Transcriptome analysis of kelp Saccharina japonica unveils its weird transcripts and metabolite shift of main components at different sporophyte developmental stages

摘要

Saccharina japonica is an economically important cold water brown alga extensively cultivated in China. It is cultivated upside down under a floating rope net with its holdfast and meristematic area facing sunlight and UV irradiation and its blade tip toward dark, and other worse cultivation environmental factors also make S. japonica face more stresses. In this study, S. japonica transcriptomes corresponding to its four developmental stages were analyzed. In total, 7800 genes predicted in the genome were transcribed. We found that 1208 of the 7800 expressed and 2697 annotated were virus associating genes. Of 778 differentially expressed genes (DEGs), 372 were annotatable and 209 were virus associating. Such portion of virus associating genes indicated that the S. japonica genome contained a large portion of active virus originating genes. It was found that the transcripts abundance associated with sugar biosynthesis was about 2.13 folds of all the expressed, indicating that the biosyntheses of structural and storage sugars were very important cellular processes. The total abundance of genes involved in the biosynthesis of alginate and laminarin were similar among all developmental stages, however, that of genes involved in the biosynthesis of mannitol increased about 2-folds from mushroom and adult stages to mature and aging stages. This trend explained our observation that the content of alginate was almost constant at different development stages, while that of mannitol increased sharply. In addition, we found that a set of defense and cell recurring genes highly expressed and many of them expressed differentially among stages. On average, the sum abundance of the transcripts of these genes at four stages were 3.40- and 4.96-folds of all the annotated and all the expressed, respectively. This indicated that S. japonica sporophytes persistently respond possible pathogen and environment stresses. The findings are important for timing S. japonica harvest and amending the current cultivation mode.