Glyphosate-Induced Branching in Leafy Spurge Involves Complex Interactions Associated with Cellular Transport, Phytohormone Signaling, and Meristem Maintenance

Leafy spurge (Euphorbia esula) is an invasive perennial weed infesting non-cultivated ecosystems in North America. Leafy spurge control generally entails a long-term management program requiring consistent and timely follow-up. Although recommended field rates of glyphosate kill above-ground shoots of leafy spurge, it does little or no damage to underground adventitious buds (referred to as crown and root buds), and leafy spurge regenerates from these buds. Application of glyphosate at greater than recommended field rates causes a sublethal effect in the underground portion of leafy spurge that induces a phenotype with reduced main stem elongation and increased branching in subsequent generations. To identify processes associated with the glyphosate-induced phenotypes, we used aerial shoots derived from crown buds of glyphosate-treated and -untreated leafy spurge to determine transcriptome, phytohormone, and shikimate profiles. Transcriptome analyses identified differential abundance among 12,918 transcripts (FDR≤0.05) and highlighted processes associated with phytohormone biosynthesis/signaling and shoot apical meristem maintenance. Shikimate abundance was increased in crown buds 7-day after foliar-treatment, indicating that the target-site of glyphosate (5-enolpyruvylshikimate-3-phosphate of shikimate pathway) was inhibited. However, shikimate abundance was similar in the new shoots derived from crown buds of treated and untreated plants, suggesting the target-site was no-longer affected. Phytohormone analyses indicated increased abundance of bioactive cytokinins and auxins, and decreased abundance of bioactive gibberellins, suggesting that the altered phenotypes were likely due to changes in balance of phytohormones. Overall, the glyphosate-induced phenotypes derived from leafy spurge crown buds appears to involve complex interactions including cellular-transport, shoot apical meristem maintenance and phytohormone biosynthesis/signaling.