Boron alleviates cadmium toxicity in Brassica napus by promoting the chelation of cadmium onto the root cell wall components

In Southern China, rice-oil rotations occur on soils with high levels of cadmium (Cd) and low levels of available boron (B). Boron can alleviate Cd toxicity, as it affects the plant cell wall structures and the components that block the entry of Cd into the cytoplasm; however, these mechanisms are not well understood. Fourier transform infrared spectroscopy (FTIR), fluorescent probe dye, electron microscope, ion abundance (inductively coupled plasma mass spectrometry), metabonomics and transcriptomics were used in the study, and we found that under Cd stress, B increased root pectin content by affecting the biosynthesis pathways and decreasing the activity of pectinase and the expression levels of related genes. The increased pectin content and pectin demethylation increased the chelation of Cd onto the cell walls and reduced the levels of Cd entering the organelles. Application of B to the roots decreased the amounts of cellulose and hemicellulose in the cell walls to normal levels and promoted the expression of genes from the expansin, xyloglucan endotransglucosylase, and alpha-xylosidase families. This contributed to cell wall integrity and root flexibility. Consequently, the accumulation of reactive oxygen species was inhibited and cell viability in the roots was increased, which reduced the destruction of root surface structures. These results have improved our understanding of how B participates in chelation of Cd onto cell walls and in maintaining cell wall integrity, thereby improving Cd toxicity resistance in rapeseed roots.