Plants exhibit phenotypical plasticity. Their general body plan is genetically determined, but plant architecture and branching patterns are variable and can be adjusted to the prevailing environmental conditions. The modular design of the plant facilitates such morphological adaptations. The prerequisite for the formation of a branch is the initiation of an axillary meristem. Here, we review the current knowledge about this process. After its establishment, the meristem can develop into a bud which can either become dormant or grow out and form a branch. Many endogenous factors, such as photoassimilate availability, and exogenous factors like nutrient availability or shading, have to be integrated in the decision whether a branch is formed. The underlying regulatory network is complex and involves phytohormones and transcription factors. The hormone auxin is derived from the shoot apex and inhibits bud outgrowth indirectly in a process termed apical dominance. Strigolactones appear to modulate apical dominance by modification of auxin fluxes. Furthermore, the transcription factor BRANCHED1 plays a central role. The exact interplay of all these factors still remains obscure and there are alternative models. We discuss recent findings in the field along with the major models. Plant architecture is economically significant because it affects important traits of crop and ornamental plants, as well as trees cultivated in forestry or on short rotation coppices. As a consequence, plant architecture has been modified during plant domestication. Research revealed that only few key genes have been the target of selection during plant domestication and in breeding programs. Here, we discuss such findings on the basis of various examples. Architectural ideotypes that provide advantages for crop plant management and yield are described. We also outline the potential of breeding and biotechnological approaches to further modify and improve plant architecture for economic needs.

Plant architecture is modified by a regulatory system that controls axillary bud outgrowth. Key components in this system are strigolactones (SLs) and BRANCHED1, which inhibit bud outgrowth. Their role has been described in herbaceous model systems, including Arabidopsis, rice and pea. However, a role in woody perennial species, including the model tree poplar, has not been unequivocally proven. In this study, we tested a role for SLs in Populus × canescens by treatment with the synthetic SL GR24. We generated MORE AXILLARY BRANCHING4 (MAX4) knockdown lines to study the architectural phenotype of poplar SL biosynthesis mutants and the expression of SL-regulated genes. We show that GR24 is perceived by the model tree poplar. MAX4 knockdown lines exhibit typical SL deficiency symptoms. The observed changes in branching pattern, internode length and plant height can be rescued by grafting. We identified putative poplar BRANCHED1 and BRANCHED2 genes and provide evidence for a regulation of BRANCHED1 by SLs. Our results suggest a conservation of major regulatory mechanisms in bud outgrowth control in the model tree poplar. This may facilitate further research, pinpointing the role of SLs and BRANCHED1 in the complex regulation of bud outgrowth in trees.

The TCP-type transcription factors BRANCHED1 and BRANCHED2 shape plant architecture by suppressing bud outgrowth, with BRANCHED2 only playing a minor role in Arabidopsis. Here, we investigated the function of orthologs of these genes in the model tree Populus. We used CRISPR/Cas9 to generate loss-of-function mutants of previously identified Populus BRANCHED1-1 and BRANCHED2-1 candidate genes. BRANCHED1-1 mutants exhibited strongly enhanced bud outgrowth. BRANCHED2-1 mutants had an extreme bud outgrowth phenotype and possessed two ectopic leaves at each node. While BRANCHED1 function is conserved in poplar, BRANCHED2, in contrast to its Arabidopsis counterpart, plays an even more critical role in bud outgrowth regulation. In addition, we identified a new, not yet reported association of this gene to leaf development.

No abstract