Abstract
Information on spatiotemporal metabolic behavior is indispensable for a precise understanding of physiological changes and responses, including those of ripening processes and wounding stress, in fruit, but such information is still limited. Here, we visualized the spatial distribution of metabolites within tissue sections of tomato (Solanum lycopersicum L.) fruit using a matrix-assisted laser desorption/ionization–mass spectrometry imaging (MALDI–MSI) technique combined with a matrix sublimation/recrystallization method. This technique elucidated the unique distribution patterns of more than 30 metabolite-derived ions, including primary and secondary metabolites, simultaneously. To investigate spatiotemporal metabolic alterations during physiological changes at the whole-tissue level, MALDI–MSI was performed using the different ripening phenotypes of mature green and mature red tomato fruits. Although apparent alterations in the localization and intensity of many detected metabolites were not observed between the two tomatoes, the amounts of glutamate and adenosine monophosphate, umami compounds, increased in both mesocarp and locule regions during the ripening process. In contrast, malate, a sour compound, decreased in both regions. MALDI–MSI was also applied to evaluate more local metabolic responses to wounding stress. Accumulations of a glycoalkaloid, tomatine, and a low level of its glycosylated metabolite, esculeoside A, were found in the wound region where cell death had been induced. Their inverse levels were observed in non-wounded regions. Furthermore, the amounts of both compounds differed in the developmental stages. Thus, our MALDI–MSI technique increased the understanding of the physiological changes and responses of tomato fruit through the determination of spatiotemporally resolved metabolic alterations.Graphical abstractᅟ
Highlights
Endogenous metabolites are important components related to fruit phenotypes, such as color, flavor, taste, and texture
The tissue sections of mature red (MR) tomato fruit were subjected to the matrix deposition and subsequent MALDI–MSI measurement
91 peaks derived from endogenous metabolites were detected (ESM Fig. S1), and 34 of these peaks had unique distributions in the MR tomato fruit (ESM Fig. S2)
Summary
Endogenous metabolites are important components related to fruit phenotypes, such as color, flavor, taste, and texture Their concentrations will dramatically and spatiotemporally change during growing and ripening processes [1,2,3]. This approach allows for the quantitative determination of each metabolite and for tracing metabolic regulation in tissues during ripening and wounding stress It can reveal potential relationships between metabolic signatures and phenotypes, such as physiological appearance and functional characteristics. Conventional metabolomics uses gas chromatography–mass spectrometry (GC–MS) and liquid chromatography–MS (LC–MS) to detect various metabolites, which appear at average levels, simultaneously in whole tissues These methods do not address the spatial aspects of more local physiological phenomena, such as metabolic responses against wounding or pest stress at tissue micro-regional levels. Information on spatiotemporal metabolic behavior is quite important to precisely understand physiological changes and responses related to the tissue microregions of the functional compartments in fruit
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