Differential levels of gene expression and molecular mechanisms between red maple (Acer rubrum) genotypes resistant and susceptible to nickel toxicity revealed by transcriptome analysis

Abstract

Knowledge of regulation of genes associated with metal resistance in higher plants is very limited. Many plant species have developed different genetic mechanisms and metabolic pathways to cope with metal toxicity. The main objectives of this study were to 1) assess gene expression dynamics of A. rubrum in response to nickel (Ni) stress and 2) describe gene function based on ontology. Certified A. rubrum genotypes were treated with 1,600 mg of Ni per 1 Kg of soil corresponding to a soil total nickel content in a metal-contaminated region in Ontario, Canada. Nickel resistant and susceptible genotypes were selected and used for transcriptome analysis. Overall, 223,610,443 bases were generated. Trinity reads were assembled to trinity transcripts. The transcripts were mapped to protein sequences and after quality controls and appropriate trimmings, 66,783 annotated transcripts were selected as expressed among the libraries. The study reveals that nickel treatment at a high dose of 1,600 mg/kg triggers regulation of several genes. When nickel-resistant genotypes were compared to water controls, 6,263 genes were upregulated and 3,142 were downregulated. These values were 3,308 and 2,176, respectively, when susceptible genotypes were compared to water control. The coping mechanism of A. rubrum to Ni toxicity was elucidated. Upregulation of genes associated with transport in cytosol was prevalent in resistant genotypes compared to controls while upregulation of genes associated with translation in the ribosome was higher in susceptible genotypes when compared to water. The analysis revealed no major gene associated with Ni resistance in A. rubrum. Overall, the results of this study suggest that the genetic mechanism controlling the resistance of this species to nickel is controlled by genes with limited expression. The subtle differences between resistant and susceptible genotypes in gene regulation were detected using water-treated genotypes as references.