Metabolomic profiling unveils metabolites that are co-regulated by the tomato fruit ripening and the cold stress response

The relationship between fruit maturity and cold resistance has been established. Nevertheless, the differentially accumulated metabolites (DAMs) in fruit of different maturities under different temperature conditions have not been thoroughly investigated. In this study, DAMs in fruit of different maturities under different temperature conditions mainly consisted of phenols, alkaloids, flavonoids, and nucleic acids and their derivatives. Compared with the DAMs produced during natural ripening, the post-ripening and senescence processes were found to give rise to additional phenylpropanoid substances. Moreover, among the DAMs co-upregulated during natural ripening, post-ripening, and senescence, one alkaloid (caffeine) and two flavonoids (2,3-dihydro-2-phenyl-4Hbenzopyran-4-one, cyanoglucose-3-rutinoside) were identified. For mature-green fruits, the DAMs induced by low temperature were mainly enriched in nucleotide metabolism, plant hormone signal transduction, galactose metabolism, and flavone and flavonol biosynthesis pathways. Regarding ripening red fruit, the DAMs induced by low temperature were predominantly enriched in nucleotide metabolism, phenylpropanoid biosynthesis, galactose metabolism, and flavonoid biosynthesis. Notably, 1-Caffeoylquinic acid showed an up-regulation pattern in both mature-green and ripening red fruit under the influence of low temperature, while caffeine was down-regulated at both stages. Intriguingly, regardless of whether the fruit were at the mature-green stage or the ripening red stage, the low-temperature induced up-regulation of metabolites did not overlap with roomtemperature induced up-regulation of metabolites. However, low-temperature induced up-regulation of metabolites intersected with the metabolites down-regulated under normal temperature, suggesting a complex and potentially antagonistic regulatory relationship between the normal and low-temperature metabolic responses in tomato fruit. These results indicate that 1-Caffeoylquinic acid and Caffeine may be key metabolites involved in fruit ripening, aging, and response to low-temperature stress.