Phenotypic and Genetic Variation in a Dominant Forage and Biofuel Grass along the Great Plains' Precipitation Gradient: a Reciprocal Garden Approach

Big bluestem Andropogon gerardii is a major forage and potential biofuel grass with wide distribution across a sharp precipitation gradient of the Great Plains (400-1200mm yr-1). We expected ecotypic variation in drought tolerance and potentially, local adaptation. We investigated phenotypic and genetic variation using reciprocal common gardens established across the precipitation gradient with sites in Carbondale, Illinois; Manhattan, Hays, and Colby, KS. Results demonstrated a phenotypic cline in drought tolerance among regional ecotypes. Several lines of evidence support genetic differentiation and structuring of regional ecotypes. Eight loci were identified to be under divergent selection with some marker frequencies shifted with precipitation. We sought to assess transcriptional response for the central KS (xeric) and Illinois (mesic) ecotypes under severe abiotic stress growing in western KS to test thresholds of tolerance. We used 454 sequencing and de novo transcriptome assembly, complemented with Illumina RNA-sequencing, followed by Bowtie, Tophat, Cufflinks pipeline. This experiment revealed 245 transcripts significantly differentially expressed (~1.8% of total transcripts) between central KS (167) and Illinois ecotypes (78). The Illinois (mesic) ecotype mounted a weaker transcriptional response consisting of up-regulated transcripts involved in abiotic stress as an inducible, short-term strategy for coping with severe water stress. In contrast, the central KS (xeric) ecotype had a stronger, more varied transcriptional response that matched its higher photosynthetic rates, higher leaf nitrogen, and increased vegetative performance and suggests a constitutive morphological drought adaptation.  These phenotypic and genetic variation studies provide new insights into how this dominant grass may withstand environmental change.