Double Is Not Trouble The doubling of the genome to create polyploidy is common among land plants, and most major flowering plant lineages exhibit some history of genome duplication. However, the physiological advantages of a doubled genome are not well understood. Chao et al. (p. 658, published online 25 July) identified accessions of the model plant Arabidopsis thaliana with naturally doubled genomes and found that the cytotype of the root, but not shoot, in these natural, as well as in artificially induced, polyploid plants appears to confer increased salt tolerance by regulating leaf potassium levels. , Certain thale cress plants collected in the wild contain a duplicated genome and can cope with salty soil. , Genome duplication (or polyploidization) has occurred throughout plant evolutionary history and is thought to have driven the adaptive radiation of plants. We found that the cytotype of the root, and not the genotype, determined the majority of heritable natural variation in leaf potassium (K) concentration in Arabidopsis thaliana . Autopolyploidy also provided resistance to salinity and may represent an adaptive outcome of the enhanced K accumulation of plants with higher ploidy.