Nickel resistance mechanisms using Arabidopsis thaliana as en experimental platform

Ni toxicity results in accumulation of ROS, interferes with Fe homeostasis, and disrupts essential functional groups in biomolecules such as DNA, cell wall and proteins.. Ni excess naturally occurs in serpentine soils, which contain high levels of heavy metals, low levels of micronutrients and a low calcium to magnesium ratio. Metal hyperaccumulators have adapted to this hostile environment and accumulate metals to concentrations of one to four orders of magnitude greater than adjacent plants. The most abundant hyperaccumulators are those that accumulate and tolerate Ni. I demonstrate here that Arabidopsis thaliana is Ni sensitive and is not adapted to serpentine soils using comparative genome analysis and measurements of plant growth at increasing metal concentrations. The genetic resources and experimental tools available make it possible to use A. thaliana as a experimental platform to test if characteristics of hyperaccumulators can induce Ni tolerance. Catechol (CA) is a compound that is accumulated in the Ni hyperaccumulator Noccaea goesingense. Col-0 was used to test if CA induces Ni resistance. I demonstrated that endogenous production via ectopic expression of the bacterial enzyme NahG or exogenous application of CA were sufficient to induce Ni resistance in A. thaliana. CA was sufficient to induce Ni resistance in Arabidopsis only in the presence of gamma-glutamyl-cysteine, which is an intermediate in the biosynthesis of glutathione (GSH). Significant increases in cysteine levels and biosynthetic enzyme activities were found when Ni and CA were both present, but no changes in total GSH were detected. Transcriptional profiling by sequencing of cDNA from roots exposed to Ni, CA and Ni+CA showed that CA increased the expression of genes related to sulfur translocation and assimilation, GSH reduction, GSH homeostasis, nicotianamine synthesis, and heat shock response. I carried out a Genome-wide association (GWA) study of primary root lengths under Ni, CA and Ni+CA treatments. This has identified natural variation in A. thaliana for Ni resistance and detected polymorphisms associated with Ni resistance linked to genes affected in expression levels by CA in the presence or absence of Ni. Analysis of mutant phenotypes for many of these genes confirmed their roles in mediating Ni resistance. The identities of these genes together with, metabolite measurements, enzyme activities, transcriptional analyses, and high resolution analysis of natural variation, I propose that CA promotes Ni resistance by inducing chelating and antioxidant metabolites through the regulation of thiol biosynthesis and homeostasis, and nicotianamine biosynthesis. Gene expression profiling also demonstrated that Ni promotes the expression of genes affected by Fe-deficiency. CA increases the expression of heat shock protein transcription factor ( HSFA2) and downstream HSFA2 genes. CA treatment also repressed the expression of genes that participate in jasmonic acid (JA) biosynthesis and JA-regulated genes. Finally, using phenylpropanoid mutants I demonstrated that disruption of specific steps of the phenylpropanoid pathway induces Ni resistance. This suggests that changes in cell wall and accumulation of diverse phenolic compounds may have an effect on Ni respons