Protein phosphorylation in carrot somatic embryos in response to abscisic acid

Patterns of protein phosphorylation in carrot (Daucus carota L.) somatic embryos and suspension cells were examined by in vivo labelling with [ 32 P]phosphate and analysis by two-dimensional gel electrophoresis and autoradiography. Nearly 100 or 200 phosphoproteins were resolved when cells were labeled in conditioned medium or phosphate-free fresh medium, respectively. Somatic embryo cultures showed only subtle differences in protein phosphorylation compared to undifferentiated cell cultures. However, in response to exogenously applied abscisic acid (ABA), several acidic phosphoproteins declined progressively in 5-day and 10-day somatic embryo cultures. These results identify several phosphoproteins as putative targets for regulation of plant embryogenesis by ABA, and indicate that a protein kinase or protein phosphatase may be involved in the ABA signal transduction pathway