Controlling branch orientation is a central challenge in tree fruit production, as it impacts light interception, pesticide use, fruit quality, yield, and labor costs. To modify branch orientation, growers use many different management practices, including tying branches to wires or applying growth regulator sprays. However, these practices are often costly and ineffective. In contrast, altering the expression of genes that control branch angles and orientations would permanently optimize tree architecture and reduce management requirements. One gene implicated in branch angle control, LAZY1, has potential for such applications as it is a key modulator of upward branch orientations in response to gravity. Here, we describe the phenotypes of transgenic plum (Prunus domestica) trees containing an antisense vector to silence LAZY1. We found that LAZY1 silencing significantly increased branch and petiole angles. LAZY1-antisense lines also displayed ‘wandering’ or weeping branch trajectories. These phenotypes were not associated with decreases in branch strength or stiffness. We evaluated the utility of LAZY1-antisense trees for use in two planar orchard systems by training them according to super slender axe and espalier methods. We found that the LAZY1-antisense trees had more open canopies and were easier to constrain to the trellis height. This work illustrates the power of manipulating gene expression to optimize plant architecture for specific horticultural applications.

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