Heck, Michelle

Abstract Delivery of therapeutic biomolecules into plant vascular tissues remains a challenge in management of vector-borne plant pathogens. The symbiont concept uses reprogrammed Agrobacterium tumefaciens galls (called symbionts) to produce biomolecules while remaining connected to host vasculature. We evaluated whether symbionts expressing antimicrobial peptides (AMPs) suppress ‘ Candidatus Liberibacter solanacearum’ (CLso), the causal agent of potato zebra chip disease. Symbionts were engineered to express a Streptococcus mutans bacteriocin associated with bacterial membrane disruption (Blp-Sm), or an AMP isolated from finger lime and associated with resistance to citrus greening disease (MaSAMP). Effects of AMP-producing symbionts on CLso titers, infection incidence, pathogen movement, and disease symptoms were evaluated in tomato and potato. In tomato, neither AMP significantly reduced CLso titers or infection incidence. However, in potato, AMP-producing symbionts reduced CLso accumulation and movement from CLso-inoculated source shoots into non-inoculated sink shoots connected through underground tubers. Blp-Sm produced the strongest reduction in CLso accumulation and infection incidence in sink tissues. In separate assays where symbionts were established directly on potato seed tubers, MaSAMP significantly reduced CLso titers in stems and tubers and reduced zebra chip symptoms in tubers, despite no reduction of CLso titers in terminal leaves. These findings demonstrate that AMP-producing symbionts suppress vascular pathogen accumulation and movement within plants and highlight the symbiont concept as a potential platform for managing diseases caused by vascular-restricted pathogens. Further, they show the potato-CLso system is a promising infection model to both refine and improve symbiont technology, and to test additional AMPs for potency against related pathogens.

This is a general protocol for the vegetative propagation of citrus material infected with the bacterium Candidatus Liberibacter asiaticus (CLas), the presumed causal agent of citrus greening disease (HLB), using an aeroponic cloning apparatus. The following procedure details a workflow using an EZ-Clone Pro Low Cloning System from Hydrobuilder.com. This system comes in varying cell-sizes (number of clones). We have found the most versatile to be the 16- or 32-cell size based on ease of preparation, keeping algae growth to a minimum, and maintenance. Cloning versus seed sowing has been useful in producing infected material for experimental assays requiring cloned, mature citrus trees as opposed to genetically variable plants from seeds. Where seed germination can take up to 12+ months to achieve plants with stem diameters ≥ 10 mm and 5+ years for flowering, infected branches from flowering trees can be directly propagated using this protocol within a few months and retain their maturity. Clone viability rate differs by citrus cultivar and general health of the cutting used, with citron and CLas negative material having the highest success rate. Infected citron cuttings have had a 95% survival rate, regardless of the diameter of the cutting. The cutting diameter ranged from 2mm-12mm and was limited to 12mm maximum, which is the size of the collar on the EZ-Clone Pro Low cloner.

Huanglongbing (HLB, or citrus greening disease) affects all citrus varieties worldwide. In the United States, Asia, and South America, the causal agent is ‘ Candidatus Liberibacter asiaticus’ ( CLas), a phloem-limited, uncultured alphaproteobacterium. The hemipteran insect vector Diaphorina citri (Asian citrus psyllid) acquires and transmits CLas in a circulative, propagative manner. In addition to CLas, D. citri hosts multiple symbiotic bacterial species including Wolbachia (wDi). In D. citri, wDi has been sequenced and studied, but specific roles in D. citri biology are unknown. Using well-established quantitative PCR methods, we measured CLas titer in D. citri collected from four groves in central Florida with distinct HLB management strategies and tested whether CLas and wDi titer were correlated in a subset of these insects. Grove site had the largest effect on CLas titer. Sex had no effect on CLas titer, whereas a higher wDi titer was correlated with noninfected insects. Our results suggest that more directed follow-up research is necessary and important to clarify whether field management tactics influence CLas titer in D. citri and to better understand gene-by-environment interactions among D. citri, wDi, and CLas. Now that millions of trees in Florida have been treated with injectable formulations of oxytetracycline, which is likely to decrease bacterial populations in D. citri, this study may represent the last biologically meaningful snapshot of grove-level vector-pathogen ecology in the state during the HLB epidemic.

