QTL mapping in an interspecific sorghum population uncovers plasma-membrane intrinsic proteins as key regulators of salinity tolerance

Soil salinity impedes crop production via both osmotic and ionic stress and is an intensifying problem affecting lands used to produce agronomically important crops.. Such stress slows plant growth and disrupts normal metabolic processes, resulting in decreased biomass and increased leaf senescence. Therefore, the ability to maintain biomass in response to salt exposure is indicative of tolerant genotypes. We used a recombinant inbred line (RIL) population derived from an interspecific cross between Sorghum propinquum and Sorghum bicolor (inbred Tx7000), two genotypes that differ for tolerance to salt exposure, to map regions of the genome associated with salt tolerance. Our high-density genetic map covers the 10 Sorghum chromosomes with 1692 segregating markers. Sixteen total quantitative trait loci (QTL) were detected for seven traits in two environmental conditions (control and salt) and when stress tolerance index (STI) values were mapped. Our results uncovered significant enrichment of genes encoding various types of aquaporins within salt specific QTL, the most interesting of which is the plasma-membrane intrinsic protein SbPIP2;6. SbPIP2;6 shares [~]85% sequence similarity with Arabidopsis PIP2;1, an aquaporin shown to co-transport both water and ions, and for which ion transport is inhibited by calcium, suggesting that it may play a role in sensing and signaling during the salt stress response. The coupling of water and ion transport may also facilitate the use of Na+ as a cheap compatible solute, reducing the energy needed to maintain osmotic equilibrium. These results indicate that PIPs play a central role in salinity tolerance in sorghum.