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.

Keywords: apical dominance; axillary bud outgrowth; cytokinins; flowering; modeling; polar auxin transport; shade avoidance; strigolactone.

FIGURE 1

Proposed model for BRC1 as an integrator of different pathways controlling axillary bud outgrowth. Dashed lines, hypothetical protein–protein interactions.

FIGURE 2

Polar auxin transport (PAT) in the main shoot is regulated by different hormones through their action on the behavior of PIN1 proteins (blue) in the cell: IAA inhibits PIN endocytosis (Paciorek et al., 2005), SL trigger PIN1 depletion from the plasma membrane (PM; Shinohara et al., 2013), GA stabilize PIN proteins to the PM (Willige et al., 2011).

FIGURE 3

Schematic representation of the main processes and players of bud outgrowth that should be represented in a model, consisted in a plant module (A) and a bud module (B). In (A), plant structure is explicitly represented and coupled with a light model to simulate the light perceived by each organ (in yellow). The level of sugar (blue rectangle), nitrogen (green rectangle) and auxin (IAA; red thin arrow) near each bud is the result of the production or assimilation by source organs (photosynthetic organs for sugars, roots or leaves for nitrogen, growing organs for auxin; large arrows), the utilization by sink organs (growing organs for sugars, nitrogen, and roots for auxin; large arrows), and possibly transport processes (red thin arrow). In (B), the concentration of sugars, nitrogen, and auxin near a bud interact with cytokinins and strigolactones in the stem, to control bud outgrowth through key integrators in the bud such as BRC1 or auxin transport.