Chapter 8 - Structure and Evolution of Plant MADS Domain Transcription Factors

Abstract

MADS domain transcription factors are involved in controlling many developmental processes in flowering plants, ranging from pollen and embryo sac development to root, flower, and fruit development. Beyond that they are probably of developmental importance in all other green plants. The developmental versatility of MADS domain proteins may depend in part on specific features of their MADS domain (i.e., highly conserved DNA-binding, dimerization, and nuclear localization domain). It makes DNA– and protein–protein interactions in quite a unique way, including recognition of a narrowed minor groove of DNA by the insertion of an arginine side chain. Presence of a MADS domain is the only common denominator uniting all MADS domain transcription factors, but a developmentally and evolutionarily important lineage of plant proteins, the MIKC-type proteins, acquired a second remarkable domain, the keratin-like (K) domain. This domain presents separate dimerization and tetramerization interfaces and thus enables combinatorial multimerization of MIKC-type MADS domain proteins. The number of MADS domain transcription factors increased by two orders of magnitude during the evolution of land plans, and acquired diverse, often complex and organ specific, expression domains. Heterotetramers of some MIKC-type proteins bind to two DNA sequence elements by looping the DNA in between. Such “floral quartets” act as important developmental switches determining floral organ identity and possibly many other developmental decisions in land plants as well. Thus, it seems that the developmental and evolutionary importance of plant MADS domain transcription factors is a consequence of their versatile mode of DNA binding and combinatorial multimerization. We are now a good way toward understanding these special features of MADS domain transcription factors in terms of the molecular details of their DNA-binding and protein–protein interaction domains.