Early events of citrus greening (Huanglongbing) disease development at the ultrastructural level

Serological detection of 'Candidatus Liberibacter asiaticus' in citrus, and identification by GeLC-MS/MS of a chaperone protein responding to cellular pathogens

We describe experiments with antibodies against 'Candidatus Liberibacter asiaticus used to detect the pathogen in infected plants. We used scFv selected to bind epitopes exposed on the surface of the bacterium in tissue prints, with secondary monoclonal antibodies directed at a FLAG epitope included at the carboxyl end of the scFv. Unexpectedly, the anti-FLAG secondary antibody produced positive results with CaLas diseased samples when the primary scFv were not used. The anti-FLAG monoclonal antibody (Mab) also identified plants infected with other vascular pathogens. We then identified a paralogous group of secreted chaperone proteins in the HSP-90 family that contained the amino acid sequence DDDDK identical to the carboxy-terminal sequence of the FLAG epitope. A rabbit polyclonal antibody against one of the same epitopes combined with a goat anti-rabbit secondary antibody produced very strong purple color in individual phloem cells, as expected for this pathogen. These results were entirely specific for CaLas-infected citrus. The simplicity, cost and ability to scale the tissue print assay makes this an attractive assay to complement PCR-based assays currently in use. The partial FLAG epitope may itself be useful as a molecular marker for the rapid screening of citrus plants for the presence of vascular pathogens.

Digital Gene Expression Analysis of Ponkan Mandarin (Citrus reticulata Blanco) in Response to Asia Citrus Psyllid-Vectored Huanglongbing Infection

Citrus Huanglongbing (HLB), the most destructive citrus disease, can be transmitted by psyllids and diseased budwoods. Although the final symptoms of the two main HLB transmission ways were similar and hard to distinguish, the host responses might be different. In this study, the global gene changes in leaves of ponkan (Citrus reticulata) mandarin trees following psyllid-transmission of HLB were analyzed at the early symptomatic stage (13 weeks post inoculation, wpi) and late symptomatic stage (26 wpi) using digital gene expression (DGE) profiling. At 13 wpi, 2452 genes were downregulated while only 604 genes were upregulated in HLB infected ponkan leaves but no pathway enrichment was identified. Gene function analysis showed impairment in defense at the early stage of infection. At late stage of 26 wpi, however, differentially expressed genes (DEGs) involved in carbohydrate metabolism, plant defense, hormone signaling, secondary metabolism, transcription regulation were overwhelmingly upregulated, indicating that the defense reactions were eventually activated. The results indicated that HLB bacterial infection significantly influenced ponkan gene expression, and a delayed response of the host to the fast growing bacteria might be responsible for its failure in fighting against the bacteria.

A review of techniques for detecting Huanglongbing (greening) in citrus

Huanglongbing (HLB) is the most destructive disease of citrus worldwide. Monitoring of health and detection of diseases in trees is critical for sustainable agriculture. HLB symptoms are virtually the same wherever the disease occurs. The disease is caused by Candidatus Liberibacter spp., vectored by the psyllids Diaphorina citri Kuwayama and Trioza erytreae. Electron microscopy was the first technique used for HLB detection. Nowadays, scientists are working on the development of new techniques for a rapid HLB detection, as there is no sensor commercially accessible for real-time assessment of health conditions in trees. Currently, the most widely used mechanism for monitoring HLB is exploration, which is an expensive, labor-intensive, and time-consuming process. Molecular techniques such as polymerase chain reaction are used for the identification of HLB disease, which requires detailed sampling and processing procedures. Furthermore, investigations are ongoing in spectroscopic and imaging techniques, profiling of plant volatile organic compounds, and isothermal amplification. This study recognizes the need for developing a rapid, cost-effective, and reliable health-monitoring sensor that would facilitate advancements in HLB disease detection. This paper compares the benefits and limitations of these potential methods for HLB detection.

Transcriptome analysis of sweet orange trees infected with 'Candidatus Liberibacter asiaticus' and two strains of Citrus Tristeza Virus

BACKGROUND

Huanglongbing (HLB) and tristeza, are diseases of citrus caused by a member of the α-proteobacteria, 'Candidatus Liberibacter asiaticus' (CaLas), and Citrus tristeza virus (CTV) respectively. HLB is a devastating disease, but CTV strains vary from very severe to very mild. Both CaLas and CTV are phloem-restricted. The CaLas-B232 strain and CTV-B6 cause a wide range of severe and similar symptoms. The mild strain CTV-B2 doesn't induce significant symptoms or damage to plants.

RESULTS

Transcriptome profiles obtained through RNA-seq revealed 611, 404 and 285 differentially expressed transcripts (DETs) after infection with CaLas-B232, CTV-B6 and CTV-B2. These DETs were components of a wide range of pathways involved in circadian rhythm, cell wall modification and cell organization, as well as transcription factors, transport, hormone response and secondary metabolism, signaling and stress response. The number of transcripts that responded to both CTV-B6 and CaLas-B232 was much larger than the number of transcripts that responded to both strains of CTV or to both CTV-B2 and CaLas-B232. A total of 38 genes were assayed by RT-qPCR and the correlation coefficients between Gfold and RT-qPCR were 0.82, 0.69, 0.81 for sweet orange plants infected with CTV-B2, CTV-B6 and CaLas-B232, respectively.

CONCLUSIONS

The number and composition of DETs reflected the complexity of symptoms caused by the pathogens in established infections, although the leaf tissues sampled were asymptomatic. There were greater similarities between the sweet orange in response to CTV-B6 and CaLas-B232 than between the two CTV strains, reflecting the similar physiological changes caused by both CTV-B6 and CaLas-B232. The circadian rhythm system of plants was perturbed by all three pathogens, especially by CTV-B6, and the ion balance was also disrupted by all three pathogens, especially by CaLas-B232. Defense responses related to cell wall modification, transcriptional regulation, hormones, secondary metabolites, kinases and stress were activated by all three pathogens but with different patterns. The transcriptome profiles of Citrus sinensis identified host genes whose expression is affected by the presence of a pathogen in the phloem without producing symptoms (CTV-B2), and host genes whose expression leads to induction of symptoms in the plant (CTV-B6, CaLas-B232).

