Natural pigments, including carotenoids, flavonoids and anthocyanidins, determine the attractive color of fruits. These natural pigments are essential secondary metabolites, which play multiple roles in the whole life cycle of plants and are characterized by powerful antioxidant activity. After decades of research and development, multiple benefits of these natural pigments to human health have been explored and recognized and have shown bright application prospects in food, medicine, cosmetics and other industries. In this paper, the research progress of natural fruit pigments in recent years was reviewed, including the structural characteristics and classification, distribution in fruits and analysis methods, biosynthetic process, antioxidant capacity and mechanism, bioaccessibility and bioavailability, and health benefits. Overall, this paper summarizes the recent advances in antioxidant activity and other biological functions of natural fruit pigments, which aims to provide guidance for future research.

Summary: Tomato (Solanum lycopersicum L.) plants are cold‐sensitive, and the fruit are susceptible to postharvest chilling injury when stored at low temperature. However, the mechanisms underlying cold stress responses in tomato are poorly understood. We demonstrate that SlGRAS4, encoding a transcription factor induced by low temperature, promotes chilling tolerance in tomato leaves and fruit. Combined genome‐wide ChIP‐seq and RNA‐seq approaches identified among cold stress‐associated genes those being direct targets of SlGRAS4 and protein studies revealed that SlGRAS4 forms a homodimer to self‐activate its own promoter. SlGRAS4 can also directly bind tomato SlCBF promoters to activate their transcription without inducing any growth retardation. The study identifies the SlGRAS4‐regulon as a new cold response pathway conferring cold stress tolerance in tomato independently of the ICE1‐CBF pathway. This provides new track for breeding strategies aiming to improve chilling tolerance of cultivated tomatoes and to preserve sensory qualities of tomato fruit often deteriorated by storage at low temperatures.

GRAS proteins are plant-specific transcription factors that play crucial roles in plant development and stress responses. However, their involvement in the ripening of economically important fruits and their transcriptional regulatory mechanisms remain largely unclear. Here, we demonstrated that SlGRAS4, encoding a transcription factor of the GRAS family, was induced by the tomato ripening process and regulated by ethylene. Overexpression of SlGRAS4 accelerated fruit ripening, increased the total carotenoid content and increased PSY1 expression in SlGRAS4-OE fruit compared to wild-type fruit. The expression levels of key ethylene biosynthesis genes (SlACS2, SlACS4, SlACO1, and SlACO3) and crucial ripening regulators (RIN and NOR) were increased in SlGRAS4-OE fruit. The negative regulator of tomato fruit ripening, SlMADS1, was repressed in OE fruit. Exogenous ethylene and 1-MCP treatment revealed that more endogenous ethylene was derived in SlGRAS4-OE fruit. More obvious phenotypes were observed in OE seedlings after ACC treatment. Yeast one-hybrid and dual-luciferase assays confirmed that SlGRAS4 can directly bind SlACO1 and SlACO3 promoters to activate their transcription, and SlGRAS4 can also directly repress SlMADS1 expression. Our study identified that SlGRAS4 acts as a new regulator of fruit ripening by regulating ethylene biosynthesis genes in a direct manner. This provides new knowledge of GRAS transcription factors involved in regulating fruit ripening.

Chlorophylls and carotenoids are essential and beneficial substances for both plant and human health. Identifying the regulatory network of these pigments is necessary for improving fruit quality. In a previous study, we identified an R2R3-MYB transcription factor, SlMYB72, that plays an important role in chlorophyll and carotenoid metabolism in tomato fruit. Here, we demonstrated that the SlMYB72-interacting protein SlZHD17, which belongs to the zinc-finger homeodomain transcription factor family, also functions in chlorophyll and carotenoid metabolism. Silencing SlZHD17 in tomato improved multiple beneficial agronomic traits, including dwarfism, accelerated flowering, and earlier fruit harvest. More importantly, downregulating SlZHD17 in fruits resulted in larger chloroplasts and a higher chlorophyll content. Dual-luciferase, yeast one-hybrid and electrophoretic mobility shift assays clarified that SlZHD17 regulates the chlorophyll biosynthesis gene SlPOR-B and chloroplast developmental regulator SlTKN2 in a direct manner. Chlorophyll degradation and plastid transformation were also retarded after suppression of SlZHD17 in fruits, which was caused by the inhibition of SlSGR1, a crucial factor in chlorophyll degradation. On the other hand, the expression of the carotenoid biosynthesis genes SlPSY1 and SlZISO was also suppressed and directly regulated by SlZHD17, which induced uneven pigmentation and decreased the lycopene content in fruits with SlZHD17 suppression at the ripe stage. Furthermore, the protein–protein interactions between SlZHD17 and other pigment regulators, including SlARF4, SlBEL11, and SlTAGL1, were also presented. This study provides new insight into the complex pigment regulatory network and provides new options for breeding strategies aiming to improve fruit quality.

Background: As the key regulators in BR signaling, BES1 family genes regulate thousands of target genes involved in various development processes. So far, the functions of BES1 family are poorly understood in tomato, and a comprehensive genomic and expressional analysis is worth to conduct for this family. Results: Here, nine SlBES1 family members were identified in tomato and classified into five groups based on the conserved motif, gene structure and phylogenetic analysis. Synteny among tomato, Arabidopsis, pepper and rice were further analyzed to obtain insights into evolutionary characteristics. Several cis-elements related to hormone, stress and plant development were exhibited in the promoter regions of SlBES1 family genes. Subcellular localization showed seven members localized both in the nucleus and cytoplasm, implying the presence of dephosphorylated and phosphorylated form of these seven proteins, furthermore, five of them possessed transcription activation activity whereas the left two functioned as transcriptional repressors. Another two members, however, neither localized in the nucleus nor had transactivation activity. Besides, SlBES1.8 showed flower-specific expression while other members expressed ubiquitously in all organs. Moreover, SlBES1 genes exhibited variational expression in response to nine principal plant hormones. Notably, the expression levels of SlBES1 genes presented a dominant downregulated trend in response to stresses. Conclusions: In this study, we systematically analyzed the genomic characterization of SlBES1 family, together with the analyses of protein functional features and expression patterns, our results lay a foundation for the functional research of SlBES1 family.