Citrus psorosis virus (CPsV) is a tripartite, negative-sense single-stranded RNA virus of the genus Ophiovirus in the family Aspiviridae, which can cause cracking of citrus trunks and even the death of the whole plant, seriously threatening the safety of the citrus industry. The construction of reverse genetics systems for negative-strand RNA viruses is challenging. This study aims to establish a full-length cDNA clone of CPsV genome and determine its infectivity, with the expectation of laying a foundation for research on its pathogenic mechanism and other aspects.

The primers were designed using software Primer 5, and the total nucleic acid of CPsV-infected plants was used as a template for RT-PCR amplification of the three strands of CPsV, RNA1, RNA2 and RNA3, respectively. Based on the dual-expression vector pXT1, cDNA clones of three RNA strands were constructed by In-Fusion homologous recombination technology, which were verified by enzyme digestion and sequencing analysis. The cDNA clones of the three strands of CPsV were screened by the Nicotiana benthamiana inoculation system, and further inoculated into the herb host Gomphrena globosa by Agrobacterium-mediated injection, and then inoculated into different citrus varieties by vacuum infiltration to observe their symptoms and perform molecular detection.

A total of two CPsV RNA1 full-length cDNA clones, two RNA2 full-length cDNA clones, and two RNA3 full-length cDNA clones were obtained, respectively. One full-length cDNA clone of RNA1, RNA2 and RNA3 was randomly selected and combined as a full-length cDNA clone of CPsV genome, eight RNA1, RNA2 and RNA3 cDNA clones were selected and inoculated on N. benthamiana through Agrobacterium-mediated inoculation and RT-PCR detection. Among the eight combinations, the positive rate of CPsV-122 was the highest (62.50%). Sequence analysis showed that CPsV-122 had the highest sequence identity with Spanish isolate P-121, and the corresponding sequence identities of RNA1, RNA2 and RNA3 were 98.06%, 97.10% and 99.32%, respectively. In a phylogenetic tree based on the amino acid sequence of coat protein, CPsV-122 and P-121 were clustered in the same clade and five isolates from China, Tunisia and Italy. CPsV-122 was inoculated by Agrobacterium-mediated inoculation to G. globosa, Citrus limon and C. paradise for symptom observation and RT-PCR test. The results showed that at 7 dpi, the positive rate of G. globosa was 16.67% (2/12), and at 25 dpi, the positive plants showed obvious reddish-brown blight, local necrosis of leaves and other CPsV infection symptoms. The RT-PCR results of C. paradise and C. limon were negative except for the positive control, but at 90 dpi, 13 out of 20 C. limon plants inoculated with CPsV-122 showed obvious CPsV infection symptoms such as dwarfing, yellowing, wilting shoots and shoot gelatinization, while no specific symptoms were observed in the empty vector group and the healthy control group.

CPsV-122 is a full-length cDNA clone of the CPsV genome, which can systematically infect G. globosa and cause typical CPsV infection symptoms on citrus, such as dwarfing and withering of shoots.

The objective of this study is to construct an expression vector based on citrus leaf blotch virus (CLBV) and systematically express antimicrobial peptides to improve plant disease resistance, which will provide a new technical means for the prevention and control of citrus canker, citrus Huanglongbing (HLB) and other diseases.

The subgenomic promoter sequence and multiple cloning sites were inserted after the terminator of the coat protein gene of CLBV to construct the viral expression vector pCLBV202 based on the previously constructed infectious clone pCY-CLBV201. Then green fluorescent protein gene (gfp) was inserted into the multiple cloning sites, and the expression of GFP was verified by Agrobacterium-mediated inoculation and fluorescence observation. The cecropin B (CB) gene of Antheraea pernyi was cloned and the recombinant vector pCLBV202-CB was constructed. Nicotiana benthamiana and citrus seedlings were inoculated by pCLBV202-CB using Agrobacterium-mediated injection and vacuum infiltration, respectively. Positive plants were screened and subjected to inoculation of Ralstonia solanacearum by injection and root irrigation or Xanthomonas citri subsp. citri (Xcc) by pricking the detached leaves. At the same time, the plants inoculated with the empty vector were set as the control. The resistance to tobacco bacterial wilt of the inoculated plants was evaluated by symptom observation, incidence rate and disease index. The citrus resistance to canker was evaluated by the detriment, incidence rate and colony concentration on leaves.