Transmission and Propagation of 'Candidatus Liberibacter asiaticus' by Grafting with Individual Citrus Leaves

Huanglongbing (HLB) is a chronic, progressive decline disease in citrus associated with a systemic infection by the bacterium 'Candidatus Liberibacter asiaticus'. Transmission of the bacterium in the field is by the Asian citrus psyllid, Diaphorina citri Kuwayama. Experimental propagation of 'Ca. L. asiaticus' is done primarily by grafting pieces of bud wood from an infected plant. To produce a small-scale model system for investigation of pathogen biology, we investigated grafting single leaves from infected citrus plants as sources of inoculum for propagation of the bacterium. In total, 162 plants ranging in age from 3 to 18 months were grafted. Grafting with intact asymptomatic and HLB-symptomatic leaves resulted in 61 of 78 (78%) and 35 of 41 (85%) of the plants infected with 'Ca. L. asiaticus', respectively. Inoculum consisting of the leaf petiole only or only an inoculum tissue remnant under the bark of the receptor tree resulted in 6 of 12 (50%) and 7 of 31 (23%) infected trees, respectively. Real-time polymerase chain reaction (qPCR) assays verified the infection in plants, a majority of which developed the foliar blotchy mottle symptom considered diagnostic for HLB, while some plants also displayed the stunted, chlorotic shoots for which the disease is named. The qPCR data together with the symptoms displayed demonstrated that individual leaves from infected trees can serve as effective inoculum sources for transmission and propagation of 'Ca. L. asiaticus' via grafting.

Huanglongbing: An overview of a complex pathosystem ravaging the world's citrus

Citrus huanglongbing (HLB) has become a major disease and limiting factor of production in citrus areas that have become infected. The destruction to the affected citrus industries has resulted in a tremendous increase to support research that in return has resulted in significant information on both applied and basic knowledge concerning this important disease to the global citrus industry. Recent research indicates the relationship between citrus and the causal agent of HLB is shaped by multiple elements, in which host defense responses may also play an important role. This review is intended to provide an overview of the importance of HLB to a wider audience of plant biologists. Recent advances on host-pathogen interactions, population genetics and vectoring of the causal agent are discussed.

Genome-Wide Characterization and Expression Analysis of Major Intrinsic Proteins during Abiotic and Biotic Stresses in Sweet Orange (Citrus sinensis L. Osb.)

The family of aquaporins (AQPs), or major intrinsic proteins (MIPs), includes integral membrane proteins that function as transmembrane channels for water and other small molecules of physiological significance. MIPs are classified into five subfamilies in higher plants, including plasma membrane (PIPs), tonoplast (TIPs), NOD26-like (NIPs), small basic (SIPs) and unclassified X (XIPs) intrinsic proteins. This study reports a genome-wide survey of MIP encoding genes in sweet orange (Citrus sinensis L. Osb.), the most widely cultivated Citrus spp. A total of 34 different genes encoding C. sinensis MIPs (CsMIPs) were identified and assigned into five subfamilies (CsPIPs, CsTIPs, CsNIPs, CsSIPs and CsXIPs) based on sequence analysis and also on their phylogenetic relationships with clearly classified MIPs of Arabidopsis thaliana. Analysis of key amino acid residues allowed the assessment of the substrate specificity of each CsMIP. Gene structure analysis revealed that the CsMIPs possess an exon-intron organization that is highly conserved within each subfamily. CsMIP loci were precisely mapped on every sweet orange chromosome, indicating a wide distribution of the gene family in the sweet orange genome. Investigation of their expression patterns in different tissues and upon drought and salt stress treatments, as well as with ‘Candidatus Liberibacter asiaticus’ infection, revealed a tissue-specific and coordinated regulation of the different CsMIP isoforms, consistent with the organization of the stress-responsive cis-acting regulatory elements observed in their promoter regions. A special role in regulating the flow of water and nutrients is proposed for CsTIPs and CsXIPs during drought stress, and for most CsMIPs during salt stress and the development of HLB disease. These results provide a valuable reference for further exploration of the CsMIPs functions and applications to the genetic improvement of both abiotic and biotic stress tolerance in citrus.

LC-MS/MS Method for the Determination and Quantitation of Penicillin G and Its Metabolites in Citrus Fruits Affected by Huanglongbing

In this study, we developed and validated a method for the extraction, identification, and quantitation of penicillin G and its metabolites (penilloic acid and penillic acid) in a variety of citrus fruits by employing sequential liquid/liquid and solid-phase extraction techniques in conjunction with UHPLC-MS/MS. Two product ion transitions per analyte were required for identification, which contributes to a high degree of selectivity. Corrected recoveries of penicillin G using an isotopically labeled internal standard were 90-100% at fortification levels of 0.1, 0.25, 1, and 10 ng/g. Absolute recoveries for penillic acid and penilloic acid were 50-75% depending on the matrix used. The limit of detection (LOD) of penicillin G and its metabolites was found to be 0.1 ng/g when 2 g of citrus was extracted. This method is useful in determining residue levels of penicillin G and its metabolites in citrus trees infected with huanglongbing bacteria after antibiotic treatment.

Localization and Distribution of 'Candidatus Liberibacter asiaticus' in Citrus and Periwinkle by Direct Tissue Blot Immuno Assay with an Anti-OmpA Polyclonal Antibody

'Candidatus Liberibacter asiaticus' (CaLas), a non-cultured member of the α-proteobacteria, is the causal agent of citrus Huanglongbing (HLB). Due to the difficulties of in vitro culture, antibodies against CaLas have not been widely used in studies of this pathogen. We have used an anti-OmpA polyclonal antibody based direct tissue blot immunoassay to localize CaLas in different citrus tissues and in periwinkle leaves. In citrus petioles, CaLas was unevenly distributed in the phloem sieve tubes, and tended to colonize in phloem sieve tubes on the underside of petioles in preference to the upper side of petioles. Both the leaf abscission zone and the junction of the petiole and leaf midrib had fewer CaLas bacteria compared to the main portions of the petiole and the midribs. Colonies of CaLas in phloem sieve tubes were more frequently found in stems with symptomatic leaves than in stems with asymptomatic leaves with an uneven distribution pattern. In serial sections taken from the receptacle to the peduncle, more CaLas were observed in the peduncle sections adjacent to the stem. In seed, CaLas was located in the seed coat. Many fewer CaLas were found in the roots, as compared to the seeds and petioles when samples were collected from trees with obvious foliar symptoms. The direct tissue blot immuno assay was adapted to whole periwinkle leaves infected by CaLas. The pathogen was distributed throughout the lateral veins and the results were correlated with results of qPCR. Our data provide direct spatial and anatomical information for CaLas in planta. This simple and scalable method may facilitate the future research on the interaction of CaLas and host plant.

