A hitchhiker's guide to the MADS world of plants

The MADS-box genes, encoding the MADS-domain family of transcription factors, are involved in controlling all major aspects of the life of land plants. The MADS-domain family is characterized by the highly conserved DNA-binding MADS domain [1] (Figure 1a). The MADS domain is about 58 amino acids long and is encoded by a DNA sequence termed the MADS box. The first MADS-box gene to be isolated was ARG80 from Saccharomyces cerevisiae (Table 1) [2]. The term MADS-box gene, however, was coined later, after four subsequently characterized 'founding family members': MINICHROMOSOME MAINTENANCE 1 (MCM1) from S. cerevisiae, AGAMOUS (AG) from Arabidopsis thaliana, DEFICIENS (DEF) from Antirrhinum majus and SERUM RESPONSE FACTOR (SRF) from Homo sapiens (Table 1) [3]. Recent data suggest that the MADS box originated from a DNA sequence encoding a region of subunit A of the DNA-winding or -unwinding topoisomerase IIA enzymes, which are involved in, for example, DNA replication [4]. There is evidence that a gene encoding topoisomerase IIA subunit A was duplicated in a common ancestor of the extant eukaryotes. One of the duplicates accumulated sequence changes such that a domain with increased sequence specificity in DNA binding originated - the MADS domain [4]. As a result of subsequent gene duplication and divergence, the two major types of MADS-box genes recognized so far - type I or SRF-like and type II or MEF2-like (after MYOCYTE ENHANCER FACTOR 2) - were established in the most recent common ancestor (MRCA) of extant eukaryotes [4]. These two types are distinguished on the basis of sequence dissimilarity and also differ in sequence specificity in DNA binding and the amount of DNA bending that they induce [1].The MADS-box family subsequently diversified in remarkably different ways in the various eukaryotic lineages during evolution. The number of MADS-box genes has remained quite small in protists, animals and fungi. In contrast, their numbers greatly increased in some plant