Acetylates histone H3 and ADA2 proteins in vitro. Acetylates 'Lys-14' of histone H3. Acetylation of histones gives a specific tag for epigenetic transcription activation. Operates in concert with certain DNA-binding transcriptional activators. Acts via the formation of large multiprotein complexes that modify the chromatin (By similarity). acetyl-CoA + L-lysyl-[protein] = CoA + H(+) + N(6)-acetyl-L-lysyl-[protein] Histone acetyltransferase activity stimulated by the presence of ADA2 proteins. Interacts in vitro with ADA2a, ADA2b and the transcriptional activator DREB1B/CBF1. Interacts with PP2C1. Binds to EML (PubMed:17151888). Expressed in roots and leaves. The N-terminal part (1-150) is not required for interactions with the ADA2 proteins. Pleiotropic effects on plant growth and development, including dwarf size, aberrant root development, and short petals and stamens in flowers. No embryonic phenotype. Increased expression of EML (PubMed:17151888). Mutations in HAG1 suppress the tpl-1 phenotype showing a transformation during embryogenesis of the shoot pole into a second root pole. Belongs to the acetyltransferase family. GCN5 subfamily.