Effect of nano-hydroxyapatite and phosphate on thorium toxicity – Arabidopsis transcriptomic study

Thorium (Th) is a ubiquitous element but elevated concentrations caused by anthropogenic or natural activities pose health and environmental risks. Nowadays, Th is obtained mainly as by-product during the mining and extraction rare earth element. However, due to its abundance in nature, which is higher than the abundance of uranium, Th is considered as a potential fuel for the reactors. Therefore, in this study, we focused on the effect of thorium on Arabidopsis plants at the transcriptomic level. Since Th and phosphorus form insoluble phosphates, we focused on the effect of Th in the presence/absence of phosphate. Furthermore, we investigated the effect of hydroxyapatite nanoparticles, which are considered as material for in situ remediation, on Th toxicity. Not surprisingly, the highest number of DEGs was recorded in the combination of P absence and Th presence followed by the presence of Th in the standard hydroponic medium, the absence of P in the medium, the presence of nano-hydroxyapatite, and the presence of Th together with hydroxyapatite. Besides a high number of genes involved in stress response, Th also induced the transcription of genes involved in response to P starvation, indicating that Th decreased the availability of P. P deficiency caused by Th contributed to the phytotoxic effect of Th. Nano-hydroxyapatite significantly reduced Th toxicity. Nano-hydroxyapatite also served as a source of inorganic P for the plants, but caused deficiency of essential metals indicated by up-regulation of genes involved in their acquisition. Comparative gene expression profiling analysis of RNA-seq data from wild type and mutants N8507, N8508, and N9416. All mutants were exposed to 1) untreated control plants, 2) plants exposed to thorium (+Th variant), 3) plants on the phosphate-free medium (-P), 4) plants on the phosphate-free medium exposed to thorium (-P+Th), 5) plants on the phosphate-free medium exposed to nano-hydroxyapatite (NanoP) and, 6) plants on the phosphate-free medium exposed to nano-hydroxyapatite and thorium (NanoP+Th) for 14 days.