Interactions among Proteins of Floral MADS-Box Genes in Basal Eudicots: Implications for Evolution of the Regulatory Network for Flower Development

Abstract

Floral MADS-box genes encode transcription factors that play critical roles in the development and evolution of the flower. Proteins of floral MADS-box genes regulate the expression of their downstream genes by forming various homodimers/heterodimers and quaternary complexes. Interactions among proteins of floral MADS-box genes have been documented in several model species, yet the information accumulated so far is still not sufficient to draw a general picture of the evolution of the interactions. We have characterized 28 putative floral MADS-box genes from three representative basal eudicots (i.e., Euptelea pleiospermum, Akebia trifoliata, and Pachysandra terminalis) and investigated the protein-protein interactions (PPIs) among the proteins encoded by these genes using yeast two-hybrid assays. We found that, although the PPIs in basal eudicots are largely consistent with those in core eudicots and monocots, there are lineage-specific features that have not been observed elsewhere. We also reconstructed the evolutionary histories of the PPIs among members of seven MADS-box gene lineages (i.e., AP1, AP3, PI, AG, STK, AGL2, and AGL9) in angiosperms. We revealed that the PPIs were extremely conserved in nine (or 32.1%) of the 28 possible combinations, whereas considerable variations existed in seven (25.0%) of them; in the remaining 12 (or 42.9%) combinations, however, no interaction was observed. Notably, most of the PPIs required for the formation of quaternary complexes, as suggested by the "quartet model," were highly conserved. This suggested that the evolutionarily conservative PPIs may have played critical roles in the establishment of the basic structure (or architecture) of the flower and experienced coevolution to maintain their functions. The evolutionarily variable PPIs, however, seem to have played subsidiary roles in flower development and have contributed to the variation in floral traits.

Loss of deeply conserved C-class floral homeotic gene function and C- and E-class protein interaction in a double-flowered ranunculid mutant

“…This result suggests that the missing eight or 13 amino acids in the K domain of 'Double White' ThtAG1 are essential for mediating interaction with ThtSEP3. Importantly, it also provides much needed biochemical evidence in a noncore eudicot for the C-E interaction predicted by the floral quartet model, in line with findings of high conservation of protein-protein interactions in other basal eudicots (53).…”

Developmental and evolutionary diversity of plant MADS-domain factors: insights from recent studies

“…Around this time, the yeast two-hybrid system was introduced as a method with which to study proteinprotein interactions and, a few years later, evidence was provided for multiple interactions between Antirrhinum floral homeotic MIKC C -type MADS-domain proteins (Davies et al, 1996). These initial studies were followed by large-scale MADS-domain protein interaction screenings in a variety of species, which provided information about MADS-domain protein dimerization potential (Immink et al, 2003;de Folter et al, 2005;Leseberg et al, 2008;Liu et al, 2010;Ruokolainen et al, 2010).…”

“…Around this time, the yeast two-hybrid system was introduced as a method with which to study proteinprotein interactions and, a few years later, evidence was provided for multiple interactions between Antirrhinum floral homeotic MIKC C -type MADS-domain proteins (Davies et al, 1996). These initial studies were followed by large-scale MADS-domain protein interaction screenings in a variety of species, which provided information about MADS-domain protein dimerization potential (Immink et al, 2003;de Folter et al, 2005;Leseberg et al, 2008;Liu et al, 2010;Ruokolainen et al, 2010).The next breakthrough in our understanding of MADS-domain protein function came from the finding that MIKC C -type proteins can assemble into higher-order complexes (Egea-Cortines et al, 1999;Honma and Goto, 2001), which led to the postulation of the 'floral quartet' model (see Box 2). According to this model, a tetrameric protein complex consisting of two dimers binds to a target DNA sequence containing two CArG boxes and thereby generates a DNA loop between the two binding sites (Theissen, 2001;Theissen and Saedler, 2001).…”

TheseirenaB Class Floral Homeotic Mutant of California Poppy (Eschscholzia californica) Reveals a Function of the Enigmatic PI Motif in the Formation of Specific Multimeric MADS Domain Protein Complexes

“…In line with this, expression of B class genes is relatively conserved in higher eudicots and grassy monocots and is found predominantly in developing stamens and petals or lodicules (Sommer et al, 1990;Bowman et al, 1991b;Ambrose et al, 2000;Nagasawa et al, 2003;Vandenbussche et al, 2004;Benlloch et al, 2009). Flowers of species outside the core eudicots often have a less well differentiated perianth; correspondingly, the B class genes show a higher degree of expression divergence and B class proteins show more variation in protein interaction partners (Kramer and Irish, 1999;Kim et al, 2005;Liu et al, 2010).…”