After inoculated by pCLBV202-GFP, green fluorescence could be observed on the systemic leaves of the N. benthamiana and Citrus limon, which was brighter in N. benthamiana, indicating that the expression vector based on CLBV was successfully constructed. After inoculation of R. solanacearum, the onset time of treatment group (pCLBV202-CB) was delayed by 4 days compared with the control group (pCLBV202). On the 24th day past inoculation (24 dpi), the incidence rate of the treatment group was 14.3%, and that of the control group was 100%. Compared with the control group, the resistance index of the treatment group was -2.66, and the resistance evaluation was high, indicating that the expression of CB by pCLBV202-CB enhanced the resistance to tobacco bacterial wilt. When C. limon leaves was inoculated by Xcc, the number of detriment in the treatment group at 7 dpi was 47, and the incidence rate was 43.5%, while that of the control group was 73, and 67.6%. The detection of colony concentration changes of Xcc showed that the number of Xcc in the treatment group was less than that in the control group, indicating that the expression of CB by pCLBV202-CB enhanced the resistance to citrus canker.

The CLBV-based viral expression vector pCLBV202 was constructed. Using pCLBV202 to systematically express CB in N. benthamiana and citrus can improve the resistance of plants to bacterial diseases, which provided a new technology for the prevention and control of citrus bacterial diseases.

An infectious clone of citrus yellow mosaic virus (CYMV) was constructed to lay a foundation for further study of its molecular characteristics and pathogenic mechanism.

The amplified CYMV genomic sequence segments were recombined with the ternary expression vector pCY using In-Fusion homologous recombination technology, and a 1.4-fold full-length genomic DNA clone of the virus was constructed and sequenced. The obtained clones were inoculated onto seedlings of Eureka lemon by Agrobacterium-mediated vacuum infiltration (AVI), and their infectivity was identified by molecular detection, presentation of symptoms and virus particles, and re-grafting determination. The obtained infectious clones were used to inoculate different citrus varieties and herbs, and the infection rate was determined by RT-PCR and the various symptoms of different citrus varieties after infection were observed. Mutants of the infectious clones with ORFⅠand ORFⅡ replaced by green fluorescent protein gene (gfp), respectively were constructed, and subjected to infectivity analysis.

Seven clones of 1.4-fold CYMV genome-length DNA were obtained by In-Fusion homologous recombination technology. Among them, CYMV-3 was 90%-100% identical to the corresponding genomic nucleotide sequences of nine CYMV isolates registered in GenBank, and had the highest homology with CYMV-SO isolate (AF347695) and was clustered on the same branch of the genetic evolutionary tree. The results of infectivity identification showed that four out of the 14 Eureka lemon plants inoculated with CYMV-3 were positive by RT-PCR detection, and punctate chlorotic symptoms were observed. DNA was extracted from the new systemic leaves of the infected plant and the expected specific fragments covering the full length of the CYMV genome were obtained by PCR amplification. CYMV-3 infected plants with symptoms were used as the virus source to graft and inoculate rough lemon seedlings, and CYMV specific bands could be detected by RT-PCR at 90 dpi. New leaves of CYMV-3 infected plants with symptoms were taken for fixation and sectioning for electron microscopy and baculovirus particles of about 130 nm × 30 nm were observed, indicating that CYMV-3 was a CYMV infectious clone. Ten and five citrus varieties were inoculated by CYMV-3 via Agrobacterium-mediated vacuum infiltration and injection, respectively, and were tested by RT-PCR. The results showed that the infection rate of the former was 11.11%-100.00% and the latter was 63.16%-90.00%. Some citrus varieties appeared CYMV symptoms such as yellowing, mosaic and yellowing patches. Nicotiana benthamiana, Zea mays, Vigna unguiculata and Sorghum bicolor were inoculated by CYMV-3 via Agrobacterium -mediated injection, and only S. bicolor plants were detected to be positive in RT-PCR detection, and chlorotic strips appeared along the veins of the systemic leaves, indicating that CYMV can infect S. bicolor. The mutants of CYMV-3 with ORFⅠand ORFⅡ replaced by gfp, respectively, were constructed and inoculated onto rough lemons by Agrobacterium-mediated injection. All the inoculated plants were negative by RT-PCR detection and showed no obvious symptoms.

