| Summary: | Pyramidal-, erect- or upright-growing plant forms are characterized by narrow branch angles of shoots and leaves. The putative advantage of upright-leaf and shoot habit could be a more efficient penetration of light into lower canopy layers. Pyramidal genotypes have already been reported for various tree genotypes including peach. The paralogous rice ortholog <i>TILLER ANGLE CONTROL 1</i> (<i>TAC1</i>) has been proposed to be the responsible gene for upright growth. However, it has not really been demonstrated for any of the pyramidal tree genotypes that a knock-out mutation of the <i>TAC1</i> gene is causing pyramidal plant growth. By in silico analyses, we have identified a putative rice <i>TAC1</i> ortholog (Potri.014G102600, “TAC-14”) and its paralog (Potri.002G175300, “TAC-2”) in the genome of <i>P. trichocarpa</i>. Two putative <i>PcTAC1</i> orthologs in the <i>P. × canescens</i> clone INRA 717-1B4 were successfully knocked-out by applying a transgenic CRISPR/Cas9-approach. The mutants were molecularly analyzed and phenotyped over a period of three years in a glasshouse. Our results indicate that the homozygous knock-out of “TAC-14” is sufficient to induce pyramidal plant growth in <i>P. × canescens</i>. If up to twice as many pyramidal individuals were planted on short rotation coppices (SRCs), this could lead to higher wood yield, without any breeding, simply by increasing the number of trees on a default field size.
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