Central roles of TOR in early embryogenesis, cell cycle control, and plant growth and development. The null tor mutants arrest at the 16–32 cell stage in early embryogenesis. Estradiol-inducible tor mutants block DNA synthesis in the root meristem, seedling development, and root hair growth. Fully differentiated leaf mesophyll cells maintain robust TOR kinase activity for S6K1-T449 phosphorylation that is sensitive to rapamycin without (upper panel) or with (lower panel) FKBP12 (F12) overexpression. Rap, rapamycin.