The infectious clone of CYMV was successfully constructed and could infected citrus plants efficiently via Agrobacterium-mediated injection, resulting different symptoms in different varieties.

Citrus yellow vein clearing virus (CYVCV) is one of the viruses mostly threatening the stable development of citrus industry in China, but its infection and pathogenic mechanism in citrus is still unclear. In this study, the coat protein (CP) of CYVCV was used as a bait to screen the Eureka lemon (Citrus limon Burm. f.) cDNA library, and the function of obtained host factors in the interaction between host and virus was analyzed by bioinformatics method.

The total RNA of Eureka lemon leaves was extracted by the Trizol method, and then reversely transcribed to the First-Stand cDNA with SMART method, which was used as a template for obtaining ds cDNA through Long-Distance PCR. After homogenization, the ds cDNA fragments were ligated to pGADT7 plasmid vector by recombination junctions to construct the primary cDNA library of Eureka lemon. The recombinant plasmids were transfected into Escherichia coli DH10B to obtain the E. coli cDNA library of Eureka lemon, and its quality was identified. Simultaneously, the CP sequence of CYVCV was amplified by PCR and ligated into the yeast two-hybrid (Y2H) bait vector pGBKT7, and the plasmids of pGBKT7-CP and pGADT7 were co-transfected into yeast Y2H Gold. The positive yeast clones were grown on the plate of SD/-Trp, SD/-Leu-Trp, SD/-Leu-Trp/X-α-Gal and SD/-Leu-Trp-His medium, respectively, and then the growth situation of the yeast was tested to identify the toxicity of pGBKT7-CP on yeast Y2H Gold and the self-activating effect of pGBKT7-CP on the reporter gene of yeast was analyzed. Then the Y2H Gold containing bait vector pGBKT7-CP was transformed with the primary cDNA library of Eureka lemon, the co-transformed yeasts were coated on the plate of SD/-Leu-Trp, SD/-Leu-Trp-His/X-α-Gal and SD/-Leu-Trp-His-Ade/X-α-Gal medium in turn. Finally, the blue and well grown positive clones were selected. The plasmids of positive yeast clones were extracted and sequenced. The candidate genes were preliminarily compared in the GenBank, and the interacted protein factors were annotated and the protein’s biological functions were analyzed with gene ontology (GO) pathway of Uniprot online websites. According to the results of the analysis, candidate factors that may be involved in host disease resistance or symptom development were selected, their CDS full-length sequences were amplified and constructed into the target vector pGADT7 and then were verified with pGBKT7-CP by one for one in yeast, respectively.

The titer of the Eureka lemon-E. coli cDNA library was 1.02×10⁸ cfu/mL, and it demonstrated that the cDNA library measured up to the experiment standard. The bait vector of pGBKT7-CP was constructed, which had no ability to activate the reporter gene and had no virulence to the yeast. The 41 positive clones were finally selected by using the SD/-Leu-Trp-His-Ade/X-α-Gal medium. After sequence similarity comparison, removing the repetitive sequences, the vector sequences and the frameshift sequences, the 32 host factors that interacted with CYVCV CP were screened. The GO pathway annotation results indicated that these host factors were involved in several chloroplast-related biological processes, including photosynthesis, metabolic process, carbohydrate metabolic process, response to light stimulus, etc. The molecular functions of the 32 host factors were multiple, including catalytic activity, hydrolase activity, transferase activity, protein binding, DNA-binding transcription factor activity, and translation factor activity, etc. Moreover, the cell components of the 32 host factors were involved in the cytoplasm, membrane, chloroplast, thylakoid, nucleolus, and golgi apparatus, etc. Validation of one-to-one yeast two-hybrid crosses of 14 important proteins selected from candidate host factors with CP showed that CP interacted with all 14 proteins.

The cDNA library of Eureka lemon was constructed successfully, and 32 host factors of Eureka lemon interacted with CYVCV CP were preliminarily screened. According to the results, the function of the important proteins was analyzed. It was presumed that CYVCV CP affects photosystem stability, thylakoid structure and chlorophyll synthesis by interacting with multiple chloroplast-related proteins such as oxygen-evolving enhancer protein 2 (PsbP), chlorophyll a-b binding protein 8 (Lhca3) and ribulose bisphosphate carboxylase small subunit (RbcS), resulting in reduced photosynthesis and damaged chloroplast morphology and function. The interaction of CP with these chloroplast-associated factors was separately verified one-to-one in yeast, which will provide a theoretical basis for revealing the molecular mechanism of CYVCV CP in viral pathogenicity.

