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.