Noccaea caerulescens populations adapted to grow in metalliferous and non-metalliferous soils: Ni tolerance, accumulation and expression analysis of genes involved in metal homeostasis

• Noccaea caerulescens populations differ in Ni tolerance and accumulation.
• Ni accumulation is associated to different expression levels of metal transporters and chelators.
• High induction of metal transporters is associated to high Ni accumulation.

Three populations of the Brassicaceae Noccaea caerulescens: (i) the metallicolous Mt. Prinzera population (Italy), which have adapted to grow on serpentinite, a soil naturally rich in Ni, Co, Cr; (ii) the metallicolous La Calamine population (Belgium), which have adapted to growth on soil highly contaminated by Cd, Zn and Pb, and (iii) the non-metallicolous population growing near the town of Rožnov pod Radhoštěm (Czech Republic), were grown in hydroponics and treated with different Ni concentrations (0, 10 and 100 μM NiSO4). Ni tolerance and accumulation were analysed along with the expression of genes belonging to different families involved in plant metal homeostasis: ZNT1, ZNT2, NRAMP3, NRAMP4 coding for non-ATP-hydrolysing plasma membrane and vacuolar metal transporters, HMA3, HMA4 coding for ATP-hydrolising metal transporters, NAS1, NAS3, NAS4 and MT1B involved in metal chelation. The three populations showed different levels of expression of some of the tested genes in condition of 0 μM Ni. In addition, the Ni hyperaccumulator Mt. Prinzera showed the highest Ni translocation capacity at 10 μM Ni and a specific up-regulation of ZNT1, ZNT2, NAS3, NRAMP3 and NRAMP4 genes. With the same Ni treatment, the La Calamine population induced HMA4 and MT1B genes, while the Rožnov pod Radhoštěm population only displayed an over-expression of all the genes at 100 μM Ni, the condition in which this population suffers heavily from Ni stress. The expression of ZNT1, ZNT2, NRAMP3, NRAMP4 was also tested on N. caerulescens plants growing in the natural environment on Mt. Prinzera. Higher transcript levels of ZNT1 and ZNT2 were associated with higher levels of plant's total Ni content. Overall these results suggested that some of the genes considered can have a role in determining metal tolerance and accumulation in metallicolous N. caerulescens populations, while they were not involved in metal homeostasis in the non-metallicolous population.