The TT8 transcription factor alleviates nickel toxicity in Arabidopsis

Nickel (Ni) is a necessary element for plants, but excessive accumulation of Ni in soil causes plant damage, one of which is oxidative stress. Plants utilize secondary metabolite polyphenols such as anthocyanins as antioxidant molecules and metal chelators to cope with heavy metal toxicity. However, the regulatory factors linking polyphenol accumulation to resistance to Ni stress in plants remain unidentified. Arabidopsis TRANSPARENT TESTA8 (TT8) is an essential transcription factor involved in anthocyanin biosynthesis, especially in seeds. Here, we reported that TT8 links polyphenol accumulation to Arabidopsis tolerance to Ni stress. TT8 knockout reduced the anthocyanin content in the seed coat, leading to several aspects of Ni toxicity: low seedling survival rates, delayed seed germination, severe membrane lipid peroxidation damage, reduced antioxidant capacity, and low polyphenol (especially epicatechin) abundance. When TT8 was overexpressed, these Arabidopsis plants tolerated extreme Ni stress with high survival rates and germination rates and accumulated more polyphenols. On the basis of these data, we concluded that TT8 maintains the antioxidant capacity of Arabidopsis to avoid Ni stress-induced oxidative damage by promoting polyphenol accumulation. Moreover, TT8 not only responds to Ni stress but also positively regulates six flavonoid biosynthesis enzyme-encoding genes (CHS, CHI, F3H, F3'H, DFR, and FLS). Therefore, TT8-mediated transcriptional regulation cascades of flavonoid biosynthesis may contribute to its effects on polyphenol accumulation. Collectively, our findings provide deep mechanical insights into how specific transcription factors alleviate excessive Ni toxicity in plants and offer new approaches for the breeding of Ni stress-tolerant crops.