Glyphosate's impact on vegetative growth in leafy spurge identifies molecular processes and hormone cross-talk associated with increased branching

Background: Leafy spurge (Euphorbia esula) is a perennial weed that is considered glyphosate tolerant, which is partially attributed to escape through establishment of new vegetative shoots from an abundance of underground adventitious buds. Leafy spurge plants treated with sub-lethal concentrations of foliar-applied glyphosate produce new vegetative shoots with reduced main stem elongation and increased branching. Processes associated with the glyphosate-induced phenotype were determined by RNAseq using aerial shoots derived from crown buds of glyphosate-treated and -untreated plants. Comparison between transcript abundance and accumulation of shikimate or phytohormones (abscisic acid, auxin, cytokinins, and gibberellins) from these same samples was also done to reveal correlations. Results: Transcriptome assembly and analyses confirmed differential abundance among 12,918 transcripts (FDR <= 0.05) and highlighted numerous processes associated with shoot apical meristem maintenance and stem growth, which is consistent with the increased number of actively growing meristems in response to glyphosate. Foliar applied glyphosate increased shikimate abundance in crown buds prior to decapitation of aboveground shoots, which induces growth from these buds, indicating that 5-enolpyruvylshikimate 3-phosphate (EPSPS) the target site of glyphosate was inhibited. However, abundance of shikimate was similar in a subsequent generation of aerial shoots derived from crown buds of treated and untreated plants, suggesting EPSPS is no longer inhibited or abundance of shikimate initially observed in crown buds dissipated over time. Overall, auxins, gibberellins (precursors and catabolites of bioactive gibberellins), and cytokinins (precursors and bioactive cytokinins) were more abundant in the aboveground shoots derived from glyphosate treated plants. Conclusion: Based on the overall data, we propose that the glyphosate-induced phenotype resulted from complex interactions involving shoot apical meristem maintenance, hormone biosynthesis and signaling (auxin, cytokinins, gibberellins, and strigolactones), cellular transport, and detoxification mechanisms.