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Proteomics Approach Identifies Adenosine Kinase as a Component in Arabidopsis Root Gravitropism.  L.-S. Young1 and P.H. Masson2 1Plant Breeding and Plant Genetics Program, and 2Lab of Genetics, Univ. of Wisconsin-Madison, Madison, WI.

   A comparative proteomics approach has been utilized here to identify Arabidopsis root tip proteins that are differentially represented early in response to gravi-stimulation (GS).  A combination of 2-DE and mass spectrometry were used to identify four of those differentially represented cytosolic protein spots as adenosine kinase (ADK), Arabidopsis Response Regulator 22 (ARR22), unknown function protein (At3g20920), and O-acetyl-serine (thiol) lyase.  ADK protein spot intensity increased 1.9-fold after 12 min of GS and reduced to very low and/or undetectable levels after 30 min of GS when the elongation zones had initiated a curvature response.  Analysis of mutations in two ADK isoforms in the Arabidopsis genome revealed that only a T-DNA insertion in the ADK1 gene resulted in a reduced reorientation kinetics in the roots as well as a decreased sensitivity to gravistimulation, as indicated by a longer presentation time.  Moreover, adk1-1 was unable to maintain a closed apical hook in dark. This hook phenotype could not be rescued by the application of ACC.  Interestingly, adk1-1 seedlings reveal distorted columella cell morphology.  PIN3 localization in adk1-1 root columella cells is mis-localized and appears to display no relocalization after GS.  The results presented here indicate that ADK1 might participate in the root gravitropic signal transduction pathway that leads to the establishment of a lateral auxin gradient. Reorientation kinetics assay of double mutants suggests that ADK1 operate in distinct pathways involving ARG1 and PGM1.

(Supported by NASA NAG2-1602)