Direct identification and metabolomic analysis of Huanglongbing associated with Candidatus Liberibacter spp. in navel orange by MALDI-TOF-MS

Identification and Characterization of Polyamine Metabolism in Citrus in Response to 'Candidatus Liberibacter asiaticus' Infection

Citrus Huanglongbing, one of the most devastating citrus diseases, is caused by 'Candidatus Liberibacter asiaticus' (CLas). Polyamines are aliphatic nitrogen-containing compounds that play important roles in disease resistance and are synthesized primarily by two pathways: an arginine decarboxylation pathway and an ornithine decarboxylation pathway. However, it is unclear whether polyamines play a role in the tolerance of citrus to infection by CLas and, if so, whether one or both of the core polyamine metabolic pathways are important. We used high-performance liquid chromatography and ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry to detect the contents of nine polyamine metabolism-related compounds in six citrus cultivars with varying levels of tolerance to CLas. We also systematically detected the changes in polyamine metabolism-related compounds and H2O2 contents and compared the gene expression levels and the activities of enzymes involved in the polyamine metabolic pathway among healthy, asymptomatic, and symptomatic leaves of Newhall navel oranges infected with CLas. The tolerant and moderately tolerant varieties showed higher polyamine metabolism-related compound levels than those of susceptible varieties. Compared with the healthy group, the symptomatic group showed significantly increased contents of arginine, ornithine, γ-aminobutyric acid, and putrescine by approximately 180, 19, 1.5, and 0.2 times, respectively, and upregulated expression of biosynthetic genes. Arginase and ornithine decarboxylase enzyme activities were the highest in the symptomatic group, whereas arginine decarboxylase and agmatine deiminase enzyme activities were the highest in the asymptomatic group. The two polyamine biosynthetic pathways showed different trends with the increase of the CLas titer, indicating that polyamines were mainly synthesized through the arginine decarboxylase pathway in the asymptomatic leaves and were synthesized via the ornithine decarboxylase pathway in symptomatic leaves. These findings provide new insight into the changes in polyamine metabolism in citrus infected with CLas.

Metabolic profiling of Oryza sativa seedlings under chilling stress using nanoliter electrospray ionization mass spectrometry

Low temperatures significantly impact on rice (Oryza sativa) yield and quality. Traditional metabolomic techniques, often involving time-consuming chromatography-mass spectrometry procedures, are currently in use. This study investigated metabolomic responses of rice seedlings under low-temperature stress using nanoliter electrospray ionization mass spectrometry (nanoESI-MS) in combination with multivariate analysis. Results revealed distinct metabolic profiles in 'Qiutianxiaoting' (japonica) and '93-11' (indica) rice seedlings. Among the 36 identified compounds in rice, seven key metabolites, comprising l-glutamic acid, asparagine, tryptophan, citric acid, α-linolenic acid, malic acid, and inositol, were identified as responsive to cold stress. Notably, malic acid content reached 1332.40 μg/g dry weight in Qiutianxiaoting and 1444.13 μg/g in 93-11. Both the qualitative and quantitative results of nanoESI-MS were further confirmed through gas chromatography-mass spectrometry validation. The findings highlight the potential of nanoESI-MS for rapidly characterizing crucial metabolites across diverse plant species under exposure to stress.

Identification of the botanical origins of honey based on nanoliter electrospray ionization mass spectrometry

The botanical origins of honey are important for the quality control and commercialization of honey. In this research, we established a nanoliter electrospray ionization mass spectrometry (Nano-ESI-MS) method to identify Castanopsis honey (CH), Eurya honey (EH), Dendropanax dentiger honey (DH), and Triadica cochinchinensis honey (TH). In total, 38 compounds were identified based on the collision-induced dissociation experiments by Nano-ESI-MS with 16 differential compounds and 7 quantified as potential differential markers. These four types of honey were distinguished from each other by their mass spectrometry data combined with multivariate analysis with three out of the 7 differential markers, i.e., phenethylamine, tricoumaroyl spermidine, and (+/-)-abscisic acid, identified as potential markers for CH, EH, and DH, respectively. Both the qualitative and quantitative results derived from Nano-ESI-MS were further verified by UPLC-Q/TOF-MS. Our studies provided the significant potential of the Nano-ESI-MS method in the identification of the botanical origins of different kinds of honey.

