Isolated aleurone cells and protoplasts of cereal grains are particularly useful model system for studying of gibberellin signaling. Up to now, the identity of the receptor protein is unknown, but its activity in cereal aleurone has been localized to the plasma membrane. The gibberellin-signaling pathways identified in aleurone cells involve heterotrimeric G proteins, 1,4,5-inositol triphosphate, Ca2+, calmodulin, cGMP, cytosolic pH, protein phosphorylation, and transcription factors. The promoters of gibberellin-inducible a-amylase genes have been sequenced and motifs essential for hormone-stimulated transcription have been determined. Barley transcription regulators (Myb, HRT, ABF) that are important trans acting elements of gibberellin-inducible a-amylase genes were identified. In Arabidopsis thaliana four gibberellin-signaling mutants (gai, grs/rga, spy, shi) have been selected. The GAI, GRS/RGA and SHI proteins function as negative regulators of gibberellin-induced growth responses. Supposedly, the product of SPY gene is O-GlcNAc transferase that modifies proteins by glycosylation.