title: The embryonic shoot: a lifeline through winter.
abstract: The tiny vascular axis of the embryo emerges post-embryonically as an elaborate and critical infrastructure, pervading the entire plant system. Its expansive nature is especially impressive in trees, where growth and development continue for extended periods. While the shoot apical meristem (SAM) orchestrates primary morphogenesis, the vascular system is mapped out in its wake in the provascular cylinder, situated just below the emerging leaf primordia and surrounding the rib meristem. Formation of leaf primordia and provascular tissues is incompatible with the harsh conditions of winter. Deciduous trees of boreal and temperate climates therefore enter a survival mode at the end of the season. However, to be competitive, they need to maximize their growth period while avoiding cellular frost damage. Trees achieve this by monitoring photoperiod, and by timely implementation of a survival strategy that schedules downstream events, including growth cessation, terminal bud formation, dormancy assumption, acquisition of freezing tolerance, and shedding of leaves. Of central importance are buds, which contain an embryonic shoot that allows shoot development and elongation in spring. The genetic and molecular processes that drive the cycle in synchrony with the seasons are largely elusive. Here, we review what is known about the signals and signal conduits that are involved, the processes that are initiated, and the developmental transitions that ensue in a terminal bud. We propose that addressing dormancy as a property of the SAM and the bud as a unique shoot type will facilitate our understanding of winter dormancy.
authors.affiliation: Department of Plant & Environmental Sciences, Norwegian University of Life Sciences, PO Box 5003, 1432 Ås, Norway.
authors.forename: Christiaan
authors.initials: C
authors.lastname: van der Schoot
authors.forename: Laju K
authors.initials: LK
authors.lastname: Paul
authors.forename: Päivi L H
authors.initials: PL
authors.lastname: Rinne
mesh_terms: D018506:Gene Expression Regulation, Plant; D018519:Meristem / Q000235:genetics* / Q000254:growth & development* / Q000378:metabolism; D057445:Plant Dormancy; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D032107:Populus / Q000254:growth & development; D012621:Seasons; D012639:Seeds / Q000254:growth & development
chemical_list: D010940:Plant Proteins
keywords: CENTRORADIALIS-LIKE1; FLOWERING LOCUS T; GH17 family proteins; deciduous perennial; dormancy cycle; gibberellic acid; lipid body; poplar; rib meristem; shoot apical meristem; terminal bud.