KNOXing on the BELL : TALE Homeobox Genes and Meristem Activity

Abstract

All plant organs are derived from meristems. The shoot apical meristem (SAM) produces the aerial part of the plant. It has two main functions: the maintenance of a group of stem cells at the center of the meristem and the initiation of organs at its periphery. The organs are initiated in a regular spatial pattern, referred to as phyllotaxy, and are separated from the surrounding tissue by a boundary domain. The KNOTTED-like homeobox (KNOX) family of transcription factors plays a key role in the control of SAM activity. These proteins belong to the three amino acid loop extension (TALE) homeodomain superclass and form heterodimers with other TALE proteins belonging to the BEL1-like (BELL) family. The KNOX proteins regulate the different activities of the SAM. They control SAM maintenance, boundary establishment, the correct patterning of organ initiation and the development of axillary meristems. They exert their effects through the regulation of several hormonal pathways. KNOX proteins repress gibberellin (GA) biosynthesis and activate cytokinin (CK) synthesis and signaling. In addition to their role in the SAM, they contribute to leaf form diversity. In plants with simple leaves, KNOX genes are expressed in the SAM and downregulated in leaf primordia, whereas in plants with dissected leaves their expression is reactivated in leaves.