Mutations in CAX1 produce phenotypes characteristic of plants tolerant to serpentine soils

Plant tolerance of serpentine soils is potentially an excellent model for studying the genetics of adaptive variation in natural populations. A large-scale viability screen of Arabidopsis thaliana mutants on a defined nutrient solution with a low Ca2+ : Mg2+ ratio (1 : 24 mol : mol), typical of serpentine soils, yielded survivors with null alleles of the tonoplast calcium-proton antiporter CAX1. cax1 mutants have most of the phenotypes associated with tolerance to serpentine soils, including survival in solutions with a low Ca2+ : Mg2+ ratio; requirement for a high concentration of Mg2+ for maximum growth; reduced leaf tissue concentration of Mg2+; and poor growth performance on 'normal' levels of Ca2+ and Mg2+. A physiological model is proposed to explain how loss-of-function cax1 mutations could produce all these phenotypes characteristic of plants adapted to serpentine soils, why 'normal' plants are unable to survive on serpentine soil, and why serpentine-adapted plants are unable to compete on 'normal' soils.