity-sensing
cells (2). We show that both ARG1 and ARL2 are required for relocalization of PIN3 to the new bottom of gravity-stimulated
columella cells. We also demonstrate that mutations in ARG1 eliminate the contribution of PIN3 to the gravitropic response.
ARG1 and ARL2 encode DnaJ-like proteins, and along with another member (ARL1), comprise a small gene family in
Arabidopsis. DnaJ proteins are known to interact with DnaK proteins and through this interaction modify the folding, activity, or
localization of other target proteins. Sub-cellular localization of an ARL2-GFP fusion is very similar to ARG1 fusions indicating
that they are both associated with several cellular membranes throughout the vesicle trafficking pathway (1 and herein).
In order to investigate the molecular function of ARG1 and ARL2 we have sought to identify physical interactors via
yeast 2 hybrid. We present the identification and characterization of a protein which interacts strongly with ARL2 in yeast.
Further characterization of this and other related proteins in Arabidopsis will help define the molecular mechanisms of ARG1
and ARL2 function.
1. Boosirichai et al., Plant Cell (2003) 11:2612-25
2. Friml et al., Nature (2002) 415:806-9
388 Characterization of a Herbivore-Inducible Arabidopsis Terpene Synthase Responsible for
the Formation of the Volatile Homoterpene TMTT
Marco Herde 1 , Katrin Gaertner 1 , Iris Camehl 1 , Benjamin Fode 1 , Dorothea Tholl 2 , Jonathan Gershenzon 3 , Christiane
Gatz 1
1
General and Developmental Plant Physiology, Georg-August-University Goettingen, Germany, 2 Department
of Biological Sciences, Virginia Tech University, Blacksburg, Virginia, United States, 3 Department of
Biochemistry, Max-Planck-Institute for Chemical Ecology, Jena, Germany
Volatiles play an important role in plant-plant and plant-insect interactions. A major component of the A. thaliana
volatile blend is the C16 homoterpene TMTT (4,8,12-trimethyltrideca-1,3,7,11-tetraene). TPS28 is as a key enzyme in
the biosynthesis of the TMTT precursor geranyl-linalool as revealed by the analysis of knock-out and overexpressing
plants. Consitutive expression of TPS28 leads to retarded growth and lesions on the first cotyledons indicating fitness
costs at this early developmental stage. TPS28 is constitutively expressed in parts of the flowers and siliques. In vegetative
tissues, TPS28 transcription is induced by the fungal elicitor alamethicin, by feeding of P. xylostella larvae, and by the
coronatine derivative coronalon. Induction requires an octadecanoid-derived signal as well as the F-box protein COI1.
In addition, TPS28 is induced by wounding. Remarkably, the wounding response occurs even in the absence of COI1. It
has been suggested that COI1 is involved in shuttling of yet unknown negative regulators to the proteasome. Inhibition
of protein biosynthesis by cycloheximide led to TPS28 transcription indicating that inhibition of the synthesis of a labile
repressor might be sufficient for induction. Interestingly, induction by cycloheximide requires COI1, indicating that COI1
is involved in the basal turnover of the postulated negative regulator. Our model suggests an accelerated degradation of
the repressor after activation of the octadecanoid pathway. In order to identify this labile repressor a screen for promoterup
mutants was established using chimeric TPS28::luciferase and TPS28:: phosphinotrycin acetyl transferase constructs.
Promoter deletion analysis using GUS as a reporter has shown that 300 bps upstream of the putative transcription start site
are sufficient for the induction by cycloheximide, alamethicin and wounding. Within this minimal promoter we identified
62 bps which are necessary for induction with cyloheximide and alamethicin. Investigation of putative cis elements is in
progress. These will be used to identify regulatory proteins using the yeast-one-hybrid approach.