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Abstract

Most organs of flowering plants develop postembryonically from groups of pluripotent cells called meristems. The shoot apical meristem (SAM) is specified during embryogenesis and is the source of aerial organs. The SAM increases in size via cell division upon emergence, and reaches its mature state prior to floral transition. In Arabidopsis, the SAM is specified by two complementary pathways. SHOOT MERISTEMLESS (STM) defines the entire SAM region. WUSCHEL (WUS), on the other hand, functions in a more restricted set of cells to promote stem cell fate, and is regulated by the CLAVATA genes in a negative feedback loop. However, little is known about how the growth of the SAM during vegetative development is regulated. Under ground, the root meristems are the origin of the root system. The primary root meristem forms during embryonic development, and most cell divisions in the root occur in the meristematic zone above the quiescent center. Till recently, there is little evidence that the shoot and root meristems share common regulatory mechanisms. We have characterized STIMPY (STIP; also called WOX9), a homeobox gene required for the growth of the vegetative SAM, and the maintenance of root growth. Loss of STIP function results in early seedling arrest due to the loss of cell division and premature differentiation of both the shoot apex and the primary root meristematic zone. Genetically, STIP interact with the CLV/WUS feedback loop in the SAM. In addition, STIP is required for lateral root initiation and the growth of other aerial organs. What sets STIP apart from known meristem mutants is that stip mutants can be fully rescued by stimulating the entry into the cell cycle using sucrose. Therefore STIP is likely to function by maintaining cell division and preventing premature differentiation in proliferating tissues. Taken together, STIP identifies a new genetic pathway integrating developmental signals with cell cycle control. We are currently investigate the mechanisms involved in STIP actions.