Chronological development of the morphological, physiological, biochemical, and transcriptomic changes provides insights into the mechanisms of gametogenesis in Saccharina japonica.

The multiannual delayed gametophyte of kelp species maintains continuous vegetative growth under unsuitable conditions and can restart gametogenesis under suitable conditions. However, the processes and mechanisms of this phenomenon are still poorly understood. To gain insight into the processes and mechanisms of gametogenesis in multiannual delayed gametophytes of the kelp Saccharina japonica, the ultrastructure changes, photosynthetic and biochemical responses, and dynamics of gene expression were investigated. Data were collected at four time points (0, 3, 6, and 9 days) during gametogenesis. After gametogenesis induction there were notable changes in the ultrastructure, including increases in mitochondria and the Golgi bodies, and changes in chloroplast and endoplasmic reticulum structures. The contents of abscisic acid and H2O2 as well as the activities of pivotal respiratory enzymes (malic dehydrogenase and phosphofructokinase) and key antioxidant enzymes (catalase and ascorbate peroxidase) all increased significantly after 3 days of gametogenesis induction, implying that these changes were closely related to the gametophyte's transition from vegetative growth to gametogenesis. Notably, photosynthetic and respiratory indicators both showed significantly upregulated trends after 6 days of induction, indicating that the gametophyte's metabolism was most active during transformation from vegetative growth into gametogenesis. Time-specific DEGs (differentially expressed genes) were identified during the vegetative and reproductive stages of gametophytes. Gene ontology analysis indicated that "cell part", "cellular process" and "binding" represented the largest categories in the DEG datasets. Moreover, the predominant transcription factor families were bHLH, MYB, and C3H in the DEG datasets. There were 8 genes related to peroxisomes, 12 genes related to proteasomes, 4 genes related to oxidative phosphorylation, and 59 genes related to ubiquitin-mediated proteolysis identified as up- or downregulated DEGs at the two key stages during gametogenesis, the initial developmental stage and the oogonia formation stage. [ABSTRACT FROM AUTHOR]