Mechanisms involved in the positive effects of high zinc exposure on growth of Sedum alfredii

Zinc (Zn) is an essential micronutrient for plant growth, but excessive Zn can lead to toxic symptoms. However, studies have reported that high concentrations of Zn have a promoting effect on the growth of Sedum alfredii Hance, a native Zn/cadmium(Cd) hyperaccumulator in China. Thus, in order to explore the underlying mechanism behind the growth-promoting effects of high Zn level, we conducted a hydroponic experiment on S. alfredii.

We compared hyperaccumulating ecotype (HE) with non-hyperaccumulating ecotype (NHE) of S. alfredii, set Zn levels to 0.5 μM, 5 μM, 100 μM, and 250 μM, and comprehensively utilized metallomics, nano-XRF analysis, and transcriptomics techniques to explore the underlying mechanism.

At 100 μM and 250 μM Zn level, the growth of HE roots was promoted while the growth of NHE was inhibited. Nano-XRF analysis showed that Zn was accumulated in the cell walls of the epidermis and cortex in HE roots, whereas scattered in the root cells in NHE. The content of auxin in HE roots was increased by 1.26-fold and the auxin oxidase activity decreased by 2.94-fold after exposure to 100 μM Zn for 12 h, while NHE exhibited the opposite trend. Transcriptomic results showed that HE up-regulated genes were related to carbohydrate metabolism, nitrogen metabolism, carbon fixation, and tryptophan biosynthesis; down-regulated genes were related to lipid metabolism and linoleic acid metabolism.

High Zn levels have positive effect on hyperaccumulating ecotype of S. alfredii growth by promoting tryptophan pathway to regulate auxin biosynthesis.