Cytokinin treatment modifies litchi fruit pericarp anatomy leading to reduced susceptibility to post-harvest pericarp browning

Comparative Transcriptome Analysis Reveals Key Functions of MiMYB Gene Family in Macadamia Nut Pericarp Formation

Macadamia nuts are one of the most important economic food items in the world. Pericarp thickness and flavonoid composition are the key quality traits of Macadamia nuts, but the underlying mechanism of pericarp formation is still unknown. In this study, three varieties with significantly different pericarp thicknesses, namely, A38, Guire No.1, and HAES 900, at the same stage of maturity, were used for transcriptome analysis, and the results showed that there were significant differences in their gene expression profile. A total of 3837 new genes were discovered, of which 1532 were functionally annotated. The GO, COG, and KEGG analysis showed that the main categories in which there were significant differences were flavonoid biosynthesis, phenylpropanoid biosynthesis, and the cutin, suberine, and wax biosynthesis pathways. Furthermore, 63 MiMYB transcription factors were identified, and 56 R2R3-MYB transcription factors were clustered into different subgroups compared with those in Arabidopsis R2R3-MYB. Among them, the S4, S6, and S7 subgroups were involved in flavonoid biosynthesis and pericarp formation. A total of 14 MiMYBs' gene expression were verified by RT-qPCR analysis. These results provide fundamental knowledge of the pericarp formation regulatory mechanism in macadamia nuts.

Cuticular wax metabolism responses to atmospheric water stress on the exocarp surface of litchi fruit after harvest

Litchi fruit is susceptible to pericarp browning, which is largely due to the oxidation of phenols in pericarp. However, the response of cuticular waxes to water loss of litchi after harvest is less mentioned. In this study, litchi fruits were stored under ambient, dry, water-sufficient, and packing conditions, while rapid pericarp browning and water loss from the pericarp were observed under the water-deficient conditions. The coverage of cuticular waxes on the fruit surface increased following the development of pericarp browning, during which quantities of very-long-chain (VLC) fatty acids, primary alcohols, and n-alkanes changed significantly. Genes involved in the metabolism of such compounds were upregulated, including LcLACS2, LcKCS1, LcKCR1, LcHACD, and LcECR for elongation of fatty acids, LcCER1 and LcWAX2 for n-alkanes, and LcCER4 for primary alcohols. These findings reveal that cuticular wax metabolism may take part in the response of litchi to water-deficient and pericarp browning during storage.

Preparation of Vanillin-Taurine Antioxidant Compound, Characterization, and Evaluation for Improving the Post-Harvest Quality of Litchi

Litchi’s post-harvest pericarp browning is one of the main constraints that drastically affect its visual attributes and market potential. Therefore, the vanillin-taurine Schiff base (VTSB) compound prepared from natural compounds of vanillin and taurine exhibited higher DPPH-radical-scavenging invitro antioxidant activity than vanillin. VTSB first-time report to mitigate the postharvest browning of litchi fruit. In this study, litchi fruits were dipped in 0.3 mM (based on pre-experiment) VTSB solution and stored at 25 ± 1 °C for six days to examine their effects on browning and postharvest quality. Fruit treated with VTSB had lower levels of browning degree (BD), browning index (BI), weight loss, soluble quinone (SQ), relative electrolyte leakage (REL), and malondialdehyde (MDA) than control fruit. Additionally, total anthocyanins and phenolic concentrations, Total soluble solids (TSS), and 2,2-diphenyl-1-picrylhydrazyl-free radical scavenging activity (DPPH-RSA) were preserved higher in VTSB-treated litchi fruit. The levels of Ascorbate peroxidase (APX), Superoxide dismutase (SOD), and Catalase (CAT) were higher in treated fruit, whereas polyphenol oxidase (PPO) and Peroxidase (POD) were decreased during the postharvest period. This study suggested that VTSB would be very useful for different post-harvest problems in the fruit and vegetable industry.

The characteristic of Yatu morphogenesis and the efficacy of exogenous hormones on the development of Yatu during fruit development in 'Yali' pear (Pyrus bretschneideri Rehd.)

