Mycorrhiza-induced resistance in citrus against Tetranychus urticae is plant species dependent and inversely correlated to basal immunity.

BACKGROUND: Mycorrhizal plants show enhanced resistance to biotic stresses, but few studies have addressed mycorrhiza-induced resistance (MIR) against biotic challenges in woody plants, particularly citrus. Here we present a comparative study of two citrus species, Citrus aurantium, which is resistant to Tetranychus urticae, and Citrus reshni, which is highly susceptible to T. urticae. Although both mycorrhizal species are protected in locally infested leaves, they show very distinct responses to MIR. RESULTS: Previous studies have indicated that C. aurantium is insensitive to MIR in systemic tissues and MIR-triggered antixenosis. Conversely, C. reshni is highly responsive to MIR which triggers local, systemic and indirect defense, and antixenosis against the pest. Transcriptional, hormonal and inhibition assays in C. reshni indicated the regulation of jasmonic acid (JA)- and abscisic acid-dependent responses in MIR. The phytohormone jasmonic acid isoleucine (JA-Ile) and the JA biosynthesis gene LOX2 are primed at early timepoints. Evidence indicates a metabolic flux from phenylpropanoids to specific flavones that are primed at 24 h post infestation (hpi). MIR also triggers the priming of naringenin in mycorrhizal C. reshni, which shows a strong correlation with several flavones and JA-Ile that over-accumulate in mycorrhizal plants. Treatment with an inhibitor of phenylpropanoid biosynthesis C4H enzyme impaired resistance and reduced the symbiosis, demonstrating that phenylpropanoids and derivatives mediate MIR in C. reshni. CONCLUSION: MIR's effectiveness is inversely correlated to basal immunity in different citrus species, and provides multifaceted protection against T. urticae in susceptible C. reshni, activating rapid local and systemic defenses that are mainly regulated by the accumulation of specific flavones and priming of JA-dependent responses. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Citrus Huanglongbing is a pathogen-triggered immune disease that can be mitigated with antioxidants and gibberellin.

Huanglongbing (HLB) is a devastating disease of citrus, caused by the phloem-colonizing bacterium Candidatus Liberibacter asiaticus (CLas). Here, we present evidence that HLB is an immune-mediated disease. We show that CLas infection of Citrus sinensis stimulates systemic and chronic immune responses in phloem tissue, including callose deposition, production of reactive oxygen species (ROS) such as H2O2, and induction of immunity-related genes. The infection also upregulates genes encoding ROS-producing NADPH oxidases, and downregulates antioxidant enzyme genes, supporting that CLas causes oxidative stress. CLas-triggered ROS production localizes in phloem-enriched bark tissue and is followed by systemic cell death of companion and sieve element cells. Inhibition of ROS levels in CLas-positive stems by NADPH oxidase inhibitor diphenyleneiodonium (DPI) indicates that NADPH oxidases contribute to CLas-triggered ROS production. To investigate potential treatments, we show that addition of the growth hormone gibberellin (known to have immunoregulatory activities) upregulates genes encoding H2O2-scavenging enzymes and downregulates NADPH oxidases. Furthermore, foliar spray of HLB-affected citrus with gibberellin or antioxidants (uric acid, rutin) reduces H2O2 concentrations and cell death in phloem tissues and reduces HLB symptoms. Thus, our results indicate that HLB is an immune-mediated disease that can be mitigated with antioxidants and gibberellin.

Extracellular DNA as an elicitor of broad-spectrum resistance in Arabidopsis thaliana.

