The influence of citrus rootstocks on the relationship between the mite Brevipalpus phoenicis and citrus leprosis disease

Brevipalpus yothersi Baker (Tenuipalpidae) development in sweet orange plants is influenced by previous mite infestation and the presence of shelters

Citrus leprosis is the most important viral disease affecting citrus. The disease is caused predominantly by CiLV-C and is transmitted by Brevipalpus yothersi Baker mites. This study brings some insight into the colonization of B. yothersi in citrus [(Citrus × sinensis (L.) Osbeck (Rutaceae)] previously infested by viruliferous or non-viruliferous B. yothersi. It also assesses the putative role of shelters on the behavior of B. yothersi. Expression of PR1 and PR4 genes, markers of plant defense mechanisms, were evaluated by RT-qPCR to correlate the role of the plant hormonal changes during the tri-trophic virus-mite-plant interplay. A previous infestation with either non-viruliferous and viruliferous mites positively influenced oviposition and the number of adult individuals in the resulting populations. Mite populations were higher on branches that had received a previous mite infestation than branches that did not. There was an increase in the expression of PR4, a marker gene in the jasmonic acid (JA) pathway, in the treatment with non-viruliferous mites, indicating a response from the plant to their feeding. Conversely, an induced expression of PR1, a marker gene in the salicylic acid (SA) pathway, was observed mainly in the treatment with viruliferous mites, which suggests the activation of a plant response against the pathogen. The earlier mite infestation, as well as the presence of leprosis lesions and a gypsum mixture as artificial shelters, all fostered the growth of the B. yothersi populations after the second infestation, regardless of the presence or absence of CiLV-C. Furthermore, it is suggested that B. yothersi feeding actually induces the JA pathway in plants. At the same time, the CiLV-C represses the JA pathway and induces the SA pathway, which benefits the mite vector.

Plant Immune System Activation Upon Citrus Leprosis Virus C Infection Is Mimicked by the Ectopic Expression of the P61 Viral Protein

Citrus leprosis virus C (CiLV-C, genus Cilevirus, family Kitaviridae) is an atypical virus that does not spread systemically in its plant hosts. Upon its inoculation by Brevipalpus mites, only localized lesions occur, and the infection remains limited to cells around mite feeding sites. Here, we aimed to gain insights into the putative causes of viral unfitness in plants by expanding the limited knowledge of the molecular mechanisms underlying plant/kitavirid interactions. Firstly, we quantified the CiLV-C viral RNAs during the infection in Arabidopsis thaliana plants using RT-qPCR and systematized it by defining three stages of distinguishing subgenomic and genomic RNA accumulation: i) 0-24 h after infestation, ii) 2-4 days after infestation (dai), and iii) 6-10 dai. Accordingly, the global plant response to CiLV-C infection was assessed by RNA-Seq at each period. Results indicated a progressive reprogramming of the plant transcriptome in parallel to the increasing viral loads. Gene ontology enrichment analysis revealed the induction of cell growth-related processes at the early stages of the infection and the triggering of the SA-mediated pathway, ROS burst and hypersensitive response (HR) at the presymptomatic stage. Conversely, infected plants downregulated JA/ET-mediated pathways and processes involved in the primary metabolism including photosynthesis. Marker genes of unfolded protein response were also induced, suggesting a contribution of the endoplasmic reticulum stress to the cell death caused by the viral infection. Finally, we transiently expressed CiLV-C proteins in Nicotiana benthamiana plants to undertake their roles in the elicited plant responses. Expression of the CiLV-C P61 protein consistently triggered ROS burst, upregulated SA- and HR-related genes, increased SA levels, reduced JA levels, and caused cell death. Mimicry of responses typically observed during CiLV-C-plant interaction indicates P61 as a putative viral effector causing the HR-like symptoms associated with the infection. Our data strengthen the hypothesis that symptoms of CiLV-C infection might be the outcome of a hypersensitive-like response during an incompatible interaction. Consequently, the locally restricted infection of CiLV-C, commonly observed across infections by kitavirids, supports the thesis that these viruses, likely arising from an ancestral arthropod-infecting virus, are unable to fully circumvent plant defenses.

Spatiotemporal association between the mite Brevipalpus yothersi and Citrus leprosis virus C in orange orchards

Citrus leprosis virus C (CiLV-C) is an economically important pathogen and the main causative agent of leprosis disease in citrus orchards. The main vector of this disease, the mite Brevipalpus yothersi, is widely distributed in Mexican orchards on a wide range of citrus species. Despite the importance of both the virus and the mite, field studies recording their occurrence and co-occurrence are practically non-existent. We systematically sampled orange orchards for both CiLV-C and B. yothersi throughout the year. The distribution of the CiLV-C and B. yothersi was evaluated on each sampling occasion and their spatiotemporal associations were determined. Specifically, 100-112 orange trees, distributed in 18 rows (five or six trees per row), were sampled monthly between March 2017 and February 2018 (11 sampling dates). Twenty leaves per tree were sampled on each occasion. The number of mites per tree and the percentage of leaves per tree with disease symptoms were recorded. On each sampling occasion, spatiotemporal associations between mites and disease were determined using the Spatial Analysis by Distance Indices (SADIE) method. CiLV-C and B. yothersi were identified using molecular methods. Throughout the study, the distribution of CiLV-C was aggregated and the distribution of B. yothersi was random. No association was found between the virus and the mite on any of the sampling dates. In total, 173 mites were collected, but only 43 mites were found to be carrying CiLV-C. The reason for this lack of association between the virus and the mite, as well as the impact of our findings on the epidemiology of the disease in orange orchards, are discussed.