Ultrastructural Changes and Putative Phage Particles Observed in Sweet Orange Leaves Infected with 'Candidatus Liberibacter asiaticus'

Huanglongbing (HLB), also known as citrus greening, is currently the most destructive citrus disease. Anatomical analyses of HLB-affected sweet orange were carried out by light and electron microscopy. As compared with healthy citrus, the phloem plasmodesmata were plugged with callose, and in some samples the phloem was collapsed. Chloroplast structures were deformed. Prophage sequences occupy a significant portion of the genome of 'Candidatus Liberibacter asiaticus' and have been used to distinguish strains from Yunnan and Guangdong provinces in China and Florida. Interestingly, a large number of possible putative phage particles were observed attached on the surface of 'Ca. L. asiaticus' cells in plants inoculated with strain FJ3 from Fujian Province, China. Phage particles have been observed previously only in periwinkle plants artificially inoculated in Florida with 'Ca. L. asiaticus' that carried the SC1-type prophage. PCR assays verified the presence of the SC1-type prophage sequences previously described from this bacterium in Florida in the FJ3 isolate. This is the first time that suspected phage particles have been observed in sweet orange trees infected with 'Ca. L. asiaticus.'

Differences in stylet sheath occurrence and the fibrous ring (sclerenchyma) between xCitroncirus plants relatively resistant or susceptible to adults of the Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae)

The Asian citrus psyllid (ACP, Diaphorina citri, Hemiptera: Liviidae), is the principal vector of the phloem-limited bacteria strongly associated with huanglongbing (HLB), the world's most serious disease of citrus. Host plant resistance may provide an environmentally safe and sustainable method of controlling ACP and/or HLB. Two xCitroncirus accessions (hybrids of Poncirus trifoliata and Citrus spp.), that are relatively resistant (UN-3881) or relatively susceptible (Troyer-1459) to ACP adults with regard to adult longevity, were compared in relation to ACP feeding behavior and some structural features of the leaf midrib. The settling (putative feeding/probing) sites of ACP adults on various parts of the leaf were not influenced primarily by plant accession. However, fewer ACP stylet sheaths were found in the midrib and fewer stylet sheath termini reached the vascular bundle (phloem and/or xylem) in UN-3881 compared to Troyer-1459 plants. Furthermore, in midribs of UN-3881 leaves the fibrous ring (sclerenchyma) around the phloem was significantly wider (thicker) compared to that in midribs of Troyer-1459 leaves. Our data indicate that feeding and/or probing by ACP adults into the vascular bundle is less frequent in the more resistant (UN-3881) than in the more susceptible (Troyer-1459) accessions. Our results also suggest that the thickness of the fibrous ring may be a barrier to stylet penetration into the vascular bundle, which is important for successful ACP feeding on the phloem and for transmitting HLB-associated bacteria. These results may help in the development of citrus plants resistant to ACP, which in turn could halt or slow the spread of the HLB-associated bacteria by this vector.

The transcriptional activator LdtR from 'Candidatus Liberibacter asiaticus' mediates osmotic stress tolerance

The causal agent of Huanglongbing disease, ‘Candidatus Liberibacter asiaticus’, is a non-culturable, gram negative, phloem-limited α-proteobacterium. Current methods to control the spread of this disease are still limited to the removal and destruction of infected trees. In this study, we identified and characterized a regulon from ‘Ca. L. asiaticus’ involved in cell wall remodeling, that contains a member of the MarR family of transcriptional regulators (ldtR), and a predicted L,D-transpeptidase (ldtP). In Sinorhizobium meliloti, mutation of ldtR resulted in morphological changes (shortened rod-type phenotype) and reduced tolerance to osmotic stress. A biochemical approach was taken to identify small molecules that modulate LdtR activity. The LdtR ligands identified by thermal shift assays were validated using DNA binding methods. The biological impact of LdtR inactivation by the small molecules was then examined in Sinorhizobium meliloti and Liberibacter crescens, where a shortened-rod phenotype was induced by growth in presence of the ligands. A new method was also developed to examine the effects of small molecules on the viability of ‘Ca. Liberibacter asiaticus’, using shoots from HLB-infected orange trees. Decreased expression of ldtR_Las and ldtP_Las was observed in samples taken from HLB-infected shoots after 6 h of incubation with the LdtR ligands. These results provide strong proof of concept for the use of small molecules that target LdtR, as a potential treatment option for Huanglongbing disease.

The rapid expansion of Huanglongbing disease (HLB) has caused a severe crisis in the citrus industry, with no solution visible in the near future. The causative agent, ‘Candidatus Liberibacter asiaticus’, is an unculturable bacterium under common laboratory conditions, which has made it difficult to gain understanding of this pathogen. Here we used a biochemical approach to identify new chemicals that could be used for the treatment of this devastating disease. These chemicals target a specific transcription factor (LdtR) in ‘Ca. Liberibacter asiaticus’. When bound to LdtR, the chemicals inactivate the protein, which disrupts a cell wall remodeling process that is critical for survival of the pathogen when exposed to osmotic stress (i.e. within the phloem of a citrus tree). Several model strains were used to confirm that the newly identified transcription factor (LdtR) and its regulated genes (ldtR and ldtP) confer tolerance to osmotic stress. The results presented in this study provide strong proof of concept for the use of small molecules that target LdtR, as a potential treatment option for Huanglongbing disease.

Spatial imaging of Zn and other elements in Huanglongbing-affected grapefruit by synchrotron-based micro X-ray fluorescence investigation

Huanglongbing (HLB) is a highly destructive, fast-spreading disease of citrus, causing substantial economic losses to the citrus industry worldwide. Nutrient levels and their cellular distribution patterns in stems and leaves of grapefruit were analysed after graft-inoculation with lemon scions containing 'Candidatus Liberibacter asiaticus' (Las), the heat-tolerant Asian type of the HLB bacterium. After 12 months, affected plants showed typical HLB symptoms and significantly reduced Zn concentrations in leaves. Micro-XRF imaging of Zn and other nutrients showed that preferential localization of Zn to phloem tissues was observed in the stems and leaves collected from healthy grapefruit plants, but was absent from HLB-affected samples. Quantitative analysis by using standard references revealed that Zn concentration in the phloem of veins in healthy leaves was more than 10 times higher than that in HLB-affected leaves. No significant variation was observed in the distribution patterns of other elements such as Ca in stems and leaves of grapefruit plants with or without graft-inoculation of infected lemon scions. These results suggest that reduced phloem transport of Zn is an important factor contributing to HLB-induced Zn deficiency in grapefruit. Our report provides the first in situ, cellular level visualization of elemental variations within the tissues of HLB-affected citrus.