Like in mammals, the plant immune system has evolved to perceive damage. Damaged-associated molecular patterns (DAMPs) are endogenous signals generated in wounded or infected tissue after pathogen or insect attack. Although extracellular DNA (eDNA) is a DAMP signal that induces immune responses, plant responses after eDNA perception remain largely unknown. Here, we report that signaling defenses but not direct defense responses are induced after eDNA applications enhancing broad-range plant protection. A screening of defense signaling and hormone biosynthesis marker genes revealed that OXI1, CML37 and MPK3 are relevant eDNA-Induced Resistance markers (eDNA-IR). Additionally, we observed that eDNA from several Arabidopsis ecotypes and other phylogenetically distant plants such as citrus, bean and, more surprisingly, a monocotyledonous plant such as maize upregulates eDNA-IR marker genes. Using 3,3'-Diaminobenzidine (DAB) and aniline blue staining methods, we observed that H2O2 but not callose was strongly accumulated following self-eDNA treatments. Finally, eDNA resulted in effective induced resistance in Arabidopsis against the pathogens Hyaloperonospora arabidopsidis, Pseudomonas syringae, and Botrytis cinerea and against aphid infestation, reducing the number of nymphs and moving forms. Hence, the unspecificity of DNA origin and the wide range of insects to which eDNA can protect opens many questions about the mechanisms behind eDNA-IR.

The transcriptome landscapes of citrus leaf in different developmental stages.

KEY MESSAGE: The temporal expression profiles of citrus leaves explain the sink-source transition of immature leaves to mature leaves and provide knowledge regarding the differential responses of mature and immature leaves to biotic stress such as citrus canker and Asian citrus psyllid (Diaphorina citri). Citrus is an important fruit crop worldwide. Different developmental stages of citrus leaves are associated with distinct features, such as differences in susceptibilities to pathogens and insects, as well as photosynthetic capacity. Here, we investigated the mechanisms underlying these distinctions by comparing the gene expression profiles of mature and immature citrus leaves. Immature (stages V3 and V4), transition (stage V5), and mature (stage V6) Citrus sinensis leaves were chosen for RNA-seq analyses. Carbohydrate biosynthesis, photosynthesis, starch biosynthesis, and disaccharide metabolic processes were enriched among the upregulated differentially expressed genes (DEGs) in the V5 and V6 stages compared with that in the V3 and V4 stages. Glucose level was found to be higher in V5 and V6 than in V3 and V4. Among the four stages, the largest number of DEGs between contiguous stages were identified between V5 and V4, consistent with a change from sink to source, as well as with the sucrose and starch quantification data. The differential expression profiles related to cell wall synthesis, secondary metabolites such as flavonoids and terpenoids, amino acid biosynthesis, and immunity between immature and mature leaves may contribute to their different responses to Asian citrus psyllid infestation. The expression data suggested that both the constitutive and induced gene expression of immunity-related genes plays important roles in the greater resistance of mature leaves against Xanthomonas citri compared with immature leaves. The gene expression profiles in the different stages can help identify stage-specific promoters for the manipulation of the expression of citrus traits according to the stage. The temporal expression profiles explain the sink-source transition of immature leaves to mature leaves and provide knowledge regarding the differential responses to biotic stress.

Walking Together: Cross-Protection, Genome Conservation, and the Replication Machinery of Citrus tristeza virus.

"Cross-protection", a nearly 100 years-old virological term, is suggested to be changed to "close protection". Evidence for the need of such change has accumulated over the past six decades from the laboratory experiments and field tests conducted by plant pathologists and plant virologists working with different plant viruses, and, in particular, from research on Citrus tristeza virus (CTV). A direct confirmation of such close protection came with the finding that "pre-immunization" of citrus plants with the variants of the T36 strain of CTV but not with variants of other virus strains was providing protection against a fluorescent protein-tagged T36-based recombinant virus variant. Under natural conditions close protection is functional and is closely associated both with the conservation of the CTV genome sequence and prevention of superinfection by closely similar isolates. It is suggested that the mechanism is primarily directed to prevent the danger of virus population collapse that could be expected to result through quasispecies divergence of large RNA genomes of the CTV variants continuously replicating within long-living and highly voluminous fruit trees. This review article provides an overview of the CTV cross-protection research, along with a discussion of the phenomenon in the context of the CTV biology and genetics.

The immunity of Meiwa kumquat against Xanthomonas citri is associated with a known susceptibility gene induced by a transcription activator-like effector.

