Unveiling targeted spatial metabolome of rice seed at the dough stage using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry imaging

Rice (Oryza sativa) seeds contain a variety of metabolites, which not only provide energy for their own growth and development, but also are an important source of nutrition for humans. It is crucial to study the distribution of metabolites in rice seeds, but the spatial metabolome of rice seeds is rarely investigated. In this study, MatrixAssisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS) imaging was used to reveal the spatial distribution of free soluble sugars (glucose, fructose, sucrose, and maltose), amino acids (9 essential amino acids and 2 amino acids affecting rice eating quality: L-aspartic acid and L-glutamic acid), and 4 metabolites in the flavonoids synthesis pathway (cinnamic acid, naringenin chalcone, naringenin, and dihydrokaempferol) in rice seed at the dough stage. It was found that the 4 free soluble sugars present similar spatial distribution, mainly distributed in the seed cortex and embryo with high abundance. The majority of amino acids are also concentrated in the rice cortex and embryo, while the others are abundant in the whole seed. Besides cinnamic acid distributed in the seed cortex and embryo, the naringenin chalcone, naringenin, and dihydrokaempferol were also found in the endosperm and had lower content. Furthermore, a colocalization phylogenetic tree according to the spatial distribution imaging of each metabolite was constructed. This study revealed the distribution diversity of metabolites in different segmentations of rice seed at the dough stage, providing clues for the nutritional differences between brown rice and white rice, and serving as a reference for people to target a healthy diet.