The development of the periderm: the final frontier between a plant and its environment.

The periderm acts as the first line of defence for a plant, protecting wood and phloem from abiotic and biotic stresses. During secondary growth, through the increase in girth of plant organs, the periderm replaces the epidermis as the outermost tissue. The phellogen, a bifacial post-embryonic meristem, forms the phelloderm inwards (toward the vasculature) and the suberized phellem outwards (toward the environment). These three tissues are collectively referred to as the periderm. Here, we summarize recent findings on the molecular mechanisms of periderm development by describing periderm formation in connection to the fate of the surrounding tissues, by discussing common regulatory hubs between the vascular cambium and the phellogen, and by highlighting transcription factors (TFs) controlling phellem differentiation.