Citrus canker caused by Xanthomonas citri subsp. citri (Xcc) is one of the most devastating diseases in citrus. Meiwa kumquat (Fortunella crassifolia) has shown a durable resistance against Xcc. Here, we aimed to characterize the mechanisms responsible for such a durable resistance by characterizing the transcriptional and physiological responses of Meiwa kumquat to Xcc. Inoculation of Meiwa kumquat with Xcc promoted immune responses such as upregulation of PR genes, accumulation of salicylic acid, hypersensitive response (HR)-like cell death and early leaf abscission. Hypertrophy and hyperplasia symptoms, which are known to be caused by Xcc-induction of the canker susceptibility gene LOB1 through the transcription activator-like effector (TALE) PthA4, always appear prior to the development of cell death. Mutation of pthA4 in Xcc abolished the induction of LOB1, canker symptoms, cell death, and leaf abscission and reduced the expression of PR genes in inoculated kumquat leaves without reducing Xcc titers in planta. Transcriptome analysis demonstrated that PthA4 promotes plant biotic and abiotic stress responses and the biosynthesis of abscisic acid. Transcriptional induction of LOB1 homologs in Meiwa kumquat by Xcc pthA4 mutant strains carrying a repertoire of designer TALEs promoted the elicitation of HR-like phenotype and leaf abscission, suggesting that kumquat response to Xcc is associated with upregulation of LOB1. Our study suggests a novel mechanism of plant resistance to Xanthomonas via elicitation of immune responses by upregulation of a host susceptibility gene.

Acid Soil Improvement Enhances Disease Tolerance in Citrus Infected by Candidatus Liberibacter asiaticus.

Huanglongbing (HLB) is a devastating citrus disease that has caused massive economic losses to the citrus industry worldwide. The disease is endemic in most citrus-producing areas of southern China, especially in the sweet orange orchards where soil acidification has intensified. In this work, we used lime as soil pH amendment to optimize soil pH and enhance the endurance capacity of citrus against Candidatus Liberibacter asiaticus (CLas). The results showed that regulation of soil acidity is effective to reduce the occurrence of new infections and mitigate disease severity in the presence of HLB disease. We also studied the associated molecular mechanism and found that acid soil improvement can (i) increase the root metabolic activity and up-regulate the expression of ion transporter-related genes in HLB-infected roots, (ii) alleviate the physiological disorders of sieve tube blockage of HLB-infected leaves, (iii) strengthen the citrus immune response by increasing the expression of genes involved in SAR and activating the salicylic acid signal pathway, (iv) up-regulate 55 proteins related to stress/defence response and secondary metabolism. This study contributes to a better understanding of the correlation between environment factors and HLB disease outbreaks and also suggests that acid soil improvement is of potential value for the management of HLB disease in southern China.

Transgenic Sweet Orange expressing hairpin CP-mRNA in the interstock confers tolerance to citrus psorosis virus in the non-transgenic scion.

The lack of naturally occurring resistance to citrus psorosis virus (CPsV) necessitates a transgenic approach for the development of CPsV-resistant citrus. To evaluate the feasibility of conferring resistance to a non-transgenic scion, we have assembled citrus plants by grafting combining a non-transgenic Sweet Orange as scion, CPsV-resistant transgenic Sweet Orange lines expressing intron-hairpin (ihp) RNA derived from the viral coat protein (ihpCP) as interstock, and a non-transgenic citrus as rootstock. We demonstrated that ihpCP-transcripts translocate through the graft from interstock to scion, triggering the silencing of coat protein mRNA target. Two independent CPsV challenge assays showed that expression of ihpCP in the interstock provides resistance against CPsV in the interstock, and different levels of protection in the non-tg scion, depending of the virus delivery site. These results indicated that grafting is a promising biotechnological alternative to protect woody plants against virus infections in vegetative propagated plants.

Comparative Genomics Screen Identifies Microbe-Associated Molecular Patterns from 'Candidatus Liberibacter' spp. That Elicit Immune Responses in Plants.

