Comparative transcriptome analysis of short-term responses to salt and glycerol hyperosmotic stress in the green alga Dunaliella salina

Dunaliella salina is a model photosynthetic organism for studying osmoregulation. To further understand the molecular basis of short-term hyperosmotic-stress responses, gene expression was investigated by RNA-Seq of D. salina cells sampled at 15, 30, 60, and 120 min following exposure to salt and glycerol stress. De novo reconstruction of the transcriptome using Illumina paired-end reads generated 155,996 contigs, out of which 31,302 transcripts could be annotated by gene ontology, assigning them to 64 functional groups. Further functional analysis of the transcripts was performed using KEGG Enzyme Commission numbers. Out of a total of 4415 transcripts with more than two-fold differential expression, 479 transcripts were identified representing 414 enzyme commission numbers that were mapped to 120 KEGG pathways. The “Chromosome and associated proteins”, “Transporters”, “Cytoskeleton proteins”, and 92 other KEGG pathways mapped for differentially expressed transcripts were common to salt stress and glycerol stress. In addition, differentially expressed transcripts mapped uniquely to 22 metabolic pathways for glycerol stress and 7 metabolic pathways mapped uniquely for salt stress. The transcripts mapping to core carbon metabolism within starch, glycolysis, Calvin cycle, and glycerol metabolic pathways were analyzed and 19 transcripts showed differential expression. Cluster analysis resulted in the identification of 939 transcripts, which showed similar expression trends. Among the fastest and most strongly induced genes, glyoxylate cycle and fatty acid desaturation-related genes implied the induction of photorespiration following rapid shifts in the osmolarity of the growth medium. These results provide novel insights for further analyses of physiological responses to short-term osmotic shock.