Protein phosphorylation is involved in the water stress-induced ABA accumulation in the roots of Malus hupehensis Rehd

Water stress-induced ABA accumulation plays a key role in the root to shoot communication and/or the cell to cell signaling under the soil stresses. T he signaling of the water stress itself that leads to the accumulation, how ever, is less known. In this study, we subjected the roots of Malus hupehen sis seedlings to water stress treatment and investigated the ABA accumulati on in relation to protein phosphorylation. Our results showed that ABA accu mulation could be substantially triggered in 40 min and reached 4 folds in 100 min after treatment with 30% PEG 6000 (polyethylene glycol). The water stress treatment also led to a substantial enhancement of total kinase acti vity, assessed with histone-III as substrate, in 15 min and a maximum enhan cement in 30 min before it declined to initial level. The Ca2+-dependent ki nase activity showed a similar, if not more sensitive, trend. When the root s were fed with labeled P-32-ATP, wa;ter stress enhanced the labeling of pr oteins, which showed a maximum labeling at 40 min. Two inhibitors of protei n kinases, Quercetin and H7, effectively diminished or completely blocked t he ABA accumulation under the stress treatment. It is therefore suggest tha t protein phosphorylation is involved in the signaling of the water stress- induced ABA accumulation.