Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake

Zaida Andrés; Javier Pérez-Hormaeche; Eduardo O. Leidi; Kathrin Schlücking; Leonie Steinhorst; Deirdre H. McLachlan; Karin Schumacher; Alistair M. Hetherington; Jörg Kudla; Beatriz Cubero; José M. Pardo

doi:10.1073/pnas.1320421111

New Research In

Edited by Jian-Kang Zhu, Purdue University, West Lafayette, IN, and approved March 25, 2014 (received for review October 30, 2013)

Significance

Rapid fluxes of K⁺ and other osmolytes in guard cells control the opening and closing of stomata and thereby gas exchange and transpiration of plants. Despite the well-established role of the plasma membrane of guard cells in stomatal function, osmolyte uptake into the cytosol represents only a transient step to the vacuole, as more than 90% of the solutes accumulate in these organelles. We show that the tonoplast-localized K⁺/H⁺ exchangers mediate the vacuolar accumulation of K⁺ in guard cells, and that activity of these transporters controls not only stomatal opening but also stomatal closure. We also establish vacuolar K⁺/H⁺ exchange as a critical component involved in vacuolar remodeling and the regulation of vacuolar pH during stomatal movements.

Abstract

Stomatal movements rely on alterations in guard cell turgor. This requires massive K⁺ bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K⁺ into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K⁺/H⁺ exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K⁺ and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K⁺ accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K⁺ is a requirement for stomatal opening and a critical component in the overall K⁺ homeostasis essential for stomatal closure, and suggest that vacuolar K⁺ fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.

Footnotes

↵¹To whom correspondence should be addressed. E-mail: jose.pardo{at}csic.es.

Author contributions: Z.A., E.O.L., K. Schumacher, A.M.H., J.K., B.C., and J.M.P. designed research; Z.A., J.P.-H., E.O.L., K. Schlücking, L.S., D.H.M., and B.C. performed research; Z.A., J.P.-H., E.O.L., K. Schlücking, L.S., D.H.M., K. Schumacher, A.M.H., J.K., B.C., and J.M.P. analyzed data; and Z.A., A.M.H., J.K., B.C., and J.M.P. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1320421111/-/DCSupplemental.

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