The induction of resistance to disease during plant development is widespread in the plant kingdom. Resistance appears at different stages of host development, varies with plant age or tissue ...maturity, may be specific or broad-spectrum and is driven by diverse mechanisms, depending on plant-pathogen interactions. Studies of these forms of resistance may help us to evaluate more exhaustively the plethora of levels of regulation during development, the variability of the defense potential of developing hosts and may have practical applications, making it possible to reduce pesticide applications. Here, we review the various types of developmental resistance in plants and current knowledge of the molecular and cellular processes involved in their expression. We discuss the implications of these studies, which provide new knowledge from the molecular to the agrosystem level. Summary 405 I. Introduction 405 II. The many forms of developmental resistance 406 III. Molecular mechanisms of developmental resistance 410 IV. Relationships between defense and development in plants 412 V. Concluding remarks 413 Acknowledgements 413 References 413
Inactivation of Arabidopsis WAT1 (Walls Are Thin1), a gene required for secondary cell‐wall deposition, conferred broad‐spectrum resistance to vascular pathogens, including the bacteria Ralstonia ...solanacearum and Xanthomonas campestris pv. campestris, and the fungi Verticillium dahliae and Verticillium albo‐atrum. Introduction of NahG, the bacterial salicylic acid (SA)‐degrading salicylate hydroxylase gene, into the wat1 mutant restored full susceptibility to both R. solanacearum and X. campestris pv. campestris. Moreover, SA content was constitutively higher in wat1 roots, further supporting a role for SA in wat1‐mediated resistance to vascular pathogens. By combining transcriptomic and metabolomic data, we demonstrated a general repression of indole metabolism in wat1‐1 roots as shown by constitutive down‐regulation of several genes encoding proteins of the indole glucosinolate biosynthetic pathway and reduced amounts of tryptophan (Trp), indole‐3‐acetic acid and neoglucobrassicin, the major form of indole glucosinolate in roots. Furthermore, the susceptibility of the wat1 mutant to R. solanacearum was partially restored when crossed with either the trp5 mutant, an over‐accumulator of Trp, or Pro35S:AFB1‐myc, in which indole‐3‐acetic acid signaling is constitutively activated. Our original hypothesis placed cell‐wall modifications at the heart of the wat1 resistance phenotype. However, the results presented here suggest a mechanism involving root‐localized metabolic channeling away from indole metabolites to SA as a central feature of wat1 resistance to R. solanacearum.
Summary
Paenibacillus larvae is the causative agent of the fatal American foulbrood disease in honeybees (Apis mellifera). Strain identification is vital for preventing the spread of the disease. To ...date, the most accessible and robust scheme to identify strains is the multilocus sequence typing (MLST) method. However, this approach has limited resolution, especially for epidemiological studies. As the cost of whole‐genome sequencing has decreased and as it becomes increasingly available to most laboratories, an extended MLST based on the core genome (cgMLST) presents a valuable tool for high‐resolution investigations. In this study, we present a standardized, robust cgMLST scheme for P. larvae typing using whole‐genome sequencing. A total of 333 genomes were used to identify, validate and evaluate 2419 core genes. The cgMLST allowed fine‐scale differentiation between samples that had the same profile using traditional MLST and allowed for the characterization of strains impossible by MLST. The scheme was successfully used to trace a localized Swedish outbreak, where a cluster of 38 isolates was linked to a country‐wide beekeeping operation. cgMLST greatly enhances the power of a traditional typing scheme, while preserving the same stability and standardization for sharing results and methods across different laboratories.
Infectious and parasitic agents (IPAs) and their associated diseases are major environmental stressors that jeopardize bee health, both alone and in interaction with other stressors. Their impact on ...pollinator communities can be assessed by studying multiple sentinel bee species. Here, we analysed the field exposure of three sentinel managed bee species (Apis mellifera, Bombus terrestris and Osmia bicornis) to 11 IPAs (six RNA viruses, two bacteria, three microsporidia). The sentinel bees were deployed at 128 sites in eight European countries adjacent to either oilseed rape fields or apple orchards during crop bloom. Adult bees of each species were sampled before their placement and after crop bloom. The IPAs were detected and quantified using a harmonised, high-throughput and semi-automatized qPCR workflow. We describe differences among bee species in IPA profiles (richness, diversity, detection frequencies, loads and their change upon field exposure, and exposure risk), with no clear patterns related to the country or focal crop. Our results suggest that the most frequent IPAs in adult bees are more appropriate for assessing the bees' IPA exposure risk. We also report positive correlations of IPA loads supporting the potential IPA transmission among sentinels, suggesting careful consideration should be taken when introducing managed pollinators in ecologically sensitive environments.
