Physiological and transcriptomic analysis reveals the potential mechanism of Morinda officinalis How in response to freezing stress.

Background: Morinda officinalis How (MO) is a vine shrub distributed in tropical and subtropical regions, known as one of the "Four Southern Herbal Medicines" in China. The unclear responsive mechanism by which MO adapt to freezing stress limits progress in molecular breeding for MO freezing tolerance.

Results: In this study, morphological, physiological and microstructure changes in MO exposed to -2℃ for 0 h, 3 h, 8 h and 24 h were comprehensively characterized. The results showed that freezing stress caused seedling dehydration, palisade cell and spongy mesophyll destruction. A significant increase in the content of proline, soluble protein and soluble sugars, as well as the activity of superoxide dismutase and peroxidase was observed. Subsequently, we analyzed the transcriptomic changes of MO leaves at different times under freezing treatment by RNA-seq. A total of 24,498 unigenes were annotated and 3252 unigenes were identified as differentially expressed genes (DEGs). Most of these DEGs were annotated in starch and sucrose metabolism, plant hormone signal transduction and MAPK signaling pathways. Family Enrichment analysis showed that the glucosyl/glucuronosyl transferases, oxidoreductase, chlorophyll a/b binding protein and calcium binding protein families were significantly enriched. We also characterized 7 types of transcription factors responding to freezing stress, among which the most abundant family was the MYBs, followed by the AP2/ERFs and NACs. Furthermore, 10 DEGs were selected for qRT-PCR analysis, which validated the reliability and accuracy of RNA-seq data.

Conclusions: Our results provide an overall view of the dynamic changes in physiology and insight into the molecular regulation mechanisms of MO in response to freezing stress. This study will lay a foundation for freezing tolerance molecular breeding and improving the quality of MO.