Identification of Autophagy-Related Genes in the Potato Psyllid, Bactericera cockerelli and Their Expression Profile in Response to ‘Candidatus Liberibacter Solanacearum’ in the Gut

Autophagy, also known as type II programmed cell death, is a cellular mechanism of “self-eating”. Autophagy plays an important role against pathogen infection in numerous organisms. Recently, it has been demonstrated that autophagy can be activated and even manipulated by plant viruses to facilitate their transmission within insect vectors. However, little is known about the role of autophagy in the interactions of insect vectors with plant bacterial pathogens. ‘Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited Gram-negative bacterium that infects crops worldwide. Two Lso haplotypes, LsoA and LsoB, are transmitted by the potato psyllid, Bactericera cockerelli and cause damaging diseases in solanaceous plants (e.g., zebra chip in potatoes). Both LsoA and LsoB are transmitted by the potato psyllid in a persistent circulative manner: they colonize and replicate within psyllid tissues. Following acquisition, the gut is the first organ Lso encounters and could be a barrier for transmission. In this study, we annotated autophagy-related genes (ATGs) from the potato psyllid transcriptome and evaluated their expression in response to Lso infection at the gut interface. In total, 19 ATGs belonging to 17 different families were identified. The comprehensive expression profile analysis revealed that the majority of the ATGs were regulated in the psyllid gut following the exposure or infection to each Lso haplotype, LsoA and LsoB, suggesting a potential role of autophagy in response to Lso at the psyllid gut interface.

Optimizing efficiencies in biosecurity surveillance and monitoring: testing colour and multi-lure traps on tomato potato psyllid, other plant-lice (Psylloidea) and stinkbugs (Pentatomoidea)

Cost efficiency in biosecurity surveillance is vital, and the ability to survey for multiple pest species using just one trap therefore highly appealing. The Psylloidea, or plantlice, contain significant horticultural pest species that act as vectors for a number of deleterious plant bacteriums. We examine the efficacy of using two different coloured sticky traps, and two lure types on the general Psylloidea and Pentatomoidea fauna, and a target extant pest psyllid; tomato potato psyllid (TPP) Bactericera cockerelli (Šulc). Specifically, we test the effect of lure (no lure, Asian citrus psyllid ACP lure, brown marmorated stink bug BMSB lure, combined lures), sticky trap color (green vs yellow), and sentinel plant (tomato vs citrus) on psyllid and stink bug species in 104 urban backyards across Perth, Australia. We found that tomato sentinel host plants and green traps significantly increased the capture rate of TPP, but that all lures decreased the capture of TPP. Green traps also increased the capture rate of all other Psylloidea. Although BMSB lures reduced TPP capture, these lures increased abundances of other Psylloidea and the pest stinkbug Plautia affinis (Dallas) on traps. Thus, our experiment demonstrates that increased efficiencies can be gained with combination traps and lures for particular groups, provided that they have been tested on focal organisms in the first instance, as reactions to non-target lures are unpredictable and species specific.

Extraction of Pest Insect Characteristics Present in a Mirasol Pepper (Capsicum annuum L.) Crop by Digital Image Processing

One of the main problems in crops is the presence of pests. Traditionally, sticky yellow traps are used to detect pest insects, and they are then analyzed by a specialist to identify the pest insects present in the crop. To facilitate the identification, classification, and counting of these insects, it is possible to use digital image processing (DIP). This study aims to demonstrate that DIP is useful for extracting invariant characteristics of psyllids (Bactericera cockerelli), thrips (Thrips tabaci), whiteflies (Bemisia tabaci), potato flea beetles (Epitrix cucumeris), pepper weevils (Anthonomus eugenii), and aphids (Myzus persicae). The characteristics (e.g., area, eccentricity, and solidity) help classify insects. DIP includes a first stage that consists of improving the image by changing the levels of color intensity, applying morphological filters, and detecting objects of interest, and a second stage that consists of applying a transformation of invariant scales to extract characteristics of insects, independently of size or orientation. The results were compared with the data obtained from an entomologist, reaching up to 90% precision for the classification of these insects.

