Physiological activities, transcriptomes and metabolomes of Pyropia yezoensis conchocelis unveil the roles of pyPGK, pyBCKDHA, and pyDLD in response to freshwater soaking

Freshwater soaking of the conchocelis is often used to reduce yellow spot, white spot, and mud red disease in Pyropia yezoensis. However, the understanding of physiological, transcriptomic, and metabolomic changes for the conchocelis under freshwater stress remains limited. Here, we comprehensively explored the dynamic changes of physiological activities, transcriptomes, and metabolomes of the conchocelis under three points of freshwater stress (0 h, 4 h, and 24 h) and one point of seawater recovery (R2h). We found that the content of photosynthetic pigments, soluble proteins, and photosynthesis performance significantly responded to freshwater stress. Metabolomic analysis identified a total of 24 metabolites, including 15 DAMs, suggesting the metabolites changes in the conchocelis in response to freshwater stress. Additionally, comparative transcriptome and metabolome analyses identified a black co-expression module that was strongly correlated with the DAMs. Furthermore, this module was predominantly enriched in carbohydrate and amino acid metabolism pathways. We found that PyDLD, PyPGK, and PyBCKDHA were key genes in hub-networks, which are potentially involved in changes of leucine, valine, isoleucine, lactate, and floridoside during freshwater stress. These findings reveal the genetic basis of the dynamic changes of physiological activities, transcriptome, and metabolome in the Py. yezoensis conchocelis during freshwater soaking for disease control.