A role for arabinogalactan proteins in gibberellin-induced α-amylase production in barley aleurone cells

Arabinogalactan proteins (AGPs) are plant proteoglycans that have been implicated in plant growth and development. The possible involvement of AGPs in the action of gibberellin (GA), a class of plant hormones, was examined by applying beta-glucosyl Yariv reagent (beta-Glc)(3)Y, a synthetic phenyl glycoside that interacts selectively with AGPs, to barley aleurone protoplasts. Gibberellin induces transcription and secretion of alpha-amylases in the protoplasts. Induction of alpha-amylase was clearly inhibited by (beta-Glc)(3)Y but not by (alpha-Gal)(3)Y, a negative control of the Yariv reagent that does not interact with AGPs. Transfection analysis, using an alpha-amylase promoter-GUS fusion gene in the protoplasts, indicated that the transcriptional activation of the alpha-amylase promoter was inhibited specifically by (beta-Glc)(3)Y. These observations are the first indication of an involvement of AGPs in a plant hormone function. The inhibitory effect of (beta-Glc)(3)Y was not observed when aleurone layers or half-seed grains were used. This result, together with the fact that protoplasts do not have cell walls, suggests that the AGPs that function in alpha-amylase induction reside at the plasma membrane. An aleurone-specific AGP was detected by reversed-phase HPLC, and supported the idea that an AGP may play an important role in aleurone-specific events. The possible mechanism of AGP function in gibberellin-induced alpha-amylase production is discussed.