Exogenous Application of Polycationic Nanobactericide on Tomato Plants Reduces the Candidatus Liberibacter Solanacearum Infection

Candidatus Liberibacter solanacearum (CaLso) is associated with diseases in tomato crops and transmitted by the tomato psyllid Bactericera cockerelli. A polymeric water-dispersible nanobactericide (PNB) was evaluated against CaLso as a different alternative. PNB is a well-defined polycationic diblock copolymer designed to permeate into the vascular system of plants. Its assessment under greenhouse conditions was carried out with tomato plants previously infected with CaLso. Using a concentration as low as 1.0 mg L−1, a small but significant reduction in the bacterial load was observed by real-time qPCR. Thus, to achieve an ecologically friendly dosage and set an optimum treatment protocol, we performed experiments to determine the effective concentration of PNB to reduce ~65% of the initial bacterial load. In a first bioassay, a 40- or 70-fold increase was used to reach that objective. At this concentration level, other bioassays were explored to determine the effect as a function of time. Surprisingly, a real reduction in the symptoms was observed after three weeks, and there was a significant decrease in the bacterial load level (~98%) compared to the untreated control plants. During this period, flowering and formation of tomato fruits were observed in plants treated with PNB.

Effectors from a Bacterial Vector-Borne Pathogen Exhibit Diverse Subcellular Localization, Expression Profiles and Manipulation of Plant Defense

ABSTRACTClimate change is predicted to increase the prevalence of vector borne disease due to expansion of insect populations. Candidatus Liberibacter solanacearum (Lso) is a phloem-limited pathogen associated with multiple economically important diseases in Solanaceous crops. Little is known about the strategies and pathogenicity factors Lso uses to colonize vector and host. We determined the Lso effector repertoire by predicting SEC secreted proteins across four different Lso haplotypes. Compared with C. Liberibacter asiaticus, the causal agent of citrus Huanglongbing, Lso possess a more variable effector repertoire, with greater similarity between haplotypes infecting the same host. The localization of Lso effectors in Nicotiana revealed diverse subcellular targets. The majority of tested effectors were unable to suppress plant immune responses, indicating they possess unique activities. Expression profiling in tomato and the psyllid Bactericera cockerelli indicated Lso differentially interacts with its vector and host and can switch effector expression in response to the environment. This study reveals Lso effectors possess complex expression patterns, target diverse host organelles and the majority are unable to suppress host immune responses, unlike effectors from foliar plant pathogenic bacteria. A mechanistic understanding of Lso effector function will reveal novel targets and provide insight into phloem biology.

Potato Zebra Chip: An Overview of the Disease, Control Strategies, and Prospects

Potato (Solanum tuberosum L.) is an important food crop worldwide. As the demand for fresh and processed potato products is increasing globally, there is a need to manage and control devastating diseases such as zebra chip (ZC). ZC disease causes major yield losses in many potato-growing regions and is associated with the fastidious, phloem-limited bacterium Candidatus Liberibacter solanacearum (CLso) that is vectored by the potato-tomato psyllid (Bactericera cockerelli Šulc). Current management measures for ZC disease mainly focus on chemical control and integrated pest management strategies of the psyllid vector to limit the spread of CLso, however, they add to the costs of potato production. Identification and deployment of CLso and/or the psyllid resistant cultivars, in combination with integrated pest management, may provide a sustainable long-term strategy to control ZC. In this review, we provide a brief overview of the ZC disease, epidemiology, current management strategies, and potential new approaches to manage ZC disease in the future.

Insecticidal Effect of Zinc Oxide and Titanium Dioxide Nanoparticles against Bactericera cockerelli Sulc. (Hemiptera: Triozidae) on Tomato Solanum lycopersicum

The use of nanoparticles (NPs) has generated an alternative pest control. The objective was to evaluate the insecticidal effect of zinc oxide nanoparticles (ZnO NPs), titanium dioxide nanoparticles (TiO2 NPs), and their combination on Bactericera cockerelli (Hemiptera: Triozidae) second-stage nymphs under laboratory and greenhouse conditions in tomato. The laboratory research was carried out with the leaf immersion bioassay method under a complete randomized design, and in the greenhouse by direct plant spraying under a randomized block design; in both designs, a control without NPs was added. Mortality was recorded every 24 h for 4 days. Both NPs in the laboratory and greenhouse showed toxicity to B. cockerelli nymphs. Results in the laboratory showed that NPs significantly caused increased mortality of 88, 99, and 100% 96 h after treatment of ZnO NPs, TiO2 NPs, and their combinations, at 1000, 100, and 250 ppm, respectively. Direct spray of plants in the greenhouse showed low mortality with 27, 32, and 23% after 96 h of ZnO NPs, TiO2 NPs, and their combinations, at 3000, 500, and 250 ppm, respectively. These results on B. cockerelli control seem promising. Nanoparticles as insecticides are a novel strategy, however, further investigation is required in field tests to obtain suitable efficacy for use in a pest management system.

