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Citrus is a globally consumed fruit with great popularity. Mandarin (Citrus reticulata cv. ‘Shatangju’) is a local variety, and its planting area and yield are the greatest regarding fruit tree planting in Guangdong Province, China. However, its resistance to Huanglongbing (HLB) is weak. After infection by HLB, the fruits cannot develop normally. In this study, four kinds of fruits were classified as HBG, XQG, ZQG, and DHG, according to the color of their peels. The metabolomes of the three abnormally colored groups (HBG, XQG, and ZQG) and the normally colored group (DHG) were compared using a UPLC-QQQ-MS-based metabolomics approach. In total, 913 metabolites were identified and classified into 23 different categories, including phenylpropanoids and flavonoids; among them, 215 (HBG, 177; XQG, 124; and ZQG, 62) metabolites showed differential accumulation in the three comparison groups (HBG/XQG/ZQG versus DHG). A total of 2 unique metabolites, O-caffeoyl maltotriose and myricetin were detected only in DHG samples. When comparing HBG with DHG, there were 109 decreased and 68 increased metabolites; comparing XQG with DHG, there were 88 decreased and 36 increased metabolites; comparing ZQG with DHG, 41 metabolites were decreased, and 21 metabolites were increased. Metabolic pathway enrichment analysis of these differential metabolites showed significant enrichment of the “phenylpropanoid biosynthesis” pathway in all comparison groups. The hierarchical cluster analysis of the differential metabolites of the four groups showed a clear grouping patterns. The relative contents of three phenylpropanoids, four flavonoids, two alkaloids, one anthocyanin, and two other metabolites were significantly different between each comparison group. This study might provide fundamental insight for the isolation and identification of functional compounds from the peels of citrus fruit infected with HLB and for in-depth research on the effect of HLB on the formation of fruits pigment and the development of HLB-resistant citrus varieties.

Color formation in Hylocereus spp. (pitayas) has been ascribed to the accumulation of betalains. However, several studies have reported the presence of anthocyanins in pitaya fruit and their potential role in color formation has not yet been explored. In this study, we profiled metabolome and transcriptome in fruit of three cultivars with contrasting flesh colors (red, pink and white) to investigate their nutritional quality and the mechanism of color formation involving anthocyanins. Results revealed that pitaya fruit is enriched in amino acid, lipid, carbohydrate, polyphenols, vitamin and other bioactive components with significant variation among the three cultivars. Anthocyanins were detected in the fruit flesh and accumulation levels of Cyanidin 3-glucoside, Cyanidin 3-rutinoside, Delphinidin 3-O-(6-O-malonyl)-beta-glucoside-3-O-beta-glucoside and Delphinidin 3-O-beta-D-glucoside 5-O-(6-coumaroyl-beta-D-glucoside) positively correlated with the reddish coloration. Transcriptome data showed that the white cultivar tends to repress the anthocyanin biosynthetic pathway and divert substrates to other competing pathways. This perfectly contrasted with observations in the red cultivar. The pink cultivar however seems to keep a balance between the anthocyanin biosynthetic pathway and the competing pathways. We identified several active transcription factors of the MYB and bHLH families which can be further investigated as potential regulators of the anthocyanin biosynthetic genes. Collectively, our results suggest that anthocyanins partly contribute to color formation in pitaya fruit. Future studies aiming at manipulating the biosynthetic pathways of anthocyanins and betalains will better clarify the exact contribution of each pathway in color formation in pitayas. This will facilitate efforts to improve pitaya fruit quality and appeal.

Citrus grandis ‘Tomentosa’, commonly known as ‘Huajuhong’ pummelo (HJH), is used in traditional Chinese medicine and can moisten the lungs, resolve phlegm, and relieve coughs. A spontaneous bud mutant, named R-HJH, had a visually attractive phenotype with red albedo tissue and red juice sacs. In this study, the content and composition of carotenoids were investigated and compared between R-HJH and wild-type HJH using HPLC–MS analysis. The total carotenoids in the albedo tissue and juice sacs of R-HJH were 4.03- and 2.89-fold greater than those in HJH, respectively. The massive accumulation of carotenoids, including lycopene, β-carotene and phytoene, led to the attractive red color of R-HJH. However, the contents of flavones, coumarins and most volatile components (mainly D-limonene and γ-terpinene) were clearly reduced in R-HJH compared with wild-type HJH. To identify the molecular basis of carotenoid accumulation in R-HJH, RNA-Seq transcriptome sequencing was performed. Among 3948 differentially expressed genes (DEGs), the increased upstream synthesis genes (phytoene synthase gene, PSY) and decreased downstream genes (β-carotene hydroxylase gene, CHYB and carotenoid cleavage dioxygenase gene, CCD7) might be the key factors that account for the high level of carotenoids in R-HJH. These results will be beneficial for determining the molecular mechanism of carotenoid accumulation and metabolism in pummelo.

