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Title: "In planta" regulation of the "Arabidopsis" Ca(2+)/H(+) antiporter CAX1

Author
item MEI, HUI - BAYLOR COLLEGE MED
item ZHAO, JIAN - BAYLOR COLLEGE MED
item PITTMAN, JON - TEXAS A&M UNIVERSITY
item LACHMANSINGH, JINESH - BAYLOR COLLEGE MED
item PARK, SUNGHUN - TEXAS A&M UNIVERSITY
item Hirschi, Kendal

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2007
Publication Date: 9/26/2007
Citation: Mei, H., Zhao, J., Pittman, J.K., Lachmansingh, J., Park, S., Hirschi, K.D. 2007. "In planta" regulation of the "Arabidopsis" Ca(2+)/H(+) antiporter CAX1. Journal of Experimental Botany. 58(12):3419-3427.

Interpretive Summary: The ability to control and regulate the activity of plant gene products is an important aspect of plant genetic engineering. If we aspire to control the nutrients within the plant to aid human nutrition, we must control the uptake of the nutrients into the edible portions of plants. In this manuscript, we demonstrate the ability to control and manipulate the activity of a plant calcium transporter. Altering the activity of this calcium transporter can be used to alter the total calcium content of agriculturally important plants. The capacity to carefully modulate the activity of a nutrient transporter is an important fundamental component to enhancing nutrient content in the food supply.

Technical Abstract: Vacuolar localized Ca(2+)/H(+) exchangers such as "Arabidopsis thaliana" cation exchanger 1 (CAX1) play important roles in Ca(2+) 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 Ca(2+)/H(+) antiport activity. Furthermore, transgenic plants expressing sCAX1 display increased Ca(2+) accumulation and heightened activity of vacuolar Ca(2+)/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 Ca(2+)/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 Ca(2+) transporter in transgenic plants. Engineering the autoinhibitory domain thus provides a strategy to enhance transport function to affect agronomic traits.