To provide a complete portrayal of the multiple factors negatively impacting insects in agricultural landscapes it is necessary to assess the concurrent incidence, magnitude, and interactions among ...multiple stressors over substantial biogeographical scales. Trans-national ecological field investigations with wide-ranging stakeholders typically encounter numerous challenges during the design planning stages, not least that the scientific soundness of a spatially replicated study design must account for the substantial geographic and climatic variation among distant sites. ‘PoshBee’ (Pan-European assessment, monitoring, and mitigation of Stressors on the Health of Bees) is a multi-partner transdisciplinary agroecological project established to investigate the suite of stressors typically encountered by pollinating insects in European agricultural landscapes. To do this, PoshBee established a network of 128 study sites across eight European countries and collected over 50 measurements and samples relating to the nutritional, toxicological, pathogenic, and landscape components of the bees’ environment. This paper describes the development process, rationale, and end-result of each aspect of the of the PoshBee field investigation. We describe the main issues and challenges encountered during the design stages and highlight a number of actions or processes that may benefit other multi-partner research consortia planning similar large-scale studies. It was soon identified that in a multi-component study design process, the development of interaction and communication networks involving all collaborators and stakeholders requires considerable time and resources. It was also necessary at each planning stage to be mindful of the needs and objectives of all stakeholders and partners, and further challenges inevitably arose when practical limitations, such as time restrictions and labour constraints, were superimposed upon prototype study designs. To promote clarity for all stakeholders, for each sub-component of the study, there should be a clear record of the rationale and reasoning that outlines how the final design transpired, what compromises were made, and how the requirements of different stakeholders were accomplished. Ultimately, multi-national agroecological field studies such as PoshBee benefit greatly from the involvement of diverse stakeholders and partners, ranging from field ecologists, project managers, policy legislators, mathematical modelers, and farmer organisations. While the execution of the study highlighted the advantages and benefits of large-scale transdisciplinary projects, the long planning period emphasized the need to formally describe a design framework that could facilitate the design process of future multi-partner collaborations.
Plant cell walls are complex structures subject to dynamic remodeling in response to developmental and environmental cues and play essential functions in disease resistance responses. We tested the ...specific contribution of plant cell walls to immunity by determining the susceptibility of a set of
cell wall mutants (
) to pathogens with different parasitic styles: a vascular bacterium, a necrotrophic fungus, and a biotrophic oomycete. Remarkably, most
mutants tested (29/34; 85.3%) showed alterations in their resistance responses to at least one of these pathogens in comparison to wild-type plants, illustrating the relevance of wall composition in determining disease-resistance phenotypes. We found that the enhanced resistance of
plants to the necrotrophic and vascular pathogens negatively impacted
fitness traits, such as biomass and seed yield. Enhanced resistance of
plants is not only mediated by canonical immune pathways, like those modulated by phytohormones or microbe-associated molecular patterns, which are not deregulated in the
tested. Pectin-enriched wall fractions isolated from
plants triggered immune responses in wild-type plants, suggesting that wall-mediated defensive pathways might contribute to
resistance. Cell walls of
plants show a high diversity of composition alterations as revealed by glycome profiling that detect specific wall carbohydrate moieties. Mathematical analysis of glycome profiling data identified correlations between the amounts of specific wall carbohydrate moieties and disease resistance phenotypes of
plants. These data support the relevant and specific function of plant wall composition in plant immune response modulation and in balancing disease resistance/development trade-offs.
•Chronic bee paralysis is diagnosed by quantifying the causative virus, CBPV, in honeybees by real-time PCR•21 laboratories assayed honeybee homogenates, adjusted to three virus loads (5, 8, and 9 ...log10 CBPV genome copies per bee)•The accuracy (trueness and precision) of the method implemented in the laboratories was evaluated according to international standards•The inter-laboratory confidence of viral quantification was about+/− 2 log10 for a diagnostic threshold of 8 log10 CBPV genome copies per bee•Taking into account this confidence, viral loads over 6 log10 CBPV RNA copies per bee may indicate probable cases of chronic paralysis
The Chronic bee paralysis virus (CBPV) is the aetiological agent of chronic bee paralysis, a contagious disease associated with nervous disorders in adult honeybees leading to massive mortalities in front of the hives. Some of the clinical signs frequently reported, such as trembling, may be confused with intoxication syndromes. Therefore, laboratory diagnosis using real-time PCR to quantify CBPV loads is used to confirm disease. Clinical signs of chronic paralysis are usually associated with viral loads higher than 108 copies of CBPV genome copies per bee (8 log10 CBPV/bee). This threshold is used by the European Union Reference Laboratory for Bee Health to diagnose the disease. In 2015, the accuracy of measurements of three CBPV loads (5, 8 and 9 log10 CBPV/bee) was assessed through an inter-laboratory study. Twenty-one participants, including 16 European National Reference Laboratories, received 13 homogenates of CBPV-infected bees adjusted to the three loads. Participants were requested to use the method usually employed for routine diagnosis. The quantitative results (n=270) were analysed according to international standards NF ISO 13528 (2015) and NF ISO 5725-2 (1994). The standard deviations of measurement reproducibility (SR) were 0.83, 1.06 and 1.16 at viral loads 5, 8 and 9 log10 CBPV/bee, respectively. The inter-laboratory confidence of viral quantification (+/− 1.96SR) at the diagnostic threshold (8 log10 CBPV/bee) was+/− 2.08 log10 CBPV/bee. These results highlight the need to take into account the confidence of measurements in epidemiological studies using results from different laboratories. Considering this confidence, viral loads over 6 log10 CBPV/bee may be considered to indicate probable cases of chronic paralysis.
