The role of calcium in ABA-induced gene expression and stomatal movements

There is much interest in the transduction pathways by which abscisic acid (ABA) regulates stomatal movements (ABA-turgor signalling) and by which thi s phytohormone regulates the pattern of gene expression in plant cells (ABA -nuclear signalling). A number of second messengers have been identified in both the ABA-turgor and ABA-nuclear signalling pathways. A major challenge is to understand the architecture of ABA-signalling pathways and to determ ine how the ABA signal is coupled to the appropriate response. We have inve stigated whether separate Ca2+-dependent and -independent ABA-signalling pa thways are present in guard cells. Our data suggest that increases in [Ca2](i) are a common component of the guard cell ABA-turgor and ABA-nuclear si gnalling pathways. The effects of Ca2+ antagonists on ABA-induced stomatal closure and the ABA-responsive CDeT6-19 gene promoter suggest that Ca2+ is involved in both ABA-turgor signalling and ABA-nuclear signalling in guard cells. However, the sensitivity of these pathways to alterations in the ext ernal calcium concentration differ, suggesting that the ABA-nuclear and ABA -turgor signalling pathways are not completely convergent. Our data suggest that whilst Ca2+-independent signalling elements are present in the guard cell, they do not form a completely separate Ca2+-independent ABA-signallin g pathway.