Citrus yellow mosaic virus (CYMV) is a member of badnaviruses that was first discovered in India and caused serious damage to its citrus industry. At present, CYMV has been listed as a quarantine pest in the United States, Japan, New Zealand, Europe and the Mediterranean region, which has the potential risk of being introduced into China.

The objective of this study is to establish a real-time quantitative PCR (qPCR) detection system for CYMV, screen sensitive citrus varieties to CYMV, elucidate the spatial and temporal distribution of CYMV in host plant, and to provide technical support for monitoring of the virus.

According to the conserved sequences of CYMV coat protein (CP) gene in NCBI, eight pairs of qPCR primers were designed using software Primer Premier 5, and were screened by conventional PCR for primer pairs with good amplification effect and high specificity. The concentration and annealing temperature of the primers were optimized to establish the CYMV qPCR detection system. The specificity of the system was evaluated by detecting different citrus pathogens. Conventional PCR and qPCR were performed in parallel with 1.98×10⁹-1.98 copies/μL of plasmid standards diluted in a gradient as template to evaluate the sensitivity of the established system. The field citrus samples were collected randomly and detected using conventional PCR and qPCR in parallel to evaluate the applicability of the method. Fifteen citrus varieties including Yuhuanyou (Citrus grandis), Chandler pomelo (C. grandis), No.22 karatachi (Poncirus trifoliata), etc., were inoculated with CYMV for symptom observation and molecular detection to screen sensitive indicator plants. Samples from different tissues of Madam Vinous orange (C. sinensis) plants were collected at different times post inoculation, and qPCR assays were performed with C. sinensis elongation factor 1 alpha as an internal reference gene to clarify the spatial and temporal distribution patterns of CYMV in the host plants.

The qPCR detection system for CYMV was established with the optimal primer of CYMV-qF7/R7, the optimal concentration 200 nmol·L^-1 and the optimal annealing temperature 63 ℃. The results of the evaluation experiments showed that the specificity of the established qPCR system is strong, and the detection sensitivity is 1 000 times that of the conventional PCR. The detection results of 660 field citrus samples from different regions showed that the qPCR assay was consistent with the conventional PCR assay, and no CYMV-positive plants were detected except for the positive control. Among the 15 citrus varieties in the inoculation experiment, Yuhuanyou and Chandler pomelo showed strong and typical yellowing mosaic symptoms of CYMV infection at the earliest, suggesting that both could be used as sensitive indicator plants. The detection results of qPCR showed that the titer of CYMV in old leaves was the highest, the titer of virus in old cortex and root was higher, and the titer in tender cortex was lower. The relative content of CYMV in plants was highest from July to September, then decreased month by month, and reached the lowest value in January of the next year. From February of the next year, the relative content of CYMV began to rise, which was consistent with the change trend of the environmental temperature.

A qPCR assay for CYMV detection with high specificity and sensitivity was established, and the spatial and temporal distribution pattern of CYMV in the host plants was determined. In addition, Yuhuanyou and Chandler pomelo can be used as CYMV-sensitive indicator plants.

Citrus Huanglongbing (HLB) is a destructive citrus disease primarily caused by Candidatus Liberibacter asiaticus (CLas). The objective of this research is to investigate the growth pattern of CLas in citrus hairy roots at the initial stage of infection, optimize the rapid evaluation system of Agrobacterium rhizogenes-mediated HLB resistance and apply it to the resistance evaluation of antimicrobial peptides.

A plant expression vector, pGNGM1300, was employed to transform different citrus varieties via the hairy root transformation system, which was mediated by A. rhizogenes K599. Subsequently, varieties with rapid induction and high transformation rate were then selected. Following inoculation with CLas, the presence of CLas in hairy roots was consistently demonstrated through the use of real-time quantitative PCR (qPCR), which allowed for the monitoring of its growth pattern. Further, the optimization of the HLB resistance evaluation system permitted the HLB resistance evaluation of antimicrobial peptides to be conducted via (RT-) qPCR analysis of CLas and related genes, quantification of callose deposition, microstructure observation, and symptom assessment.

