JXB Advance Access published online on September 26, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm190
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RESEARCH PAPER |
In planta regulation of the Arabidopsis Ca2+/H+ antiporter CAX1
1Molecular and Environmental Plant Science, Texas A&M University, College Station, Texas 77843, USA
2United States Department of Agriculture–Agricultural Research Service Children's Nutrition Research Center, Baylor College of Medicine, 1100 Bates Street, Houston, Texas 77030, USA
3Faculty of Life Sciences, University of Manchester, 3.614 Stopford Building, Oxford Road, Manchester M13 9PT, UK
4Vegetable and Fruit Improvement Center, Texas A&M University, College Station, Texas 77845, USA
* To whom correspondence should be addressed at the Baylor College of Medicine: E-mail: kendalh{at}bcm.tmc.edu
Vacuolar localized Ca2+/H+ exchangers such as Arabidopsis thaliana cation exchanger 1 (CAX1) play important roles in Ca2+ homeostasis. When expressed in yeast, CAX1 is regulated via an N-terminal autoinhibitory domain. In yeast expression assays, a 36 amino acid N-terminal truncation of CAX1, termed sCAX1, and variants with specific mutations in this N-terminus, show CAX1-mediated Ca2+/H+ antiport activity. Furthermore, transgenic plants expressing sCAX1 display increased Ca2+ accumulation and heightened activity of vacuolar Ca2+/H+ antiport. Here the properties of N-terminal CAX1 variants in plants and yeast expression systems are compared and contrasted to determine if autoinhibition of CAX1 is occurring in planta. Initially, using ionome analysis, it has been demonstrated that only yeast cells expressing activated CAX1 transporters have altered total calcium content and fluctuations in zinc and nickel. Tobacco plants expressing activated CAX1 variants displayed hypersensitivity to ion imbalances, increased calcium accumulation, heightened concentrations of other mineral nutrients such as potassium, magnesium and manganese, and increased activity of tonoplast-enriched Ca2+/H+ transport. Despite high in planta gene expression, CAX1 and N-terminal variants of CAX1 which were not active in yeast, displayed none of the aforementioned phenotypes. Although several plant transporters appear to contain N-terminal autoinhibitory domains, this work is the first to document clearly N-terminal-dependent regulation of a Ca2+ transporter in transgenic plants. Engineering the autoinhibitory domain thus provides a strategy to enhance transport function to affect agronomic traits.
Key words: Arabidopsis, autoinhibition, calcium, ionome, tobacco, transport
Received 8 May 2007; Revised 19 June 2007 Accepted 10 July 2007
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