Portable and low-cost analytical devices are essential for rapid detection of bioactive substrates in agricultural products. This study presents the first highly integrated microelectrochemical sensor based on pencil graphite for rapid and sensitive detection of hesperidin in Citrus reticulate ‘Chachi’ peel. The surface morphology and characterization as well as the electrochemical property of pencil graphite was investigated and discussed. A high electrocatalytic efficiency of hesperidin has been found at used pencil graphite-based microelectrodes. Kinetic analysis was carried out to further understand the electrochemical process of hesperidin at a pencil graphite microelectrode. Consequently, a portable and highly-integrated microelectrochemical sensor exhibits a sensitivity of 0.7251 μA cm−2 μM−1 and a detection limit as low as 25 nM (S/N = 3), and high selectivity was fabricated. Proposed microelectrochemical sensors were applied to electrochemically determinate the hesperidin content in the extract of Citrus reticulata “chachi” peel. As a result, the concentration of hesperidin in the actual real sample detected electrochemically with the proposed portable and low-cost microelectrochemical sensors is highly consistent to that obtained with a common chromatographic method, thus indicating the good reliability and that it can be used in practical applications.

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera: Liviidae), is a vector of Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus Huanglongbing (HLB). We examine the effectiveness of spraying adhesive on new shoots of orange jasmine (Murraya paniculata) to trap ACP in laboratory and field conditions and for the monitoring of ACP population dynamics and directional preferences. After 36 h of observation, orange jasmine plants with new shoots, with and without adhesive, are significantly (p < 0.05) more attractive to ACP than plants without new shoots. In field trials, orange jasmine with new shoots attracted more ACP, particularly females, than plants without new shoots. A male-biased ACP sex ratio occurred in a near-natural population. Orange jasmine with new shoots coated with adhesive more effectively trapped ACP than yellow sticky traps, particularly during the winter and early spring, when ACP densities were low. ACP has a strong phototropic response, preferring to feed and rest in south- and east-facing positions. Adhesive trapping shows potential for attracting adult ACP, especially in citrus orchards during cooler seasons, when host trees lack new shoots, and it may be particularly effective in doing so in urban areas and unmanaged citrus refugia (the primary sources of ACP infestations for commercial groves).

This article has been retracted. Please see the Retraction Notice for more detail: https://doi.org/10.1186/s12870-020–02566-2

Harmful substances in food and agricultural environments pose significant risks to human health, necessitating the development of sensitive detection technologies. Electrochemical sensors are ideal for rapid monitoring because of their low cost, high efficiency, and portability. Recently developed laser-induced graphene (LIG)-based electrochemical sensors have demonstrated exceptional potential owing to the unique structural properties and outstanding electrochemical performance of LIG. In this review, the key factors influencing the LIG material characteristics during fabrication are discussed. Then, LIG-based electrochemical sensors are systematically categorized as pristine LIG and nanomaterial-functionalized, biomaterial-modified, and polymer-functionalized electrochemical sensors, and their application in the detection of functional components, additives, and agrochemicals in food products, and the detection of environmental pollutants, is comprehensively analyzed. Finally, the current challenges and the directions for future development are discussed.

Exocarpium Citri Grandis (ECG) is a distinctive medicinal and edible product originating from southern China and is often covered with a layer of characteristic “white frost” (WF). This study investigated the composition, formation mechanism, microbial safety, and anti-inflammatory activity of the WF. Multi-technique analyses revealed that WF mainly consisted of crystalline naringin (~80% of total mass). Drying-induced shrinkage and rupture of oil glands on ECG suggested metabolite migration and surface crystallization as the key mechanisms for WF formation. Microbial profiling revealed no significant differences in fungal and bacterial communities between WF and non-frost (NF) samples, and none of eight common mycotoxins was detected, confirming its microbial safety. Brewing tests demonstrated that water boiling for 30 min achieved efficient extraction of naringin, with higher yields in WF samples than in NF samples. In RAW264.7 cells, both WF and NF extract significantly inhibited lipopolysaccharide-induced NO production as well as the secretion and transcription of TNF-α, IL-6, IL-1β, iNOS, and NF-κB, with WF extract showing a stronger effect. Overall, these findings indicate that WF originates from endogenous flavonoid crystallization rather than microbial contamination and enhances the anti-inflammatory activity. This study provides a scientific basis for quality evaluation, processing optimization, and standardization of ECG products.

