Full-length transcriptome sequences of ephemeral plant Arabidopsis pumila provides insight into gene expression dynamics during continuous salt stress

Background: Arabidopsis pumila is native to the desert region of northwest China and it is extraordinarily well adapted to the local semi-desert saline soil, thus providing a candidate plant system for environmental adaptation and salt-tolerance gene mining. However, understanding of the salt-adaptation mechanism of this species is limited because of genomic sequences scarcity. In the present study, the transcriptome profiles of A. pumila leaf tissues treated with 250 mM NaCl for 0, 0.5, 3, 6, 12, 24 and 48 h were analyzed using a combination of second-generation sequencing (SGS) and third-generation single-molecule real-time (SMRT) sequencing. Results: Correction of SMRT long reads by SGS short reads resulted in 59,328 transcripts. We found 8075 differentially expressed genes (DEGs) between salt-stressed tissues and controls, of which 483 were transcription factors and 1157 were transport proteins. Most DEGs were activated within 6 h of salt stress and their expression stabilized after 48 h; the number of DEGs was greatest within 12 h of salt stress. Gene annotation and functional analyses revealed that expression of genes associated with the osmotic and ionic phases rapidly and coordinately changed during the continuous salt stress in this species, and salt stress-related categories were highly enriched among these DEGs, including oxidation-reduction, transmembrane transport, transcription factor activity and ion channel activity. Orphan, MYB, HB, bHLH, C3H, PHD, bZIP, ARF and NAC TFs were most enriched in DEGs; ABCB1, CLC-A, CPK30, KEA2, KUP9, NHX1, SOS1, VHA-A and VP1 TPs were extensively up-regulated in salt-stressed samples, suggesting that they play important roles in slat tolerance. Importantly, further experimental studies identified a mitogen-activated protein kinase (MAPK) gene MAPKKK18 as continuously up-regulated throughout salt stress, suggesting its crucial role in salt tolerance. The expression patterns of the salt-responsive 24 genes resulted from quantitative real-time PCR were basically consistent with their transcript abundance changes identified by RNA-Seq. Conclusion: The full-length transcripts generated in this study provide a more accurate depiction of gene transcription of A. pumila. We identified potential genes involved in salt tolerance of A. pumila. These data present a genetic resource and facilitate better understanding of salt-adaptation mechanism for ephemeral plants.