Extreme drought deactivates ABA biosynthesis

The phytohormone abscisic acid (ABA) is synthesized by plants during drought to close stomata and regulate desiccation tolerance pathways. In conifers and a few angiosperms with embolism resistant xylem a peaking-type (p-type) response in ABA levels has been observed, in which ABA levels increase early in drought then decrease as drought progresses, declining to pre-stressed levels. The mechanism behind this dynamic remains unknown. Here we sought to characterize the mechanism driving p-type ABA dynamics in the conifer Callitris rhomboidea and the highly drought resistant angiosperm Umbellularia californica. We measured leaf water potentials (Ψ ), stomatal conductance, ABA, conjugates and phaseic acid (PA) levels in potted plants during a prolonged but non-fatal drought. Both species displayed a p-type ABA dynamic during prolonged drought. Measuring ABA levels in bench dried, rehydrated branches collected before and after the peak in ABA levels revelated that in both species ABA biosynthesis is deactivated in leaves that have been dehydrated beyond leaf turgor loss point. Considerable conversion of ABA to conjugates was found to occur during drought, but not catabolism to PA. The mechanism driving the decline in ABA levels in p-type species appears to be conserved across seed plants and is mediated by sustained conjugation of ABA and the deactivation of ABA biosynthesis as Ψ becomes more negative than turgor loss.