Sierla et al. show that GHR1 is a receptor-like pseudokinase that acts in stomatal closure through its scaffolding functions rather than by directly phosphorylating its target proteins. Plant Cell https://doi.org/10.1105/tpc.18.00441.
By Maija Sierla
Organismal and Evolutionary Biology Research Programme, University of Helsinki, Finland
Background: Leaf surfaces contain small holes known as stomatal pores. These are responsible for gas-exchange in plants and are formed by two guard cells that surround the pore. The most important roles of stomata are to maximize the intake of carbon dioxide (CO2) needed for photosynthesis and to minimize water loss. Guard cells can sense changing environmental conditions and respond by opening or closing the stomatal pore. Stomata close in response to drought, darkness, low air humidity, high CO2 levels and the air pollutant ozone. In response to these cues, the guard cells lose water and this decrease in turgor closes the stomatal pore. The turgor decrease is caused by the activation of ion transport across the guard cell plasma membranes. Activation of anion efflux through the SLAC1 anion channel plays a central role during stomatal closure.
Question: We investigated how the leucine-rich repeat receptor-like kinase GHR1 controls stomatal closure. We wanted to understand the molecular mechanisms by which GHR1 controls SLAC1 anion channel activity.
Findings: We identified an Arabidopsis mutant that was sensitive to ozone treatment and discovered that this was due to a mutation in the GHR1 gene. We found that the stomata of the ghr1 mutants are gaping open day and night and do not close in response to the stimuli that usually close the stomata. We found that GHR1 is a pseudokinase and that ATP binding and thus kinase activity is not required for GHR1 function in stomatal closure in Arabidopsis or SLAC1 activation in Xenopus oocytes. Our results suggest that GHR1-mediated activation of SLAC1 occurs via interacting kinases and that CPK3 interacts with GHR1. We propose that GHR1 activates SLAC1 and acts in stomatal closure through scaffolding functions rather than by direct phosphorylation of target proteins.
Next steps: Next, we aim to identify additional components of the GHR1 receptor complex. These will likely include co-receptors, additional pathway-specific cytosolic effectors, and other channels and transporters that regulate stomatal function. Later on, manipulation of GHR1 in crop plants could be used to modulate plant gas-exchange, water-use efficiency, and thus productivity.
Key words: stomata, guard cell, pseudokinase
Feature image: Arabidopsis stomata, artwork by Elina Savolainen
Maija Sierla, Hanna Hõrak, Kirk Overmyer, Cezary Waszczak, Dmitry Yarmolinsky, Tobias Maierhofer, Julia P. Vainonen, Jarkko Salojärvi, Konstantin Denessiouk, Kristiina Laanemets, Kadri Tõldsepp, Triin Vahisalu, Adrien Gauthier, Tuomas Puukko, Lars Paulin, Petri Auvinen, Dietmar Geiger, Rainer Hedrich, Hannes Kollist, Jaakko Kangasjärvi. (2018). The Receptor-like Pseudokinase GHR1 Is Required for Stomatal Closure. Plant Cell 30: 2813-2837; DOI: https://doi.org/10.1105/tpc.18.00441.