ABSCISIC-ACID SIGNAL-TRANSDUCTION IN EPIDERMAL-CELLS OF PISUM-SATIVUML ARGENTEUM - BOTH DEHYDRIN MESSENGER-RNA ACCUMULATION AND STOMATAL RESPONSES REQUIRE PROTEIN-PHOSPHORYLATION AND DEPHOSPHORYLATION

The effects of a number of treatments, both in planta and in vitro, on the accumulation of mRNA encoding dehydrin, an abscisic acid (ABA)-in ducible protein, were determined for guard cells and mesophyll cells p repared from leaves of the Argenteum mutant of Pisum sativum L. Guard cells and mesophyll cells treated for 10 d with ABA in planta accumula ted dehydrin mRNA. However, after 1 or 3 d treatment, dehydrin mRNA wa s induced only in guard cells. Wilting induced dehydrin mRNA accumulat ion in leaves and epidermal cells. Induction of mRNA in epidermal cell s was correlated with an increased ABA content after either ABA applic ation or following wilting. Isolated mesophyll and guard cells respond ed to ABA in vitro by the induction of dehydrin mRNA. However, osmotic stress, imposed by incubation in mannitol, had no effect on ABA and d ehydrin mRNA synthesis in mesophyll cells, and only a slight effect on guard cells. Both protein phosphorylation and dephosphorylation were shown to be required for ABA-induced dehydrin gene expression and stom atal movements. Inhibitors of protein kinases (K-252a) and protein pho sphatases 1/2A (okadaic acid) and protein phosphatase 2B (cyclosporin A) all inhibited ABA-induced dehydrin mRNA accumulation. They also red uced the inhibitory effects of ABA on stomatal opening, as did the pro tein kinase activator phorbol myristate acetate (PMA). While K-252a, c yclosporin A and PMA also inhibited ABA-induced stomatal closure, okad aic acid enhanced the ABA effect, indicating the involvement of multip le protein phosphorylation/dephosphorylation steps in ABA signal trans duction in guard cells.