Activation of apoplastic sugar at the transition stage may be essential for axillary bud outgrowth in the grasses

Abstract

Shoot branches develop from buds in leaf axils. Once formed from axillary meristems, the buds enter a transition stage before growing into branches. The buds may transition into dormancy if internal and environmental factors limit sucrose supply to the buds. A fundamental question is why sucrose can be limiting at the transition stage for bud outgrowth, whereas new buds continue to be formed. Sucrose is transported to sink tissues through symplastic or apoplastic pathways and a shift from symplastic to apoplastic pathway is common during seed and fruit development. In addition, symplastic connected tissues are stronger sinks than symplastically isolated tissues that rely on sugars effluxed to the apoplast. Recent studies in sorghum, sugarcane, and maize indicate activation of apoplastic sugar in buds that transition to outgrowth but not to dormancy, although the mode of sugar transport during bud formation is still unclear. Since the apoplastic pathway in sorghum buds was specifically activated during bud outgrowth, we posit that sugar for axillary bud formation is most likely supplied through the symplastic pathway. This suggests a key developmental change at the transition stage, which alters the sugar transport pathway of newly-formed buds from symplastic to apoplastic, making the buds a less strong sink for sugars. We suggest therefore that bud outgrowth that relies on overflow of excess sucrose to the apoplast will be more sensitive to internal and environmental factors that enhance the growth of sink tissues and sucrose demand in the parent shoot; whereas bud formation that relies on symplastic sucrose will be less affected by these factors.

Lessons from a century of apical dominance research

“…In contrast to perennial plants where callose deposition is often associated with bud endodormancy (including the apical bud; Wu et al , 2018 ), axillary bud release in Arabidopsis was not significantly affected by increased callose deposition, and subsequent growth of Arabidopsis axillary buds was only somewhat suppressed ( Paterlini et al , 2021 ). This observation and other studies in grass crops led Kebrom and Doust (2022) to propose that perhaps apoplastic transport to dormant buds in annual plants may be more important than symplastic transport.…”

“…Sugar levels increase rapidly after decapitation in the node and bud at a long distance from the site of decapitation ( Mason et al , 2014 ; Fichtner et al , 2017 ). Exogenous sugar supply in intact plants can also rapidly promote bud outgrowth, even in the presence of an actively growing shoot tip ( Mason et al , 2014 ; F. Wang et al , 2020 ; Kebrom and Doust, 2022 ). By sensing sugar status, such as after decapitation, plants with a long distance between the shoot tip and axillary buds can trigger bud release prior to the longer term drop in auxin content ( Mason et al , 2014 ; Barbier et al , 2015a , b ; Fichtner et al , 2017 , 2021a , b ; Bertheloot et al , 2020 ; M. Wang et al , 2021 ).…”