The RAPTOR/KOG1 proteins are binding partners of the target of rapamycin kinase that is present in all eucaryotes and plays a central role in the stimulation of cell growth and metabolism in response to nutrients. We show in this report that two genes are coding for RAPTOR/KOG1 homologs in the Arabidopsis and rice genomes. Disruption of the Arabidopsis AtRaptor1 gene leads to a very early arrest of embryo development whereas disruption of the AtRaptor2 gene, which is expressed at a lower level than AtRaptor1, has no visible effects on embryo and plant development. We also observed that mutations in the AtRaptor1 gene result in an earlier halt of embryo development than disruption of the AtTor gene. (C) 2004 Elsevier Inc. All rights reserved.