Salt and heavy metal tolerance and expression levels of candidate tolerance genes among four extremophile Cochlearia species with contrasting habitat preferences

To test the concept of a general “mineral stress tolerance”, we compared four extremophile Cochlearia species for salt (NaCl), zinc (Zn) and cadmium (Cd) tolerance and accumulation, and for expression of candidate tolerance genes for salt and Zn tolerance. Salt tolerance decreased in the order C. anglica > C. x hollandica > C. danica > C. pyrenaica, corresponding with the average salinity levels in the species' natural environments. The glycophytic metallophyte, C. pyrenaica, showed a relatively high level of salt tolerance, compared to other glycophytic Brassicaceae. Salt tolerance was positively correlated with HKT1 expression and the K⁺ concentration in roots under salt exposure, but uncorrelated with the Na⁺ concentrations in roots and shoots. All the species accumulated Na⁺ primarily in their leaves, and exhibited a high NHX1 expression in leaves, in comparison with other glycophytic Brassicaceae, suggesting that salt tolerance in Cochlearia is based on an efficient vacuolar sequestration of Na⁺ in leaves. The metallicolous C. pyrenaica population was hypertolerant to Zn, but not to Cd, in comparison with the other Cochlearia species. All the Cochlearia species accumulated Zn and Cd primarily in roots, and showed high levels of Cd and Zn tolerance, with unusually low rates of metal accumulation, in comparison with non-metallophytes, or non-metallicolous metallophyte populations, of species belonging to other genera or families. Although Cochlearia, as a genus, shows relatively high levels of tolerance to both salt and heavy metals, this is most probably not due to a common ‘mineral stress tolerance’ mechanism.