Yatu is a protuberance formed on the base part of 'Yali' pear fruit, near the pedicel, causing a shape like a duck head termed Yatu. It is a typical phenotypic trait to evaluate fruit appearance quality. The mechanism for Yatu formation has not been clear yet. Here, 90.8% of fruits with Yatu generated in outer base part of fruits. Primitive cells of Yatu were found at 10 days after pollination (DAP). There were higher expression levels of PbGA20ox2, PbIPT7a, and PbIPT5a, lower transcription levels of PbGA2ox1, PbNCED1, and PbNCED3 in outer base part of fruits at 10 DAP, accompanied with significantly higher levels of GA₃, ZR, (GA₃+ ZR)/ABA, and lower ABA content compared to that in the inner base part of fruits. GA₃ + 6-BA promoted Yatu development by increasing GA₃ content at 10 and 20 DAP, and ZR content at 20 DAP. PAC suppressed Yatu morphogenesis and development by increasing ABA level at 10 DAP. These results suggest that Yatu usually generated in outer base part of fruits, relatively higher GA₃ and ZR contents, lower ABA content promoted Yatu morphogenesis and development.

Residue Analysis and the Effect of Preharvest Forchlorfenuron (CPPU) Application on On-Tree Quality Maintenance of Ripe Fruit in "Feizixiao" Litchi (Litchi chinensis Sonn.)

Litchi is a highly perishable fruit. Ripe litchi fruit loses quality quickly as they hang on tree, giving a very short hanging life and thus harvest period. This study attempted to explore the roles of cytokinin in regulating fruit ripening and senescence of litchi and examine the possibility of applying cytokinin in "on-tree storage" of the fruit. Exogenous cytokinin, forchlorfenuron (CPPU), was applied at 20 mg L^-1 7 weeks after full bloom on litchi (Litchi chinensis cv. Feizixiao) fruit clusters. Color parameters, chlorophylls, anthocyanins, fruit and fruit part weights, total soluble solutes (TSSs), soluble sugars, organic acids, non-anthocyanin flavonoids, ethanol, and also CPPU residue in fruit were traced. CPPU residue was higher but decreased faster in the pericarp than in the aril, where it maintained < 10 μg kg^-1. CPPU had no significant effect on fruit weight but tended to increase pericarp weight. The treatment suppressed chlorophyll loss and anthocyanin accumulation in the pericarp, increased non-anthocyanin flavonoids in the aril, but had no significant effects on non-anthocyanin flavonoids in the pericarp and total sugar and organic acids in the aril. As the commercially ripe fruit hanged on tree, TSSs, total sugar, and sucrose decreased with ethanol and acetic acid accumulation in the aril. CPPU significantly suppressed the loss of sucrose and total sugar and the accumulation of ethanol and acetic acid in the aril and inhibited malondialdehyde accumulation in the pericarp of the overripe fruit. Soluble invertase, alcohol dehydrogenase, and pyruvate decarboxylase (PDC) activity and gene expression in the aril were downregulated by CPPU. The results suggest that cytokinin partially suppresses the ripening process in litchi and is effective to slow quality loss in the overripe fruit on tree.

Cytokinins is involved in regulation of tomato pericarp thickness and fruit size

Although cytokinins (CKs) regulate fruit development, no direct genetic evidence supports the role of endogenous CKs in pericarp growth or development or fruit size. Here, we report that the reduction in endogenous active CKs level via overexpression of a CKs-inactivating enzyme gene AtCKX2 specifically in fruit tissues resulted in reduced pericarp thickness and smaller fruit size, compared to wild-type control fruits. The pericarp thickness and single fruit weight in transgenic plants were significantly reduced. Analysis of paraffin sections showed that the reduced pericarp thickness was due largely to a decreased number of cells, and thus decreased cell division. Transcriptome profiling showed that the expression of cell division- and expansion-related genes was reduced in AtCKX2-overexpressing fruits. In addition, the expression of auxin-signaling and gibberellin-biosynthetic genes was repressed, whereas that of gibberellin-inactivating genes was enhanced, in AtCKX2-overexpressing fruits. These results demonstrate that endogenous CKs regulate pericarp cell division and, subsequently, fruit size. They also suggest that CKs interact with auxin and gibberellins in regulating tomato pericarp thickness and fruit size.

