MADS-box genes are a family of transcription factors found in plants, animals and fungi and are characterized by a conserved MADS domain. MADS-box genes have been found in all major plant lineages studied to date including green algae, mosses, ferns, gymnosperms and angiosperms. In Arabidopsis, MADS-box genes can be divided into three groups: Type I and Type II MADS box genes and MADS-like genes. Type II MADS box genes in plants can be further subdivided into the MIKCC and MIKC* types (reviewed in (Becker and Theissen, 2003)). Type I MADS-box genes contain a conserved MADS-box gene, but lack the relatively conserved I and K and highly divergent C domains found in the Type II MIKC type MADS-box genes.
Function has been characterized for less than one fifth of the members of the MADS-box gene family. However, in recent years several thorough expression and evolutionary studies have been published investigating the underlying similarities and differences among these genes, especially the TYPE II MIKCC genes.
Among the TYPE II MIKC genes there have been approximately 19 genes that have been functionally characterized. These genes include many of the so called floral MADS-box genes (Theissen et al., 2000). These genes, along with other genes that have been less well characterized but are hypothesized to preferentially affect floral organ development, includes AGAMOUS (AG), AGL6, AGL13, APETALA1 (AP1), APETALA3 (AP3), CAULIFLOWER (CAL), PISTILLATA (PI), SEEDSTICK (STK), SEPALLATA1,2,3,4 (SEP1,2,3,4), SHATTERPROOF1 and 2 (SHP1, 2) and TRANSPARENT TESTA16 (TT16). Additionally, several genes with either identified or hypothesized function regulating floral timing and inflorescence development have been identified including AGL24, AGL27, FLOWERING LOCUS C (FLC) (also known as FLOWERING LOCUS F (FLF)), FLC1, FLC2, FLOWERING LOCUS M (FLM) (also known as MADS AFFECTING FLOWERING1 (MAF1), FRUITFUL (FUL), SHORT VEGETATIVE PHASE (SVP) and SUPRESSOR OF CONSTANS (SOC1). Lastly, there are the non-floral genes, a group that mostly have currently unknown function. These genes have diverse expression that most likely reflects a multitude of function including floral development as well as root, seed, seedling, stem and leaf development. These genes include AGL12, AGL14, AGL15, AGL16, AGL17, AGL18, AGL19, AGL21 and ANR1.
In contrast to the many studies of TYPE II MIKC MADS box genes, only one Type I MADS-box gene, PHERES1 (formerly AGL37), has been investigated to date. The function of PHERES1 has not yet been described, but based on expression studies it is maternally regulated by MEDEA and appears to be involved in proper seed and embryo development (Kohler et al., 2003; Kohler et al., 2005). With the multitude of recent advances in analyses of gene expression and genomics we can expect many more studies of the pathways involving previously unidentified MADS-box genes along with functional characterization of these genes.
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