The meristem-to-organ boundary: more than an extremity of anything.

In plant shoot meristems, cells with indeterminate fate are separated from determinate organ founder cells by morphological boundaries. Organ founder cells are selected at sites of auxin accumulation. Auxin is channeled between cells via efflux carrier proteins, but influx carriers are needed to concentrate auxin in the outer meristem layer. The genetic programmes executed by organs and meristems are established by mutual repression of transcription factors, involving the sequestration of enhancer elements into DNA loops. Boundary cells play a dual role in separating and maintaining meristem and organ domains, and express unique genes that reduce cell division and auxin efflux carrier activity, but activate meristematic gene expression. Boundary positions depend on signals emitted from indeterminate cells at the meristem center.