Journal Article

Calcium specificity signaling mechanisms in abscisic acid signal transduction in Arabidopsis guard cells

Alemán, Fernando, Brandt, Benjamin, Yong, Taiming, Poretsky, Elly, Nguyen, Desiree, Yang, Paul G, Munemasa, Shintaro, Wang, Cun, Waadt, Rainer, Schroeder, Julian I, Belknap, Thomas F

2015

Overview

A central question is how specificity in cellular responses to the eukaryotic second messenger Ca2+ is achieved. Plant guard cells, that form stomatal pores for gas exchange, provide a powerful system for in depth investigation of Ca2+-signaling specificity in plants. In intact guard cells, abscisic acid (ABA) enhances (primes) the Ca2+-sensitivity of downstream signaling events that result in activation of S-type anion channels during stomatal closure, providing a specificity mechanism in Ca2+-signaling. However, the underlying genetic and biochemical mechanisms remain unknown. Here we show impairment of ABA signal transduction in stomata of calcium-dependent protein kinase quadruple mutant plants. Interestingly, protein phosphatase 2Cs prevent non-specific Ca2+-signaling. Moreover, we demonstrate an unexpected interdependence of the Ca2+-dependent and Ca2+-independent ABA-signaling branches and the in planta requirement of simultaneous phosphorylation at two key phosphorylation sites in SLAC1. We identify novel mechanisms ensuring specificity and robustness within stomatal Ca2+-signaling on a cellular, genetic, and biochemical level. Plant leaves have tiny openings or pores called stomata, which allow carbon dioxide, water vapor and other gases to diffuse in and out of the plant. Two cells called guard cells surround each stoma and control the opening and closing of the pore. If a plant is losing excessive amounts of water, for example during a drought, the plant produces a hormone called abscisic acid that promotes the closure of its stomata. When abscisic acid is present, the guard cells are sensitive to changes in their internal concentration of calcium ions so that calcium ions can activate a protein called SLAC1. This leads to responses in the guard cells that close the stoma. The calcium ions activate SLAC1 by stimulating enzymes called calcium-dependent protein kinases (CPKs). However, abscisic acid can also trigger other enzymes that can activate SLAC1 independently of the calcium ions. Calcium ions are also reported to be involved in the opening of stomata, when abscisic acid is not present. Therefore, it is not clear how abscisic acid works to specifically ‘prime’ guard cells to close the stomata in response to increases in calcium ions during drought. Brandt, Munemasa et al. studied stomata in a plant called Arabidopsis thaliana. The experiments show that, in the presence of abscisic acid, mutant plants that lack four different CPK enzymes are impaired in the activation of SLAC1 and the closing of stomata in response to increases in calcium ions. Further experiments found that other enzymes called the PP2Cs—which are switched off by abscisic acid—are responsible for regulating the Ca2+ sensitivity of guard cells. Switching off PP2Cs enables closing of the stomata in response to calcium ions. It has been suggested previously that the CPKs and the calcium-independent enzymes are involved in two separate pathways that promote the closure of stomata. However, Brandt, Munemasa et al. found that the calcium-independent enzymes are required for calcium ions to activate SLAC1 in guard cells, revealing that these two pathways are linked. Brandt, Munemasa et al.'s findings reveal how abscisic acid is able to specifically prime guard cells to close stomata in response to calcium ions. The next challenge is to understand how the CPKs and calcium-independent enzymes work together during the closure of stomata.

• Publisher: eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Abscisic acid / Abscisic Acid - metabolism / Anion channels / Arabidopsis / Arabidopsis - cytology / Arabidopsis - metabolism / Arabidopsis - physiology / Arabidopsis Proteins - metabolism / BASIC BIOLOGICAL SCIENCES / Calcium - metabolism / Calcium signalling / calcium specificity / calcium-dependent protein kinase / Channel pores / Developmental biology / Focal Adhesion Kinase 2 - genetics / Focal Adhesion Kinase 2 - metabolism / Gas exchange / Gene Deletion / Guard cells / Kinases / Lasers / Membrane Proteins - metabolism / Phosphoprotein Phosphatases - metabolism / Phosphorylation / Plant Biology / Plant cells / Plant Cells - metabolism / Plant Cells - physiology / Plant Stomata - metabolism / Plant Stomata - physiology / Protein kinase / Protein phosphatase / Protein Phosphatase 2C / Protein Processing, Post-Translational / Proteins / Signal Transduction / SLAC1 / SnRK / Stomata / Xenopus