Brevipalpus-transmitted viruses: parallelism beyond a common vector or convergent evolution of distantly related pathogens?

Although diseases caused by Brevipalpus-transmitted viruses (BTV) became relevant for agriculture a century ago, their causal agents have been only recently characterized and classified in two new genera of plant-infecting viruses: Cilevirus and Dichorhavirus. In this review, we highlight both similarities and differences between these viruses emphasizing their current taxonomy and historical classification, phylogeny, genomic organization, gene expression, and the latest research developments on BTVs. Additionally, we stress particular features of interactions with their mite vectors and plant hosts that support, from an evolutionary perspective, the potential convergence of both viral groups.

Space-time variability of citrus leprosis as strategic planning for crop management

BACKGROUND

Citrus leprosis is the most important viral disease of citrus. Knowledge of its spatiotemporal structure is fundamental to a representative sampling plan focused on the disease control approach. Such a well-crafted sampling design helps to reduce pesticide use in agriculture to control pests and diseases.

RESULTS

Despite the use of acaricides to control citrus leprosis vector (Brevipalpus spp.) populations, the disease has spread rapidly through experimental areas. Citrus leprosis has an aggregate spatial distribution, with high dependence among symptomatic plants. Temporal variation in disease incidence increased among symptomatic plants by 4% per month.

CONCLUSIONS

Use of acaricides alone to control the vector of leprosis is insufficient to avoid its incidence in healthy plants. Preliminary investigation into the time and space variation in the incidence of the disease is fundamental to select a sampling plan and determine effective strategies for disease management. © 2018 Society of Chemical Industry.

Citrus leprosis virus C Infection Results in Hypersensitive-Like Response, Suppression of the JA/ET Plant Defense Pathway and Promotion of the Colonization of Its Mite Vector

Leprosis is a serious disease of citrus caused by Citrus leprosis virus C (CiLV-C, genus Cilevirus) whose transmission is mediated by false spider mites of the genus Brevipalpus. CiLV-C infection does not systemically spread in any of its known host plants, thus remaining restricted to local lesions around the feeding sites of viruliferous mites. To get insight into this unusual pathosystem, we evaluated the expression profiles of genes involved in defense mechanisms of Arabidopsis thaliana and Citrus sinensis upon infestation with non-viruliferous and viruliferous mites by using reverse-transcription qPCR. These results were analyzed together with the production of reactive oxygen species (ROS) and the appearance of dead cells as assessed by histochemical assays. After interaction with non-viruliferous mites, plants locally accumulated ROS and triggered the salicylic acid (SA) and jasmonate/ethylene (JA/ET) pathways. ERF branch of the JA/ET pathways was highly activated. In contrast, JA pathway genes were markedly suppressed upon the CiLV-C infection mediated by viruliferous mites. Viral infection also intensified the ROS burst and cell death, and enhanced the expression of genes involved in the RNA silencing mechanism and SA pathway. After 13 days of infestation of two sets of Arabidopsis plants with non-viruliferous and viruliferous mites, the number of mites in the CiLV-C infected Arabidopsis plants was significantly higher than in those infested with the non-viruliferous ones. Oviposition of the viruliferous mites occurred preferentially in the CiLV-C infected leaves. Based on these results, we postulated the first model of plant/Brevipalpus mite/cilevirus interaction in which cells surrounding the feeding sites of viruliferous mites typify the outcome of a hypersensitive-like response, whereas viral infection induces changes in the behavior of its vector.

Infestation dynamics of Brevipalpus phoenicis (Geijskes) (Acari: Tenuipalpidae) in citrus orchards as affected by edaphic and climatic variables

Brevipalpus phoenicis (Geijskes) is a cosmopolitan and polyphagous mite that transmits important phytoviruses, such as coffee ringspot virus, passion fruit green spot virus and Citrus leprosis virus C. To characterise the dynamics of the probability and the rate of B. phoenicis infestation in response to edaphic and climatic factors, monthly inspections were performed in nine orchards in a citrus region of the State of Bahia, Brazil, for 35 months. Three fruits per plant were examined using a magnifying glass (10×) on 21 plants distributed along a "W"-shaped path in each orchard. Meteorological data were collected from a conventional station. To determine the correlations among the climatic variables, the data were analysed using Spearman correlations. Variables were selected by principal component analysis, and those that contributed the most to differentiate the groups were evaluated via a Mann-Whitney test. Using the quantile-quantile method, the limit values for the following climatic variables were determined: temperature (24.5 °C), photoperiod (12 h), relative humidity (83%), evapotranspiration (71 mm) and rainy days (14 days). The combination of longer days, higher temperatures, lower relative humidity levels and lower evapotranspiration increased the probability of B. phoenicis infestation, whereas successive rain events decreased that risk. Infestation rates were negatively affected by relative humidity levels above 83% and were positively affected by a decreasing available soil-water fraction and increasing insolation and photoperiod.