Integrated Metabolomic and Transcriptomic Analysis Reveals That Amino Acid Biosynthesis May Determine Differences in Cold-Tolerant and Cold-Sensitive Tea Cultivars.

Cold stress is one of the major abiotic stresses limiting tea production. The planting of cold-resistant tea cultivars is one of the most effective measures to prevent chilling injury. However, the differences in cold resistance between tea cultivars remain unclear. In the present study, we perform a transcriptomic and metabolomic profiling of Camellia sinensis var. "Shuchazao" (cold-tolerant, SCZ) and C. sinensis var. assamica "Yinghong 9" (cold-sensitive, YH9) during cold acclimation and analyze the correlation between gene expression and metabolite biosynthesis. Our results show that there were 51 differentially accumulated metabolites only up-regulated in SCZ in cold-acclimation (CA) and de-acclimation (DA) stages, of which amino acids accounted for 18%. The accumulation of L-arginine and lysine in SCZ in the CA stage was higher than that in YH9. A comparative transcriptomic analysis showed an enrichment of the amino acid biosynthesis pathway in SCZ in the CA stage, especially "arginine biosynthesis" pathways. In combining transcriptomic and metabolomic analyses, it was found that genes and metabolites associated with amino acid biosynthesis were significantly enriched in the CA stage of SCZ compared to CA stage of YH9. Under cold stress, arginine may improve the cold resistance of tea plants by activating the polyamine synthesis pathway and CBF (C-repeat-binding factor)-COR (cold-regulated genes) regulation pathway. Our results show that amino acid biosynthesis may play a positive regulatory role in the cold resistance of tea plants and assist in understanding the cold resistance mechanism differences among tea varieties.

CsCBF5 depletion impairs cold tolerance in tea plants.

CBFs play important roles in tea plant cold tolerance. In our study, 16 tea varieties were used to investigate the relationship between the expression level of CsCBFs and cold tolerance in field experiments. A strong and positive correlation was found between cold stress-regulated CsCBF1, CsCBF3 and CsCBF5 expression levels (R2 > 0.8) in tea mesophyll cells and cold tolerance in 16 tea varieties. A previous study reported that CsCBF1 and CsCBF3 were important components associated with cold tolerance in tea plants; thus, the function of CsCBF5 in the CsCBF family was targeted. Our previous study reported that CsCBF5 was localized in the nucleus and exhibited transcriptional activity. In the current study, MDA content in leaves was significantly increased in CsCBF5-silenced leaves, which exhibited poor cold tolerance, compared with WT plants under cold stress. In contrast, increased germination rates and antioxidant enzyme activities under cold conditions compared with WT plants. Furthermore, CsCBF5 overexpression in Arabidopsis promoted the expression levels of the cold-regulated genes AtCOR15a, AtCOR78, AtERD4 and AtRD29B; however, the expression levels of downstream genes, including CsCOR47, CsCOR413, CsERD4 and CsRD29B, were significantly reduced in CsCBF5-silenced tea leaves. Taken together, our results indicated that CsCBF5 could function as a positive regulator in the cold stress response.