CELL LINEAGE PATTERNS AND HOMEOTIC GENE ACTIVITY DURING ANTIRRHINUM FLOWER DEVELOPMENT

Background: Homeotic genes controlling the identity of newer organs ha ve been characterized in several plant species. To determine whether c ells expressing these genes are specified to follow particular develop mental fates, we have studied the pattern of cell lineages in developi ng flowers of Antirrhinum. Each flower has four whorls of organs, and progenitor cells of these can be marked at particular stages of develo pment using a temperature-sensitive transposon. This allows the cell l ineages in the newer to be followed, as well as giving information abo ut rates of cell division. Results: We show here that, prior to the em ergence of organ primordia, cells in the floral meristem have not been allocated organ identities. After this time, lineage restrictions ari se between whorls, correlating with the onset of expression of genes t hat control organ identity. A further lineage restriction appears slig htly later on, between the dorsal and ventral surfaces of the petal. O ur results further suggest that the rates of cell division fluctuate d uring key stages of meristem development, perhaps as a consequence of meristem-identity gene expression. Conclusions: The patterns of lineag e restriction and organ-identity gene expression in early floral meris tems are consistent with some cells being allocated specific identitie s at about this stage of development. Plant cells cannot move relative to each other, so lineage restrictions in plants may reflect particul ar orientations and/or rates of growth at boundary regions.