Host Plant Adaptation Drives Changes in Diaphorina citri Proteome Regulation, Proteoform Expression, and Transmission of 'Candidatus Liberibacter asiaticus', the Citrus Greening Pathogen

The Hidden Secrets of Psylloidea: Biology, Behavior, Symbionts, and Ecology

Psyllids constitute a diverse group of sap-feeding Sternorrhyncha that were relatively obscure until it was discovered that a handful of species transmit bacterial plant pathogens. Yet the superfamily Psylloidea is much richer than the sum of its crop-associated vectors, with over 4,000 described species exhibiting diverse life histories and host exploitation strategies. A growing body of research is uncovering fascinating insights into psyllid evolution, biology, behavior, and species interactions. This work has revealed commonalities and differences with better-studied Sternorrhyncha, as well as unique evolutionary patterns of lineage divergence and host use. We are also learning how psyllid evolution and foraging ecology underlie life history traits and the roles of psyllids in communities. At finer scales, we are untangling the web of symbionts across the psyllid family tree, linking symbiont and psyllid lineages, and revealing mechanisms underlying reciprocal exchange between symbiont and host. In this review, we synthesize and summarize key advances within these areas with a focus on free-living (nongalling) Psylloidea.

A Perspective on Current Therapeutic Molecule Screening Methods Against 'Candidatus Liberibacter asiaticus', the Presumed Causative Agent of Citrus Huanglongbing

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.

Variations on a theme: factors regulating interaction between Diaphorina citri and "Candidatus Liberibacter asiaticus" vector and pathogen of citrus huanglongbing

Diaphorina citri, the Asian citrus psyllid, is a vector of Candidatus Liberibacter asiaticus (CLas), the causal agent of huanglongbing (HLB), the world's most serious disease of citrus. Owing to the relevancy and urgency of HLB research, the study of transmission biology in the HLB pathosystem has been a significant area of research. The focus of this article is to summarize and synthesize recent advancements in transmission biology between D. citri and CLas to create an updated view of the research landscape and to identify avenues for future research. Variability appears to play an important role in the transmission of CLas by D. citri. We advocate that it is important to understand the genetic basis for and environmental factors contributing to CLas transmission and how that variation may be exploited to develop and improve HLB control tactics.

Lessons learned about the biology and genomics of Diaphorina citri infection with "Candidatus Liberibacter asiaticus" by integrating new and archived organ-specific transcriptome data

Background

Huanglongbing, a devastating disease of citrus, is caused by the obligate, intracellular bacterium “Candidatus Liberibacter asiaticus” (CLas). CLas is transmitted by Diaphorina citri, the Asian citrus psyllid. Development of transmission-blocking strategies to manage huanglongbing relies on knowledge of CLas and D. citri interactions at the molecular level. Prior transcriptome analyses of D. citri point to changes in psyllid biology due to CLas infection but have been hampered by incomplete versions of the D. citri genome, proper host plant controls, and/or a lack of a uniform data analysis approach. In this work, we present lessons learned from a quantitative transcriptome analysis of excised heads, salivary glands, midguts, and bacteriomes from CLas-positive and CLas-negative D. citri using the chromosomal length D. citri genome assembly.

Results

Each organ had a unique transcriptome profile and response to CLas infection. Though most psyllids were infected with the bacterium, CLas-derived transcripts were not detected in all organs. By analyzing the midgut dataset using both the Diaci_v1.1 and v3.0 D. citri genomes, we showed that improved genome assembly led to significant and quantifiable differences in RNA-sequencing data interpretation.

Conclusions

Our results support the hypothesis that future transcriptome studies on circulative, vector-borne pathogens should be conducted at the tissue-specific level using complete, chromosomal-length genome assemblies for the most accurate understanding of pathogen-induced changes in vector gene expression.