The box bordered by the black dotted line roughly corresponds to the region imaged by confocal microscopy, and the white dotted lines marks incision line. Insert in (B) shows scanning electron micrograph of a rosette leaf axil of similar stage. Arrowheads highlight the approximate center of the incision line. The white line indicates the incision line at the leaf axil. Selected progenitor cells are color-coded, and the same color has been used for each progenitor cell and its descendants. (B-G) Reconstructed view of the L1 layer of a P 9 leaf axil with STM-Venus (green) expression and FM4-64 stain (red) showing location and lineage of AM progenitor cells, with (B) being the first time point and elapsed time in (C-G). The black square at the end of the petiole disproportionately highlights the region of imaging. (A) Schematic flow showing isolation of a rosette leaf primordium for AM live imaging. Our data favor a threshold model for axillary meristem initiation, in which low levels of STM maintain meristematic competence and high levels of STM lead to meristem initiation.Įxistence of a meristematic cell population with a fixed developmental fate in leaf axils. Cell type-specific binding of REV to the STM region correlates with epigenetic modifications. In more mature leaves, REVOLUTA (REV) directly up-regulates STM expression in leaf axil meristematic cells, but not in differentiated cells, to establish axillary meristems. This suggests that some cells undergo irreversible commitment to a developmental fate. Ectopic expression of STM is insufficient to activate axillary buds formation from plants that have lost leaf axil STM expressing cells. The maintenance of STM expression depends on the leaf axil auxin minimum. Using live-cell imaging of leaf axil cells, we show that the initiation of axillary meristems requires a meristematic cell population continuously expressing the meristem marker SHOOT MERISTEMLESS (STM). In seed plants, these new meristems initiate in leaf axils to enable lateral shoot branching. Shoot branching requires the establishment of new meristems harboring stem cells this phenomenon raises questions about the precise regulation of meristematic fate.
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