The phloem-sap feeding mealybug (Ferrisia virgata) carries 'Candidatus Liberibacter asiaticus' populations that do not cause disease in host plants

'Candidatus Liberibacter asiaticus' (Las) is the primary causal agent of huanglongbing (HLB), the most devastating disease of citrus worldwide. There are three known insect vectors of the HLB-associated bacteria, and all are members of the Hemiptera: Diaphorina citri (Psyllidae), Trioza erytreae (Triozidae), and Cacopsylla (Psylla) citrisuga (Psyllidae). In this study, we found that another hemipteran, the striped mealybug Ferrisia virgata (Cockerell) (Hemiptera: Pseudococcidae), was able to acquire and retain Las bacteria. The bacterial titers were positively correlated with the feeding acquisition time on Las-infected leaf discs, with a two-weeks feeding period resulting in Ct values ranging from 23.1 to 36.1 (8.24 × 10(7) to 1.07 × 10(4) Las cells per mealybug). We further discovered that the prophage/phage populations of Las in the mealybugs were different from those of Las in psyllids based on Las prophage-specific molecular markers: infected psyllids harbored the Las populations with prophage/phage FP1 and FP2, while infected mealybugs carried the Las populations with the iFP3 being the dominant prophage/phage. As in the psyllids, Las bacteria were shown to move through the insect gut wall to the salivary glands after being ingested by the mealybug based on a time-course quantitative polymerase chain reaction (qPCR) assay of the dissected digestive systems. However, Las populations transmitted by the mealybugs did not cause disease in host plants. This is the first evidence of genetic difference among Las populations harbored by different insect vectors and difference among Las populations with respect to whether or not they cause disease in host plants.

Viability of 'Candidatus Liberibacter asiaticus' prolonged by addition of citrus juice to culture medium

Huanglongbing, or citrus greening disease, is associated with infection by the phloem-limited bacterium 'Candidatus Liberibacter asiaticus'. Infection with 'Ca. L. asiaticus' is incurable; therefore, knowledge regarding 'Ca. L. asiaticus' biology and pathogenesis is essential to develop a treatment. However, 'Ca. L. asiaticus' cannot currently be successfully cultured, limiting its study. To gain insight into the conditions conducive for growth of 'Ca. L. asiaticus' in vitro, 'Ca. L. asiaticus' inoculum obtained from seed of fruit from infected pomelo trees (Citrus maxima 'Mato Buntan') was added to different media, and cell viability was monitored for up to 2 months using quantitative polymerase chain reaction in conjunction with ethidium monoazide. Media tested included one-third King's B (K), K with 50% juice from the infected fruit, K with 50% commercially available grapefruit juice, and 100% commercially available grapefruit juice. Results show that juice-containing media dramatically prolong viability compared with K in experiments reproduced during 2 years using different juice sources. Furthermore, biofilm formed at the air-liquid interface of juice cultures contained 'Ca. L. asiaticus' cells, though next-generation sequencing indicated that other bacterial genera were predominant. Chemical characterization of the media was conducted to discuss possible factors sustaining 'Ca. L. asiaticus' viability in vitro, which will contribute to future development of a culture medium for 'Ca. L. asiaticus'.

An HPLC-MS characterization of the changes in sweet orange leaf metabolite profile following infection by the bacterial pathogen Candidatus Liberibacter asiaticus

Huanglongbing (HLB) presumably caused by Candidatus Liberibacter asiaticus (CLas) threatens the commercial U.S. citrus crop of an annual value of $3 billion. The earliest shift in metabolite profiles of leaves from greenhouse-grown sweet orange trees infected with Clas, and of healthy leaves, was characterized by HPLC-MS concurrently with PCR testing for the presence of Clas bacteria and observation of disease symptoms. Twenty, 8-month-old ‘Valencia’ and ‘Hamlin’ trees were grafted with budwood from PCR-positive HLB source trees. Five graft-inoculated trees of each variety and three control trees were sampled biweekly and analyzed by HPLC-MS and PCR. Thirteen weeks after inoculation, Clas was detected in newly growing flushes in 33% and 55% of the inoculated ‘Hamlin’ and ‘Valencia’ trees, respectively. Inoculated trees remained asymptomatic in the first 20 weeks, but developed symptoms 30 weeks after grafting. No significant differences in the leaf metabolite profiles were detected in Clas-infected trees 23 weeks after inoculation. However, 27 weeks after inoculation, differences in metabolite profiles between control leaves and those of Clas-infected trees were evident. Affected compounds were identified with authentic standards or structurally classified by their UV and mass spectra. Included among these compounds are flavonoid glycosides, polymethoxylated flavones, and hydroxycinnamates. Four structurally related hydroxycinnamate compounds increased more than 10-fold in leaves from ‘Hamlin’ and ‘Valencia’ sweet orange trees in response to Clas infection. Possible roles of these hydroxycinnamates as plant defense compounds against the Clas infection are discussed.

Validation of 'variable number of tandem repeat'-based approach for examination of 'Candidatus Liberibacter asiaticus' diversity and its applications for the analysis of the pathogen populations in the areas of recent introduction

Citrus greening (Huanglongbing, HLB) is one of the most destructive diseases of citrus worldwide. In South Asia HLB has been known for more than a century, while in Americas the disease was found relatively recently. HLB is associated with three species of 'Candidatus Liberibacter' among which 'Ca. Liberibacter asiaticus' (CLas) has most wide distribution. Recently, a number of studies identified different regions in the CLas genome with variable number of tandem repeats (VNTRs) that could be used for examination of CLas diversity. One of the objectives of the work presented here was to further validate the VNTR analysis-based approach by assessing the stability of these repeats upon multiplication of the pathogen in a host over an extended period of time and upon its passaging from a host to a host using CLas populations from Florida. Our results showed that the numbers of tandem repeats in the four loci tested display very distinguishable "signature profiles" for the two Florida-type CLas haplotype groups. Remarkably, the profiles do not change upon passage of the pathogen in citrus and psyllid hosts as well as after its presence within a host over a period of five years, suggesting that VNTR analysis-based approach represents a valid methodology for examination of the pathogen populations in various geographical regions. Interestingly, an extended analysis of CLas populations in different locations throughout Florida and in several countries in the Caribbean and Central America regions and in Mexico where the pathogen has been introduced recently demonstrated the dispersion of the same haplotypes of CLas. On the other hand, these CLas populations appeared to differ significantly from those obtained from locations where the disease has been present for a much longer time.