Effects of sex ratio on different biological parameters of Engytatus varians (Distant) (Hemiptera: Miridae) adults and their offspring: prey preference for Bactericera cockerelli (Sulcer) (Hemiptera: Triozidae)

In the present study, the influence of three sex ratios (1:1, 1:2, and 1:3; female:male) of the mirid Engytatus varians (Distant) (Hemiptera) on different biological parameters and on its offspring was evaluated. The prey preference of different developmental stages of this predator for different nymphal instars (N) of Bactericera cockerelli (Sulcer) (Hemiptera: Triozidae) was also evaluated. The fertility was significantly higher (24 nymphs/female) in the 1:3 sex ratio than in the 1:1 and 1:2 sex ratios (14 and 16 nymphs/female, respectively). The females in the 1:1 and 1:2 sex ratios lived 1.14 and 1.43 days more (27 and 28 days, respectively) than those in the 1:3 sex ratio (26 days). The nymphs derived from the females of the three sex ratios (first filial generation, F1) had five instars and a duration of 17 or 18 days. The ratio of the F1 generation females was not affected by the sex ratio of their parents. In choice tests, independent of whether the preys were placed on a single or multiple tomato (Solanum lycopersicum L.) leaflets, the consumption of females and males and N3, N4, and N5 nymphs of E. varians on B. cockerelli, generally showed the order of N2>N3>N4>N5. In conclusion, the findings revealed in this study can help to improve the rearing methodology for increasing populations of E. varians. In addition, they can serve as a guideline for releasing this predator in times when there is an abundance of early instar nymphs of B. cockerelli.

Improving Suppression of Hemipteran Vectors and Bacterial Pathogens of Citrus and Solanaceous Plants: Advances in Antisense Oligonucleotides (FANA)

We report on the development, evaluation, and efficient delivery of antisense oligonucleotide FANA (2′-deoxy-2′-fluoro-arabinonucleotide) RNA-targeting technology into citrus trees and potato plants for management of bacterial pathogens and arthropod pests. The FANA ASO technology is a single nucleotide strand of 20–24 nt in length that incorporates 2′F- chemically modifications of nucleotides, along with a phosphorothioate backbone and modified flanking nucleotides, in their structure called “gapmers,” produced by AUM LifeTech., Inc. These unique modified structures of FANA “triggers” enables gymnotic activity that self-delivers into cells, moving systemically in treated plants and insects, with significant suppression of their RNA targets. Reported is the FANA suppression of two plant-infecting bacterium Candidatus Liberibacter asiaticus, CLas (in citrus trees), and C. Liberibacter solanacearum, CLso (in potato and tomato). The CLas pathogen is associated with huanglongbing (a.k.a. Citrus Greening Disease), which causes severe loss of citrus trees, threatening global citrus production. The CLas bacterium is transmitted during feeding by the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae). CLso causes Zebra-Chip disease in potato and is transmitted by the potato psyllid, Bactericera cockerelli (Hemiptera: Triozidae). Infected citrus trees or potato plants were treated with aqueous FANA solutions applied as a soil drench, root-infusion, topical spray, tree trunk injection or by absorption into cuttings, detached leaves, and leaf disks. Plants showed significant reduction of each pathogen or symptom development in response to FANA treatments. Similarly, ingestion of FANA solutions designed specifically to CLas by insects via artificial diets produced significant titer reductions in infected citrus psyllid adults that resulted in reduction of CLas transmission. The unique properties of FANA ASO solves many of the problems of stability, cell entry, and binding affinity that plagues exogenous RNAi strategies. Breakthroughs in production methods are reducing costs enabling these ASO to expand beyond medical applications into agricultural treatments. Thus, FANA ASO may provide viable treatments in the response to crop pandemics, like huanglongbing in citrus that threatens global food production.