Unexpected Connections: Salicinoid Biosynthesis in Poplar




Gordon, Harley Oliver William

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Populus is a genus distributed across the northern hemisphere. Poplars (Salicaceae) are subject to stresses in their environment such as herbivory, drought, and fire. These perennial hardwoods produce abundant phenylpropanoid derived anti-herbivory molecules called salicinoids. Understanding salicinoid function and biosynthesis is crucial for understanding the chemical ecology, carbon balance, and adaptability of poplars to changing ecosystems. The full biosynthetic pathway of salicinoids is unknown; however, recent progress has identified a biosynthetic gene for salicinoids. The gene is a UDP-dependent glycosyl transferase called UGT71L1. Using CRISPR/Cas9 genome editing, UGT71L1 was disrupted in the hybrid poplar Populus tremula x Populus alba. Through metabolomic, transcriptomic, and biochemical techniques the co-dependent nature of growth, defence and salicinoid biosynthesis in poplars was explored. Following the elimination of UGT71L1, the exogenous application of deuterated benzenoids and mass spectroscopic analysis was used to examine biochemical connections across metabolic pathways. A carbon limited growth experiment was used to assess the capacity for glucosylated salicinoids to contribute to carbon reserves in resprouting trees. In addition, a second glycosyltransferase gene, UGT78M1, was disrupted in genome-edited poplars. Interruption of UGT71L1 disrupted salicinoid biosynthesis. UGT71L1 knockout plants had small crinkled leaves, reduced growth, and were preferred by insect herbivores. Growth impacts were caused by the abundance of salicylic acid, which increased in concentration following salicinoid biosynthesis interruption. Furthermore, we determined that benzyl benzoate is a precursor to salicortin biosynthesis. Salicinoids are also an inaccessible carbon sink in poplar that cannot be remobilized during carbon starvation. The hypothesized salicinoid biosynthetic gene UGT78M1 does not contribute to salicinoid biosynthesis; however, UGT78M1 is crucial for salicyl benzoate glucoside homeostasis. This dissertation highlights the small molecule trichotomy of plant biochemistry and identifies connections between specialized metabolites, phytohormones, and primary metabolites.



Poplar, salicinoid, metabolomics, stable isotope, chemical ecology, phenolic glycoside