Abstract

Multicellular control circuits can conveniently be studied in plants. Plant stem cells reside in niches and are maintained by short-range signals emanating from organizing centres. The Arabidopsis PLETHORA genes encode transcription factors required for root stem cell specification. PLT expression is induced by the plant hormone auxin. The PLT gene clade regulates expression of the PIN facilitators of polar auxin transport, which contributes to a specific auxin transport route that maintains stem cells at the appropriate position. PLT genes also regulate auxin biosynthesis, which is relevant for root development and phyllotaxis. We address the properties of PLT -auxin feedback loops by gene and protein network analysis and computational modelling.

Stem cells and their daughters in the root display specific asymmetric divisions at fixed locations. We investigate how such divisions are spatially regulated. The SHORTROOT -SCARECROW transcription factor pathway provides mitotic potential to the stem cell daughters that form the proximal meristem. This activity involves the conserved RETINOBLASTOMA -RELATED (RBR) pocket protein, and we established molecular links between the RBR pathway and SCR action that form a feedback control system. Formal analysis of this feedback circuit indicates that it acts as a bistable switch ensuring asymmetric divisions at fixed positions. Our work illustrates how divisions that shape tissues are robustly positioned by dynamic regulatory circuits combining intracellular and extracellular loops.