Gene Expression in the Early Floral Meristem.

The differential expression of genetic information facilitates the regulation of complex developmental pathways. Thus, to understand the control of morphogenesis, it is essential that the patterns of developmentally regulated gene activity and the basis for this regulation be determined. Perhaps the clearest illustration of the potential of this type of investigation comes from the analysis of early embryogenesis in Drosophila (reviewed by Akam, 1987; lngham and Arias, 1992; St Johnston ’ and Nüsslein-Volhard, 1992; Wilkens, 1993). The descriptions of gene expression patterns during Drosophila embryogenesis revealed the genetic mechanisms underlying the control of anterior-posterior axis determination and showed that the cellular specification of segment boundaries in the embryo occurs long before segmentation is manifest. In recent years, the Drosophila studies have demonstrated that the molecular basis for the developmental regulation of embryonic gene activity is complex and diverse, including transcriptional, post-transcriptional, and translational control mechanisms as well as mechanisms related to the structural organization of chromatin (see Wilkens, 1993). The analysis of developmentally regulated gene expression in plants should be equally exciting and informative, especially when applied to the study of meristem function. Meristems are the site of all postembryonic organ initiation in angiosperms, yet we understand very little about how meristems function. In this review, I will discuss severa1 patterns of meristem gene expression that occur during the vegetative-to-floral transition, and I will examine what these patterns reveal about the control of floral meristem initiation. Severa1 other recent reviews discuss the evidence that alterations in meristem gene activity are associated with the transition from vegetative to floral development (Bernier, 1988; Kelly and Meeks-Wagner, 1993).