Evidence is provided that plant transcription factors of the ABA response element binding protein (AREBP) class are phosphorylated by sucrose non-fermenting-1-related protein kinase-1 (SnRK1) and a calcium-dependent protein kinase at highly conserved target sites. Two target sites for SnRK1 were identified in AREBPs using a specially developed motif search pipeline. Peptides containing the AREBP sites were phosphorylated by purified SnRK1 in vitro and by calcium-dependent and calcium-independent activities present in soluble protein extracted from Arabidopsis seedlings grown in liquid culture. Most (69-77%) of the calcium-independent phosphorylation of these peptides was removed by immunoprecipitation using anti-SnRK1 antisera, linking this activity unambiguously to SnRK1. It is possible that the protein kinase responsible for the calcium-dependent activity was SnRK3, a protein kinase that is related to SnRK1 and which has similar requirements for target site recognition. However, the involvement of other calcium-dependent protein kinases cannot be ruled out.