Gene regulatory networks elucidating huanglongbing disease mechanisms

Next-generation sequencing was exploited to gain deeper insight into the response to infection by Candidatus liberibacter asiaticus (CaLas), especially the immune disregulation and metabolic dysfunction caused by source-sink disruption. Previous fruit transcriptome data were compared with additional RNA-Seq data in three tissues: immature fruit, and young and mature leaves. Four categories of orchard trees were studied: symptomatic, asymptomatic, apparently healthy, and healthy. Principal component analysis found distinct expression patterns between immature and mature fruits and leaf samples for all four categories of trees. A predicted protein - protein interaction network identified HLB-regulated genes for sugar transporters playing key roles in the overall plant responses. Gene set and pathway enrichment analyses highlight the role of sucrose and starch metabolism in disease symptom development in all tissues. HLB-regulated genes (glucose-phosphate-transporter, invertase, starch-related genes) would likely determine the source-sink relationship disruption. In infected leaves, transcriptomic changes were observed for light reactions genes (downregulation), sucrose metabolism (upregulation), and starch biosynthesis (upregulation). In parallel, symptomatic fruits over-expressed genes involved in photosynthesis, sucrose and raffinose metabolism, and downregulated starch biosynthesis. We visualized gene networks between tissues inducing a source-sink shift. CaLas alters the hormone crosstalk, resulting in weak and ineffective tissue-specific plant immune responses necessary for bacterial clearance. Accordingly, expression of WRKYs (including WRKY70) was higher in fruits than in leaves. Systemic acquired responses were inadequately activated in young leaves, generally considered the sites where most new infections occur.

Transcriptional and microscopic analyses of citrus stem and root responses to Candidatus Liberibacter asiaticus infection

Huanglongbing (HLB) is the most destructive disease that affects citrus worldwide. The disease has been associated with Candidatus Liberibacter. HLB diseased citrus plants develop a multitude of symptoms including zinc and copper deficiencies, blotchy mottle, corky veins, stunting, and twig dieback. Ca. L. asiaticus infection also seriously affects the roots. Previous study focused on gene expression of leaves and fruit to Ca. L. asiaticus infection. In this study, we compared the gene expression levels of stems and roots of healthy plants with those in Ca. L. asiaticus infected plants using microarrays. Affymetrix microarray analysis showed a total of 988 genes were significantly altered in expression, of which 885 were in the stems, and 111 in the roots. Of these, 551 and 56 were up-regulated, while 334 and 55 were down-regulated in the stem and root samples of HLB diseased trees compared to healthy plants, respectively. Dramatic differences in the transcriptional responses were observed between citrus stems and roots to Ca. L. asiaticus infection, with only 8 genes affected in both the roots and stems. The affected genes are involved in diverse cellular functions, including carbohydrate metabolism, cell wall biogenesis, biotic and abiotic stress responses, signaling and transcriptional factors, transportation, cell organization, protein modification and degradation, development, hormone signaling, metal handling, and redox. Microscopy analysis showed the depletion of starch in the roots of the infected plants but not in healthy plants. Collapse and thickening of cell walls were observed in HLB affected roots, but not as severe as in the stems. This study provides insight into the host response of the stems and roots to Ca. L. asiaticus infection.

Quantification of Live 'Candidatus Liberibacter asiaticus' Populations Using Real-Time PCR and Propidium Monoazide

Huanglongbing (HLB) is a devastating citrus disease. It is associated with a phloem-restricted bacterium, 'Candidatus Liberibacter asiaticus', and primarily transmitted by Asian citrus psyllid in Florida. Because Liberibacter cannot be cultured, early diagnosis of HLB relies on DNA-based polymerase chain reaction (PCR), including real-time quantitative (q)PCR. Although estimating genomes from live bacteria (GLB) is critical for HLB research, PCR does not distinguish between live and dead cells and, thus, does not estimate GLB in hosts. Propidium monoazide (PMA), a novel DNA-binding dye, has been successfully used on many bacterial pathogens to effectively remove DNA from dead cells but there is no report of its use on uncultured bacteria. In this study, PMA-qPCR protocols were first optimized to work with plant and psyllid samples, respectively. Both TissueLyser treatment and plant tissue were demonstrated to have an insignificant impact on the GLB detected by PMA-qPCR. Finally, a standard curve for GLB determination was successfully established between PMA-qPCR results and microscopic counts and then applied in two studies with different greenhouse plant samples. This rapid qPCR method provides a more accurate way to determine GLB in HLB hosts which, in turn, should benefit disease epidemiology studies and serve as a crucial component in HLB management.

Citrus huanglongbing: a newly relevant disease presents unprecedented challenges

Citrus huanglongbing (HLB) is one of the oldest citrus diseases and has been known for over a century. HLB is caused by 'Candidatus Liberibacter' spp. that are phloem-limited, fastidious α-proteobacteria and infect hosts in different Kingdoms (i.e., Animalia and Plantae). When compared with well-characterized, cultivatable plant-pathogenic Gram-negative bacteria, the interactions of uncultured insect-vectored plant-pathogenic bacteria, including 'Ca. Liberibacter' spp., with their hosts remain poorly understood. 'Ca. Liberibacter' spp. have been known to cause HLB, which has been rapidly spreading worldwide, resulting in dramatic economic losses. HLB presents an unprecedented challenge to citrus production. In this review, we focus on the most recent research on citrus, 'Candidatus Liberibacter asiaticus', and psyllid interactions, specifically considering the following topics: evolutionary relationships among 'Ca. Liberibacter' spp., genetic diversity, host range, genome analysis, transmission, virulence mechanisms, and the ecological importance of HLB. Currently, no efficient management strategy is available to control HLB, although some promising progress has been made. Further studies are needed to understand citrus, 'Ca. L. asiaticus', and psyllid interactions to design innovative management strategies. Although HLB has been problematic for over a century, we can only win the battle against HLB with a coordinated and deliberate effort by the citrus industry, citrus growers, researchers, legislatures, and governments.

Visualization of 'Candidatus Liberibacter asiaticus' cells in the vascular bundle of citrus seed coats with fluorescence in situ hybridization and transmission electron microscopy

'Candidatus Liberibacter asiaticus' is the bacterium implicated as a causal agent of the economically damaging disease of citrus called huanglongbing (HLB). Vertical transmission of the organism through seed to the seedling has not been demonstrated. Previous studies using real-time polymerase chain reaction assays indicated abundant bacterial 16S rRNA sequences in seed coats of citrus seed but the presence of intact bacterial cells was not demonstrated. We used microscopy to verify that intact bacterial cells were present in citrus seed coats. Bacterial cells with the morphology and physical dimensions appropriate for 'Ca. L. asiaticus' were seen in phloem sieve elements in the vascular bundle of grapefruit seed coats using transmission electron microscopy (TEM). Fluorescence in situ hybridization (FISH) analyses utilizing probes complementary to the 'Ca. L. asiaticus' 16S rRNA gene revealed bacterial cells in the vascular tissue of intact seed coats of grapefruit and pummelo and in fragmented vascular bundles excised from grapefruit seed coats. The physical measurements and the morphology of individual bacterial cells were consistent with those ascribed in the literature to 'Ca. L. asiaticus'. No bacterial cells were observed in preparations of seed from fruit from noninfected trees. A small library of clones amplified from seed coats from a noninfected tree using degenerate primers targeting prokaryote 16S rRNA gene sequences contained no 'Ca. L. asiaticus' sequences, whereas 95% of the sequences in a similar library from DNA from seed coats from an infected tree were identified as 'Ca. L. asiaticus', providing molecular genetic corroboration that the bacterial cells observed by TEM and FISH in seed coats from infected trees were 'Ca. L. asiaticus'.

