Plant development: a TALE story

Plant development depends on the activity of a group of dividing cells called the meristem. Extensive genetic analyses have identified the major regulators of the shoot apical meristem (SAM), which control the development of all aerial organs. Among them, the three-amino-acid-loop-extension (TALE) class of homeoproteins has been shown to control meristem formation and/or maintenance, organ morphogenesis, organ position, and several aspects of the reproductive phase. This family contains the KNOTTED-like homeodomain (KNOX) and BEL1-like Homeodomain (BELL) members, which function as heterodimers. In this review, we have reported the functions of the TALE members throughout the Arabidopsis life cycle. Genetic analyses revealed a complex network, as TALE members exhibit both overlapping and antagonistic activities. The characterization of a new KNOX member (KNATM), which lacks a homeodomain and interacts with other members to modulate their activities, adds another layer of complexity to this network. While the mode of action of these transcription factors is still largely unknown, they have been implicated in the regulation of several hormonal pathways, providing a link between gene regulatory networks and signaling in the SAM.