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Huanglongbing (HLB) is one of the most destructive citrus diseases, mainly caused by the Gram-negative bacteria

Huanglongbing (HLB), presumably caused by Candidatus Liberibacter asiaticus (CaLas), is a devastating citrus disease worldwide. While all citrus are affected by HLB, some cultivars display greater tolerance; however, the underlying mechanisms are not fully understood. Here, volatile changes in HLB-tolerant LB8-9 Sugar Belle (SB) and HLB-sensitive Murcott mandarins after CaLas infection were comprehensively compared to determine if specific volatiles are associated with HLB responses and to discern the underlying tolerance mechanisms. These cultivars emitted qualitatively and quantitatively different volatiles in response to HLB induced by artificial graft or natural psyllid inoculation. Increasing amounts of total volatiles and de novo-synthesized new volatiles were two key responses to HLB of both cultivars. Markers potentially associated with HLB and host susceptibility were identified. Terpenoid biosynthetic pathway, green leaf volatile, and thymol metabolic pathways responsive to CaLas infection were dramatically altered. SB mandarin allows simultaneous defense and growth, contributing to its greater HLB tolerance.

With the high demand for a healthy diet, it is necessary and important to explore natural sweeteners used in food that enhance palatability but minimize calories. Citrus is considered a good potential source of noncaloric sweeteners, but to date, only one sweetness modulator has been found in this most common fruit crop. Herein, an efficient strategy based on an in-house database and the untargeted and targeted metabolomics analyses was proposed to screen sweeteners or sweet-enhancing compounds from citrus. Eight sweeteners or sweetness-enhancing compounds were screened out, seven of which were newly identified from the genus

The bacterial disease Huanglongbing (HLB) has been causing large economic losses in the citrus industry worldwide. Aimed at unraveling the mechanisms of scion/rootstock combination on improving HLB-affected orange juice quality, a specific scion/rootstock combination field trial was designed using three sibling rootstocks and two late-maturing sweet orange scion cultivars. Scion/rootstock combination significantly improved the overall consumer liking of orange juice from the HLB-affected trees. Rootstocks showed significant effects on the consumer liking and overall flavor, while scions had significant effects on the freshness and overall orange flavor intensity of the juice. A PLS-DA model combined with KEGG pathway enrichment analysis and some biomarker metabolites further indicated that scions mainly affected metabolism of alanine, aspartate, and glutamate in orange fruits. Meanwhile, rootstocks had an impact on the biosynthetic pathways of secondary metabolites. Sugars and organic acids were not closely correlated with the overall liking and sensory perception of orange juice. Rather, flavonoids, terpenoids, and volatile aromas played important roles in improving consumer overall liking. These results indicated that an optimum tolerant scion/rootstock combination can make a positive contribution toward improved fruit or juice quality from HLB-affected citrus trees.

Candidatus Liberibacter asiaticus (CLas) is the presumed causal agent of Huanglongbing, one of the most destructive diseases in citrus. However, the lipid metabolism component of host response to this pathogen has not been investigated well. Here, metabolic profiling of a variety of long-chain fatty acids and their oxidation products was first performed to elucidate altered host metabolic responses of disease. Fatty acid signals were found to decrease obviously in response to disease regardless of cultivar. Several lipid oxidation products strongly correlated with those fatty acids were also consistently reduced in the diseased group. Using a series of statistical methods and metabolic pathway mapping, we found significant markers contributing to the pathological symptoms and identified their internal relationships and metabolic network. Our findings suggest that the infection of CLas may cause the altered metabolism of long-chain fatty acids, possibly leading to manipulation of the host's defense derived from fatty acids.

Pioneering investigations referring to citrus flavor have been intensively conducted. However, the characteristic flavor difference between sweet orange and mandarin has not been defined. In this study, sensory analysis illustrated the crucial role of aroma in the differentiation between orange flavor and mandarin flavor. To study aroma, Valencia orange and LB8-9 mandarin were used. Their most aroma-active compounds were preliminarily identified by aroma extract dilution analysis (AEDA). Quantitation of key volatiles followed by calculation of odor activity values (OAVs) further detected potent components (OAV ≥ 1) impacting the overall aromatic profile of orange/mandarin. Follow-up aroma profile analysis revealed that ethyl butanoate, ethyl 2-methylbutanoate, octanal, decanal, and acetaldehyde were essential for orange-like aroma, whereas linalool, octanal, α-pinene, limonene, and (E,E)-2,4-decadienal were considered key components for mandarin-like aroma. Furthermore, an unreleased mandarin hybrid producing fruit with orange-like flavor was used to validate the identification of characteristic volatiles in orange-like aroma.