Elucidation of the mechanism by which the foliar application of triacontanol enhances Cd enrichment in Tagetes patula L. through morphological, metabolomic, and transcriptomic analyses

Abstract

Phytoremediation is an effective technology for removing heavy metal cadmium (Cd) from soil without harming the soil; however, it is limited by its long remediation time and low efficiency. In this study, a plant growth regulator (PGR), triacontanol, was sprayed on the leaves of the hyperaccumulator Tagetes patula L. at different growth stages to enhance the accumulation of soil Cd, thereby ultimately enhancing the efficiency of phytoremediation. Results showed that leaves were the main site of Cd accumulation in T. patula, and foliar application of triacontanol increased the leaf biomass and Cd content, with maximum values of 14.69% and 15.44%, respectively. Furthermore, the Cd removal rate in the soil increased to 11.53%. The effect of a single application of triacontanol on Cd accumulation was better than that of two applications, and the bloom period was found to be the best application stage. The proportion of Cd in the cell walls increased, enhancing Cd fixation ability. The photosynthetic efficiency and antioxidant capacity of T. patula improved significantly. In the roots, metabolomic and transcriptomic analyses indicated that triacontanol promoted the metabolism of low-molecular-weight organic acids, leading to an increase in the available and exchangeable Cd in soil, with maximum values of 14.72% and 2.29%, respectively. The upregulation of Cd transport-related genes and pathways in the roots strengthened their ability to absorb Cd and resist Cd stress. These findings systematically elucidated the molecular mechanism of triacontanol-enhanced Cd accumulation in T. patula and provide technical support for its wide application.