Tissue regeneration after bark girdling: an ideal research tool to investigate plant vascular development and regeneration

Regeneration is a common strategy for plants to survive the intrinsic and extrinsic challenges they face through their life cycle, and it may occur upon wounding. Bark girdling is applied to improve fruit production or harvest bark as medicinal material. When tree bark is removed, the cambium and phloem will be peeled off. After a small strip of bark is removed from trees, newly formed periderm and wound cambium develop from the callus on the surface of the trunk, and new phloem is subsequently derived from the wound cambium. However, after large-scale girdling, the newly formed sieve elements (SEs) appear earlier than the regenerated cambium, and both of them derive from differentiating xylem cells rather than from callus. This secondary vascular tissue regeneration mainly involves three key stages: callus formation and xylem cell dedifferentiation; SEs appearance and wound cambium formation. The new bark is formed within 1 month in poplar, Eucommia; thus, it provides high temporal resolution of regenerated tissues at different stages. In this review, we will illustrate the morphology, gene expression and phytohormone regulation of vascular tissue regeneration after large-scale girdling in trees, and also discuss the potential utilization of the bark girdling system in studies of plant vascular development and tissue regeneration.