MADS life on MAdLand II: Changes in DNA-binding specificity of MADS-domain transcription factors during the transition of plants to land

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The overarching goal of our projects in both phases of MAdLand is to investigate the role of MIKC-type MADS-domain transcription factors (MTFs) during the transition of plants to land. MTFs constitute tetrameric proteins termed floral quartet-like complexes (FQCs) that control major developmental processes in land plants. The high combinatorial potential and cooperative DNA binding of FQCs enables the control of target gene activity in an especially efficient and specific way. This feature of FQCs is probably causally linked to the evolutionary origin of complex novelties in land plants and thus is of great biological interest. MTFs originated in the stem group of extant streptophytes, comprising all land plants and their closest relatives (charophyte algae). During the first phase of MAdLand we demonstrated that in the stem group of extant land plants important changes within the protein-protein interaction domain of MTFs occurred. These changes followed a gene duplication that led to two subfamilies of MTFs, termed MIKCC- and MIKC*-type proteins. MIKCC- and MIKC*-type proteins are major constituents of two largely independent gene regulatory networks that control important aspects of sporophyte and gametophyte development in land plants, respectively. In the second phase of MAdLand we want to study the biological relevance of FQC formation of the only MIKCC-type protein MpMADS2 in the liverwort M. polymorpha, reflecting the functional gain achieved by the ability of FQC formation during the transition of plants to land. We will do that by studying MpMADS2 mutants unable of FQC formation that we generated during the first phase. In addition, we want to determine the impact of FQC formation on genome-wide DNA-binding of MTFs for selected charophyte species. This will also provide useful insight into potential target genes of MIKC-type proteins in charophytes, for which no such data are available so far. Moreover, we want to complement our findings about the evolution of the protein-protein interaction domain by investigating major evolutionary changes within the DNA-binding MADS-domain of MTFs. Preliminary data suggest that ancestral MTFs that existed prior to the transition of plants to land, had a more promiscuous mode of DNA-binding than MIKCC- and MIKC*-type proteins. In our project we want to determine the mutations in the MADS-domain of MTFs that brought about these changes in DNA-recognition, and the consequences for the activity of those genes that are controlled by the different types of MTFs.