Development of the aerial parts of

Development of the aerial parts of higher plants depends center of shoot and floral meristems, resulting in a someon the activity of meristems, the formative regions that times massive size increase of the shoot meristem (refareable 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 wildcommunicate
it to others? Recent reports have shown type plants, only the shoot apical meristem is indetermithat
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 silique (latin clava 5 club). Genetic analysis using depends on a signal that is secreted from the stem double mutants indicated that the CLV gene products
cells and perceived by a receptor-kinase complex. The act in the same pathway to restrict stem cell fate or
intertwined activities of both pathways allow mainte- division in the meristem. Clonal studies using an unstanance
of a group of permanent stem cells in the meri- ble clv3 allele caused by a transposon insertion revealed
stem throughout plant life