Molecular characterization of metal tolerance mechanisms in Thlaspi goesingense (Halacsy) with comparison to other Thlaspi nickel hyperaccumulators

In Brassicaceae family members collected from serpentine soils, including Thlaspi species which hyperaccumulate Ni up to 3% of shoot dry weight, we observed that shoot tissue concentrations of glutathione, Cys, and O-acetyl-L-serine (OAS), strongly correlate with hyperaccumulation of Ni. These species included Thlaspi goesingense, T. oxyceras, and T. rosulare, and nonaccumulator relatives, T. perfoliatum, T. arvense. The closely related model plant Arabidopsis thaliana was also examined for comparison. The high concentrations of OAS, Cys, and GSH observed in the model hyperaccumulator T. goesingense coincided with constitutively high activity of serine acetyltransferase (SAT) and glutathione reductase. The increases in both Cys and GSH also coincided with the ability of T. goesingense to hyperaccumulate Ni without apparent oxidative damage. Overproduction of T. goesingense SAT in the non-accumulator Arabidopsis was found to result in the accumulation of OAS, Cys, and glutathione. In these transgenic Arabidopsis, glutathione concentrations appeared to ameliorate Ni induced growth inhibition and oxidative stress. This evidence suggests that elevated GSH concentrations resulting from constitutively elevated SAT activities enhance the Ni tolerance of Thlaspi Ni hyperaccumulators. Additionally, we found that over expression of TgSATm conferred increased resistance to Co and Zn with only marginal Cd resistance. Resistance to these metals is ranked Ni = Co, \u3eZn \u3e Cd. Metal resistances correlate with the accumulation of cysteine (Cys) and glutathione (GSH) and the ability to resist the differential oxidative damage caused by these metals. The molecular signalling pathways that control these mechanisms were not elucidated, however, elevated concentrations of salicylic acid, a molecule known to be involved in signaling induced pathogen defense responses in plants, emerged as a strong predictor of Ni/Zn hyperaccumulation in the Thlaspi species investigated. The salicylic acid metabolites phenylalanine, cinnamic acid, salicyloyl-glucose and catechol are also elevated in T. goesingense compared to the non-accumulators A. thaliana and T. arvense. Increased levels of salicylic acid in A. thaliana shoots, caused either by genetic mutation or by exogenous feeding in media, enhances the specific activity of serine acetyltransferase, leading to elevated glutathione and increased Ni resistance. Such salicylic acid mediated Ni resistance in A. thaliana phenocopies the glutathione based Ni tolerance previously observed in Thlaspi, suggesting a biochemical linkage between salicylic acid and Ni tolerance. Intriguingly, the hyperaccumulator T. goesingense also shows enhanced sensitivity to the pathogen Erysiphe cruciferarum (powdery mildew), and exhibits no induction of salicylic acid biosynthesis after infection. Ni treatment and hyperaccumulation reverses this pathogen hypersensitivity, suggesting that the interaction between pathogen resistance and Ni tolerance and hyperaccumulation may have played a critical role in the evolution of metal hyperaccumulation in the Thlaspi genus