Inhibition of Tiller Bud Outgrowth in thetinMutant of Wheat Is Associated with Precocious Internode Development

Abstract

Tillering (branching) is a major yield component and, therefore, a target for improving the yield of crops. However, tillering is regulated by complex interactions of endogenous and environmental signals, and the knowledge required to achieve optimal tiller number through genetic and agronomic means is still lacking. Regulatory mechanisms may be revealed through physiological and molecular characterization of naturally occurring and induced tillering mutants in the major crops. Here we characterize a reduced tillering (tin, for tiller inhibition) mutant of wheat (Triticum aestivum). The reduced tillering in tin is due to early cessation of tiller bud outgrowth during the transition of the shoot apex from the vegetative to the reproductive stage. There was no observed difference in the development of the main stem shoot apex between tin and the wild type. However, tin initiated internode development earlier and, unlike the wild type, the basal internodes in tin were solid rather than hollow. We hypothesize that tin represents a novel type of reduced tillering mutant associated with precocious internode elongation that diverts sucrose (Suc) away from developing tillers. Consistent with this hypothesis, we have observed upregulation of a gene induced by Suc starvation, downregulation of a Suc-inducible gene, and a reduced Suc content in dormant tin buds. The increased expression of the wheat Dormancy-associated (DRM1-like) and Teosinte Branched1 (TB1-like) genes and the reduced expression of cell cycle genes also indicate bud dormancy in tin. These results highlight the significance of Suc in shoot branching and the possibility of optimizing tillering by manipulating the timing of internode elongation.

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

“…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). The molecular mechanism regulating Suc levels, and the specific signaling pathways mediating sugar sensing in buds, remain to be discovered (Barbier et al, 2015a).…”

“…The proposed function of Elevated expression of the TFL1-like gene SbCN2 in phyB-1 sorghum tiller buds is to protect these vegetative axillary meristems from florigenic signals in a manner similar to perennial plants. In annual crops, such as wheat, the transition from vegetative to the reproductive phase converts all apical and axillary meristems from vegetative to inflorescence meristems (Hay and Kirby, 1991;Evers et al, 2006;Kebrom et al, 2012). In perennial plants, vegetative meristems in axillary buds do not transition to flowering before forming a vegetative shoot during the next growth cycle, which ensures a perennial growth habit (Rohde and Bhalerao, 2007).…”

“…Numerous genes with expression kinetics similar to DRM1 in phyB-1 buds were identified (i.e. DIN1), including genes encoding transcription factors previously shown to regulate bud outgrowth that will be described below (Kebrom et al, 2006(Kebrom et al, , 2012. Increased expression of ASN (Sb01g038460; approximately 5-8-fold) and decreased expression of a geneencoding asparaginase (Sb06g030910; 51-fold) is consistent with increased N-storage/decreased amino-acid utilization in growth-inhibited phyB-1 buds at 8 DAP (Fig.…”

“…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).…”