Anthocyanin synthesis is critical for melatonin-induced chromium stress tolerance in tomato

Chromium (Cr) is a toxic heavy metal for both animals and plants. The multifunctional signaling molecule melatonin can confer plant tolerance to heavy metal stress, but the mechanisms remain largely unknown. Here, we unveiled the critical role of the secondary metabolite anthocyanin in melatonin-induced Cr stress tolerance. Excess Cr caused severe phytotoxicity, which was manifested by leaf yellowing, stunted growth, reduced Fv/Fm, and increased accumulation of reactive oxygen species and malondialdehyde in a dose-dependent manner. Interestingly, leaf anthocyanin content increased under Cr stress and was the highest under 100 mu M Cr (7.67 -fold), while exogenous melatonin further increased anthocyanin accumulation with the highest being with 100 mu M melatonin (by 90.72 %). In addition, exogenous melatonin increased endogenous melatonin content and alleviated Cr stress; however, suppression of melatonin accumulation aggravated Cr phytotoxicity and inhibited anthocyanin accumulation by downregulating the transcript levels of key structural genes. Melatonin also reduced the Cr content in roots and leaves. Crucially, suppression of anthocyanin biosynthesis by silencing an anthocyanin biosynthetic gene ANTHOCYANIDIN SYNTHASE (ANS) significantly compromised melatonin- induced anthocyanin accumulation and alleviation of Cr phytotoxicity, suggesting that anthocyanin potentially acts downstream of melatonin and its accumulation is essential for melatonin-induced Cr stress tolerance in tomato plants.

Automated summary

Chromium is a toxic heavy metal for animals and plants. The multifunctional signaling molecule melatonin can confer plant tolerance to chromium stress by inducing the accumulation of anthocyanin, a secondary metabolite. Melatonin increases both endogenous and exogenous anthocyanin content, alleviates chromium phytotoxicity, and reduces chromium accumulation in roots and leaves. Suppression of anthocyanin biosynthesis compromises melatonin-induced anthocyanin accumulation and alleviation of chromium phytotoxicity, suggesting that anthocyanin acts downstream of melatonin and is essential for melatonin-induced chromium stress tolerance in tomato plants.