Citrus huanglongbing (HLB), caused by phloem-limited 'Candidatus Liberibacter' bacteria, is a destructive disease threatening the worldwide citrus industry. The mechanisms of pathogenesis are poorly understood and no efficient strategy is available to control HLB. Here, we used a comparative genomics screen to identify candidate microbe-associated molecular patterns (MAMPs) from 'Ca. Liberibacter' spp. We identified the core genome from multiple 'Ca. Liberibacter' pathogens, and searched for core genes with signatures of positive selection. We hypothesized that genes encoding putative MAMPs would evolve to reduce recognition by the plant immune system, while retaining their essential functions. To efficiently screen candidate MAMP peptides, we established a high-throughput microtiter plate-based screening assay, particularly for citrus, that measured reactive oxygen species (ROS) production, which is a common immune response in plants. We found that two peptides could elicit ROS production in Arabidopsis and Nicotiana benthamiana. One of these peptides elicited ROS production and defense gene expression in HLB-tolerant citrus genotypes, and induced MAMP-triggered immunity against the bacterial pathogen Pseudomonas syringae. Our findings identify MAMPs that boost immunity in citrus and could help prevent or reduce HLB infection.

Targeted transcriptional and proteomic studies explicate specific roles of Bacillus subtilis iturin A, fengycin, and surfactin on elicitation of defensive systems in mandarin fruit during stress.

Application of Bacillus cyclic lipopeptides (CLPs); fengycin, iturin A and surfactin has shown a great potential in controlling the spread of green mold pathogen invasion (Penicillium digitatum) in wounded mandarin fruit during postharvest period. The limited defensive protein profiles followed specific expression of pivotal genes relating to plant hormone mediating signaling pathways of the CLPs' action on stimulating host plant resistance have been exhibited. The present study aimed to elucidate the specific effect of individual CLP obtained from Bacillus subtilis ABS-S14 as elicitor role on activation of plant defensive system at transcriptional and proteomic levels with and without P. digitatum co-application in mandarin fruit. Fengycin and iturin A elevated the gene expression of PAL, ACS1, ACO, CHI, and GLU while significantly stimulating plant POD transcription was only detected in the treatments of surfactin both with and without following P. digitatum. An increase of LOX and PR1 gene transcripts was determined in the treatments of individual CLP with fungal pathogen co-application. Fengycin activated production of unique defensive proteins such as protein involved in ubiquinone biosynthetic process in treated flavedo without P. digitatum infection. Proteins involved in the auxin modulating pathway were present in the iturin A and surfactin treatments. CLP-protein binding assay following proteome analysis reveals that iturin A attached to 12-oxophytodienoate reductase 2 involved in the oxylipin biosynthetic process required for jasmonic acid production which is implicated in induced systemic resistance (ISR). This study suggests specific elicitor action of individual CLP, particularly iturin A showed the most powerful in stimulating the ISR system in response to stresses in postharvest mandarins.

Global Transcriptomic Analysis Reveals Insights into the Response of 'Etrog' Citron (Citrus medica L.) to Citrus Exocortis Viroid Infection.

Citrus exocortis viroid (CEVd) is the causal agent of citrus exocortis disease. We employed CEVd-infected 'Etrog' citron as a system to study the feedback regulation mechanism using transcriptome analysis in this study. Three months after CEVd infection, the transcriptome of fresh leaves was analyzed, and 1530 differentially expressed genes were detected. The replication of CEVd in citron induced upregulation of genes encoding key proteins that were involved in the RNA silencing pathway such as Dicer-like 2, RNA-dependent RNA polymerase 1, argonaute 2, argonaute 7, and silencing defective 3, as well as those genes encoding proteins that are related to basic defense responses. Many genes involved in secondary metabolite biosynthesis and chitinase activity were upregulated, whereas other genes related to cell wall and phytohormone signal transduction were downregulated. Moreover, genes encoding disease resistance proteins, pathogenicity-related proteins, and heat shock cognate 70 kDa proteins were also upregulated in response to CEVd infection. These results suggest that basic defense and RNA silencing mechanisms are activated by CEVd infection, and this information improves our understanding of the pathogenesis of viroids in woody plants.

Temporal and spatial detection of Candidatus Liberibacter asiaticus putative effector transcripts during interaction with Huanglongbing-susceptible, -tolerant, and -resistant citrus hosts.

