The crosstalk between ABA, nitric oxide, hydrogen peroxide, and calcium in stomatal closing of Arabidopsis thaliana

Guard cells can integrate and process multiple complex signals from the environment and respond by opening and closing stomata in order to adapt to the environmental conditions changing. Over the past several years, considerable research progress has been made in our understanding of the role of calcium (Ca2+), nitric oxide (NO) and hydrogen peroxide (H2O2) as essential signal molecules that mediate abscisic acid (ABA)-induced stomatal closure. However, the sequence or epistasis of signaling cascade elements remains unknown. Here, we report that there are some relationships between extracellular Ca2+ induced cytosolic free Ca2+ increases (CICI) and ABA signal transduction in guard cell. ABA-induced stomatal closure in CASas mutant indicates that CAS activity is not required for ABA-induced stomatal closure, while exogenous Ca2+ failed to induce stomatal closure in CASas mutant and ABA-insensitive mutant, abi1 and abi2, indicating that extracellular Ca2+-induced stomatal closure via CAS requires some components of ABA signal pathway in guard cells. Hydrogen peroxide (H2O2), as a secondary messenger in the ABA-induced stomatal closure pathway, could not induce stomatal closure in CASas guard cells, it indicates that the functional CAS is required for H2O2 to induce stomatal closure. Nitric oxide (NO), another component of ABA-induced stomatal closure pathway, could induce stomatal closure in CASas guard cells while NO overproduction mutant-nox1 did not respond to exogenous high Ca2+, this demonstrates that NO acts downstream of CAS. Our results on the analysis of various Arabidopsis mutants indicate that NO and H2O2 maybe the points of connection between CICI and ABA signaling cascade in guard cell.