In plants, flowering is triggered by endogenous and environmental signals. CONSTANS (CO) promotes flowering of Arabidopsis in response to day length. Four early target genes of CO were identified using a steroid-inducible version of the protein. Two of these genes, SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and FLOWERING LOCUS T (FT), are required for CO to promote flowering; the others are involved in proline or ethylene biosynthesis. The SOC1 and FT genes are also regulated by a second flowering-time pathway that acts independently of CO. Thus, early target genes of CO define common components of distinct flowering-time pathways.

Deficiens (defA+) is a homeotic gene involved in the genetic control of Antirrhinum majus flower development. Mutation of this gene (defA‐1) causes homeotic transformation of petals into sepals and of stamina into carpels in flowers displaying the ‘globifera’ phenotype, as shown by cross sections and scanning electronmicroscopy of developing flowers. A cDNA derived from the wild type defA+ gene has been cloned by differential screening of a subtracted ‘flower specific’ cDNA library. The identity of this cDNA with the defA+ gene product has been confirmed by utilizing the somatic and germinal instability of defA‐1 mutants. According to Northern blot analyses the defA+ gene is expressed in flowers but not in leaves, and its expression is nearly constant during all stages of flower development. The 1.1 kb long mRNA has a 681 bp long open reading frame that can code for a putative protein of 227 amino acids (mol. wt 26.2 kd). At its N‐terminus the DEF A protein reveals homology to a conserved domain of the regulatory proteins SRF (activating c‐fos) in mammals and GRM/PRTF (regulating mating type) in yeast. We discuss the structure and the possible function of the DEF A protein in the control of floral organogenesis.

Homeotic mutants have been useful for the study of animal development. Such mutants are also known in plants. The isolation and molecular analysis of several homeotic genes in Antirrhinum majus provide insights into the underlying molecular regulatory mechanisms of flower development. A model is presented of how the characteristic sequential pattern of developing organs, comprising the flower, is established in the process of morphogenesis.

GLOBOSA (GLO) is a homeotic gene whose mutants show sepaloid petals and carpelloid stamens. The similarity of Glo mutants to those of the DEFICIENS (DEFA) gene suggests that the two genes have comparable functions in floral morphogenesis. The GLO cDNA has been cloned by virtue of its homology to the MADS‐box, a conserved DNA‐binding domain also contained in the DEFA gene. We have determined the structure of the wild type GLO gene as well as of several glo mutant alleles which contain transposable element insertions responsible for somatic and germinal instability of Glo mutants. Analyses of the temporal and spatial expression patterns of the DEFA and GLO genes during development of wild type flowers and in flowers of various stable and unstable defA and glo alleles indicate independent induction of DEFA and GLO transcription. In contrast, organ‐specific up‐regulation of the two genes in petals and stamens depends on expression of both DEFA and GLO. In vitro DNA‐binding studies were used to demonstrate that the DEFA and GLO proteins specifically bind, as a heterodimer, to motifs in the promoters of both genes. A model is presented which proposes both combinatorial and cross‐regulatory interactions between the DEFA and GLO genes during petal and stamen organogenesis in the second and third whorls of the flower. The function of the two genes controlling determinate growth of the floral meristem is also discussed.

We have determined the structure of the floral homeotic deficiens (defA) gene whose mutants display sepaloid petals and carpelloid stamens, and have analysed its spatial and temporal expression pattern. In addition, several mutant alleles (morphoalleles) were studied. The results of these analyses define three functional domains of the DEF A protein and identify in the deficiens promoter a possible cis‐acting binding site for a transcription factor which specifically upregulates expression of deficiens in petals and stamens. In vitro DNA binding studies show that DEF A binds to specific DNA motifs as a heterodimer, together with the protein product of the floral homeotic globosa gene, thus demonstrating that the protein encoded by deficiens is a DNA binding protein. Furthermore, Northern analysis of a temperature sensitive allele at permissive and non‐permissive temperatures provides evidence for autoregulation of the persistent expression of deficiens throughout flower development. A possible mechanism of autoregulation is discussed.