Exogenous EBR mediated the plant growth and absorption and accumulation of Cu, Fe and Zn in tomato seedlings under Cu stress

As an essential element and a heavy metal, copper (Cu) occurs in various vital movements and metabolic processes of plant. In previous industrial and agricultural production, excess Cu²⁺ or Cu compounds were frequently intentionally or unintentionally discharged into the environment. In some polluted soils, Cu content exceeded ten times the original content of the soil. This led to fatal damages to plant growth and food security, and thus developing new phytoremediation pathways were imperative. Our early studies showed that exogenous 2,4-epibrassinolide (EBR) alleviated Cu stress and increased Cu accumulation in tomato seedlings. 'Gailiang Maofen 802F1' was used as the plant material to explore the alleviation mechanism of EBR on tomato seedlings under Cu stress. Through nutrient solution cultivation in greenhouse conditions, we investigated the effects of EBR on tomato growth and mineral elements contents under Cu stress. The results showed that exogenous EBR effectively alleviated growth strains induced by Cu stress. 50 umol·L^-1 of CuCl2 obviously restrained tomato growth and increased Cu content in tomato seedlings. Compared with seedlings exposed to Cu stress, 0.1 mg·L^-1 of exogenous EBR effectively increased leaf chlorophyll content and plant biomass respectively by 39.6% and 20.0%. Under Cu stress, exogenous EBR observably reduced Cu absorption and transport, simultaneously increasing contents of iron (Fe) and zinc (Zn) in roots. EBR efficiently regulated different chemical forms and Cu, Fe, Zn distributions among subcellular organelles and reduced Cu biotoxicity in cells. This was helpful to the normal growth of tomato and ensured regular physiological and biochemical metabolism of roots and leaves. Cu stress increased contents of different chemical species of Cu in tomato leaves and root. Addition of EBR under Cu stress increased contents of all forms of Cu except NaCl extracted Cu in leaf, compared with Cu stress. Proportion of movable Cu increased in leaves, and decreased in roots under Cu stress. Under Cu stress, EBR application made the proportions of movable and un-moveable Cu similar to those of CK, which indicated exogenous EBR obviously regulated the Cu chemical form and subcellular distribution in plant.