AbstractHuanglongbing (HLB, or citrus greening disease) affects all citrus varieties world-wide. In the USA, Asia, and South America the causal agent is “CandidatusLiberibacter asiaticus” (CLas), a phloem-limited, uncultured, alphaproteobacterium. The hemipteran insect vector,Diaphorina citri(Asian citrus psyllid) acquires and transmitsCLas in a circulative, propagative manner. In addition toCLas,D. citrihosts multiple symbiotic bacterial species includingWolbachia(wDi). InD. citri, wDi has been sequenced and studied but specific roles inD. citribiology are unknown. Using well established quantitative PCR methods we measuredCLas titer inD. citricollected from four groves in central Florida with distinct HLB management strategies and tested whetherCLas and wDi titer were correlated in a sub-set of these insects. Grove site had the largest effect onCLas titer. Sex had no effect onCLas titer, while higher wDi titer was correlated with non-infected insects. Our results suggest that more directed follow-up research is necessary and important to clarify whether field management tactics influenceCLas titer inD. citriand to better understand gene-by-environment interactions amongD. citri, wDi andCLas. Now that millions of trees in Florida have been treated with injectable formulations of oxytetracycline, which is likely to decrease bacterial populations inD. citri, this study may represent the last biologically meaningful snapshot of grove-level vector-pathogen ecology in the state during the HLB epidemic.

Huanglongbing (HLB), referred to as citrus greening disease, is a bacterial disease impacting citrus production worldwide and is fatal to young trees and mature trees of certain varieties. In some areas, the disease is devastating the citrus industry. A successful solution to HLB will be measured in economics: citrus growers need treatments that improve tree health, fruit production, and most importantly, economic yield. The profitability of citrus groves is the ultimate metric that truly matters when searching for solutions to HLB. Scientific approaches used in the laboratory, greenhouse, or field trials are critical to the discovery of those solutions and to estimate the likelihood of success of a treatment aimed at commercialization. Researchers and the citrus industry use a number of proxy evaluations of potential HLB solutions; understanding the strengths and limitations of each assay, as well as how best to compare different assays, is critical for decision-making to advance therapies into field trials and commercialization. This perspective aims to help the reader compare and understand the limitations of different proxy evaluation systems based on the treatment and evaluation under consideration. The researcher must determine the suitability of one or more of these metrics to identify treatments and predict the usefulness of these treatments in having an eventual impact on citrus production and HLB mitigation. As therapies advance to field trials in the next few years, a reevaluation of these metrics will be useful to guide future research efforts on strategies to mitigate HLB and vascular bacterial pathogens in other perennial crops.

AbstractThe Asian citrus psyllid,Diaphorina citri, is a vector of ‘CandidatusLiberibacter asiaticus’ (CLas), a gram-negative, obligate biotroph whose infection inCitrusspecies is associated with citrus greening disease, or Huanglongbing (HLB). Strategies to blockCLas transmission byD. citriremain the best way to prevent the spread of the disease into new citrus growing regions. However, identifying control strategies to block HLB transmission poses significant challenges, such as the discovery and delivery of antimicrobial compounds targeting the bacterium and overcoming consumer hesitancy towards accepting the treatment. Here, we computationally identified and tested a series of 20-mer nodule-specific cysteine-rich peptides (NCRs) derived from the Mediterranean legume,Medicago truncatulaGaertn. (barrelclover) to identify those peptides that could effectively prevent or reduceCLas infection in citrus leaves and/or preventCLas acquisition by the bacterium’s insect vector,D. citri. A set of NCR peptides were tested in a screening pipeline involving three distinct assays: a bacterial culture assay, aCLas-infected excised citrus leaf assay, and aCLas-infected nymph acquisition assay that includedD. citrinymphs, the only stage ofD. citri’s life-cycle that can acquireCLas leading to the development of vector competent adult insects. We demonstrate that a subset ofM. truncatula-derived NCRs inhibit bothCLas growth in citrus leaves andCLas acquisition byD. citrifromCLas-infected leaves. These findings reveal NCR peptides as a new class and source of biopesticide molecules to controlCLas for the prevention and/or treatment of HLB.