A reverse-ecology framework to uncover the potential metabolic interplay among 'Candidatus Liberibacter' species, Citrus hosts and psyllid vector

'Candidatus Liberibacter' species have developed a dependency on essential nutrients and metabolites from the host cell, as a result of substantial genome reduction. Still, it is difficult to state which nutrients they acquire and whether or not they are metabolically reliant. We used a reverse-ecology model to investigate the potential metabolic interactions of 'Ca Liberibacter' species, Citrus, and the psyllid Diaphorina citri in the huanglongbing disease pyramid. Our findings show that hosts (citrus and psyllid) tend to support the nutritional needs of 'Ca. Liberibacter' species, implying that the pathogen's metabolism has become tightly linked to hosts, which may reflect in the parasite lifestyle of this important genus.

Combining MALDI-MS with machine learning for metabolomic characterization of lung cancer patient sera

An increasing amount of evidence has proven that serum metabolites can instantly reflect disease states. Therefore, sensitive and reproducible detection of serum metabolites in a high-throughput manner is urgently needed for clinical diagnosis. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a high-throughput platform for metabolite detection, but it is hindered by significant signal fluctuations because of the "sweet spot" effect of organic matrices. Here, by screening two transformation methods and four normalization techniques to reduce the significant signal fluctuations of the DHB matrix, an integrated MALDI-MS data processing approach combined with machine learning methods was established to reveal metabolic biomarkers of lung cancer. In our study, 13 distinctive features with statistically significant differences (p < 0.001) between 34 lung cancer patients and 26 healthy controls were selected as significant potential biomarkers of lung cancer. 6 out of the 13 distinctive features were identified as intact metabolites. Our results demonstrate the potential for clinical application of MALDI-MS in serum metabolomics for biomarker screening in lung cancer.

The Impact of Diaphorina citri-Vectored 'Candidatus Liberibacter asiaticus' on Citrus Metabolism

'Candidatus Liberibacter asiaticus' is associated with the devastating citrus disease Huanglongbing (HLB). It is transmitted by grafting infected material to healthy plants and by the feeding of the Asian citrus psyllid (Diaphorina citri). Previously, we demonstrated that a metabolomics approach using proton-nuclear magnetic resonance spectroscopy discriminates healthy from diseased plants via grafting. This work assessed the capability of this technology in discriminating healthy and diseased plants when the bacterium is vectored by psyllids. One-year-old greenhouse-grown 'Lisbon' lemon trees were exposed to either carrier psyllids (exposed, n = 10), or psyllids that were free of 'Candidatus Liberibacter asiaticus' (control, n = 6). Leaf metabolites were tracked for 1 year and disease diagnosis was made using quantitative PCR. Overall, 31 water-soluble metabolites were quantified in leaves, including four sugars and 12 amino acids. Analysis via nonmetric multidimensional scaling and principal component analysis revealed significant differences between the leaf metabolome of control versus infected trees beginning at 8 weeks postexposure, including alterations in glucose and quinic acid concentrations. These findings provide a longitudinal overview of the metabolic effects of HLB during the early phases of disease, and confirm previous experimental work demonstrating that infection elicits changes in the leaf metabolome that enables discrimination between healthy and infected plants. Here we demonstrate that the mode of inoculation (i.e., graft versus psyllid) results in a similar pathology.

Metabolomic analysis elucidates how shade conditions ameliorate the deleterious effects of greening (Huanglongbing) disease in citrus

