Sugar demand, not auxin, is the initial regulator of apical dominance

Abstract

For almost a century the plant hormone auxin has been central to theories on apical dominance, whereby the growing shoot tip suppresses the growth of the axillary buds below. According to the classic model, the auxin indole-3-acetic acid is produced in the shoot tip and transported down the stem, where it inhibits bud growth. We report here that the initiation of bud growth after shoot tip loss cannot be dependent on apical auxin supply because we observe bud release up to 24 h before changes in auxin content in the adjacent stem. After the loss of the shoot tip, sugars are rapidly redistributed over large distances and accumulate in axillary buds within a timeframe that correlates with bud release. Moreover, artificially increasing sucrose levels in plants represses the expression of BRANCHED1 (BRC1), the key transcriptional regulator responsible for maintaining bud dormancy, and results in rapid bud release. An enhancement in sugar supply is both necessary and sufficient for suppressed buds to be released from apical dominance. Our data support a theory of apical dominance whereby the shoot tip's strong demand for sugars inhibits axillary bud outgrowth by limiting the amount of sugar translocated to those buds.shoot branching | sink demand | decapitation | girdling | long-distance signaling

Abscisic acid signaling is controlled by a BRANCHED1/HD-ZIP I cascade in Arabidopsis axillary buds

“…Sucrose treatments on buds caused down-regulation of BRC1 (SI Appendix, Fig. S3B) as described in pea and rose (8,9). Likewise, HB21 and HB53 (but not HB40) mRNA levels decreased after a sugar treatment (SI Appendix, Fig.…”

Transcriptome Profiling of Tiller Buds Provides New Insights into PhyB Regulation of Tillering and Indeterminate Growth in Sorghum

“…The dormancy-versus-growth fates of axillary buds in diverse species have been linked to the level of Suc/ sugar-signaling and to changes in the expression of Sucresponsive genes in buds (Kebrom et al, 2012;Rabot et al, 2012;Kebrom and Mullet, 2014;Mason et al, 2014). Sugar signaling occurs through pathways involving hexokinase, trehalose-6-phosphate/SnRK1, and TOR, and by direct effects of sugars on growth and metabolism (Rolland et al, 2006;Lunn et al, 2014;Ruan, 2014).…”

“…In the tiller inhibition (tin) mutant of wheat, diversion of Suc away from buds to elongating internodes inhibits bud outgrowth (Kebrom et al, 2012). Sugar also stimulates bud outgrowth in pea and rose Mason et al, 2014;Barbier et al, 2015b). The expression of the Suc-starvation-inducible Gln-dependent Asn synthase (ASN) gene was increased and Suc-inducible PFP-gene encoding pyrophosphate-Fru-6-phosphate 1-phosphotransferase was down-regulated in dormant buds of tin in wheat and in buds repressed by defoliation in sorghum (Kebrom et al, 2012;Kebrom and Mullet, 2014).…”

Mechanisms of vegetative propagation in bulbs : a molecular approach

“…Changes in the sink to source dynamics in the plant also play an important role in the activation of axillary bud outgrowth. Sucrose has been proposed as an early activator of axillary bud outgrowth when the main sink of the plant, the apex, is removed, making the dormant buds the new sink organs (Mason et al 2014).…”