Physiological and molecular mechanisms of boron in alleviating cadmium toxicity in Capsicum annuum

Soil cadmium (Cd) contamination threatens food safety and human health, particularly in developing countries. Previously, we have proposed that boron (B) could reduce Cd uptake and accumulation in hot peppers (Capsicum annuum) by regulating the expression of genes related to Cd transport in roots. However, only few studies have examined the role of B in plant leaves under Cd stress. It is unclear how B induces the expression of relevant genes and metabolites in hot pepper leaves and to what extent B is involved in leaf growth and Cd accumulation. The purpose of this study was to investigate the effects of B on growth and Cd accumulation in hot pepper leaves by determining physiological parameters and transcriptome sequencing. The results showed that B application significantly improved the concentration of chlorophyll a and intercellular CO₂, stomatal conductance, and photosynthetic and transpiration rates by 18-41 % in Cd-stressed plants. Moreover, B enhanced Cd retention in the cell wall by upregulating the expression levels of pectin-, lignin-, and callose-related genes and improving the activity of pectin methylesterase by 30 %, resulting in an approximate 31 % increase in Cd retention in the cell wall. Furthermore, B application not only enhanced the expression levels of genes related to antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and their activities by 28-40 %, thereby counteracting Cd-induced oxidative stress, but also improved Cd chelation, sequestration, and exclusion by upregulating the expression levels of genes related to sulfur metabolism, heavy metal-associated isoprenylated plant protein (HIPP), and transporters such as vacuolar cation/proton exchanger (CAX3), metal-nicotianamine transporter (YSL), ATP-binding cassette (ABC), zinc/iron transporters (ZIP) and oxic-compound detoxification (DTX), ultimately reinforcing Cd tolerance. Together, our results suggest that B application reduces the negative effects of Cd on leaf growth, promotes photosynthesis, and decreases Cd transfer to fruits through its sequestration and retention.