Global gene expression changes in Candidatus Liberibacter asiaticus during the transmission in distinct hosts between plant and insect

Huanglongbing (HLB) or citrus greening disease is a destructive disease of citrus worldwide, which is associated with Candidatus Liberibacter asiaticus. This phloem-limited fastidious pathogen is transmitted by the Asian citrus psyllid, Diaphorina citri, and appears to be an intracellular pathogen that maintains an intimate association with the psyllid or the plant throughout its life cycle. The molecular basis of the interaction of this pathogen with its hosts is not well understood. We hypothesized that, during infection, Ca. L. asiaticus differentially expresses the genes critical for its survival and/or pathogenicity in either host. To test this hypothesis, quantitative reverse transcription-polymerase chain reaction was performed to compare the gene expression of Ca. L. asiaticus in planta and in psyllid. Overall, 381 genes were analysed for their gene expression in planta and in psyllid. Among them, 182 genes were up-regulated in planta compared with in psyllid (P < 0.05), 16 genes were up-regulated in psyllid (P < 0.05) and 183 genes showed no statistically significant difference (P ≥ 0.05) in expression between in planta and in psyllid. Our study indicates that the expression of the Ca. L. asiaticus genes involved in transcriptional regulation, transport system, secretion system, flagella assembly, metabolic pathway and stress resistance are changed significantly in a host-specific manner to adapt to the distinct environments of plant and insect. To our knowledge, this is the first large-scale study to evaluate the differential expression of Ca. L. asiaticus genes in a plant host and its insect vector.

Candidatus Liberibacter americanus induces significant reprogramming of the transcriptome of the susceptible citrus genotype

BACKGROUND

Citrus huanglongbing (HLB) disease is caused by endogenous, phloem-restricted, Gram negative, uncultured bacteria named Candidatus Liberibacter africanus (CaLaf), Ca. L. asiaticus (CaLas), and Ca. L. americanus (CaLam), depending on the continent where the bacteria were first detected. The Asian citrus psyllid vector, Diaphorina citri, transmits CaLas and CaLam and both Liberibacter species are present in Brazil. Several studies of the transcriptional response of citrus plants manifesting HLB symptoms have been reported, but only for CaLas infection. This study evaluated the transcriptional reprogramming of a susceptible genotype of sweet orange challenged with CaLam, using a customized 385K microarray containing approximately 32,000 unigene transcripts. We analyzed global changes in gene expression of CaLam-infected leaves of sweet orange during the symptomatic stage of infection and compared the results with previously published microarray studies that used CaLas-infected plants. Twenty candidate genes were selected to validate the expression profiles in symptomatic and asymptomatic PCR-positive leaves infected with CaLas or CaLam.

RESULTS

The microarray analysis identified 633 differentially expressed genes during the symptomatic stage of CaLam infection. Among them, 418 (66%) were upregulated and 215 (34%) were down regulated. Five hundred and fourteen genes (81%) were orthologs of genes from Arabidopsis thaliana. Gene set enrichment analysis (GSEA) revealed that several of the transcripts encoded transporters associated with the endomembrane system, especially zinc transport. Among the most biologically relevant gene transcripts in GSEA were those related to signaling, metabolism and/or stimulus to hormones, genes responding to stress and pathogenesis, biosynthesis of secondary metabolites, oxidative stress and transcription factors belonging to different families. Real time PCR of 20 candidate genes validated the expression pattern of some genes in symptomatic and asymptomatic leaves infected with CaLam or CaLas.

CONCLUSIONS

Many gene transcripts and biological processes are significantly altered upon CaLam infection. Some of them had been identified in response to CaLas infection, while others had not been previously reported. These data will be useful for selecting target genes for genetic engineering to control HLB.

Stylet morphometrics and citrus leaf vein structure in relation to feeding behavior of the Asian citrus psyllid Diaphorina citri, vector of citrus huanglongbing bacterium

The Asian citrus psyllid (ACP), Diaphorina citri (Hemiptera: Psyllidae), is the primary vector of the phloem-limited bacterium Candidatus Liberibacter asiaticus (LAS) associated with huanglongbing (HLB, citrus greening), considered the world's most serious disease of citrus. Stylet morphometrics of ACP nymphs and adults were studied in relation to citrus vein structure and to their putative (histologically verified) feeding sites on Valencia orange leaves. ACP nymphs preferred to settle and feed on the lower (abaxial) side of young leaves either on secondary veins or on the sides of the midrib, whereas adults preferred to settle and feed on the upper (adaxial) or lower secondary veins of young or old leaves. Early instar nymphs can reach and probe the phloem probably because the distance to the phloem is considerably shorter in younger than in mature leaves, and is shorter from the sides of the midrib compared to that from the center. Additionally, the thick-walled 'fibrous ring' (sclerenchyma) around the phloem, which may act as a barrier to ACP stylet penetration into the phloem, is more prominent in older than in younger leaves and in the center than on the sides of the midrib. The majority (80-90%) of the salivary sheath termini produced by ACP nymphs and adults that reached a vascular bundle were associated with the phloem, whereas only 10-20% were associated with xylem vessels. Ultrastructural studies on ACP stylets and LAS-infected leaves suggested that the width of the maxillary food canal in first instar nymphs is wide enough for LAS bacteria to traverse during food ingestion (and LAS acquisition). However, the width of the maxillary salivary canal in these nymphs may not be wide enough to accommodate LAS bacteria during salivation (and LAS inoculation) into host plants. This may explain the inability of early instar nymphs to transmit LAS/HLB in earlier reports.

