Comparative Metabolomics Reveals Two Metabolic Modules Affecting Seed Germination in Rice (Oryza sativa)

The process of seed germination is crucial not only for the completion of the plant life cycle but also for agricultural production and food chemistry; however, the underlying metabolic regulation mechanism involved in this process is still far from being clearly revealed. In this study, one indica variety (Zhenshan 97, with rapid germination) and one japonica variety (Nipponbare, with slow germination) in rice were used for in-depth analysis of the metabolome at different germination stages (0, 3, 6, 9, 12, 24, 36, and 48 h after imbibition, HAI) and exploration of key metabolites/metabolic pathways. In total, 380 annotated metabolites were analyzed by using a high-performance liquid chromatography (HPLC)-based targeted method combined with a nontargeted metabolic profiling method. By using bioinformatics and statistical methods, the dynamic changes in metabolites during germination in the two varieties were compared. Through correlation analysis, coefficient of variation analysis and differential accumulation analysis, 74 candidate metabolites that may be closely related to seed germination were finally screened. Among these candidates, 29 members belong to the ornithine-asparagine-polyamine module and the shikimic acid-tyrosine-tryptamine-phenylalanine-flavonoid module. As the core member of the second module, shikimic acid's function in the promotion of seed germination was confirmed by exogenous treatment. These results told that nitrogen flow and antioxidation/defense responses are potentially crucial for germinating seeds and seedlings. It deepens our understanding of the metabolic regulation mechanism of seed germination and points out the direction for our future research.