Integrated Physiological and Transcriptomic Analysis Reveals ABA-Mediated Drought Responses and Suppression of Pyrethrin Biosynthesis in Tanacetum cinerariifolium

Tanacetum cinerariifolium (pyrethrum) is a commercially valuable ornamental and economically important industrial crop that produces pyrethrins, a mixture of six insecticidal esters with potent insecticidal activity and a relatively low toxicity to mammals. Drought stress is a major constraint on pyrethrum cultivation, impairing vegetative growth and leaf physiological function. However, the effects of drought on pyrethrin biosynthesis and the underlying drought-response mechanisms remain poorly understood. In this study, we investigated drought-induced changes in pyrethrum leaves through integrated phenotypic observation, physiological and biochemical analyses, and transcriptome sequencing. Drought stress caused leaf dehydration, wilting, and chlorosis, accompanied by increased activities of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), together with enhanced proline accumulation, indicating the activation of reactive oxygen species (ROS) scavenging and osmotic adjustment. Under severe drought stress, membrane lipid peroxidation increased further, indicating damage to cellular integrity. Transcriptomic analysis revealed extensive drought-induced transcriptional reprogramming in pyrethrum leaves, with the number of differentially expressed genes increasing as the stress severity increased. Abscisic acid (ABA) biosynthesis pathway was significantly activated, with 9-cis-epoxycarotenoid dioxygenase 3 (NCED3) strongly upregulated under severe drought stress, whereas NCED9 and abscisic aldehyde oxidase 3 (AAO3) were primarily responsive to mild drought stress and downregulated under severe conditions. Concurrently, the pyrethrin biosynthesis pathway was partially inhibited, indicating that pyrethrum prioritizes ABA-mediated drought tolerance over pyrethrin production. These findings provide insight into drought-induced physiological and transcriptional responses and may inform the development of drought-tolerant pyrethrum germplasm with an enhanced quality.