Sporobolus stapfianus transcriptome: Insights into the unique adaptations of the resurrection grasses to the dehydration stress

Sporobolus stapfianus is a desiccation tolerant (DT) grass and a member of the Poaceae family alongside prominent crop and forage species. Despite the worldwide distribution of this family, most of its species are vulnerable to water loss within their vegetative tissues. As a DT species, S. stapfianus, could, therefore, serve as a model species for identifying the molecular changes that enabled such a rare occurrence of the DT phenotype in resurrection grasses and the implications of such a unique adaptation for agriculture. A comprehensive gene expression profiling was performed in plants subjected to a dehydration/rehydration cycle using NimbleGen microarrays hybridization method. Our results showed that most transcripts were high in the hydrated state of S. stapfianus leaf tissues and that minor dehydration (> 60% relative water content; RWC) did not induce most transcripts, but did repress photosynthetic activity, which show the importance of curtailing the production of toxic elements to the DT phenotype. It took a loss of 40% RWC for induction of most transcripts. This shows that this species is pre-equipped to deal with mild dehydration, but requires stress-induced activation mechanisms to prepare for desiccation. In agreement with our transcriptomic data, a global metabolic analysis conducted in this species has shown that most metabolites accumulate to their highest levels below 45% RWC, which suggests the importance of those late stages of dehydration in preparing resurrection grasses for desiccation and for early stages of rehydration. In most cases, a 12-h rehydration was sufficient to reinstate pre-stress expression levels, which shows that only partial damage occurred during drying. Unlike during dehydration, our metabolomic data profiled after rehydration showed that there is no clear correlation between gene expression and metabolic abundance, which suggests that most of the compounds that are produced during rehydration are quickly utilized in active metabolism of restoration. The findings of the paper show that S. stapfianus is primed to respond to mild dehydration, although severe dehydration require inducible response, with 40-30% RWC being the peak of gene expression. As for photosynthesis, this species cease its photosynthetic activity even at high water levels (85% RWC) alleviating the need to activate antioxidants activity during mild dehydration. Overall design: Total RNA was extracted from the grass, Sporobolus stapfianus leaf tissues at six hydration levels (3.0, 2.0, 1.5, 1.0, 0.75, and 0.50 g water/ g tissue) as well as at 12 and 24 h after rehydration. Each treatment was conducted in triplicates; making a total of 24 samples. Each chip measures the expresion level of 50,690 genes with each gene model represented by 7x60 mer probes.