De novo transcriptomics analysis revealed a global reprogramming towards dehydration and hyposalinity in Bangia fuscopurpurea gametophytes (Rhodophyta)

Bangia fuscopurpurea is a farmed species in the ancient family Bangiales. It inhabits upper intertidal zones and suffers periodical desiccation and osmotic stress. The transcriptomic regulation under dehydration and hyposalinity was investigated. The differentially expressed genes (DEGs) accounted for 18.7% of the unigenes obtained by de novo transcriptome assembly (|log2fold-change| ≥ 1, FDR ≤ 0.001). Over 72% of the DEGs were downregulated under stress. The DEGs were predominantly enriched into the KEGG pathways “metabolic pathways,” “ribosome,” “biosynthesis of secondary metabolites,” “protein processing in endoplasmic reticulum,” and “oxidative phosphorylation.” The optimum photosynthetic efficiency (Fv/Fm) and photochemical quenching (qP) dropped significantly with 89% relative water loss and recovered rapidly after being rehydrated. Most DEGs regarding “photosynthesis” and “C3 carbon fixation” were upregulated in the dehydrated thalli, which may enable the thalli to gain photosynthetic recovery once being rehydrated. Fv/Fm and qP decreased significantly with 1 h of 90% freshwater treatment and then recovered to the control level 1 day later. With 6 h hyposaline treatment, expression of plasma membrane H+-ATPase genes was strongly and predominantly induced while the mRNA abundance of vacuolar, chloroplastic, and mitochondrial H+-ATPase genes decreased or showed no significant change. Some transporter, ion channel, and transmembrane protein genes together with the gene-encoding key enzymes involving in proline and heteroside metabolism were upregulated under hyposalinity. The results indicated that transmembrane exchange of ion and osmolytes was induced under hyposalinity to balance the osmotic fluctuation, which seemed to be triggered by plasma membrane H+-ATPases. These findings will facilitate elucidating the stress acclimation mechanism of B. fuscopurpurea.