Small RNA profiling reveals phosphorus deficiency as a contributing factor in symptom expression for citrus huanglongbing disease

Huanglongbing (HLB) is a devastating citrus disease that is associated with bacteria of the genus 'Candidatus Liberibacter' (Ca. L.). Powerful diagnostic tools and management strategies are desired to control HLB. Host small RNAs (sRNA) play a vital role in regulating host responses to pathogen infection and are used as early diagnostic markers for many human diseases, including cancers. To determine whether citrus sRNAs regulate host responses to HLB, sRNAs were profiled from Citrus sinensis 10 and 14 weeks post grafting with Ca. L. asiaticus (Las)-positive or healthy tissue. Ten new microRNAs (miRNAs), 76 conserved miRNAs, and many small interfering RNAs (siRNAs) were discovered. Several miRNAs and siRNAs were highly induced by Las infection, and can be potentially developed into early diagnosis markers of HLB. miR399, which is induced by phosphorus starvation in other plant species, was induced specifically by infection of Las but not Spiroplasma citri that causes citrus stubborn-a disease with symptoms similar to HLB. We found a 35% reduction of phosphorus in Las-positive citrus trees compared to healthy trees. Applying phosphorus oxyanion solutions to HLB-positive sweet orange trees reduced HLB symptom severity and significantly improved fruit production during a 3-year field trial in south-west Florida. Our molecular, physiological, and field data suggest that phosphorus deficiency is linked to HLB disease symptomology.

Convenient detection of the citrus greening (huanglongbing) bacterium 'Candidatus Liberibacter asiaticus' by direct PCR from the midrib extract

A phloem-limited bacterium, 'Candidatus Liberibacter asiaticus' (Las) is a major pathogen of citrus greening (huanglongbing), one of the most destructive citrus diseases worldwide. The rapid identification and culling of infected trees and budwoods in quarantine are the most important control measures. DNA amplification including conventional polymerase chain reaction (PCR) has commonly been used for rapid detection and identification. However, long and laborious procedures for DNA extraction have greatly reduced the applicability of this method. In this study, we found that the Las bacterial cells in the midribs of infected leaves were extracted rapidly and easily by pulverization and centrifugation with mini homogenization tubes. We also found that the Las bacterial cells in the midrib extract were suitable for highly sensitive direct PCR. The performance of direct PCR using this extraction method was not inferior to that of conventional PCR. Thus, the direct PCR method described herein is characterized by its simplicity, sensitivity, and robustness, and is applicable to quarantine testing.

The destructive citrus pathogen, 'Candidatus Liberibacter asiaticus' encodes a functional flagellin characteristic of a pathogen-associated molecular pattern

Huanglongbing (HLB) is presently the most devastating citrus disease worldwide. As an intracellular plant pathogen and insect symbiont, the HLB bacterium, 'Candidatus Liberibacter asiaticus' (Las), retains the entire flagellum-encoding gene cluster in its significantly reduced genome. Las encodes a flagellin and hook-associated protein (Fla) of 452 amino acids that contains a conserved 22 amino acid domain (flg22) at positions 29 to 50 in the N-terminus. The phenotypic alteration in motility of a Sinorhizobium meliloti mutant lacking the fla genes was partially restored by constitutive expression of Fla(Las). Agrobacterium-mediated transient expression in planta revealed that Fla(Las) induced cell death and callose deposition in Nicotiana benthamiana, and that the transcription of BAK1 and SGT1, which are associated with plant innate immunity, was upregulated. Amino acid substitution experiments revealed that residues 38 (serine) and 39 (aspartate) of Fla(Las) were essential for callose induction. The synthetic flg22(Las) peptide could not induce plant cell death but retained the ability to induce callose deposition at a concentration of 20 µM or above. This demonstrated that the pathogen-associated molecular pattern (PAMP) activity of flg22 in Las was weaker than those in other well-studied plant pathogenic bacteria. These results indicate that Fla(Las) acts as a PAMP and may play an important role in triggering host plant resistance to the HLB bacteria.

Predictive sequence analysis of the Candidatus Liberibacter asiaticus proteome

Candidatus Liberibacter asiaticus (Ca. L. asiaticus) is a parasitic Gram-negative bacterium that is closely associated with Huanglongbing (HLB), a worldwide citrus disease. Given the difficulty in culturing the bacterium and thus in its experimental characterization, computational analyses of the whole Ca. L. asiaticus proteome can provide much needed insights into the mechanisms of the disease and guide the development of treatment strategies. In this study, we applied state-of-the-art sequence analysis tools to every Ca. L. asiaticus protein. Our results are available as a public website at http://prodata.swmed.edu/liberibacter_asiaticus/. In particular, we manually curated the results to predict the subcellular localization, spatial structure and function of all Ca. L. asiaticus proteins (http://prodata.swmed.edu/liberibacter_asiaticus/curated/). This extensive information should facilitate the study of Ca. L. asiaticus proteome function and its relationship to disease. Pilot studies based on the information from our website have revealed several potential virulence factors, discussed herein.

Modeling huanglongbing transmission within a citrus tree

The citrus disease huanglongbing (HLB), associated with an uncultured bacterial pathogen, is threatening the citrus industry worldwide. A mathematical model of the transmission of HLB between its psyllid vector and citrus host has been developed to characterize the dynamics of the vector and disease development, focusing on the spread of the pathogen from flush to flush (a newly developing cluster of very young leaves on the expanding terminal end of a shoot) within a tree. This approach differs from that of prior models for vector-transmitted plant diseases where the entire plant is the unit of analysis. Dynamics of vector and host populations are simulated realistically as the flush population approaches complete infection. Model analysis indicates that vector activity is essential for initial infection but is not necessary for continued infection because infection can occur from flush to flush through internal movement in the tree. Flush production, within-tree spread, and latent period are the most important parameters influencing HLB development. The model shows that the effect of spraying of psyllids depends on time of initial spraying, frequency, and efficacy of the insecticides. Similarly, effects of removal of symptomatic flush depend on the frequency of removal and the time of initiation of this practice since the start of the epidemic. Within-tree resistance to spread, possibly affected by inherent or induced resistance, is a major factor affecting epidemic development, supporting the notion that alternate routes of transmission besides that by the vector can be important for epidemic development.

