Arabidopsis: Flower Development and Patterning

Abstract The development of flowers and floral organs is directed by intricate genetic programmes, many aspects of which appear to be shared among angiosperms. Early acting genes establish floral meristem identity in flower primordia initiated at the periphery of the inflorescence meristem. Later, floral organ primordia arise at precise positions within these floral meristems and take on one of the four distinct identities (sepals, petals, stamens and carpels). The ABCE model, supported by both molecular and genetic experiments inArabidopsis, explains how a small number of regulatory genes (called floral homeotic genes or floral organ identity genes) act in different combinations to specify these different organ types. The floral organ identity genes encode transcription factors that form distinct higher order protein complexes in different regions of a flower primordium to control the expression of target genes responsible for organogenesis. Key Concepts Lateral organs produced by the shoot apical meristem during reproductive development acquire their identity as flowers through the action of floral meristem identity genes such asLEAFYandAPETALA1. The identities of each of the four organ types of a flower (sepal, petal, stamen and carpel) is conferred by a unique combination of floral organ identity gene activities, referred to as class A, B, C and E in the ABCE model. The activities of the class A, B and C genes are restricted to particular regions within a developing flower primarily, but not exclusively, through transcriptional regulation. The MADS domain transcription factors encoded by the class A, B, C and E genes form unique tetrameric transcriptional regulatory complexes in cells of each floral whorl. The transcriptional regulatory complexes formed by the A, B, C and E proteins regulate distinct sets of genes at different stages of flower development. Many aspects of the genetic programmes conferring floral meristem identity and floral organ identity are conserved among all angiosperms.