Transcriptome analysis of the tea oil camellia (Camellia oleifera) reveals candidate drought stress genes

Abstract

Background: The tea-oil camellia (Camellia oleifera) is the most important oil plant in southern China, and has a strong resistance to drought and barren soil. Understanding the molecular mechanisms of drought tolerance would greatly promote its cultivation and molecular breeding.

Results: In total, we obtained 76,585 unigenes with an average length of 810 bp and an N50 of 1,092 bp. We mapped all the unigenes to the NCBI 'nr' (non-redundant), SwissProt, KEGG, and clusters of orthologous groups (COG) databases, where 52,531 (68.6%) unigenes were functionally annotated. According to the annotation, 46,171 (60.8%) unigenes belong to 338 KEGG pathways. We identified a series of unigenes that are related to the synthesis and regulation of abscisic acid (ABA), the activity of protective enzymes, vitamin B6 metabolism, the metabolism of osmolytes, and pathways related to the biosynthesis of secondary metabolites. After exposed to drought for 12 hours, the number of differentially-expressed genes (DEGs) between treated plants and control plants increased in the G4 cultivar, while there was no significant increase in the drought-tolerant C3 cultivar. DEGs associated with drought stress responsive pathways were identified by KEGG pathway enrichment analysis. Moreover, we found 789 DEGs related to transcription factors. Finally, according to the results of qRT-PCR, the expression levels of the 20 unigenes tested were consistent with the results of next-generation sequencing.

Conclusions: In the present study, we identified a large set of cDNA unigenes from C. oleifera annotated using public databases. Further studies of DEGs involved in metabolic pathways related to drought stress and transcription will facilitate the discovery of novel genes involved in resistance to drought stress in this commercially important plant.

Fig 1. Length distribution of unigenes in Camellia oleifera.

Fig 2. Distribution of gene ontology (GO) function in Camellia oleifera.

The y-axis indicates the number of unigenes.

Fig 3. Clusters of orthologous groups (COG) classification of differentially expressed genes in Camellia oleifera.

Fig 4. Global metabolic pathway in Camellia oleifera.

Fig 5. Distribution of gene expression.

The distribution of gene expression in Camellia oleifera.

Fig 6. Genes from the C3 and G4 cultivars of Camellia oleifera showing treatment-specific expression during exposure to conditions of drought.

Venn diagram showing the numbers of unigenes expressed as a result of exposure to conditions of drought over time (in C3 and G4 cultivars). The number of expressed genes shared between the four treatments is represented by overlapping circles.

Fig 7. Number of differentially expressed genes in two Camellia oleifera cultivars.

Fig 8. Number of differently expressed genes specific to the C3 and G4 cultivars of Camellia oleifera.

Fig 9. Principle component analysis (PCA) based on FPKM.

PC1 and PC2 accounted for 32.9% and 18.05% of the principle components, respectively.

Fig 10. Transcription factors (TFs) identified in Camellia oleifera.

Fig 11. Physiological changes in the leaves of two Camellia oleifera cultivars in response conditions of drought.

(A) Relative water content; (B) Relative conductivity; (C) Chlorophyll content; (D) Peroxidase activity; (E) MDA content; and (F) Soluble sugar content, were investigated under 20% PEG at four time points (0, 12, 24, and 36 h).

Fig 12. Vitamin B6 metabolism in Camellia oleifera.

Fig 13. Flavonoid biosynthesis in Camellia oleifera.