Growth in continuous high air humidity increases the expression of CYP707A-genes and inhibits stomatal closure

• Arabidopsis developed in high RH had lower ABA levels during growth.
• The lower ABA levels in plants from high RH were due to increased ABA catabolism.
• CYP707A-genes were up-regulated during darkness in high RH.
• Inhibiting the oxidation of ABA or spraying with ABA reduced the stomatal aperture.
• Detached leaves from high RH produce high levels of ABA, but have high water loss.

To conserve water plants close the stomata in response to increased abscisic acid (ABA) content. Previous studies have shown that plants developed in high relative air humidity (RH > 85%) develop malfunctioning stomata and have lower ABA content. It has therefore been hypothesized that low ABA content during development results in malfunctioning stomata. In this study the stomatal functioning of Arabidopsis thaliana was evaluated and the content, biosynthesis and catabolism of ABA were quantified. It was found that even though they have lower ABA content during growth, plants developed under high RH were able to produce large amounts of ABA in detached leaves, but still had high water loss. Plants developed in high RH had increased ABA 8′-hydroxylase activity of cytochrome P450CYP707A. Also, plants developed in high RH that were sprayed with ABA or Abscinazole-E1, which inhibits the ABA 8′-hydroxylase activity, had reduced stomatal apertures. ABA deficient mutants had higher water loss in detached leaves than wild type plants in both high and moderate RH. From these results we therefore conclude that continuous low ABA content in high RH is due to increased ABA catabolism by the ABA 8′-hydroxylase activity of cytochrome P450CYP707A. The continuous low ABA content result in reduced ability to close the stomata.