Hormonal orchestration of root apical meristem formation and maintenance in Arabidopsis.

Plant hormones are key regulators of a number of developmental and adaptive responses in plants, integrating the control of intrinsic developmental regulatory circuits with environmental inputs. Here we provide an overview of the molecular mechanisms underlying hormonal regulation of root development. We focus on key events during both embryonic and postembryonic development including specification of the hypophysis as a future organizer of the root apical meristem (RAM), hypophysis asymmetric division, specification of the quiescent centre (QC) and the stem cell niche (SCN), RAM maturation and maintenance of QC/SCN activity and RAM size. We address both well-established as well as newly-proposed concepts, highlight potential ambiguities in recent terminology and classification criteria of longitudinal root zonation and point to contrasting results and alternative scenarios for recent models. In the concluding remarks, we summarise the common principles of hormonal control during root development, the mechanisms potentially explaining often antagonistic outputs of hormone action and propose possible future research directions on hormones in the root.