Among the 11 tested citrus varieties, citron (Citrus medica) exhibited the fastest induction of hairy roots (15 d), accompanied by the highest induction rate (73.75%) and a substantial transgene positivity rate (53.54%) in hairy roots. The results of regular qPCR monitoring indicated that CLas began to colonize roots on 20-30 days post-inoculation (dpi). At 30-50 dpi, there was a notable increase in the CLas content. The content of the CLas exhibited a slight fluctuation at a range of 60 to 120 dpi, though the discrepancy with 50 dpi was not statistically significant. The results of the antimicrobial peptide resistance evaluation, conducted on MaSAMP (stable antimicrobial peptide), HBD-4 (homo sapiens defensin beta 4), and CB (cecropin B), revealed that at 50-120 dpi, the content of CLas in both MaSAMP- and CB-expressing plants was significantly lower than that of the control. However, except that at 90 dpi, the CLas content was also significantly lower in HBD-4-expressing plants than the control. The callose content in all three plants expressing antimicrobial peptides was significantly lower than that of the control (60 dpi). There was no visible cell wall thickening in the phloem, and no significant deposition of starch grains or callus was observed. Furthermore, none of the plants exhibited signs of root death at 90 dpi.

This study elucidates the growth dynamics of CLas in citrus hairy roots during the early stages of infection and optimizes a rapid scoring system for A. rhizogenes-mediated HLB resistance. In addition, the results show that overexpression of MaSAMP, HBD-4 and CB effectively suppresses CLas proliferation, reduces callose deposition and alleviates HLB symptoms. These findings have the potential to be used in the prevention and control of HLB.

Citrus yellow vein clearing virus (CYVCV) is a novel virus that poses a significant threat to the citrus industry. Currently, there is no effective therapeutic agent. The primary strategies for prevention and management involve utilizing virus-free seedlings and implementing stringent control measures against insect vectors. The objective of this study is to investigate the technology of virus-induced gene silencing (VIGS) as a means of developing antiviral “vaccines” for CYVCV, and to offer innovative approaches for the prevention and management of citrus viral diseases.

Based on the previously constructed VIGS vector pCLBV201 in the laboratory, a series of recombinant vectors were designed and developed to target the conserved regions of the CYVCV genome, specifically open reading frame 1 (ORF1), open reading frame 6 (ORF6), coat protein (CP), and triple gene block (TGB). Agrobacterium-mediated vacuum infiltration was inoculated on Eureka lemon, followed by RT-PCR detection. After acquiring several positive plants including pCLBV201-ORF1, pCLBV201-ORF6, pCLBV201-CP, and pCLBV201-TGB, CYVCV infectious clones were inoculated via Agrobacterium-mediated injection, with plants infiltrated with the pCLBV201 empty vector serving as controls. Subsequent RT-qPCR, Western blot analysis, symptom observation, and disease index statistics were conducted to elucidate the preventive and control effects of the various VIGS recombinant vectors on CYVCV.

A series of recombinant vectors of pCLBV201 were constructed, and the results of RT-PCR following inoculation indicated that multiple positive plants for pCLBV201-ORF1, pCLBV201-ORF6, pCLBV201-CP, and pCLBV201-TGB were obtained, respectively. CYVCV infectious clones were inoculated by injection. The results of RT-qPCR at 7, 14, 28, 70, and 150 dpi (days post infection) showed that the relative expression of CYVCV in the pCLBV201-CP and pCLBV201-TGB treatment groups was significantly lower than that in the control group. Western blot analyses conducted at 70 and 150 dpi showed a significant decrease in the expression of CP protein in the pCLBV201-CP and pCLBV201-TGB treatment groups. Observational symptoms indicated that the control group exhibited typical manifestations of CYVCV infection, such as severe vein clearing and leaf distortion, whereas the pCLBV201-CP treatment group displayed mild symptoms, and the pCLBV201-TGB treatment group showed no apparent symptoms. The disease index statistics at 70 and 150 dpi demonstrated that the disease indices of pCLBV201-CP and pCLBV201-TGB were the lowest, recorded at 17.6, 41.2 and 15.6, 29.1, respectively, while the control group had indices of 52.1 and 80.0, showing obvious differences.

The VIGS-based technology for the prevention and control of CYVCV was developed, which clarified that pCLBV201-CP and pCLBV201-TGB could significantly reduce the virus titers and alleviate the symptoms caused by the virus. They have the potential to be used as “vaccines” for CYVCV prevention and control.