Publications

4980

Nowadays, Huanglongbing (HLB) disease, associated with Candidatus Liberibacter asiaticus (CLas), seriously affects citriculture worldwide, and no cure is currently available. Transcriptomic analysis of host–pathogen interaction is the first step to understand the molecular landscape of a disease. Previous works have reported the transcriptome profiling in response to HLB in different susceptible citrus species; however, similar studies in tolerant citrus species, including Mexican lime, are limited. In this work, we have obtained an RNA-seq-based differential expression profile of Mexican lime plants challenged against CLas infection, at both asymptomatic and symptomatic stages. Typical HLB-responsive differentially expressed genes (DEGs) are involved in photosynthesis, secondary metabolism, and phytohormone homeostasis. Enrichment of DEGs associated with biotic response showed that genes related to cell wall, secondary metabolism, transcription factors, signaling, and redox reactions could play a role in the tolerance of Mexican lime against CLas infection. Interestingly, despite some concordance observed between transcriptional responses of different tolerant citrus species, a subset of DEGs appeared to be species-specific. Our data highlights the importance of studying the host response during HLB disease using as model tolerant citrus species, in order to design new and opportune diagnostic and management methods.

AbstractHuanglongbing, a highly destructive disease of citrus species, is associated with a fastidious, gram-negative, phloem-limited bacteria (Candidatus Liberibacter spp.). In Florida, the causative agent of Huanglongbing (HLB) is C. Liberibacter asiaticus (CLas) and it is transmitted by the insect vector, Asian citrus psyllid (Diaphorina citri Kuwayama). Previous investigations have revealed systemic infection of CLas with an erratic and uneven distribution of pathogen in tree phloem. However, previous investigations did not consider the potential impact of plant vegetative growth on presence/absence of CLas in planta. Our objectives were to determine: 1) the effect of vegetative growth of Citrus sinensis (L.) Osbeck cv Valencia on detection of CLas in mature leaves, and 2) the impact of CLas inoculation frequency on progression of CLas titer in citrus leaves through the first year of infection. Temporal dynamics of CLas detection were associated with vegetative flush growth. Surprisingly, there was no difference in CLas titer detected between plants exposed to infected vectors for a one-time 7 d inoculation access period, as compared with plants exposed to continuously breeding CLas-infected insects over the course of an entire year of plant infection. Our results suggest that the CLas bacterium is transported through phloem during annual movement of carbon compounds needed for vegetative plant growth, including transportation from roots to mature leaves. These results highlight the importance of vegetative growth on temporal dynamics of CLas in citrus, and suggest a critical role of the sink-source interaction on presence/absence of CLas in leaves.

Globally, citrus is threatened by huanglongbing (HLB), and the lack of effective control measures is a major concern of farmers, markets, and consumers. There is compelling evidence that plant health is a function of the activities of the plant's associated microbiome. Using Liberibacter crescens , a culturable surrogate for the unculturable HLB-associated bacterium “ Candidatus Liberibacter asiaticus,” we tested the hypothesis that members of the citrus microbiome produce potential anti-“ Ca . Liberibacter asiaticus” natural products with potential anti-“ Ca . Liberibacter asiaticus” activity. A subset of isolates obtained from the microbiome inhibited L. crescens growth in an agar diffusion inhibition assay. Further fractionation experiments linked the inhibitory activity of the fungus Cladosporium cladosporioides to the fungus-produced natural products cladosporols A, C, and D, demonstrating dose-dependent antagonism to L. crescens .

Specific scopeThis Standard describes a diagnostic protocol for ‘Candidatus Phytoplasma mali’, ‘Ca. P. pyri’ and ‘Ca. P. prunorum’.This Standard should be used in conjunction with PM 7/76 Use of EPPO diagnostic protocolsSpecific approval and amendmentApproved as PM 7/62 Candidatus Phytoplasma mali and PM 7/63 Ca. P. pyri in 2006. First revision in 2017‐02 as a single Standard as PM 7/62 (2) with the addition of ‘Ca. P. prunorum’. Second revision in 2019‐06.

Abstract Anaerobic oxidation of methane (AOM) is a major biological process that reduces global methane emission to the atmosphere. Anaerobic methanotrophic archaea (ANME) mediate this process through the coupling of methane oxidation to different electron acceptors, or in concert with a syntrophic bacterial partner. Recently, ANME belonging to the archaeal family Methanoperedenaceae (formerly known as ANME-2d) were shown to be capable of AOM coupled to nitrate and iron reduction. Here, a freshwater sediment bioreactor fed with methane and Mn(IV) oxides (birnessite) resulted in a microbial community dominated by two novel members of the Methanoperedenaceae, with biochemical profiling of the system demonstrating Mn(IV)-dependent AOM. Genomic and transcriptomic analyses revealed the expression of key genes involved in methane oxidation and several shared multiheme c-type cytochromes (MHCs) that were differentially expressed, indicating the likely use of different extracellular electron transfer pathways. We propose the names “Candidatus Methanoperedens manganicus” and “Candidatus Methanoperedens manganireducens” for the two newly described Methanoperedenaceae species. This study demonstrates the ability of members of the Methanoperedenaceae to couple AOM to the reduction of Mn(IV) oxides, which suggests their potential role in linking methane and manganese cycling in the environment.

ScopeThis Standard describes procedures for official control with the aim of detecting, containing and eradicating those ‘Candidatus Liberibacter’ species which are causal agents of Huanglongbing (also known as citrus greening disease) and their vectors Trioza erytreae and Diaphorina citri. NPPOs may draw on this guidance when developing national contingency plans for outbreaks of ‘Candidatus Liberibacter’ species and their vectors.Approval and amendmentFirst approved in 2019‐09.