Other news in citrus greening research
Effect of Cyantraniliprole, a Novel Insecticide, on the Inoculation of Candidatus Liberibacter Asiaticus Associated with Citrus Huanglongbing by the Asian Citrus Psyllid
Candidatus Liberibacter asiaticus-inoculation by Diaphorina citri adults (CaLas-exposed, reared on infected plants) was tested by feeding them on excised healthy citrus leaves with dry residues of cyantraniliprole, a novel insecticide, in comparison with fenpropathrin, an insecticide commonly used against D. citri. Inoculation rates of CLas into healthy citrus leaves treated with cyantraniliprole or fenpropathrin (2.5–12.5%) were significantly lower than untreated leaves (47.5–85%). This study presented the effects of new pesticides in reducing CLas inoculation or transmission by psyllid vectors and also an excised leaf assay method for testing other treatments.
Ammar, E.-D., Hall, D. G. & Alvarez, J. M. (2015). Effect of Cyantraniliprole, a Novel Insecticide, on the Inoculation of Candidatus Liberibacter Asiaticus Associated with Citrus Huanglongbing by the Asian Citrus Psyllid (Hemiptera: Liviidae). Journal of Economic Entomology 108, 399-404.
Asymptomatic spread of huanglongbing and implications for disease control
Very little is known about the spread of citrus greening during the period between initial infection and the onset of disease symptoms. Lee et al. demonstrated that trees become infectious within 15 days of initial infection and entire groves can become infected within 1 year. It was also shown that insecticide applications that reduce the psyllid population by 75% during flushing periods only can delay the spread of disease throughout a grove. This strategy will delay to the effects of citrus greening, but there is still a need for a definitive treatment to prevent the occurrence in the first place.
Lee, J. A., Halbert, S. E., Dawson, W. O., Robertson, C. J., Keesling, J. E. & Singer, B. H. (2015). Asymptomatic spread of huanglongbing and implications for disease control. Proc Natl Acad Sci U S A 112, 7605-7610.
Antimicrobial Nanoemulsion Formulation with Improved Penetration of Foliar Spray through Citrus Leaf Cuticles to Control Citrus Huanglongbing
Yang et al. developed a water-in-oil nanoemulsion in order to aid in the transportation of an antimicrobials through the thick cuticle of the citrus leaf and into the tree phloem. The nanoemulsion contained ampicillin as the antimicrobial and Brij 35 as an adjuvant to increase penetration of the solution through the cuticle. Although there was great success in improving the transport of the foliar spray into the phloem, there is little practical application of ampicillin as an antimicrobial due to concerns regarding human consumption and the development of resistant bacteria in the environment.
Yang, C., Powell, C. A., Duan, Y., Shatters, R. & Zhang, M. (2015). Antimicrobial Nanoemulsion Formulation with Improved Penetration of Foliar Spray through Citrus Leaf Cuticles to Control Citrus Huanglongbing. PLoS One 10, e0133826.
Effective Antibiotics against ‘Candidatus Liberibacter asiaticus’ in HLB-Affected Citrus Plants Identified via the Graft-Based Evaluation
31 antibiotics were screened for effectiveness and phytotoxicity using the optimized graft-based screening system with Candidatus Liberibacter asiaticus-infected citrus scions. A number of antibiotics such as, ampicillin, carbenicillin, penicillin, cefalexin, rifampicin and sulfadimethoxine were all highly effective in eliminating or suppressing ‘Candidatus Liberibacter asiaticus’ indicated by both the lowest CLas infection rate and titers of the treated scions and inoculated rootstock. It would be of great value to cure CLas-infected citrus trees; however, development and/or refinement of application techniques will be necessary prior to commercial application. The economic feasibility and regulatory requirements need to be determined as well.
Zhang, M., Guo, Y., Powell, C. A., Doud, M. S., Yang, C. & Duan, Y. (2014). Effective antibiotics against ‘Candidatus Liberibacter asiaticus’ in HLB-affected citrus plants identified via the graft-based evaluation. PLoS One 9, e111032.
