Abstract
Axillary buds (AXBs) of hybrid aspen (Populus tremula×P. tremuloides) contain a developing dwarfed shoot that becomes para-dormant at the bud maturation point. Para-dormant AXBs can grow out after stem decapitation, while dormant AXBs pre-require long-term chilling to release them from dormancy. The latter is mediated by gibberellin (GA)-regulated 1,3-β-glucanases, but it is unknown if GA is also important in the development, activation, and outgrowth of para-dormant AXBs. The present data show that para-dormant AXBs up-regulate GA receptor genes during their maturation, but curtail GA biosynthesis by down-regulating the rate-limiting GIBBERELLIN 3-OXIDASE2 (GA3ox2), which is characteristically expressed in the growing apex. However, decapitation significantly up-regulated GA3ox2 and GA4-responsive 1,3-β-glucanases (GH17-family; α-clade). In contrast, decapitation down-regulated γ-clade 1,3-β-glucanases, which were strongly up-regulated in maturing AXBs concomitant with lipid body accumulation. Overexpression of selected GH17 members in hybrid aspen resulted in characteristic branching patterns. The α-clade member induced an acropetal branching pattern, whereas the γ-clade member activated AXBs in recurrent flushes during transient cessation of apex proliferation. The results support a model in which curtailing the final step in GA biosynthesis dwarfs the embryonic shoot, while high levels of GA precursors and GA receptors keep AXBs poised for growth. GA signaling, induced by decapitation, reinvigorates symplasmic supply routes through GA-inducible 1,3-β-glucanases that hydrolyze callose at sieve plates and plasmodesmata.
Highlights
The architecture and three-dimensional shape of a tree arise and higher order branches exponentially enhances the comgradually during multiple seasons
The latter is mediated by gibberellin (GA)-regulated 1,3-β-glucanases, but it is unknown if gibberellic acid (GA) is important in the development, activation, and outgrowth of para-dormant axillary bud (AXB)
The results suggest that AXBs are GA deficient but highly sensitive to GA owing to low expression of the rate-limiting GA biosynthesis gene GIBBERELLIN 3-OXIDASE2 (GA3ox2) and high expression of two GIBBERELLIN INSENSITIVE DWARF1-like (GID1-like) GA receptor genes
Summary
The architecture and three-dimensional shape of a tree arise and higher order branches exponentially enhances the comgradually during multiple seasons. Phyllotaxis as branches grow out of axillary meristems (AXMs) that arise in the axils of leaves. The molecular mechanisms that regulate branching are thought to be conserved between annuals and woody perennials, there are some distinct differences in the initiation of AXMs, axillary bud (AXB) formation, and branching. In the annual Arabidopsis, AXM initiation is delayed, and the AXBs they produce are simple and lack scales (Grbic and Bleecker, 2000; Long and Barton, 2000; Greb et al, 2003). Deciduous woody perennials produce AXMs by default, in conjunction with nascent leaves, and in continuity with the shoot apical meristem (SAM) (Garrison, 1955). The first products of these AXMs are bud scales which confine an emerging dwarfed shoot (Romberger, 1963; Brunner et al, 2014; Rinne et al, 2015). Whereas syllepsis is highly variable due to its environmental responsiveness, the more robust phenomenon of prolepsis reflects strong apical dominance (Ceulemans et al, 1990; Cline, 1997; Wu and Stettler, 1998)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
