Abstract
Based on their evolutionary origin, MADS box transcription factor genes have been divided into two classes, namely, type I and II. The plant-specific type II MIKC MADS box genes have been most intensively studied and shown to be key regulators of developmental processes, such as meristem identity, flowering time, and fruit and seed development. By contrast, very little is known about type I MADS domain transcription factors, and they have not attracted interest for a long time. A number of recent studies have now indicated a key regulatory role for type I MADS box factors in plant reproduction, in particular in specifying female gametophyte, embryo, and endosperm development. These analyses have also suggested that type I MADS box factors are decisive for setting reproductive boundaries between species.