The floral quartet model of floral organ specification poses that different tetramers of MIKC-type MADS-domain transcription factors control gene expression and hence the identity of floral organs during development. Here, we provide a brief history of the floral quartet model and review several lines of recent evidence that support the model. We also describe how the model has been used in contemporary developmental and evolutionary biology to shed light on enigmatic topics such as the origin of land and flowering plants. Finally, we suggest a novel hypothesis describing how floral quartet-like complexes may interact with chromatin during target gene activation and repression.

Summary: Wheat (Triticum aestivum) is one of the most important crops worldwide. Given a growing global population coupled with increasingly challenging cultivation conditions, facilitating wheat breeding by fine‐tuning important traits is of great importance. MADS‐box genes are prime candidates for this, as they are involved in virtually all aspects of plant development. Here, we present a detailed overview of phylogeny and expression of 201 wheat MIKC‐type MADS‐box genes. Homoeolog retention is significantly above the average genome‐wide retention rate for wheat genes, indicating that many MIKC‐type homoeologs are functionally important and not redundant. Gene expression is generally in agreement with the expected subfamily‐specific expression pattern, indicating broad conservation of function of MIKC‐type genes during wheat evolution. We also found extensive expansion of some MIKC‐type subfamilies, especially those potentially involved in adaptation to different environmental conditions like flowering time genes. Duplications are especially prominent in distal telomeric regions. A number of MIKC‐type genes show novel expression patterns and respond, for example, to biotic stress, pointing towards neofunctionalization. We speculate that conserved, duplicated and neofunctionalized MIKC‐type genes may have played an important role in the adaptation of wheat to a diversity of conditions, hence contributing to the importance of wheat as a global staple food.

Understanding the mode and mechanisms of the evolution of the angiosperm flower is a long-standing and central problem of evolutionary biology and botany. It has essentially remained unsolved, however. In contrast, considerable progress has recently been made in our understanding of the genetic basis of flower development in some extant model species. The knowledge that accumulated this way has been pulled together in two major hypotheses, termed the ‘ABC model’ and the ‘floral quartet model’. These models explain how the identity of the different types of floral organs is specified during flower development by homeotic selector genes encoding transcription factors. We intend to explain how the ‘ABC model’ and the ‘floral quartet model’ are now guiding investigations that help to understand the origin and diversification of the angiosperm flower. Investigation of orthologues of class B and class C floral homeotic genes in gymnosperms suggest that bisexuality was one of the first innovations during the origin of the flower. The transition from dimer to tetramer formation of floral homeotic proteins after establishment of class E proteins may have increased cooperativity of DNA binding of the transcription factors controlling reproductive growth. That way, we hypothesize, better ‘developmental switches’ originated that facilitated the early evolution of the flower. Expression studies of ABC genes in basally diverging angiosperm lineages, monocots and basal eudicots suggest that the ‘classical’ ABC system known from core eudicots originated from a more fuzzy system with fading borders of gene expression and gradual transitions in organ identity, by sharpening of ABC gene expression domains and organ borders. Shifting boundaries of ABC gene expression may have contributed to the diversification of the angiosperm flower many times independently, as may have changes in interactions between ABC genes and their target genes.

The organs of a eudicot flower are specified by four functional classes, termed class A, B, C and E, of MADS domain transcription factors. The combinatorial formation of tetrameric complexes, so called ‘floral quartets’, between these classes is widely believed to represent the molecular basis of floral organ identity specification. As constituents of all complexes, the class E floral homeotic proteins are thought to be of critical relevance for the formation of floral quartets. However, experimental support for tetrameric complex formation remains scarce. Here we provide physico-chemical evidence that in vitro homotetramers of the class E floral homeotic protein SEPALLATA3 from Arabidopsis thaliana bind cooperatively to two sequence elements termed ‘CArG boxes’ in a phase-dependent manner involving DNA looping. We further show that the N-terminal part of SEPALLATA3 lacking K3, a subdomain of the protein–protein interactions mediating K domain, and the C-terminal domain, is sufficient for protein dimerization, but not for tetramer formation and cooperative DNA binding. We hypothesize that the capacity of class E MADS domain proteins to form tetrameric complexes contributes significantly to the formation of floral quartets. Our findings further suggest that the spacing and phasing of CArG boxes are important parameters in the molecular mechanism by which floral homeotic proteins achieve target gene specificity.

Homeotic MADS box genes encoding transcription factors specify the identity of floral organs by interacting in a combinatorial way. The ‘floral quartet model’, published several years ago, pulled together several lines of evidence suggesting that floral homeotic proteins bind as tetramers to two separated DNA sequence elements termed ‘CArG boxes’ by looping the intervening DNA. However, experimental support for ‘floral quartet’ formation remains scarce. Recently, we have shown that the class E floral homeotic protein SEPALLATA3 (SEP3) is sufficient to loop DNA in floral-quartet-like complexes in vitro . Here, we demonstrate that the class B floral homeotic proteins APETALA3 (AP3) and PISTILLATA (PI) do only weakly, at best, form floral-quartet-like structures on their own. However, they can be incorporated into such complexes together with SEP3. The subdomain K3 of SEP3 is of critical importance for the DNA-bound heterotetramers to be formed and is capable to mediate floral quartet formation even in the sequence context of AP3 and PI. Evidence is presented suggesting that complexes composed of SEP3, AP3 and PI form preferentially over other possible complexes. Based on these findings we propose a mechanism of how target gene specificity might be achieved at the level of floral quartet stability.