ACTIVATION OF BASAL CELLS OF THE APICAL MERISTEM DURING SEPAL FORMATION IN TOMATO

Although great advances have been made in research on the regulation o f primordium fate in the floral meristem, our understanding of the mol ecular events occurring during the floral transition remains incomplet e. Via a careful analysis of the expression patterns of five genes enc oding housekeeping functions during the floral transition in tomato (u sing both in situ hybridization and enzyme histochemistry), we identif ied a particular phase of floral development (sepal initiation) at whi ch cells located toward the base of the meristem show a high level of cellular metabolism, whereas cells at the tip of the meristem dome sho w little activity. At other stages of floral development, the probes u sed showed gene-specific patterns of expression generally consistent w ith our previous investigation of the vegetative apical meristem. Our data, in conjunction with other reports in the literature, enabled us to postulate that relative activation of basal cells of the meristem m ay be of general occurrence during the transition to flowering. Such a hypothesis could account for recent observations using periclinal chi meras on the effect of L3 genotypes on flower development and have a b earing on the expected mechanism by which the number of primordia gene rated by a floral meristem is regulated.