CCS52A2/FZR1, a cell cycle regulator, is an essential factor for shoot apical meristem maintenance in Arabidopsis thaliana

Phenotypic analysis of the xcm9 mutant reveals concomitant premature termination of floral shoots with frequent bifurcation of the shoot apices, stems, and flowers. Microscopic observations show irregular cell organization in shoot apical meristems of xcm9. Positional cloning revealed that xcm9 is a loss of function allele of the CCS52A2/FZR1 gene, which has previously been implicated in root development. Expression analysis demonstrated that CCS52A2 maintains a higher transcriptional expression level in actively dividing tissue. Genetic studies indicated that the CCS52A2 gene functions together with WUSCHEL (WUS) and CLAVATA3 (CLV3) in regulating the development of the shoot meristem, and also contributes to this regulation together with the chromatin remodeling pathway. In addition, fewer xcm9 cells express CYCLIN B1:1, showing that cell cycle progression is disrupted in the mutant.We propose that the CCS52A2 gene is a mediator that functions together with meristematic genes to regulate meristem organization, and cross-functions with chromatin regulators in cell cycle progression during shoot apical meristem development.In higher plants, the vast majority of structures and organs, such as leaves, stems, roots, and flowers, are formed postembryonically from groups of undifferentiated cells, called meristems. In many species of plants, certain meristems are maintained throughout life. The activities of the root apical meristem (RAM) and the shoot apical meristem (SAM) determine root and shoot structure and function, respectively. Stem cells are confined to the centers of shoot and root apices, and their proliferation is maintained by signals that cells receive from the local environment [1]. They undergo precisely controlled division, which must be rapid enough to replenish cells lost to differentiation, but restricted enough to prevent overproliferation of undifferentiated cells. Adjacent to the stem cells, several cells in the SAM and RAM form the organizing cente