Under tropical and subtropical environments, citrus leaves are exposed to excess sunlight, inducing photoinhibition. Huanglongbing (HLB, citrus greening), a devastating phloem-limited disease putatively caused by Candidatus Liberibacter asiaticus, exacerbates this challenge with additional photosynthetic loss and excessive starch accumulation. A combined metabolomics and physiological approach was used to elucidate whether shade alleviates the deleterious effects of HLB in field-grown citrus trees, and to understand the underlying metabolic mechanisms related to shade-induced morpho-physiological changes in citrus. Using metabolite profiling and multinomial logistic regression, we identified pivotal metabolites altered in response to shade. A core metabolic network associated with shade conditions was identified through pathway enrichment analysis and metabolite mapping. We measured physio-biochemical responses and growth and yield characteristics. With these, the relationships between metabolic network and the variables measured above were investigated. We found that moderate-shade alleviates sink limitation by preventing excessive starch accumulation and increasing foliar sucrose levels. Increased growth and fruit yield in shaded compared with non-shaded trees were associated with increased photosystem II efficiency and leaf carbon fixation pathway metabolites. Our study also shows that, in HLB-affected trees under shade, the signaling of plant hormones (auxins and cytokinins) and nitrogen supply were downregulated with reducing new shoot production likely due to diminished needs of cell damage repair and tissue regeneration under shade. Overall, our findings provide the first glimpse of the complex dynamics between cellular metabolites and leaf physiological functions in citrus HLB pathosystem under shade, and reveal the mechanistic basis of how shade ameliorates HLB disease.

Dual Transcriptional Profiling of Carrot and 'Candidatus Liberibacter solanacearum' at Different Stages of Infection Suggests Complex Host-Pathogen Interaction

The interactions between the phloem-limited pathogen 'Candidatus Liberibacter solanacearum' haplotype C and carrot (Daucus carota subsp. sativus) were studied at 4, 5, and 9 weeks postinoculation (wpi), by combining dual RNA-Seq results with data on bacterial colonization and observations of the plant phenotype. In the infected plants, genes involved in jasmonate biosynthesis, salicylate signaling, pathogen-associated molecular pattern- and effector-triggered immunity, and production of pathogenesis-related proteins were up-regulated. At 4 wpi, terpenoid synthesis-related genes were up-regulated, presumably as a response to the psyllid feeding, whereas at 5 and 9 wpi, genes involved in both the terpenoid and flavonoid production were down-regulated and phenylpropanoid genes were up-regulated. Chloroplast-related gene expression was down-regulated, in concordance with the observed yellowing of the infected plant leaves. Both the RNA-Seq data and electron microscopy suggested callose accumulation in the infected phloem vessels, likely to impair the transport of photosynthates, while phloem regeneration was suggested by the formation of new sieve cells and the upregulation of cell wall-related gene expression. The 'Ca. L. solanacearum' genes involved in replication, transcription, and translation were expressed at high levels at 4 and 5 wpi, whereas, at 9 wpi, the Flp pilus genes were highly expressed, suggesting adherence and reduced mobility of the bacteria. The 'Ca. L. solanacearum' genes encoding ATP and C4-dicarboxylate uptake were differentially expressed between the early and late infection stages, suggesting a change in the dependence on different host-derived energy sources. HPE1 effector and salicylate hydroxylase were expressed, presumably to suppress host cell death and salicylic acid-dependent defenses during the infection.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The hidden treasures of citrus: finding Huanglongbing cure where it was lost

Huanglongbing (HLB), a deadly citrus disease which has significantly downsized the entire industry worldwide. The intractable and incurable disease has brought the citriculture an enormous loss of productivity. With no resistant varieties available, failure of chemical treatments despite repeated applications, and hazardous consequences to environmental health, have led to large-scale research to find a sustainable cure. Inside plants, the key determinants of health and safety, live the endophytic microbes. Endophytes possess unrivaled plant benefiting properties. The progression of HLB is known to cause disturbance in endophytic bacterial communities. Given the importance of the plant endophytic microbiome in disease progression, the notion of engineering microbiomes through indigenous endophytes is attracting scientific attention which is considered revolutionary as it precludes the incompatibility concerns associated with the use of alien (microbes from other plant species) endophytes. In this review, we briefly discuss the transformation of the plant-pathogen-environment to the plant-pathogen-microbial system in a disease triangle. We also argue the employment of indigenous endophytes isolated from a healthy state to engineer the diseased citrus endophytic microbiomes that can provide sustainable solution for vascular pathogens. We evaluated the plethora of microbiomes responses to the re-introduction of endophytes which leads to disease resistance in the citrus host. The idea is not merely confined to citrus-HLB, but it is globally applicable for tailoring a customized cure for general plant-pathogen systems particularly for the diseases caused by the vascular system-restricted pathogens.