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.

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, gamma-carotene, beta-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.

Huanglongbing (HLB), caused by the Candidatus Liberibacter spp., is the most devastating disease in the citrus industry. HLB significantly affects and alters the microbial community structure or potential function of the microbial community of leaves and roots. However, it is unknown how the microbial community structure of the pericarp with different pigments is affected by Candidatus Liberibacter asiaticus (CLas). This study identified the enriched taxa of the microbial community in the citrus pericarp with normal or abnormal pigment and determine the effects of HLB on the pericarp microbial community using 16S rRNA-seq. The alpha and beta diversity and composition of microbial communities were significantly different between normal and abnormal pigment pericarp tissues of ripe fruits infected by CLas. Firmicutes, Actinobacteriota, Bacteroidota, Acidobacteriota, and Desulfobacterota dominated the pericarp microbiota composition in WDYFs (whole dark yellow fruits) samples. The relative abundance of most genera in WDYFs was higher than 1%, such as Burkholderia, and Pelomonas. However, with the exception of the HLB pathogen, the relative abundance of most genera in the abnormal-colored pericarp samples was less than 1%. CLas decreased the relative abundance of pericarp taxonomic. The predicted function of microbial was more plentiful and functional properties in the WDYF sample, such as translation, ribosomal structure and biogenesis, amino acid transport and metabolism, energy production and conversion, and some other clusters of orthologous groups (COG) except for cell motility. The results of this study offer novel insights into understanding the composition of microbial communities of the CLas-affected citrus pericarps and contribute to the development of biological control strategies for citrus against Huanglongbing.

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).

The intercropping system is used for weed control in orchards, but the intercrops need to be well-designed to fit into the row spaces of fruit trees. In this study, the citrus (Citrus reticulata cv. Chachiensis) row spaces were intercropped with either soybean (Glycine max (L.) Merr.) or sweet potato (Ipomoea batatas (L.) Lam.), and their effects on weed control, soil physiochemical properties, and soil microbiome were compared to the natural weeds. Both plant species were effective in reducing the orchard weeds, and their different varieties commonly improved soil organic matter, available P and K, and beneficial metal elements compared to the weeds. Even though the soil fungal and bacterial richness and diversity of the intercrops were not significantly altered, their composition, structure, and function were distinctive to those of the weeds. The soils of the intercrops generally enriched with the fungal genera of Talaromyces and Penicillium and the bacterial genera Sphingomonas, Knoellia, and Nocardioides. Accordingly, the altered microbial communities, in taxonomy, correlated to the enriched cellular functional pathways of glycolysis and gluconeogenesis, homologous recombination, nitrogen metabolism, lipoic acid metabolism, mismatch repair, DNA replication, nicotinate and nicotinamide metabolism. Taken together, these results imply that intercrops and weeds exert distinct effects on soil nutrient accumulation, and these effects are associated with their differential impacts on soil microbiomes—which are likely driven by the rhizosphere activities of the intercrops.

Introduction: Understanding the molecular mechanisms underlying fruit color variation is crucial for citrus breeding. The variegated ‘Gonggan’ mandarin ( Citrus reticulata Blanco) exhibits distinct phenotypic differences compared to the normal cultivar. However, the physiological and genetic basis for this variation remains unclear. Methods: In this study, we integrated physiological measurements, liquid chromatography-mass spectrometry (LC-MS), and transcriptome sequencing to characterize the pericarp of the variegated ‘Gonggan’ mandarin during early fruit development stage. Results: Compared with the normal ‘Gonggan’ mandarin, lightness and chromaticity values of the pericarp color exhibited significantly elevated in the variegated ‘Gonggan’ mandarin. Physiological assays revealed a drastic reduction in chlorophyll a and b contents (by 75.76% and 58.33%, respectively) and a moderate increase in total carotenoids (28.61%) in the variegated ‘Gonggan’ mandarin. Notably, phytoene accumulation in the pericarp of the variegated ‘Gonggan’ mandarin was 44.35-fold higher than that in the normal fruit, while metabolites in the middle and downstream pathway of carotenoid biosynthesis such as β-carotene and lutein were significantly reduced. Transcriptome analysis identified 1,438 differentially expressed genes (DEGs), which were predominantly enriched in metabolic pathways, secondary metabolite biosynthesis and photosynthesis. Specifically, 3 chlorophyll degradation genes encoding chlorophyll b reductase gene, chlorophyllase and SGR protein, 23 photosynthesis genes ( PsaD , PsaE , PsbO , etc .), and 6 photosynthesis-antenna proteins ( Lhca1 , Lhca2 , Lhcb1 , etc .) were significantly upregulated. Furthermore, 13 DEGs associated with plant hormone signal transduction exhibited differential expression patterns. Discussion: Our findings demonstrate that the disruption of the chlorophyll degradation pathway and carotenoid biosynthesis pathway are the primary causes of the pericarp color variation in the variegated ‘Gonggan’ mandarin. This study provides novel insights into the regulatory network governing pericarp coloration and establishes a foundation for future molecular breeding strategies targeting citrus appearance quality.