Zsuzsanna Schwarz-Sommer

Genetic Control of Flower Development by Homeotic Genes in Antirrhinum majus

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.

Deficiens, a homeotic gene involved in the control of flower morphogenesis in Antirrhinum majus: the protein shows homology to transcription factors

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.

GLOBOSA: a homeotic gene which interacts with DEFICIENS in the control of Antirrhinum floral organogenesis.

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.

Characterization of the Antirrhinum floral homeotic MADS-box gene deficiens: evidence for DNA binding and autoregulation of its persistent expression throughout flower development.

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.

Molecular analysis of the waxy locus of Zea mays

SummaryThe structure of the wild-type waxy (wx+) locus was determined by sequence analysis of both a genomic and an almost full-size cDNA clone. The coding region comprises 3,718 bp and is composed of 14 exons and 13 small introns. The exons and the promoter region are G/C rich (60%–80%). All three waxy transcripts analysed so far reveal different polyadenylation sites and corresponding polyadenylation signals. The smallest of these mRNAs has a size of 2,263 nucleotides. Northern blot analysis suggests that the tissue-specific expression of the locus is due to transcriptional control. The insertion sites of all transposable element induced waxy mutations analysed have been mapped precisely within the locus. N-terminal sequencing of the mature wx+ protein leads to the identification of a maize amyloplast-specific transit peptide of 72 aminoacid residues.

PLENA and FARINELLI: redundancy and regulatory interactions between two Antirrhinum MADS‐box factors controlling flower development

We report the discovery of an Antirrhinum MADS‐box gene, FARINELLI (FAR), and the isolation of far mutants by a reverse genetic screen. Despite striking similarities between FAR and the class C MADS‐box gene PLENA (PLE), the phenotypes of their respective mutants are dramatically different. Unlike ple mutants, which show homeotic conversion of reproductive organs to perianth organs and a loss of floral determinacy, far mutants have normal flowers which are partially male‐sterile. Expression studies of PLE and FAR, in wild‐type and mutant backgrounds, show complex interactions between the two genes. Double mutant analysis reveals an unexpected, redundant negative control over the B‐function MADS‐box genes. This feature of the two Antirrhinum C‐function‐like genes is markedly different from the control of the inner boundary of the B‐function expression domain in Arabidopsis, and we propose and discuss a model to account for these differences. The difference in phenotypes of mutants in two highly related genes illustrates the importance of the position within the regulatory network in determining gene function.

Molecular cloning of the a1 locus of Zea mays using the transposable elements En and Mu1.

The a1 locus of Zea mays has been cloned using transposable elements as gene tags. The strategy was to make genomic libraries from maize stocks with a1 mutations induced either by En(Spm) or by Robertsons Mutator‐system. These libraries were then screened with either Spm‐I8 and En1, for the En‐containing mutant, or with Mu1 for the Mu‐induced mutation. There are many En and Mu1 hybridizing sequences present in the maize genome, however, by a process of cross‐screening of the positives from the two libraries and by molecular analysis of the En‐positive clones it was possible to identify clones in both libraries carrying all or part of the a1 gene.

Floral organ identity: 20 years of ABCs

One of the early successes of the application of molecular genetics to study plant development was the discovery of a series of genes that act together, in an apparently simple combinatorial model, to specify the identity of the different organs of a flower. Widely known as the ABC model, this framework for understanding has been investigated and modified over the course of the last two decades. The cast list of genes has been defined and, as other chapters in this volume will show, great progress has been made in understanding how they are regulated, how they act together, what they do and how they have contributed to the evolution of the flower in its varied forms. In this introductory review to the volume we will review the derivation and elaboration of the most current version of the ABC model, highlighting the modifications that have been necessary to ensure it fits the available experimental data. We will highlight the remaining difficulties in fitting the current model to the experimental data and propose a further modification to enable it to regain its applicability.

Influence of transposable elements on the structure and function of the A1 gene of Zea mays

The structure of the A1 gene of Zea mays was determined by sequencing cDNA and genomic clones. The gene is composed of four exons and three short introns. The 40.1‐kd A1 protein is an NADPH‐dependent reductase. Germinal derivatives of the mutable a1‐ml allele with either recessive or wild‐type phenotype have been isolated. Sequence analysis of these revertant alleles indicates that frame‐shift mutations abolish A1 gene function, whereas one additional amino acid within the protein sequence still allows wild‐type gene expression. The presence of a second, promoter‐like structure, upstream of the functional A1 gene promoter is discussed with respect to its possible involvement in differential expression of the A1 gene. The structure of the a1‐m2 8004, 3456 and 4412 alleles, featuring distinguishable phenotypes in the presence of Spm(En), was also determined. In all alleles the 1080‐bp‐long inhibitor (I) element is located 15 bp upstream of the CAAT box of the A1 gene promoter. The unusual response of al‐m2 alleles to trans‐active signals of the Spm(En) element is discussed with respect to the position of the I inserts. Also presented are data on the structure and insertion sites of transposable elements determined by cloning and sequencing of the mutable a1 alleles a1‐mpapu, a1‐mr 102 and al‐ml.

Molecular analysis of the En/Spm transposable element system of Zea mays

The nucleotide sequence of the autonomous transposable element En‐1 isolated from the wx‐844::En‐1 allele has been determined. En‐1 is 8287 bp long. The structure of the mosaic gene 1, coding for the major En transcript, has been established. The promoter of gene 1 is located in the highly structured left end of the element and the gene spans almost the entire length of En‐1. The first intron of gene 1 is 4434 nucleotides long and contains two large open reading frames, 2714 bp and 761 bp in size, which hybridize to minor RNA species in Northern blot experiments.