Calcium-sensing receptor regulates stomatal closure through hydrogen peroxide and nitric oxide in response to extracellular calcium in Arabidopsis

Abstract

The Arabidopsis calcium-sensing receptor CAS is a crucial regulator of extracellular calcium-induced stomatal closure. Free cytosolic Ca²⁺ (Ca²⁺_i) increases in response to a high extracellular calcium (Ca²⁺_o) level through a CAS signalling pathway and finally leads to stomatal closure. Multidisciplinary approaches including histochemical, pharmacological, fluorescent, electrochemical, and molecular biological methods were used to discuss the relationship of hydrogen peroxide (H₂O₂) and nitric oxide (NO) signalling in the CAS signalling pathway in guard cells in response to Ca²⁺_o. Here it is shown that Ca²⁺_o could induce H₂O₂ and NO production from guard cells but only H₂O₂ from chloroplasts, leading to stomatal closure. In addition, the CASas mutant, the atrbohD/F double mutant, and the Atnoa1 mutant were all insensitive to Ca²⁺_o-stimulated stomatal closure, as well as H₂O₂ and NO elevation in the case of CASas. Furthermore, it was found that the antioxidant system might function as a mediator in Ca²⁺_o and H₂O₂ signalling in guard cells. The results suggest a hypothetical model whereby Ca²⁺_o induces H₂O₂ and NO accumulation in guard cells through the CAS signalling pathway, which further triggers Ca²⁺_i transients and finally stomatal closure. The possible cross-talk of Ca²⁺_o and abscisic acid signalling as well as the antioxidant system are discussed.