Besides the systemic acquired resistance (SAR) induced in response to microbial stimulation, host plants may also acquire resistance to pathogens in response to endogenous stimuli associated with ...their own development. In tobacco (Nicotiana tabacum), the vegetative-to-flowering transition comes along with a susceptibility-to-resistance transition to the causal agent of black shank disease, the oomycete Phytophthora parasitica. This resistance affects infection effectiveness and hyphal expansion and is associated with extracellular accumulation of a cytotoxic activity that provokes in vitro cell death of P. parasitica zoospores. As a strategy to determine the extracellular events important for restriction of pathogen growth, we screened the tobacco genome for genes encoding secreted or membrane-bound proteins expressed in leaves of flowering plants. Using a signal sequence trap approach in yeast (Saccharomyces cerevisiae), 298 clones were selected that appear to encode for apoplastic, cell wall, or membrane-bound proteins involved in stress response, in plant defense, or in cell wall modifications. Microarray and northern-blot analyses revealed that, at late developmental stages, leaves were characterized by the coordinate up-regulation of genes involved in SAR and in peroxidative cross-linking of structural proteins to cell wall. This suggests the potential involvement of these genes in extracellular events that govern the expression of developmental resistance. The analysis of the influence of salicylic acid on mRNA accumulation also indicates a more complex network for regulation of gene expression at a later stage of tobacco development than during SAR. Further characterization of these genes will permit the formulation of hypotheses to explain resistance and to establish the connection with development.
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•Nosema microsporidia are prevalent endoparasites of managed and wild bees.•Current molecular methods do not enable accurate Nosema quantification.•We developed qPCR methods to ...quantify N. apis, N. ceranae, and N. bombi.•Accuracy of parasite loads was validated on honey bee, bumble bee, and mason bee.•qPCR methods open perspectives in nosemosis diagnosis and in bee epidemiology.
Among stressors affecting bee health, Nosema microsporidia are prevalent intracellular parasites. Nosema apis and Nosema ceranae have been described in honey bees (Apis spp.), while Nosema bombi has been described in bumble bees (Bombus spp.). Although available molecular methods serve as a complement to microscopic diagnosis of nosemosis, they do not enable accurate quantification of these three Nosema species. We developed three quantitative real-time PCRs (qPCRs) starting from in silico design of specific primers, probes, and recombinant plasmids, to target the RNA polymerase II subunit B1 (RPB1) gene in the three species. The complete methods, including bee grinding, DNA purification, and qPCR, were validated in honey bee (Apis mellifera) homogenate. Specificity was assessed in silico and in vitro with several types of bee samples. The limit of detection was estimated at 4 log10 copies/honey bee. A small, systematic method bias was corrected for accurate quantification up to 10 log10 copies/honey bee. Method accuracy was also verified in bumble bee (Bombus terrestris) and mason bee (Osmia bicornis) homogenates in the range of 5 to 10 log10 copies/bee. These validated qPCR methods open perspectives in nosemosis diagnosis and in the study of the parasite’s eco-dynamics in managed and wild bees.
•The accuracy of 5 RT-qPCRs was assessed for their ability to quantify Europe’s most frequently-detected viruses.•Honeybees collected in France (2012–2019) were tested to determine the frequency of ...each viral load.•Viral load thresholds were suggested to differentiate covert from overt infections.
Acute bee paralysis virus (ABPV), Black queen cell virus (BQCV), Chronic bee paralysis virus (CBPV), Deformed wing virus (DWV), Sacbrood virus (SBV) and Varroa destructor virus 1 (VDV1) are the six main honeybee viruses reported in Europe. We assessed the accuracy (trueness and precision) of reverse transcriptase quantitative TaqMan® PCR methods (RT-qPCR) for quantifying ABPV, BQCV, DWV, VDV1 and SBV loads. Once the systematic bias in quantitative results had been corrected (overestimation in ABPV and BQCV quantification and underestimation in that of SBV and VDV1), measurements were taken to determine the viral load ranges for which quantification uncertainty was below ± 1 log10 equivalent of genome copies per bee (hereafter reported as genome copies/bee). The accuracy range of RT-qPCR was found to be between 6.4 and 10.4 log10 genome copies/bee for ABPV, between 3.0 and 10.0 log10 genome copies/bee for BQCV, between 2.4 and 10.4 log10 genome copies/bee for DWV and between 3.4 and 10.4 log10 genome copies/bee for SBV. Outside these ranges, the results’ uncertainty is higher. VDV1 RT-qPCR accuracy was outside accuracy limits for all viral loads. Using these RT-qPCR methods, we quantified viral loads in naturally-infected honeybees. The viral load distribution and clinical signs reported with the honeybee samples allowed us to define a threshold that could be used to differentiate between covert and overt infections. These methods will be useful in diagnosing the main viral infections impairing honeybee health.
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