Factors with a graded distribution can program fields of cells in a dose-dependent manner, but no evidence has hitherto surfaced for such mechanisms in plants. In the Arabidopsis thaliana root, two PLETHORA (PLT) genes encoding AP2-domain transcription factors have been shown to maintain the activity of stem cells. Here we show that a clade of four PLT homologues is necessary for root formation. Promoter activity and protein fusions of PLT homologues display gradient distributions with maxima in the stem cell area. PLT activities are largely additive and dosage dependent. High levels of PLT activity promote stem cell identity and maintenance; lower levels promote mitotic activity of stem cell daughters; and further reduction in levels is required for cell differentiation. Our findings indicate that PLT protein dosage is translated into distinct cellular responses.