Roles of boron in preventing cadmium uptake by Capsicum annuum root tips: Novel insights from ultrastructural investigation and single-cell RNA sequencing.

Sufficient boron (B) can reduce cadmium (Cd) accumulation in crops; however, the underlying mechanism remains unclear. Ultrastructural analysis and single-cell RNA sequencing were used to investigate the changes of the Casparian strip of hot pepper (Capsicum annuum L.) root tips exposed to Cd under different B supplements, lignin formation-related gene expression and regulation in the endodermis to deeper understand the molecular mechanisms by which B inhibits root Cd uptake. The results showed that the Casparian strip widths significantly increased in hot pepper root tips exposed to Cd under B-sufficient (B1Cd1) conditions compared to those under B-deficient conditions (B0Cd1). Additionally, more cerium precipitates, which indicate HO accumulation, were observed in the Casparian strip region in B0Cd1 than that in B1Cd1. B supplementation markedly enhanced the expression of certain lignin formation-related genes only in the endodermis cells. These genes included transcription factor genes, WRKY (WRKY7/40/41/53) and ERF (ERF2/109), and two types of genes related to lignin formation, namely, PER genes (PER3/9/49/6472 and LAC3) and dirigent protein (DIR) genes (DIR16/21/24/25). It suggests that the main reason for B reducing Cd accumulation in hot peppers is that B-induced endodermal lignification of root tips under Cd exposure is beneficial to prevent Cd influx into the stele via the apoplastic pathway.