Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch (Betula pendula). Using a candidate gene approach, we identified a bushy kanttarelli variant with a loss-of-function mutation in the BpMAX1 gene required for strigolactone (SL) biosynthesis. While kanttarelli is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied kanttarelli in transgenic BpMAX1::RNAi birch lines. SL profiling confirmed that both kanttarelli and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and BpMAX1::RNAi. In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture.

Biosciences Division Oak Ridge National Laboratory Oak Ridge TN 37830

Center for Bioscience Research and Education Utsunomiya University Utsunomiya 321 8505 Japan

Center of Plant Systems Biology and Biotechnology 4000 Plovdiv Bulgaria

Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University and Institute of Experimental Botany of the Academy of Sciences of the Czech Republic Olomouc 78371 Czech Republic

Department of Forest Genetics and Plant Physiology Umeå Plant Science Centre Swedish University of Agricultural Sciences 90183 Umeå Sweden

Institute of Biotechnology Helsinki Institute of Life Science University of Helsinki Helsinki 00014 Finland

Key Laboratory of Horticultural Plant Biology of Ministry of Education College of Horticulture and Forestry Sciences Huazhong Agricultural University Wuhan 430070 China

Laboratoire de Reproduction et Développement des Plantes École Normale Supérieure de Lyon Institut National de la Recherche Agronomique Lyon 69342 France

Laboratory of Growth Regulators Institute of Experimental Botany of the Czech Academy of Sciences Faculty of Science of Palacký University Olomouc CZ 78371 Czech Republic

Mathematics and Computer Science Adam Mickiewicz University Poznań 61 614 Poland

Mathematics Tampere University Tampere 33720 Finland

Max Planck Institute of Molecular Plant Physiology Potsdam Golm 14476 Germany

Molecular and Integrative Biosciences Research Program Faculty of Biological and Environmental Sciences and Viikki Plant Science Centre University of Helsinki Helsinki 00014 Finland

National Plant Phenotyping Infrastructure Helsinki Institute of Life Science University of Helsinki Biocenter Finland Helsinki 00014 Finland

Organismal and Evolutionary Biology Research Program Faculty of Biological and Environmental Sciences Viikki Plant Science Centre University of Helsinki Helsinki 00014 Finland

Production Systems Natural Resources Institute Finland Helsinki 00790 Finland

Sainsbury Laboratory University of Cambridge Cambridge CB2 1LR United Kingdom

School of Biological Sciences Nanyang Technological University Singapore 637551 Singapore

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