Establishment of the Embryonic Shoot Meristem Involves Activation of Two Classes of Genes with Opposing Functions for Meristem Activities

The shoot meristem, a stem-cell-containing tissue initiated during plant embryogenesis, is responsible for continuous shoot organ production in postembryonic development. Although key regulatory factors includingKNOXgenes are responsible for stem cell maintenance in the shoot meristem, how the onset of such factors is regulated during embryogenesis is elusive. Here, we present evidence that the twoKNOXgenesSTMandKNAT6together with the two other regulatory genesBLRandLASare functionally important downstream genes ofCUC1andCUC2, which are a redundant pair of genes that specify the embryonic shoot organ boundary. Combined expression ofSTMwith any ofKNAT6,BLR, andLAScan efficiently rescue the defects of shoot meristem formation and/or separation of cotyledons incuc1cuc2double mutants. In addition,CUC1andCUC2are also required for the activation ofKLU, a cytochrome P450-encoding gene known to restrict organ production, andKLUcounteractsSTMin the promotion of meristem activity, providing a possible balancing mechanism for shoot meristem maintenance. Together, these results establish the roles forCUC1andCUC2in coordinating the activation of two classes of genes with opposite effects on shoot meristem activity.