Transgenic Citrus Expressing an Arabidopsis NPR1 Gene Exhibit Enhanced Resistance against Huanglongbing (HLB; Citrus Greening)
Considering the current lack of fast-acting treatments to alleviate citrus greening disease, these authors posit that prevention of infection is paramount in controlling the disease and planting trees that are tolerant or resistant to HLB can attain this. Dutt et al. target the citrus tree’s systemic acquired resistance (SAR), which conveys protection against pathogens through salicylic acid production and expression of pathogenesis-related (PR) genes. Specifically, the signal transduction pathway regulator encoded by the NPR1 gene was utilized for genetic engineering of existing citrus cultivars. The susceptibility of sweet orange trees engineered to overexpress AtNPR1 to HLB differed between transgenic lines, but there was a reduction in disease severity in many lines. Although the use of these transgenic lines has great potential in the future, the non-uniform phenotype and amount of time it would take to grow a full crop to maturity limit the use of this method in the near future.
Dutt, M., Barthe, G., Irey, M. & Grosser, J. (2015). Transgenic Citrus Expressing an Arabidopsis NPR1 Gene Exhibit Enhanced Resistance against Huanglongbing (HLB; Citrus Greening). PLoS One 10, e0137134.
Field evaluation of plant defense inducers for the control of citrus Huanglongbing
Natural plant defense mechanisms are activated under a number of conditions and are effective in the resistance of many pathogens. Exogenous agents that mimic inducers of these defense mechanisms have been identified, including acibenzolar-S-methyl (ASM), benzothiadiazole (BTH), 2,6-Dichloroisonicotinic acid (INA), ß-aminobutyric acid (BABA), oligosaccharide from plant and fungal cell walls, and probenazole. These authors have shown that BTH, BABA, and INA are effective in suppressing HLB disease and reducing disease severity 15 to 30% compared to non-treated trees with the best results in young trees with mild HLB. These positive results require more in depth research before application in the field. ?
Li, J., Trivedi, P. & Wang, N. (2015). Field evaluation of plant defense inducers for the control of citrus Huanglongbing. Phytopathology 4, PHYTO08150196R
Comprehensive meta-analysis, co-expression, and miRNA nested network analysis identifies gene candidates in citrus against Huanglongbing disease
Several individual transcriptomic studies of the citrus tree-HLB interaction have been completed; however, there is limited overlap between studies. These authors performed a comprehensive meta-analysis of 22 studies as well as co-expression and miRNA nested network approaches to fill in missing information between the individual studies. Through this approach, they identified gene sets that contribute most to citrus-HLB interactions. Specifically, 65 common and 30 resistance-specific HLB-responsive gene sets were identified. The results suggested that the most important classes of probe sets regulated by HLB infection in citrus were carbohydrate metabolism, phloem plugging, transport, cell wall, hormone, defense, and stress-related pathways with a large difference depending on the tissue examined and stage of infection. Genes of greatest importance include: (1) genes coding for MYB, NAC and WRKY transcription factor in carbohydrate metabolism; (2) csi-miR167 in phloem-plugging and nutrient transport; (3) Cu/Zn superoxide dismutase and protease inhibitor in the hormone and stress response. The gibberellin signaling pathway was indicated as a vital pathway in resistance defenses. Further research into the roles of these genes are necessary but they may be important candidates for HLB treatment in the future.
Rawat, N., Kiran, S. P., Du, D., Gmitter, F. G., Jr. & Deng, Z. (2015). Comprehensive meta-analysis, co-expression, and miRNA nested network analysis identifies gene candidates in citrus against Huanglongbing disease. BMC Plant Biol 15, 184.
Development of single chain variable fragment (scFv) antibodies against surface proteins of Candidatus Liberibacter Asiaticus
The polymerase chain reaction has long been the most widely used assay for the detection or identification of Candidatus Liberibacter asiaticus. However, PCR has drawbacks, notably the requirement for a DNA extract prepared from the plants to be tested. The authors of this article utilized psyllids with high concentrations of ‘Ca. L. asiaticus’ to immunize mice and a scFv antibody library was made from mRNA isolated from spleens of immunized mice. Cloned surface antigens from ‘Ca. L. asiaticus’ were used to select scFv. Several selected scFv detected ‘Ca. L. asiaticus’ in plant extracts in a dot blot format. All of these antibodies not only are potentially useful in diagnostic assays, and also may be useful to answer important biological questions about CaLas, such as whether or not the pilus and flagellae are produced, and if so, under what circumstances and in which host.
Yuan, Q., Jordan, R., Brlansky, R. H., Minenkova, O., & Hartung, J. (2015). Development of single chain variable fragment (scFv) antibodies against surface proteins of ‘Candidatus Liberibacter asiaticus’. Journal of Microbiological Methods In Press