BACKGROUND: Citrus Huanglongbing (HLB) is a bacterial disease with high economic significance. The associated agent Candidatus Liberibacter asiaticus is a fastidious, phloem-limited, intracellular bacterium that is transmitted by an insect vector the Asian citrus psyllid (ACP). The genome of Ca. L. asiaticus contains protein secretion machinery that suggests host cell modulation capacity of this bacterium. RESULTS: A total of 28 candidate effectors, an important class of secreted proteins, were predicted from the Ca. L. asiaticus genome. Sequence specific primers were designed for reverse transcription (RT) and quantitative PCR (qPCR), and expression was validated for 20 of the effector candidates in infected citrus with multiple genetic background. Using detached leaf inoculation, the mRNA of effectors was detected from 6 h to 7 days post ACP exposure. It was observed that higher bacterial titers were associated with a larger number of effectors showing amplification across all samples. The effectors' expression were compared in citrus hosts with various levels of HLB tolerance, including susceptible Duncan grapefruit and Washington navel orange, tolerant citron and Cleopatra mandarin, and resistant Pomeroy trifoliate and Carrizo citrange. Across all genotypes relatively high expression was observed for CLIBASIA_03695, CLIBASIA_00460, CLIBASIA_00420, CLIBASIA_04580, CLIBASIA_05320, CLIBASIA_04425, CLIBASIA_00525 and CLIBASIA_05315 in either a host-specific or -nonspecific manners. The two genotypes in each HLB-response group also show effector-expression profiles that seem to be different. In a companion study, the expression of effectors was compared between leaves and roots of own-rooted citrus that had been Ca. L. asiaticus-infected for more than a year. Results indicated relatively high expression of CLIBASIA_03875, CLIBASIA_04800 and CLIBASIA_05640 in all leaf and some root tissues of citron, Duncan and Cleopatra. CONCLUSION: This temporal and spatial expression analysis of Ca. L. asiaticus effectors identified candidates possibly critical for early bacterial colonization, host tolerance suppression and long-term survival which are all worthy of further investigation.

'Candidatus Liberibacter asiaticus' peroxiredoxin (LasBCP) suppresses oxylipin-mediated defense signaling in citrus.

The Lasbcp (CLIBASIA_RS00445) 1-Cys peroxiredoxin gene is conserved among all 13 sequenced strains of Candidatus Liberibacter asiaticus, the causal agent of Huanglongbing or "citrus greening" disease. LasBCP was previously characterized as a secreted peroxiredoxin with substrate specificity for organic peroxides, and as a potential pathogenicity effector. Agrobacterium-mediated transient expression of LasBCP in citrus leaves provided significant protection against peroxidation of free and membrane-bound lipids, thereby preserving the molecular integrity of the chlorophyll apparatus and reducing accumulation of lipid peroxidation products (oxylipins) following exposure to tert-butyl hydroperoxide (tBOOH, an organic peroxide). Oxylipins extracted from GUS-expressing citrus leaves reduced viability of L. crescens, the only Liberibacter species cultured to date. However, similar extracts obtained from LasBCP-expressing leaves were less inhibitory to L. crescens growth and viability in culture. Quantitative RT-PCR analyses showed coordinated transcriptional downregulation of oxylipin biosynthetic (CitFAD, CitLOX, CitAOS and CitAOC), and jasmonic acid (JA) (CitJAR1, CitCOI1 and CitJIN1) and salicylic acid (SA) (CitPAL, CitICS and CitPR1) signaling pathway genes in citrus leaves expressing LasBCP and treated with tBOOH. The negative response regulator of jasmonic acid CitJAZ1 was upregulated in LasBCP-expressing citrus leaves under similar conditions. These data clearly demonstrated a protective role of secreted LasBCP in favor of Las survival and colonization by alleviating ROS-induced lipid peroxidation in citrus host, preventing accumulation of antimicrobial oxylipins, and suppressing both localized and systemic immune responses in planta.

A Secreted 'Candidatus Liberibacter asiaticus' Peroxiredoxin Simultaneously Suppresses Both Localized and Systemic Innate Immune Responses In Planta.

