Regulation mechanisms of humic acid on Pb stress in tea plant (Camellia sinensis L.)

Abstract

Though the interaction between humic acid (HA) and heavy metals has been widely reported, the effects of HA on the toxicity of heavy metals to plants are still in debate. In this study, the regulation mechanisms of HA on Pb stress in tea plant (Camellia sinensis L.) was investigated through hydroponic experiments, and the experimental results were explained by using transmission electron microscope (TEM), scanning transmission X-ray microscopes (STXM) and isobaric tags for relative and absolute quantitation (iTRAQ) differential proteomics. Significant alleviation of Pb stress was found with HA coexistence. TEM results showed that HA greatly mitigated the damage of cells caused by Pb stress. Compared with sole Pb treatment, the addition of HA increased the contents of pectin and pectic acid in the cell wall by 10.5% and 30.5%, while arabinose (Ara) and galactose (Gal) decreased by 20.5% and 15.9%, respectively, which were beneficial for increasing Pb adsorption capacity of the cell wall and promoting cell elongation. Moreover, iTRAQ differential proteomics analysis proved that HA strengthened the antioxidant system, promoted the synthesis of cell wall, and stabilized protein and sulfur-containing substance metabolism in molecular level. Notably, the concentration of calcium (Ca) in the cell wall of HA coexistence treatment was 47.4% higher than Pb treatment. STXM results also indicated that the distribution of Ca in the cell wall was restored with the presence of HA. This might promote the formation of the egg-box model, thus alleviating Pb stress in cells. Our results reveal the regulation mechanisms of HA on Pb detoxification in plants and provide useful information for improving the safety of agricultural products.

Keywords: Humic acid; Pb stress; Regulation mechanism; Tea plant.