Drought has become a major threat to sustainable sweetpotato production, resulting in huge yield losses. Therefore, the present study was conducted to identify drought stressresponsive genes using next-generation (NGS) and third-generation sequencing (TGS) techniques. Five cDNA libraries were constructed from seedling leaf segments treated with a 30% solution of polyethylene glycol (PEG-6000) for 0, 1, 6, 12, and 48 h for second-generation sequencing. Leaf samples taken from upper third of sweet potato seedlings after 1, 6, 12, and 48 h of drought stress were used for the construction of cDNA libraries for third-generation sequencing; however, leaf samples from untreated plants were collected as controls. A total of 184,259,679 clean reads were obtained using second and third-generation sequencing and then assembled into 17,508 unigenes with an average length of 1,783 base pairs. Out of 17,508 unigenes, 642 (3.6%) unigenes failed to hit any homologs in any databases, which might be considered novel genes. A total of 2, 920, 1578, and 2,418 up-regulated unigenes and 3,834, 2,131, and 3,337 down-regulated unigenes from 1 h, 6 h, 12 h, and 48 h library were identifed, respectively in drought stress versus control. In addition, after 6, 12, and 48 h of drought stress, 540 up-regulated unigenes, 486 down-regulated unigenes and 414 signifcantly diferentially expressed unigenes were detected. It was found that several gene families including Basic Helix-loop-helix (bHLH), basic leucine zipper (bZIP), Cystein2/Histidine2 (C2H2), C3H, Ethylene-responsive transcription factor (ERF), Homo domain-leucine zipper (HD-ZIP), MYB, NAC (NAM, ATAF1/2, and CUC2), Thiol specifc antioxidant and WRKY showed responses to drought stress. In total, 17,472 simple sequence repeats and 510,617 single nucleotide polymorphisms were identifed based on transcriptome sequencing of the PFSP. About 96.55% of the obtained sequences are not available online in sweet potato genomics resources. Therefore, it will enrich annotated sweet potato gene sequences and enhance understanding of the mechanisms of drought tolerance through genetic manipulation. Moreover, it represents a sequence resource for genetic and genomic studies of sweet potato.
Figure 1: (A) Assembly result sequence length distribution map of transcripts and unigenes in Xuzi-8 sweetpotato cultivar under drought stress conditions. Te horizontal axis represents the length intervals of the transcripts and unigenes, and the vertical axis represents the number of transcripts and unigenes. (B) Species distribution of the top BlastX matches of the transcriptome unigenes in the non-redundant protein database (Nr) data base under drought stress conditions.