The oxidative (H2O2) burst is a seminal feature of the basal plant defense response to attempted pathogen invasions. In 'Candidatus Liberibacter asiaticus' UF506, expression of the SC2 prophage-encoded secreted peroxidase (F489_gp15) increases bacterial fitness and delays symptom progression in citrus. Two chromosomal 1-Cys peroxiredoxin genes, CLIBASIA_RS00940 (Lasprx5) and CLIBASIA_RS00445 (Lasbcp), are conserved among all sequenced 'Ca. L. asiaticus' strains, including those lacking prophages. Both LasBCP and LasdPrx5 have only a single conserved peroxidatic Cys (CP/SH) and lack the resolving Cys (CR/SH). Lasprx5 appeared to be a housekeeping gene with similar moderate transcript abundance in both 'Ca. L. asiaticus'-infected psyllids and citrus. By contrast, Lasbcp was expressed only in planta, similar to the expression of the SC2 peroxidase. Since 'Ca. L. asiaticus' is uncultured, Lasbcp and Lasprx5 were functionally validated in a cultured surrogate species, Liberibacter crescens, and both genes significantly increased oxidative stress tolerance and cell viability in culture. LasBCP was nonclassically secreted and, in L. crescens, conferred 214-fold more resistance to tert-butyl hydroperoxide (tBOOH) than wild type. Transient overexpression of Lasbcp in tobacco suppressed H2O2-mediated transcriptional activation of RbohB, the key gatekeeper of the systemic plant defense signaling cascade. Lasbcp expression did not interfere with the perception of 'Ca. L. asiaticus' flagellin (flg22Las) but interrupted the downstream activation of RbohB and stereotypical deposition of callose in tobacco. Critically, LasBCP also protected against tBOOH-induced peroxidative degradation of lipid membranes in planta, preventing subsequent accumulation of antimicrobial oxylipins that can also trigger the localized hypersensitive cell death response.

An effector from the Huanglongbing-associated pathogen targets citrus proteases.

The citrus industry is facing an unprecedented challenge from Huanglongbing (HLB). All cultivars can be affected by the HLB-associated bacterium 'Candidatus Liberibacter asiaticus' (CLas) and there is no known resistance. Insight into HLB pathogenesis is urgently needed in order to develop effective management strategies. Here, we use Sec-delivered effector 1 (SDE1), which is conserved in all CLas isolates, as a molecular probe to understand CLas virulence. We show that SDE1 directly interacts with citrus papain-like cysteine proteases (PLCPs) and inhibits protease activity. PLCPs are defense-inducible and exhibit increased protein accumulation in CLas-infected trees, suggesting a role in citrus defense responses. We analyzed PLCP activity in field samples, revealing specific members that increase in abundance but remain unchanged in activity during infection. SDE1-expressing transgenic citrus also exhibit reduced PLCP activity. These data demonstrate that SDE1 inhibits citrus PLCPs, which are immune-related proteases that enhance defense responses in plants.

The Asian Citrus Psyllid Host Murraya koenigii Is Immune to Citrus Huanglongbing Pathogen 'Candidatus Liberibacter asiaticus'.

The Asian citrus psyllid (ACP) Diaphorina citri, vector of 'Candidatus Liberibacter asiaticus' (CLas), the putative causal agent of citrus Huanglongbing (HLB), is controlled by application of insecticides, which, although effective, has resulted in serious biological imbalances. New management tools are needed, and the technique known as "trap crop" has been attracting attention. A potential plant for use as a trap crop in the management of the ACP is Murraya koenigii (curry leaf). However, for this plant to be used in the field, it needs to be attractive for the vector and must not harbor CLas. To verify the potential of curry leaf as trap crop for the management of HLB, we investigated the ability of D. citri to transmit CLas to M. koenigii, and to other test plants, including M. paniculata (orange jasmine) and cultivar Valencia sweet-orange seedlings. For the tests, the insects were reared on a symptomatic CLas-infected plant and allowed to feed on the three test plant species. The overall maximum transmission rate for the citrus seedlings was 83.3%, and for orange jasmine was 33.3%. Successful transmission of CLas by ACP to the curry-leaf seedlings was not observed, and it was treated as immune to CLas. Supported by the previous results that M. koenigii is attractive for ACP, these results indicate that curry leaf is an excellent candidate for use as a trap crop, to improve the management of the insect vector and consequently of HLB.

