Molecular networks of periderm development in Arabidopsis

Abstract

A periderm is developed by most dicotyledons and gymnosperms to protect the vascular cylinder against the environment and replaces the epidermis once it cannot accommodate the radial thickening during secondary growth anymore. The periderm consists of the cork cambium producing inwards the phelloderm and outwards the phellem or cork. Phellem cells are highly suberized and develop into dead cells. Despite its biological and economic importance, the molecular mechanisms of periderm development are largely unknown. To shed light on the periderm development of Arabidopsis, marker lines for phellem and cork cambium cells were developed. Six stages of periderm growth in the root and hypocotyl were defined in this study. At stage 0 the pericycle is surrounded by the endodermis, cortex and epidermis cell layers while at stage 6 the differentiated periderm is the outside tissue of root and hypocotyl. In addition to the periderm development, these stages reveal the fate of the outside layers (endodermis, cortex and epidermis). The loosening of the outside layers follows a specific pattern that involves programmed cell death and cell abscission. Furthermore, this study shows that perturbing the cell number and identity and cell wall composition of these outside layers has an effect on the periderm development, demonstrating that a mechanical interaction occurs between the periderm cells and the outer layers. In contrast, there seems to be some kind of competition between the lateral root formation and the periderm development. In this regard, it was demonstrated that auxin plays an important role in the periderm development and many auxin-dependent early LR regulators are expressed in the periderm and are involved in its development.