Multiple pathways regulate shoot branching

Abstract

Shoot branching patterns result from the spatio-temporal regulation of axillary bud outgrowth. Numerous endogenous, developmental and environmental factors are integrated at the bud and plant levels to determine numbers of growing shoots. Multiple pathways that converge to common integrators are most probably involved. We propose several pathways involving not only the classical hormones auxin, cytokinins and strigolactones, but also other signals with a strong influence on shoot branching such as gibberellins, sugars or molecular actors of plant phase transition. We also deal with recent findings about the molecular mechanisms and the pathway involved in the response to shade as an example of an environmental signal controlling branching. We propose the TEOSINTE BRANCHED1, CYCLOIDEA, PCF transcription factor TB1/BRC1 and the polar auxin transport stream in the stem as possible integrators of these pathways. We finally discuss how modeling can help to represent this highly dynamic system by articulating knowledges and hypothesis and calculating the phenotype properties they imply.

IPA1: a direct target of SL signaling

“…Consistent with this, tb1 enhances branching in the ipa1-1D gain of function mutant [6]. A role for BRC1 downstream of SL has been implied in pea and Arabidopsis where BRC1 expression is upregulated by SL, and SL treatment cannot inhibit shoot branching in the brc1 mutant [7]. However, transcriptional activation of TB1 by SL has not been observed in rice and indeed, TB1 expression is not always anti-correlated with shoot branching in rice SL mutants [1].…”

“…IPA1 regulation of TB1 gene expression in rice has also been confirmed [6] even though SL regulation of TB1 has not been reported in rice [1]. TB1 can then inhibit shoot branching/tillering via unknown mechanisms [7]. miR156 integrates many environmental and endogenous signals [11] and post-transcriptionally regulates IPA1 gene expression [3].…”

“…Furthermore, the miR156/SPL regulatory hub is also a key target of many external and internal signals including CO 2 , sugar, temperature, light and different stresses [11] ( Figure 1). Many of these signals also regulate TB1 and its homologues [7], raising the possibility that miR156/SPL is a key integrator of external signals that influence SL signaling.…”

“…FINE CULM1 (FC1), TEOSINTE BRANCHED1 (TB1) and BRANCHED 1 (BRC1) are well established in several species as homologous transcription factors that suppress shoot branching [7]. TB1 has been shown to be a direct transcriptional target of IPA1 in rice [6] (Figure 1).…”

SUPPRESSOR OF APICAL DOMINANCE1 of Sporisorium reilianum changes inflorescence branching at early stages in di- and monocot plants and induces fruit abortion in Arabidopsis thaliana

“…6 The process of branching is controlled by many environmental and endogenous factors. 7 Hormones like auxin and strigolactones are main players in control of bud outgrowth. 8 If SAD1 modulated branching by interfering with these hormones, we would expect to observe additional phenotypes, i.e.…”