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Heavy metal pollution not only decreases crop yield and quality, but also affects human health via the food chain. Ubiquitination-dependent protein degradation is involved in plant growth, development, and environmental interaction, but the functions of ubiquitin-ligase (E3) genes are largely unknown in tomato (Solanum lycopersicum L.). Here, we functionally characterized a RING E3 ligase gene, SlRING1, which positively regulates cadmium (Cd) tolerance in tomato plants. An in vitro ubiquitination experiment shows that SlRING1 has E3 ubiquitin ligase activity. The determination of the subcellular localization reveals that SlRING1 is localized at both the plasma membrane and the nucleus. Overexpression of SlRING1 in tomato increased the chlorophyll content, the net photosynthetic rate, and the maximal photochemical efficiency of photosystem II (Fv/Fm), but reduced the levels of reactive oxygen species and relative electrolyte leakage under Cd stress. Moreover, SlRING1 overexpression increased the transcript levels of CATALASE (CAT), DEHYDROASCORBATE REDUCTASE (DHAR), MONODEHYDROASCORBATE REDUCTASE (MDHAR), GLUTATHIONE (GSH1), and PHYTOCHELATIN SYNTHASE (PCS), which contribute to the antioxidant and detoxification system. Crucially, SlRING1 overexpression also reduced the concentrations of Cd in both shoots and roots. Thus, SlRING1-overexpression-induced enhanced tolerance to Cd is ascribed to reduced Cd accumulation and alleviated oxidative stress. Our findings suggest that SlRING1 is a positive regulator of Cd tolerance, which can be a potential breeding target for improving heavy metal tolerance in horticultural crops., (© 2020 Scandinavian Plant Physiology Society.)

Both selenium (Se) and melatonin reduce cadmium (Cd) uptake and mitigate Cd toxicity in plants. However, the relationship between Se and melatonin in Cd detoxification remains unclear. In this study, we investigated the influence of three forms of Se (selenocysteine, sodium selenite, and sodium selenate) on the biosynthesis of melatonin and the tolerance against Cd in tomato plants. Pretreatment with different forms of Se significantly induced the biosynthesis of melatonin and its precursors (tryptophan, tryptamine, and serotonin); selenocysteine had the most marked effect on melatonin biosynthesis. Furthermore, Se and melatonin supplements significantly increased plant Cd tolerance as evidenced by decreased growth inhibition, photoinhibition, and electrolyte leakage (EL). Se-induced Cd tolerance was compromised in melatonin-deficient plants following tryptophan decarboxylase (TDC) gene silencing. Se treatment increased the levels of glutathione (GSH) and phytochelatins (PCs), as well as the expression of GSH and PC biosynthetic genes in nonsilenced plants, but the effects of Se were compromised in TDC-silenced plants under Cd stress. In addition, Se and melatonin supplements reduced Cd content in leaves of nonsilenced plants, but Se-induced reduction in Cd content was compromised in leaves of TDC-silenced plants. Taken together, our results indicate that melatonin is involved in Se-induced Cd tolerance via the regulation of Cd detoxification., (© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

• Cadmium (Cd) stress induced oxidative stress and reduced photosynthesis in tomato. • E3 ubiquitin ligase gene SlRING1 highly expressed in roots in response to Cd stress. • Silencing SlRING1 aggravated Cd phytotoxicity, H 2 O 2 content and oxidative damage. • SlRING1 deficiency reduced Cd-induced expression of CAT , DHAR , MDHAR , GSH , and PCS. • Suppression of SlRING1 expression increased Cd accumulation in shoots and roots. The E3 ubiquitin ligases participate in the degradation of plant proteins and play a regulatory role in stress response. However, the role of tomato E3 ubiquitin ligase genes in plant response to heavy metal stress remains elusive. Here, we identified 17 tomato E3 ubiquitin ligase genes using blast analysis of highly expressed E3 ubiquitin ligase genes of Arabidopsis thaliana. Through organ expression analysis, three E3 ubiquitin ligase genes with higher expression levels in roots were further screened out, and they were named Sl1 , SlRHE1, and SlRING1. Among these three genes, SlRING1 expression was the highest in response to cadmium (Cd) stress. Silencing SlRING1 significantly decreased chlorophyll content, Fv/Fm, photosynthetic rate, and biomass accumulation under Cd stress. The levels of H 2 O 2 , electrolyte leakage, and malondialdehyde significantly increased in SlRING1 –silenced plants under Cd stress compared with that in non-silenced tomato plants. Cd stress-induced increases in the transcript levels of antioxidant and detoxification genes such as CAT, DHAR, MDHAR, GSH, and PCS were compromised by SlRING1 silencing. Moreover, Cd accumulation in shoots and roots significantly increased in SlRING1 -silenced plants compared with non-silenced tomato plants. These findings suggest that SlRING1 plays a positive role in plant tolerance to Cd stress in tomato. [ABSTRACT FROM AUTHOR]