Dandan Zhang

Litchi (Litchi chinensis Sonn.) is a high-value subtropical fruit. The shelf life of litchi fruit after harvest is largely limited during transportation and commercial circulation due to rapid pericarp browning, which drastically reduces consumer acceptability. Pericarp browning is a complex senescence process jointly modulated by internal genetic characteristics and external postharvest environmental factors. This review systematically summarizes multi-layered research advances in litchi pericarp browning, covering cultivar and fruit traits, storage environmental regulations (temperature, gas composition, relative humidity and water migration), browning reaction, energy homeostasis, hormone signal, multi-omics metabolic regulatory module, transcriptional factor network, and miRNA-mediated post-transcriptional regulation. The core roles of polyphenol oxidase, peroxidase and especially laccase in anthocyanin degradation and phenolic oxidation are highlighted, while the relationships among water, energy metabolism, membrane integrity and oxidative damage are discussed carefully. Currently, the priority research directions include to establish integrated water-redox-energy regulatory frameworks, construct multi-scale browning quantitative models, select browning-resistant varieties via genetic improvement, and develop safe and low-cost integrated postharvest handling to extend the shelf life and mitigate loss of postharvest litchi fruit.

Penicillium italicum is the principal pathogen causing blue mold of citrus. Searching for novel antifungal agents is an important aspect of the post-harvest citrus industry because of the lack of higher effective and low toxic antifungal agents. Herein, the effects of 2-methoxy-1,4-naphthoquinone (MNQ) on P. italicum and its mechanism were carried out by a series of methods. MNQ had a significant anti-P. italicum effect with an MIC value of 5.0 µg/mL. The label-free protein profiling under different MNQ conditions identified a total of 3037 proteins in the control group and the treatment group. Among them, there were 129 differentially expressed proteins (DEPs, up-regulated > 2.0-fold or down-regulated < 0.5-fold, p < 0.05), 19 up-regulated proteins, 26 down-regulated proteins, and 67 proteins that were specific for the treatment group and another 17 proteins that were specific for the control group. Of these, 83 proteins were sub-categorized into 23 hierarchically-structured GO classifications. Most of the identified DEPs were involved in molecular function (47%), meanwhile 27% DEPs were involved in the cellular component and 26% DEPs were involved in the biological process. Twenty-eight proteins identified for differential metabolic pathways by KEGG were sub-categorized into 60 classifications. Functional characterization by GO and KEGG enrichment results suggests that the DEPs are mainly related to energy generation (mitochondrial carrier protein, glycoside hydrolase, acyl-CoA dehydrogenase, and ribulose-phosphate 3-epimerase), NADPH supply (enolase, pyruvate carboxylase), oxidative stress (catalase, glutathione synthetase), and pentose phosphate pathway (ribulose-phosphate 3-epimerase and xylulose 5-phosphate). Three of the down-regulated proteins selected randomly the nitro-reductase family protein, mono-oxygenase, and cytochrome P450 were verified using parallel reaction monitoring. These findings illustrated that MNQ may inhibit P. italicum by disrupting the metabolic processes, especially in energy metabolism and stimulus response that are both critical for the growth of the fungus. In conclusion, based on the molecular mechanisms, MNQ can be developed as a potential anti-fungi agent against P. italicum.

Thioredoxins (Trxs) are small proteins with a conserved redox active site WCGPC and are involved in a wide range of cellular redox processes. However, little information on the role of Trx in regulating low-temperature stress of harvested fruit is available. In this study, three full-length Trx cDNAs, designated MaTrx6, MaTrx9 and MaTrx12, were cloned from banana (Musa acuminata) fruit. Phylogenetic analysis and protein sequence alignments showed that MaTrx6 was grouped to h2 type with a typical active site of WCGPC, whereas MaTrx9 and MaTrx12 were assigned to atypical cys his-rich Trxs (ACHT) and h3 type with atypical active sites of GCAGC and WCSPC, respectively. Subcellular localization indicated that MaTrx6 and MaTrx12 were located in the plasma membrane and cytoplasm, respectively, whereas MaTrx9 showed a dual cytoplasmic and chloroplast localization. Application of ethylene induced chilling tolerance of harvested banana fruit, whereas 1-MCP, an inhibitor of ethylene perception, aggravated the development of chilling injury. RT-qPCR analysis showed that expression of MaTrx12 was up-regulated and down-regulated in ethylene- and 1-MCP-treated banana fruit at low temperature, respectively. Furthermore, heterologous expression of MaTrx12 in cytoplasmic Trx-deficient Saccharomyces cerevisiae strain increased the viability of the strain under H₂O₂. These results suggest that MaTrx12 plays an important role in the chilling tolerance of harvested banana fruit, possibly by regulating redox homeostasis.