Immunoblotting Quantification Approach for Identifying Potential Hypoallergenic Citrus Cultivars.

The inherent allergens of citrus fruits, such as Cit s 1, Cit s 2, Cit s 3 can cause allergic reactions. A better understanding of the genetic factors (cultivar to cultivar) affecting the allergenic potential of citrus fruits would be beneficial for further identification of hypoallergenic genotypes. In the present study, an immunoblotting quantification approach was adopted to assess the potential allergenicity of 21 citrus cultivars, including nine subgroups (tangerine, satsuma, orange, pummelo, grapefruit, lemon, kumquat, tangor, and tangelo). To prepare highly sensitive and specific rabbit polyclonal antibodies, antigenicity of purified rCit s 1.01, rCit s 2.01, and rCit s 3.01 peptides were enhanced with high epitope density in a single protein molecule. The data integration of three citrus allergen quantifications demonstrated that the four pummelo cultivars (Kao Phuang Pummelo, Wanbai Pummelo, Shatian Pummelo, and Guanxi Pummelo) were potential hypoallergenic, compared with other 8 subgroups. Moreover, the immunological analyses with sera of allergic subjects revealed that Shatian Pummelo and Guanxi Pummelo showed the lowest immunoreactivity in 8 representative citrus cultivars. These potential hypoallergenic genotypes are of great significance to not only allergic consumers but also citrus breeders in the genetic improvement of hypoallergenic citrus as breeding resources.

[A CASE OF A 7-YEAR-OLD GIRL WITH ANAPHYLAXIS CAUSED BY PECTIN IN ALBEDO OF CITRUS UNSHIU BUT NOT BY THE FRUIT ITSELF].

Pectin is used in several foods as an additive and a thickner. But some cases of anaphylaxis have been reported. Most of these are induced by occasional exposures; however, no cases of anaphylaxis after eating a Citrus unshiu, the albedo of which is rich in pectin, have been reported.A 7-year-old girl developed barking cough and pruritus approximately two hours after eating a frozen Citrus unshiu. She had a history of anaphylaxis induced by consuming cashew nuts. Skin testing and basophil activation tests were performed using a commercially available pectin product. Both tests were positive. In an oral food challenge test, she felt abdominal pain and nausea only after eating fruit, along with the albedo, of Citrus unshiu. We concluded that this case was induced by pectin present in the albedo of Citrus unshiu, but not by the fruit itself. We should consider that patients with cashew nut allergies have a possibility of pectin allergies as well, and that pectin in the albedo of Citrus unshiu may induce anaphylaxis.

'Candidatus Liberibacter asiaticus' Encodes a Functional Salicylic Acid (SA) Hydroxylase That Degrades SA to Suppress Plant Defenses.

Pathogens from the fastidious, phloem-restricted 'Candidatus Liberibacter' species cause the devastating Huanglongbing (HLB) disease in citrus worldwide and cause diseases on many solanaceous crops and plants in the Apiaceae family. However, little is known about the pathogenic mechanisms due to the difficulty in culturing the corresponding 'Ca. Liberibacter' species. Here, we report that the citrus HLB pathogen 'Ca. L. asiaticus' uses an active salicylate hydroxylase SahA to degrade salicylic acid (SA) and suppress plant defenses. Purified SahA protein displays strong enzymatic activity to degrade SA and its derivatives. Overexpression of SahA in transgenic tobacco plants abolishes SA accumulation and hypersensitive response (HR) induced by nonhost pathogen infection. By degrading SA, 'Ca. L. asiaticus' not only enhances the susceptibility of citrus plants to both nonpathogenic and pathogenic Xanthomonas citri but also attenuates the responses of citrus plants to exogenous SA. In addition, foliar spraying of 2,1,3-benzothiadiazole and 2,6-dichloroisonicotinic acid, SA functional analogs not degradable by SahA, displays comparable (and even better) effectiveness with SA in suppressing 'Ca. L. asiaticus' population growth and HLB disease progression in infected citrus trees under field conditions. This study demonstrates one or more pathogens suppress plant defenses by degrading SA and establish clues for developing novel SA derivatives-based management approaches to control the associated plant diseases.