GC-MS metabolomic differentiation of selected citrus varieties with different sensitivity to citrus huanglongbing

Huanglongbing (HLB) is the most destructive disease of citrus worldwide. The rapid identification of tolerant varieties is considered a critical step towards controlling HLB. GC-MS metabolite profiles were used to differentiate HLB-tolerant citrus varieties 'Poncirus trifoliata' (TR) and 'Carrizo citrange' (CAR) from HLB-sensitive varieties 'Madam Vinous sweet orange' (MV) and 'Duncan' grapefruit (DG). PCR analyses revealed that MV was the most sensitive variety followed by DG and the tolerant varieties CAR and TR. Metabolomic multivariate analysis allowed classification of the cultivars in apparent agreement with PCR results. Higher levels of the amino acids l-proline, l-serine, and l-aspartic acid, as well as the organic acids butanedioic and tetradecanoic acid, and accumulation of galactose in healthy plants were characteristic of the most sensitive variety MV when compared to all other varieties. Only galactose was significantly higher in DG when compared to the tolerant varieties TR and CAR. The tolerant varieties showed higher levels of l-glycine and mannose when compared to sensitive varieties MV and DG. Profiling of the sensitive varieties MV and DG over a 20-week period after inoculation of those with the HLB-containing material revealed strong responses of metabolites to HLB infection that differed from the response of the tolerant varieties. Significant changes of l-threonine level in the leaves from old mature flushes and l-serine, l-threonine, scyllo-inositol, hexadecanoic acid, and mannose in the leaves from young developing flushes were observed in MV. Significant changes in myo-inositol in old flushes and l-proline, indole, and xylose in new flushes were observed in DG.

Transcriptional response of susceptible and tolerant citrus to infection with Candidatus Liberibacter asiaticus

Candidatus Liberibacter asiaticus (Las), a non-culturable phloem-limited bacterium, is the suspected causal agent of huanglongbing (HLB) in Florida. HLB is one of the most devastating diseases of citrus and no resistant cultivars have been identified to date, though tolerance has been observed in the genus Poncirus and some of its hybrids. This study compares transcriptional changes in tolerant US-897 (Citrus reticulata Blanco×Poncirus trifoliata L. Raf.) and susceptible 'Cleopatra' mandarin (C. reticulata) seedlings in response to infection with Las using the Affymetrix GeneChip citrus array, with the main objective of identifying genes associated with tolerance to HLB. Microarray analysis identified 326 genes which were significantly upregulated by at least 4-fold in the susceptible genotype, compared with only 17 genes in US-897. Exclusively upregulated in US-897 was a gene for a 2-oxoglutarate (2OG) and Fe(II)-dependant oxygenase, an important enzyme involved in the biosynthesis of plant secondary metabolites. More than eight hundred genes were expressed at much higher levels in US-897 independent of infection with Las. Among these, genes for a constitutive disease resistance protein (CDR1) were notable. The possible involvement of these and other detected genes in tolerance to HLB and their possible use for biotechnology are discussed.

Characterizing the citrus cultivar Carrizo genome through 454 shotgun sequencing

The citrus cultivar Carrizo is the single most important rootstock to the US citrus industry and has resistance or tolerance to a number of major citrus diseases, including citrus tristeza virus, foot rot, and Huanglongbing (HLB, citrus greening). A Carrizo genomic sequence database providing approximately 3.5× genome coverage (haploid genome size approximately 367 Mb) was populated through 454 GS FLX shotgun sequencing. Analysis of the repetitive DNA fraction indicated a total interspersed repeat fraction of 36.5%. Assembly and characterization of abundant citrus Ty3/gypsy elements revealed a novel type of element containing open reading frames encoding a viral RNA-silencing suppressor protein (RNA binding protein, rbp) and a plant cytokinin riboside 5′-monophosphate phosphoribohydrolase-related protein (LONELY GUY, log). Similar gypsy elements were identified in the Populus trichocarpa genome. Gene-coding region analysis indicated that 24.4% of the nonrepetitive reads contained genic regions. The depth of genome coverage was sufficient to allow accurate assembly of constituent genes, including a putative phloem-expressed gene. The development of the Carrizo database ( http://citrus.pw.usda.gov/ ) will contribute to characterization of agronomically significant loci and provide a publicly available genomic resource to the citrus research community.

Colonization of citrus seed coats by 'Candidatus Liberibacter asiaticus': implications for seed transmission of the bacterium

Huanglongbing is an economically damaging disease of citrus associated with infection by 'Candidatus Liberibacter asiaticus'. Transmission of the organism via infection of seeds has not been demonstrated but is a concern since some citrus varieties, particularly those used as rootstocks in commercial plantings are propagated from seed. We compared the incidence of detection of 'Ca. Liberibacter asiaticus' DNA in individual fruit peduncles, seed coats, seeds, and in germinated seedlings from 'Sanguenelli' sweet orange and 'Conners' grapefruit fruits sampled from infected trees. Using real-time quantitative PCR (qPCR) we detected pathogen DNA in nucleic acid extracts of 36 and 100% of peduncles from 'Sanguenelli' and from 'Conners' fruits, respectively. We also detected pathogen DNA in extracts of 37 and 98% of seed coats and in 1.6 and 4% of extracts from the corresponding seeds of 'Sanguenelli' and 'Conners', respectively. Small amounts of pathogen DNA were detected in 10% of 'Sanguenelli' seedlings grown in the greenhouse, but in none of 204 extracts from 'Conners' seedlings. Pathogen DNA was detected in 4.9% and in 89% of seed coats peeled from seeds of 'Sanguenelli' and 'Conners' which were germinated on agar, and in 5% of 'Sanguenelli' but in none of 164 'Conners' seedlings which grew from these seeds on agar. No pathogen DNA was detected in 'Ridge Pineapple' tissue at 3 months post-grafting onto 'Sanguenelli' seedlings, even when pathogen DNA had been detected initially in the 'Sanguenelli' seedling. Though the apparent colonization of 'Conners' seeds was more extensive and nearly uniform compared with 'Sanguenelli' seeds, no pathogen DNA was detected in 'Conners' seedlings grown from these seeds. For either variety, no association was established between the presence of pathogen DNA in fruit peduncles and seed coats and in seedlings.

Fluorescence images combined to statistic test for fingerprinting of citrus plants after bacterial infection

The citrus greening (or huanglongbing) disease has caused serious problems in citrus crops around the world. An early diagnostic method to detect this malady is needed due to the rapid dissemination of Candidatus Liberibacter asiaticus (CLas) in the field. This analytical study investigated the fluorescence responses of leaves from healthy citrus plants and those inoculated with CLas by images from a stereomicroscope and also evaluated their potential for the early diagnosis of the infection caused by this bacterium. The plants were measured monthly, and the evolution of the bacteria on inoculated plants was monitored by real-time quantitative polymerase chain reaction (RT-qPCR) amplification of CLas sequences. A statistical method was used to analyse the data. The selection of variables from histograms of colours (colourgrams) of the images was optimized using a paired Student's t-test. The intensity of counts for green colours from images of fluorescence had clearly minor variations for healthy plants than diseased ones. The darker green colours were the indicators of healthy plants and the light colours for the diseased. The method of fluorescence images is novel for fingerprinting healthy and diseased plants and provides an alternative to the current method represented by PCR and visual inspection. A new, non-subjective pattern of analysis and a non-destructive method has been introduced that can minimize the time and costs of analyses.