Comparative molecular analyses between red maple (Acer rubrum) and trembling aspen (Populus tremuloides) exposed to soil metal contamination: metal translocation, gene expression, and DNA methylation.

Abstract

he main objectives of the present study were to 1) compare the physiological responses and gene expression in red maple (Acer rubrum) and trembling aspen (Populus tremuloides) exposed to metal contamination, 2) determine if epigenetic events are associated with metal resistance in A. rubrum, and 3) assess global gene expression in A. rubrum exposed to different doses of nickel. Metal analyses of soil and plant tissues revealed that P. tremuloides is an accumulator of Mg, Zn, and Ni while A. rubrum does not accumulate these metals in the leaves as it shows avoidance as the main mechanism of coping with soil metal accumulation. Comparative analysis of gene expression revealed that the four genes tested (Nramps4, Nas3, At2G, and MRP4) were more upregulated in P. tremuloides compared to A. rubrum in a field study. AT2G and MRP4 genes were significantly down regulated in A. rubrum from the targeted metal contaminated sites compared to those from uncontaminated areas but environmental factors driving this differential gene expression couldn’t be established. The growth chamber experiment, showed differential gene expression based on p values when the effects of nickel doses were compared. There were more upregulated than down regulated genes in resistant genotypes compared to susceptible genotypes. Most of these genes are associated with coping with abiotic stressors and involves tolerance and detoxification mechanisms. There was a significant variation in the level of cytosine methylation among the metal-contaminated sites, with significant negative correlations between bioavailable nickel / copper content and cytosine methylation being observed. In Conclusion, the present study reveals that A. rubrum exhibits the avoidance strategy as the main mechanism of coping with soil metal accumulation. However, a more informative and indepth analysis of this mechanism would be very beneficial in deciding whether it is strictly physiological or genetic. The role of epigenetics can be further understood by determining the distribution of DNA methylation in both A. rubrum and P. tremuloides.