Laccase-Mediated Flavonoid Polymerization Leads to the Pericarp Browning of Litchi Fruit

Litchi pericarp turns brown rapidly after fruit harvest, while the mechanism remains obscure. The contents of (-)-epicatechin (EC) and procyanidins (PCs) A2/B1/B2/C1 decreased during the pericarp browning, and a previously identified laccase (ADE/LAC) showed activity to these compounds, with brown products observed in the reactions. By UPLC-DAD-QTOF-MS/MS, isomers of dimeric, trimeric, and tetrameric PCs were detected in the EC-ADE/LAC reaction. In the presence of cyanidin-3-O-rutiside and rutin, anthocyanin-EC and rutin-EC adducts were, respectively, produced, and darker brown precipitation was observed in these reactions relatively to the EC-ADE/LAC reaction alone. ADE/LAC catalyzed the conversion of PC B2 to A-type PC dimers and B-type PC tetramers. ADE/LAC complemented the transparent testa of Arabidopsis LAC15-loss-of-function mutant (tt10) to wild-type dark brown seed coat. The results demonstrated that ADE/LAC-mediated flavonoid polymerization played an important role in the browning of pericarp.

Litchi (Litchi chinensis Sonn.): a comprehensive review of phytochemistry, medicinal properties, and product development

Since ancient times, litchi has been well recognized as a functional food for the management of various ailments. Many bioactives, including flavanoids, anthocyanins, phenolics, sesquiterpenes, triterpenes, and lignans, have been identified from litchi with a myriad of biological properties both in vitro and in vivo. In spite of the extensive research progress, systemic reviews regarding the bioactives of litchi are rather scarce. Therefore, it is crucial to comprehensively analyze the pharmacological activities and the structure-activity relationships of the abundant bioactives of litchi. Besides, more and more studies have focused on litchi preservation and development of its by-products, which is significant for enhancing the economic value of litchi. Based on the analysis of published articles and patents, this review aims to reveal the development trends of litchi in the healthcare field by providing a systematic summary of the pharmacological activities of its extracts, its phytochemical composition, and the nutritional and potential health benefits of litchi seed, pulp and pericarp with structure-activity relationship analysis. In addition, its by-products also exhibited promising development potential in the field of material science and environmental protection. Furthermore, this study also provides an overview of the strategies of the postharvest storage and processing of litchi.

Applications of Cytokinins in Horticultural Fruit Crops: Trends and Future Prospects

Cytokinins (CKs) are a chemically diverse class of plant growth regulators, exhibiting wide-ranging actions on plant growth and development, hence their exploitation in agriculture for crop improvement and management. Their coordinated regulatory effects and cross-talk interactions with other phytohormones and signaling networks are highly sophisticated, eliciting and controlling varied biological processes at the cellular to organismal levels. In this review, we briefly introduce the mode of action and general molecular biological effects of naturally occurring CKs before highlighting the great variability in the response of fruit crops to CK-based innovations. We present a comprehensive compilation of research linked to the application of CKs in non-model crop species in different phases of fruit production and management. By doing so, it is clear that the effects of CKs on fruit set, development, maturation, and ripening are not necessarily generic, even for cultivars within the same species, illustrating the magnitude of yet unknown intricate biochemical and genetic mechanisms regulating these processes in different fruit crops. Current approaches using genomic-to-metabolomic analysis are providing new insights into the in planta mechanisms of CKs, pinpointing the underlying CK-derived actions that may serve as potential targets for improving crop-specific traits and the development of new solutions for the preharvest and postharvest management of fruit crops. Where information is available, CK molecular biology is discussed in the context of its present and future implications in the applications of CKs to fruits of horticultural significance.