ABA-induced stomatal closure through Ca²⁺-dependent and -independent signaling pathways.

Shouguang Huang, Triinu Arjus, Hannes Kollist, Dietmar Geiger, Rainer Hedrich and Rob Roelfsema
Plant Molecular Biology and Biophysics, Biocenter, Julius-von-Sachs Institute for Biosciences, University of Würzburg, 97082 Würzburg, Germany

During drought, the stress hormone ABA causes activation of SLAC-type anion channels inguard cells, which leads to stomatal closure. This ABA-response of guard cells was linked to cytosolic Ca²⁺ signals in guard cells by pioneering work on stomata of Commelina communis (McAinsh et al., 1990). However, later work with Vicia faba showed that ABA can also activate SLAC-type anion channels in the absence of Ca²⁺ signals (Levchenko et al., 2005). We therefore studied the role of Ca ²⁺ signals in ABA-induced stomatal closure with Arabidopsis thaliana plants that express the Ca²⁺ reporter R-GECO1-mTurquoise (Waadt et al., 2017; Huang et al. , 2019). ABA induced stomatal closure in approximately 10 min. and in 3 out of 4 guard cells, we observed a transient increase of the cytosolic Ca²⁺ concentration during this response. The remaining stomata closed in the absence of a cytosolic Ca²⁺ signals, showing that these Ca²⁺ signals are dispensable for stomatal closure. Nevertheless, in the majority of the guard cells, the cytosolic Ca²⁺ signal rose during stomatal closure, indicating that it was provoked by the stomatal closure response, instead of initiating this response. Based on these and other data, an ABA-signaling pathway will be discussed in which the activation of OST1 will cause a direct activation of the SLAC1 anion channel. Due to stomatal movements, Ca²⁺ signals can be elicited that activate the SLAH3 channel and TPK channels in the vacuole. These Ca²⁺-dependent responses will enhance the efficiency by which ABA provokes stomata closure. The conclusion is that ABA induced stomatal closure is strictly dependent activation of SLAC1 by OST1, but the response is enhanced by the Ca²⁺-dependent activation of additional ion release mechanisms in guard cells.

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