Phytohormone Profiling of Potato ( Solanum tuberosum L.) Exposed to French Marigold ( Tagetes patula L.) Essential Oil

French marigold ( Tagetes patula L.) is an aromatic plant known for its repellent effects on pests. It is traditionally grown in fields near many vegetable crops, including potato ( Solanum tuberosum L.). However, the effects of marigold essential oil (EO) on the physiology of neighboring crops have been neglected in research. The aim of this study was to establish, for the first time, a comprehensive phytohormone profile of potato plants exposed to French marigold EO for different time periods (4, 8 and 12 h). Endogenous levels of all major phytohormone groups, determined by HPLC–MS analysis, showed altered phytohormone responses of EO-exposed potato plants. The most affected were stress-related phytohormones: abscisic acid–glucose ester, 1-aminocyclopropane-1-carboxylic acid, and jasmonic acid. Increased levels of jasmonic acid, cytokinins, storage form of abscisic acid together with decreased levels of indole-3-acetic acid and ethylene precursor were observed. In most of the analyzed phytohormone groups similar response pattern was observed—an increase in levels after short time exposure (4 h), followed by a decrease to control values after prolonged exposure (8 h and 12 h). Expression levels of genes involved in biosynthesis and catabolism of stress-related phytohormones, obtained by de novo bioinformatic processing of data from cDNA microarray analysis, revealed that jasmonic acid biosynthetic pathway was the most affected, with the highest number of altered transcripts and with one of the genes from this pathway ( 12-oxophytodienoate reductase 1-like ) exhibiting the highest expression rate.