Higher Cd-accumulating oilseed rape has stronger Cd tolerance due to stronger Cd fixation in pectin and hemicellulose and higher Cd chelation

Abstract

Oilseed rape (Brassica napus) has potential as a hyperaccumulator in the phytoremediation of cadmium (Cd)-contaminated soils. Oilseed rape varieties with higher Cd accumulation ability and Cd tolerance are ideal candidates for the hyperaccumulation of excess Cd. To explore the physiological and molecular mechanisms underlying Cd tolerance and high Cd accumulation in oilseed rape leaves, we examined two genotypes, “BN067” (Cd-sensitive with lower Cd accumulation in leaves) and “BN06” (Cd-tolerant with higher Cd accumulation in leaves). We characterized the physiological morphology, structure, subcellular distribution of Cd, cell wall components, cell chelates, and the transcriptional levels of the related genes. Greater Cd accumulation was observed in the cell walls and vacuoles of Cd-tolerant leaves, reducing Cd toxicity to the lamellar structure of the chloroplast thylakoid and leaf stomata. Higher expression of PMEs genes and lower expression of pectin methylesterase inhibitors (PMEI) genes improved pectin methylesterase (PME) activity in leaves of Cd-tolerant genotype. Stronger demethylation of pectin along with higher pectin and hemicellulose levels induced by lower pectinase and hemicellulose activities in the leaves of the Cd-tolerant genotype, resulting in higher Cd retention in the cell walls. Under Cd toxicity, higher Cd sequestration within the vacuoles of Cd-tolerant leaves was closely related to greater accumulation of Cd chelates with stronger biosynthesis in protoplasts. The results highlight the importance of using hyperaccumulation by plants to remediate our environment, and also provide a theoretical basis for the development of Cd-tolerant varieties.