The Asian citrus psyllid, Diaphorina citri, is an invasive insect and a vector of ‘ Candidatus Liberibacter asiaticus’ ( CLas), a bacterium whose growth in Citrus species results in huanglongbing (HLB), also known as citrus greening disease. Methods to enrich and sequence CLas from D. citri often rely on biased genome amplification and nevertheless contain significant quantities of host DNA. To overcome these hurdles, we developed a simple pretreatment DNase and filtration (PDF) protocol to remove host DNA and directly sequence CLas and the complete, primarily uncultivable microbiome from D. citri adults. The PDF protocol yielded CLas abundances upward of 60% and facilitated direct measurement of CLas and endosymbiont replication rates in psyllids. The PDF protocol confirmed our lab strains derived from a progenitor Florida CLas strain and accumulated 156 genetic variants, underscoring the utility of this method for bacterial strain tracking. CLas genetic polymorphisms arising in lab-reared psyllid populations included prophage-encoding regions with key functions in CLas pathogenesis, putative antibiotic resistance loci, and a single secreted effector. These variants suggest that laboratory propagation of CLas could result in different phenotypic trajectories among laboratories and could confound CLas physiology or therapeutic design and evaluation if these differences remain undocumented. Finally, we obtained genetic signatures affiliated with Citrus nuclear and organellar genomes, entomopathogenic fungal mitochondria, and commensal bacteria from laboratory-reared and field-collected D. citri adults. Hence, the PDF protocol can directly inform agricultural management strategies related to bacterial strain tracking, insect microbiome surveillance, and antibiotic resistance screening.

AbstractThe Asian citrus psyllid, Diaphorina citri is an invasive insect 1 and a vector of ’Candidatus Liberibacter asiaticus’ (CLas), a bacterium whose growth in Citrus species results in citrus greening disease 2,3. Methods to enrich and sequence CLas from D. citri often rely on biased genome amplification 4 and nevertheless contain significant quantities of host DNA 5,6. To overcome these hurdles, we developed a simple pre-treatment DNase and filtration (hereafter PDF) protocol to directly sequence CLas and the complete, primarily uncultivable, microbiome from D. citri adults. The PDF protocol yielded CLas abundances upwards of 60% and enabled detection of 156 genetic variants in these strains compared to progenitor strains in Florida, which included prophage encoding regions with key functions in CLas pathogenesis, putative antibiotic resistance loci, and a single secreted effector. These variants suggest laboratory propagation of CLas may result in different phenotypic trajectories among laboratories, and may confound CLas physiology or therapeutic design and evaluation if these differences remain undocumented. Finally, we obtained genetic signatures affiliated with Citrus nuclear and organellar genomes, entomopathogenic fungal mitochondria, and commensal bacteria from laboratory-reared and field-collected D. citri adults. Hence, the PDF protocol can inform agricultural management strategies related to pathogen evolution 7, insect microbiome surveillance 8, antibiotic resistance screening 9, and gut content analysis 10.

The Asian citrus psyllid (Diaphorina citri) is a pest of citrus and the primary insect vector of the bacterial pathogen, ‘Candidatus Liberibacter asiaticus’ (CLas), which is associated with citrus greening disease. The citrus relative Murraya paniculata (orange jasmine) is a host plant of D. citri but is more resistant to CLas compared with all tested Citrus genotypes. The effect of host switching of D. citri between Citrus medica (citron) and M. paniculata plants on the acquisition and transmission of CLas was investigated. The psyllid CLas titer and the proportion of CLas-infected psyllids decreased in the generations after transfer from CLas-infected citron to healthy M. paniculata plants. Furthermore, after several generations of feeding on M. paniculata, pathogen acquisition (20 to 40% reduction) and transmission rates (15 to 20% reduction) in psyllids transferred to CLas-infected citron were reduced compared with psyllids continually maintained on infected citron. Top-down (difference gel electrophoresis) and bottom-up (shotgun MS/MS) proteomics methods were used to identify changes in D. citri protein expression resulting from host plant switching between Citrus macrophylla and M. paniculata. Changes in expression of insect metabolism, immunity, and cytoskeleton proteins were associated with host plant switching. Both transient and sustained feeding on M. paniculata induced distinct patterns of protein expression in D. citri compared with psyllids reared on C. macrophylla. The results point to complex interactions that affect vector competence and may lead to strategies to control the spread of citrus greening disease.