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Published on March 2, 2007; 10.1104/pp.106.092155

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Received November 2, 2006
Accepted February 22, 2007

Participation of an Endomembrane Cation/H+ Exchanger AtCHX20 in Osmoregulation of Guard Cells

Senthilkumar Padmanaban , Salil Chanroj , June M. Kwak , Xiyan Li , John M. Ward , and Heven Sze *

Department of Cell Biology & Molecular Genetics, H. J. Patterson Hall, University of Maryland, College Park, MD 20742-5815, USA; Department Plant Biology, 250 Biological Sciences Center, 1445 Gortner Ave., University of Minnesota, St. Paul, MN 55108, USA

* Corresponding author; email: hsze{at}umd.edu.

Guard cell movement is induced by environmental and hormonal signals that cause changes in turgor through changes in uptake or release of solutes and water. Several transporters mediating these fluxes at the plasma membrane have been characterized, however less is known about transport at endomembranes. CHX20, a member of a poorly-understood cation/H+ exchanger gene family in Arabidopsis thaliana, is preferentially and highly expressed in guard cells as shown by promoter::GUS activity and by whole-genome microarray. Interestingly, three independent homozygous mutants carrying T-DNA insertions in CHX20 showed 35% reduction in light-induced stomatal opening compared to wild type plants. To test the biochemical function of CHX20, the cDNA was expressed in a yeast mutant which lacks Na+(K+)/H+ antiporters ({Delta}nhx1{Delta}nha1{Delta}kha1) and plasma membrane Na+ pumps ({Delta}ena1-4). Curiously, CHX20 did not enhance tolerance of mutants to moderate Na+ or high K+ stress. Instead it restored growth of the mutant on medium with low K+ at slightly alkaline pH, but had no effect on growth at acidic pH. GFP-tagged CHX20 expressed in mesophyll protoplasts was localized mainly to intracellular membranes, like endosomes. Furthermore, light-induced stomatal opening of the Arabidopsis mutants was insensitive to external pH and was impaired at high KCl. The results are consistent with the idea that in exchanging K+ for H+, CHX20 maintains K+ homeostasis and influences pH under certain conditions. Together these results provide genetic and biochemical evidence that one CHX protein plays a critical role in osmoregulation through K+ fluxes and possibly pH modulation of an active endomembrane system in guard cells.




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J. Morris, H. Tian, S. Park, C. S. Sreevidya, J. M. Ward, and K. D. Hirschi
AtCCX3 Is an Arabidopsis Endomembrane H+-Dependent K+ Transporter
Plant Physiology, November 1, 2008; 148(3): 1474 - 1486.
[Abstract] [Full Text] [PDF]


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