OPEN ACCESS ARTICLE

This Article Free via Open Access: OA OA Full Text Full Text (PDF) Supplemental Data OA All Versions of this Article: 144/1/82    most recent pp.106.092155v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via ISI Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Padmanaban, S. Articles by Sze, H. Search for Related Content PubMed PubMed Citation Articles by Padmanaban, S. Articles by Sze, H. Agricola Articles by Padmanaban, S. Articles by Sze, H.

CELL BIOLOGY AND SIGNAL TRANSDUCTION

Participation of Endomembrane Cation/H⁺ Exchanger AtCHX20 in Osmoregulation of Guard Cells^1,[W],[OA]

Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742–5815 (S.P., S.C., J.M.K., X.L., H.S.); and Department Plant Biology, University of Minnesota, St. Paul, Minnesota 55108 (J.M.W.)

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 (Arabidopsis thaliana), is preferentially and highly expressed in guard cells as shown by promoter::-glucuronidase 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, cDNA was expressed in a yeast (Saccharomyces cerevisiae) mutant that lacks Na⁺(K⁺)/H⁺ antiporters (nhx1 nha1 kha1) and plasma membrane Na⁺ pumps (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. Green fluorescent protein-tagged CHX20 expressed in mesophyll protoplasts was localized mainly to membranes of the endosomal system. 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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Heven Sze (hsze{at}umd.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.092155

^* Corresponding author; e-mail hsze{at}umd.edu; fax 301–314–9081.