Mechanisms by which plant species respond to drought are an essential component in predicting ecosystem responses to more variable climate. Of particular interest are mechanistic responses in dominant species, plants which comprise the largest proportion of cover, production, or function in their constituent communities. Blue grama grass (Bouteloua gracilis), dominates shortgrass steppes of the Central USA, where it comprises up to 90% of plant species cover. Despite its ecological significance and economic importance (i.e., forage value), little is known about the molecular responses to drought stress in blue grama. Specifically, there have been no studies on gene expression in this species. We quantified the transcriptome responses before and after a controlled greenhouse dry-down in three distinct populations of blue grama using RNA-seq and the de novo transcriptome assembler Trinity. We selected populations that we previously determined to be genetically distinct based on SNP analysis, originating from South Dakota, Southern Colorado, and New Mexico. We expected that geographic isolation and different evolutionary pressures relating to climate would lead to different molecular strategies for coping with our imposed drought stress.
Results/Conclusions
Overall, we observed the drydown treatment had the greatest effect on gene expression, leading to over 4,000 genes differentially expressed between treatments. The proportion of up- and down-regulated genes was similar, indicating an active strategy for responding to drought (rather than dormancy). Many drought downregulated genes mapped to cell wall biosynthesis, suggesting a common tradeoff between growth and response to stress found in other plant species. We also observed differences among population gene expression, with the New Mexico population emerging as most distinct. South Dakota and Southern Colorado populations were very similar pre-drydown and gene expression was overall lower than New Mexico, suggesting more active growth in plants sourced farther south. In contrast to trait-based analysis, we found that New Mexico plants had the least gene expression plasticity following drydown, while Southern Colorado had the greatest plasticity. This could indicate that plants sourced from the Colorado population have a consistent plastic strategy for regulating drought response, while those from New Mexico favor constitutive expression. Understanding the mechanisms of drought response in blue grama may help inform climate change and distribution models in the future. Shortgrass steppe management and restoration will also benefit from incorporating mechanistic differences among populations.