Boron mitigates cadmium toxicity to rapeseed (Brassica napus) shoots by relieving oxidative stress and enhancing cadmium chelation onto cell walls

Abstract

In southern China, Brassica napus (rapeseed) is a widely planted oilseed crop in rice–rapeseed rotation systems with characteristically high levels of cadmium (Cd) and low levels of available boron (B). Current knowledge of the ameliorative effects of B on Cd toxicity in plants mainly concerns plant growth, Cd uptake, and Cd translocation, while little attention has been paid to the role of B on plant antioxidant enzyme systems and cell wall chelation of Cd. We explored the mechanisms whereby B improves rapeseed Cd resistance. Application of B alleviated Cd-induced oxidative stress caused by reactive oxygen species (ROS) in the shoots of Cd-treated plants, by increasing the activity of the major antioxidant enzymes, superoxide dismutase, peroxidase, and catalase. Moreover, the shoots of rapeseed plants supplied with B under Cd toxicity had higher ionic soluble pectin (ISP) content, thereby providing more Cd-binding sites in pectin, as well as higher methylesterase activity. However, no changes in covalent soluble pectin were observed. In addition, B also induced higher cellulose in Cd-toxic shoots, thus promoting Cd chelation onto cell walls. Fourier infrared spectrum analysis confirmed that the addition of B increased protein, pectin, cellulose, and carbohydrate content in the cell walls of Cd-toxic leaves. In conclusion, B can mitigate Cd phytotoxicity by alleviating oxidative stress and immobilizing Cd on the ISP and cellulose of shoot cell walls, thereby playing a potential role in improving the growth potential of crops and Cd phytoremediation. The results also provide a theoretical basis for alleviating Cd toxicity in crops and development of Cd-tolerant varieties.