cv. 'Chunhongtangju' was mutated from Mandarin 'Shatangju', which has been identified as a new citrus variety. Mandarin 'Chunhongtangju' fruits were late-ripening for about two months than Mandarin 'Shatangju'. To understand the pericarp differential development mechanism in Mandarin 'Shatangju' (CK) and 'Chunhongtangju' (LM), hormones and transcriptome profiles of pericarps were performed in different development stages: Young fruit stage (CK1/LM1), Expansion and Turning color stage (CK2), Expansion stage (LM2), Turning color stage (LM3), and Maturity stage (CK3/LM4). In this study, the development of LM was significantly slower, and the maturity was significantly delayed. At the same stage, most hormones in Mandarin 'Chunhongtangju' pericarps were higher than that in 'Shatangju' such as gibberellin A24, cis(+)-12-oxophytodienoic acid, and L-phenylalanine. The deficiency of hormones in late-maturing pericarps was mainly manifested in ABA, 12-OHJA, MeSAG, and ABA-GE. Differences in transcriptome profiles between the two citrus varieties are primarily observed in energy metabolism, signal transduction such as MAPK signaling pathway and plant hormone signaling, and biosynthesis of secondary metabolites. After analyzing the hormones and transcriptome data, we found that the top genes and hormones, such as Cs_ont_5g020040 (transcription elongation factor, ), Cs_ont_7g021670 (BAG family molecular chaperone regulator 5, ), Cs_ont_2g025760 (40S ribosomal protein S27, ), 5-deoxystrigol, salicylic acid 2-O-β-glucosid, and gibberellin A24, contributed significantly to gene transcription and hormone synthesis. This study suggests that the variances of pericarp development between the two varieties are linked to variations in the transcription levels of genes associated with energy and secondary metabolism, signal transduction related genes. These findings expand our understanding of the complex transcriptional and hormonal regulatory hierarchy during pericarp development.

Background Mandarin ‘Shatangju’ is susceptible to Huanglongbing (HLB) and the HLB-infected fruits are small, off-flavor, and stay-green at the maturity period. To understand the relationship between pericarp color and HLB pathogen and the effect mechanism of HLB on fruit pericarp coloration, quantitative analyses of HLB bacterial pathogens and carotenoids and also the integrative analysis of metabolome and transcriptome profiles were performed in the mandarin ‘Shatangju’ variety with four different color fruits, whole green fruits (WGF), top-yellow and base-green fruits (TYBGF), whole light-yellow fruits (WLYF), and whole dark-yellow fruits (WDYF) that were infected with HLB. Results the HLB bacterial population followed the order WGF > TYBGF > WLYF > WDYF. And there were significant differences between each group of samples. Regarding the accumulation of chlorophyll and carotenoid, the chlorophyll-a content in WGF was the highest and in WDYF was the lowest. The content of chlorophyll-b in WGF was significantly higher than that in other three pericarps. There were significant differences in the total content of carotenoid between each group. WGF and TYBGF pericarps were low in phytoene, γ-carotene, β-cryptoxanthin and apocarotenal, while other kinds of carotenoids were significantly higher than those in WDYF. And WLYF was only short of apocarotenal. We comprehensively compared the transcriptome and metabolite profiles of abnormal (WGF, TYBGF and WLYF) and normal (WDYF, control) pericarps. In total, 2,880, 2,782 and 1,053 differentially expressed genes (DEGs), including 121, 117 and 43 transcription factors were identified in the three comparisons, respectively. The qRT-PCR confirmed the expression levels of genes selected from transcriptome. Additionally, a total of 77 flavonoids and other phenylpropanoid-derived metabolites were identified in the three comparisons. Most (76.65 %) showed markedly lower abundances in the three comparisons. The phenylpropanoid biosynthesis pathway was the major enrichment pathway in the integrative analysis of metabolome and transcriptome profiles. Conclusions Synthesizing the above analytical results, this study indicated that different color pericarps were associated with the reduced levels of some carotenoids and phenylpropanoids derivatives products and the down-regulation of proteins in flavonoids, phenylpropanoids derivatives biosynthesis pathway and the photosynthesis-antenna proteins.