Tolerance to salinity stress
We assessed salinity tolerance in a Sorghum diversity panel that included S. bicolor landraces and improved lines, in addition to several wild Sorghum species (Henderson et al, 2020). By measuring several biomass traits under control and salt stress conditions, we found that both tolerant and sensitive accessions maintain growth trajectories for some traits. Therefore, salinity tolerance in Sorghum appears to correspond more closely with the ability to overcome early osmotic stress rather than the ability to exclude or compartmentalize ions during the delayed phase of Na+ toxicity. We also found that tolerant accessions are better able to maintain Na+:K+ homeostasis.
When our results were considered in the context of known phylogenetic relationships (Mace et al., 2013, Nature Communications), we discovered that individuals that predated the domestication event were particularly sensitive to salt exposure, while individuals that originated shortly after domestication and occupy ancestral positions in the phylogeny, primarily within the “durra” landrace, were the most tolerant. From this, we concluded that the most parsimonious explanation for our observations was that salinity tolerance was acquired early during domestication. Salinity tolerance, however, was polyphyletically distributed in the derived genotypes, having been maintained or lost in a lineage-specific manner during improvement.
Because our initial findings demonstrated that the highest levels of salinity tolerance are observed in the earliest domesticated sorghum lineages, and that wild Sorghum species are especially sensitive, we utilized an interspecific recombinant inbred line (RIL) mapping population derived from a cross between S. bicolor inbred Tx7000 (durra) and a weedy undomesticated relative, S. propinquum, to determine the genetic underpinnings of the salt stress response in Sorghum (Hostetler 2021, Plant Stress, in revision). Through the measurement of growth trajectories for 177 RILs grown under both control and salt conditions, we identified quantitative trait loci (QTL) that play a role in salinity tolerance . Candidate genes that lie within these QTL included genes previously shown to aid in the alleviation of salinity stress, including genes that encode aquaporins, CDPKs (calcium-dependent protein kinases), SAPK3 (stress-activated protein kinases), heat shock proteins, and ionic detoxification proteins.
We are currently experimenting with the use of high-throughput image-capture to measure growth rates, transpiration, photosynthesis, and leaf senescence. Advantages of this approach are that data collection is less labor intensive and that the resulting data are synchronized and can be normalized based on temperature fluctuations and circadian effects. We plan to integrate the imaging measurements with other approaches such as ionomics, QTL mapping, gene expression, and phylogenetic analysis at both inter- and intra-specific levels and across temporal and developmental stages to dissect the underpinnings and evolution of salinity tolerance in Sorghum.