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Published on August 4, 2006; 10.1104/pp.106.084798

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Received June 7, 2006
Accepted July 26, 2006

Adenosine Kinase Modulates Root Gravitropism and Cap Morphogenesis in Arabidopsis

Li-Sen Young , Benjamin R. Harrison , Narayana Murthy UM , Barbara A. Moffatt , Simon Gilroy , and Patrick H. Masson *

Laboratory of Genetics, University of Wisconsin-Madison, 425-G Henry Mall, Madison, WI 53706, USA; Plant Breeding and Plant Genetics Program, University of Wisconsin-Madison, 425-G Henry Mall, Madison, WI 53706, USA
Laboratory of Genetics, University of Wisconsin-Madison, 425-G Henry Mall, Madison, WI 53706, USA
Department of Biology, University of Waterloo, Ontario, Canada N2L 3G1
Biology Department, Pennsylvania State University, 208 Mueller Laboratory, University Park, PA 16802, USA

* Corresponding author; email: phmasson{at}wisc.edu.

Adenosine kinase (ADK) is a key enzyme that regulates intra- and extra-cellular levels of adenosine, thereby modulating methyltransferase reactions, production of polyamines and secondary compounds, and cell signaling in animals. Unfortunately, little is known about ADK's contribution to the regulation of plant growth and development. Here, we show that ADK is a modulator of root cap morphogenesis and gravitropism. Upon gravistimulation soluble ADK levels and activity increase in the root tip. Mutation in one of two Arabidopsis ADK genes, ADK1, results in cap morphogenesis defects, along with alterations in root sensitivity to gravistimulation and slower kinetics of root gravitropic curvature. The kinetics defect can be partially rescued by adding spermine to the growth medium, whereas the defects in cap morphogenesis and gravitropic sensitivity cannot. The root morphogenesis and gravitropism defects of adk1-1 are accompanied by altered expression of the PIN3 auxin efflux facilitator in the cap, and decreased expression of the auxin-responsive DR5-GUS reporter. Furthermore, PIN3 fails to re-localize to the bottom membrane of statocytes upon gravistimulation. Consequently, adk1-1 roots cannot develop a lateral auxin gradient across the cap, necessary for the curvature response. Interestingly, adk1-1 does not affect gravity-induced cytoplasmic alkalinization of the root statocytes, suggesting either that ADK1 functions between cytoplasmic alkalinization and PIN3 relocalization in a linear pathway, or that the pH and PIN3-relocalization responses to gravistimulation belong to distinct branches of the pathway. Our data are consistent with a role for ADK and the S-adenosyl-L-methionine (AdoMet) pathway in the control of root gravitropism and cap morphogenesis.




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