Signaling Cell Fate in Plant Meristems Three Clubs on One Tousle

cells in the central zone and cell displacement to the periphery. But how are permanent stem cells maintained in the meristem? The CLAVATA Genes Restrict Stem Cell Number The first insights into the underlying regulatory mechanisms came from studies of mutations that disrupt meri-Germany stem organization. Loss-of-function mutations in the CLAVATA1, 2 or 3 genes (CLV1, 2 and 3) of Arabidopsis cause an accumulation of undifferentiated cells in the Development of the aerial parts of higher plants depends center of shoot and floral meristems, resulting in a some-on the activity of meristems, the formative regions that times massive size increase of the shoot meristem (ref-are able to continuously initiate new organs at their erences can be found in Fletcher et al., 1999) (Figures flanks. New meristematic cells are created from stem 1A and 1B). Since cell division rates were not altered in cells that are positioned at the summit of the meristem. clv mutants (Laufs et al., 1998), these genes must act Maintenance of a functional meristem requires a coordi-to restrict stem cell number in the meristem. The buildup nation between stem cell divisions and loss of cells of stem cells in the central zone causes a concomitant by differentiation. In plants, cell fate is determined by increase in size of the surrounding peripheral zone positional cues, but not by lineage. How can cells in a where organs are initiated, resulting in the formation of meristem interpret their position, decide their fate, and supernumerary, but otherwise normal, organs. In wild-communicate it to others? Recent reports have shown type plants, only the shoot apical meristem is indetermi-that keeping the correct number of stem cells is nate and maintains stem cells throughout the life of achieved by negative feedback regulation of two path-the plant; floral meristems cease their activity after the ways. The stem cell promoting pathway is controlled by formation of a discrete number of organs. The stem cells the homeobox protein WUS, acting from cells in deeper are then consumed during the formation of carpels in regions of the meristem to induce and maintain stem the center of the flower. However, floral meristems of cells at the tip. A second pathway acts antagonistically clv mutants maintain more stem cells, and additional and restricts the number of stem cells by repressing the carpels are made that fuse to form a club-shaped pod activity of the WUS gene. This counteracting pathway or …