Carotenoids are indispensable plant secondary metabolites that are involved in photosynthesis, antioxidation, and phytohormone biosynthesis. Carotenoids are likely involved in other biological functions that have yet to be discovered. In this study, we integrated genomic, biochemical, and cellular studies to gain deep insight into carotenoid-related biological processes in citrus calli overexpressing CrtB (phytoene synthase from Pantoea agglomerans). Fortunella hindsii Swingle (a citrus relative) and Malus hupehensis (a wild apple) calli were also utilized as supporting systems to investigate the effect of altered carotenoid accumulation on carotenoid-related biological processes. Transcriptomic analysis provided deep insight into the carotenoid-related biological processes of redox status, starch metabolism, and flavonoid/anthocyanin accumulation. By applying biochemical and cytological analyses, we determined that the altered redox status was associated with variations in O2 - and H2O2 levels. We also ascertained a decline in starch accumulation in carotenoid-rich calli. Furthermore, via an extensive cellular investigation of the newly constructed CrtB overexpressing Fortunella hindsii Swingle, we demonstrated that starch level reducation occurred in parallel with significant carotenoid accumulation. Moreover, studying anthocyanin-rich Malus hupehensis calli showed a negative effect of carotenoids on anthocyanin accumulation. In citrus, altered carotenoid accumulation resulted in dramatic effects on metabolic processes involved in redox modification, starch degradation, and flavonoid/anthocyanin biosynthesis. These findings provided new perspectives to understand the biological importance of carotenogenesis and of the developmental processes associated with the nutritional and sensory qualities of agricultural products that accumulate carotenoids.

Background: Flavor contributes to the sensory quality of fruits, including taste and aroma aspects. The quality of foods is related to their flavor-associated compounds. Pear fruits have a fruity sense of smell, and esters are the main contributor of the aroma. Korla pear are well known due to its unique aroma, but the mechanism and genes related to volatile synthesis have not been fully investigated. Results: Flavor-associated compounds, including 18 primary metabolites and 144 volatiles, were characterized in maturity fruits of ten pear cultivars from five species, respectively. Based on the varied metabolites profiles, the cultivars could be grouped into species, respectively, by using orthogonal partial least squares discrimination analysis (OPLS-DA). Simultaneously, 14 volatiles were selected as biomarkers to discriminate Korla pear (Pyrus sinkiangensis) from others. Correlation network analysis further revealed the biosynthetic pathways of the compounds in pear cultivars. Furthermore, the volatile profile in Korla pear throughout fruit development was investigated. Aldehydes were the most abundant volatiles, while numerous esters consistently accumulated especially at the maturity stages. Combined with transcriptomic and metabolic analysis, Ps5LOXL, PsADHL, and PsAATL were screened out as the key genes in ester synthesis. Conclusion: Pear species can be distinguished by their metabolic profiles. The most diversified volatiles as well as esters was found in Korla pear, in which the enhancement of lipoxygenase pathway may lead to the high level of volatile esters at maturity stages. The study will benefit the fully usage of pear germplasm resources to serve fruit flavor breeding goals.

Hexokinase (HXK) plays a crucial role in plants, catalyzing the phosphorylation of hexose substances, which is one of the key steps in sugar metabolism and energy production. While HXK genes have been well-studied in model plants, the evolutionary and functional characteristics of HXK gene family in jujube is unknow. In this study, the HXK gene family members were identified by bioinformatics methods, the key members regulating glucose metabolism were identified by transcriptome data, and finally the function of the key genes was verified by instantaneous and stable genetic transformation. Our results showed that seven HXK genes were identified in the jujube genome, all of which were predict located in the chloroplast and contain Hexokinase-1 (PF00349) and Hexokinase-2 (PF03727) conserved domains. Most of HXK proteins were transmembrane protein with stable, lipid-soluble, hydrophilic. The secondary structure of ZjHXK proteins main α-helix, and contains two distinct tertiary structure. All ZjHXK genes contain nine exons and eight introns. Predictions of cis-regulatory elements indicate that the promoter region of ZjHXK contains a large number of MeJA responsive elements. Finally, combined with the analysis of the relationship between the expression and glucose metabolism, found that ZjHXK5 and ZjHXK6 may the key genes regulating sugar metabolism. Transient overexpression of ZjHXK5 and ZjHXK6 on jujube, or allogeneic overexpression of ZjHXK5 and ZjHXK6 on tomato would significantly reduce the content of total sugar and various sugar components. Transient silencing of ZjHXK5 and ZjHXK6 genes results in a significant increase in sucrose and total sugar content. Interestingly, the expression of ZjHXK5 and ZjHXK6 were also affected by methyl jasmonate.

Soil mineral elements are the key factors affecting fruit quality, but which soil elements affect the sugars and organic acids of jujube fruit is still not clear. In this study, the fruit and soil of 18 major producing areas of the Tarim Basin were studied. By measuring the sugar and acid content in the fruits and element contents in the soil, the main soil mineral elements affecting the sugar and acid content in jujube fruits were identified. The results showed that the sugar components were mainly sucrose, glucose and fructose, and the organic acid components were mainly succinic acid, citric acid and malic acid. The fruits near the Kunlun Mountains had a higher ratio of sugar-to-acid and sweet-to-acid. Some elements in the soil of the Tarim Basin are abundant, such as the elements Ca, Fe, Mn and B, but the contents of the elements NO₃-N, NH₄-N, Zn and Mo are low. The contents of Ca, Mg and Mn were positively correlated with the contents of glucose, fructose and galactose. And the Fe, Ca, Mg and Mn were the main factors affecting the sugars and organic acid contents. Our study provides theoretical support for rational fertilization and efficient cultivation management of jujube.

Apple (Malus × domestica) trees suffer from low-temperature-induced cold stress, leading to huge production and economic losses. Understanding the molecular mechanism of apple plants under cold stress is the basis of cold-resistance improvement. Here, we report that a basic region/leucine zipper (bZIP) transcription factor gene, ABSCISIC ACID INSENSITIVE 5 (MdABI5), involved in ABA signaling can positively contribute to cold tolerance in apple. MdABI5 directly binds to the promoters of C-Repeat Binding Factor 3 (MdCBF3) and jasmonate ZIM domain 1/3 (MdJAZ1/3), activating the expression of MdCBF3 but inhibiting the expression of MdJAZ1/3 to regulate cold responses in apple. Because previous studies have verified that MdABI5 can interact with the transcriptional factor TEOSINTE BRANCHED/CYCLOIDEA/PROLIFERATING CELL FACTOR 15 (MdTCP15) and histone deacetylase 6 (MdHDA6), we investigated the coordinated regulations of MdTCP15-MdABI5 and MdHDA6-MdABI5 on MdCBF3 and MdJAZ1/3. MdTCP15 suppressed the transcriptional regulation of MdABI5 on MdCBF3 and MdJAZ1/3, corresponding with the negative regulatory role of MdTCP15 in response to cold stress. MdHDA6 enhanced the repression of MdABI5 on MdJAZ1/3 expression to positively regulate cold response in apple. However, MdABI5 also interacted with and recruited MdHDA6 to the promoter regions of MdJAZ1/3, resulting in histone deacetylation around MdJAZ1/3, reinforcing the repression of MdABI5 on MdJAZ1/3, and enhancing cold tolerance in apple. Our findings provide insights into the involvement of MdABI5 in apple response to cold stress, providing the basis of genetic improvement for cold-resistant germplasm using a molecular approach.