Tryptophan pretreatment adjusts transcriptome and metabolome profiles to alleviate cadmium toxicity in Arabidopsis

Cadmium (Cd) is highly toxic to all organisms including plants, and recently tryptophan (Trp) pretreatment of plant seedlings is shown to improve Cd tolerance. But the underlying mechanism remains largely unknown. In this study, we used Arabidopsis (Arabidopsis thaliana) to determine the physiological relevance of Trp pretreatment in alleviating Cd toxicity in plants and explore its molecular mechanism with a focus on the metabolic pathways. The results showed that Trp pretreatment maintained the biomass and root lengths, relieved Cd-induced lipid peroxidation, and reduced Cd transport to the shoots, and eventually improved the response against Cd in Arabidopsis seedlings. The integrative analyses of the transcriptome and metabolome further revealed that Trp pretreatment alleviated Cd toxicity not only through a known mechanism of producing a major auxin indole-3-acetic acid and maintaining its levels, but also through two previously unrecognized mechanisms: increasing the area and strength of cell walls by promoting lignification to further reduce Cd entry, and fine-tuning Cd detoxification products derived from sulfur-containing amino acid metabolism. Our findings thereby provide deep mechanical insights into how Trp alleviates Cd toxicity in plants.