Dear Editor,It has been known since the work of Francis Darwin that,in response to a reduction in atmospheric relative humidity (rh),stomatal aperture decreases.Screening for Arabidopsis mutants compromised in stomatal responses to reduced rh resulted in the identification of two genes,OST1 and ABA2,that are involved in stomatal response to low rh conditions.Interestingly both encode proteins previously known to be involved in ABA signaling (Xie et al.,2006,and references therein).These findings strongly suggested that,at least in part,the stomatal response to low rh is mediated by ABA and the intracellular ABA signaling pathway.Our most recent data show that low rh-induced stomatal closure can proceed by guard cell autonomous ABA synthesis (Bauer et al.,2013),suggesting that there is significant overlap between the guard cell low air humidity-induced signaling pathway and the guard cell ABA signaling pathway.Such convergence has been noted before (Xie et al.,2006,and references therein) and the example of Open STomata 1 (OST1/SRK2E)that encodes a serine-threonine protein kinase and the SLow Anion Channel SLAC1 may represent a common mechanism for most other closure-inducing signals,too.