Heck, Michelle Lynn

AbstractHuanglongbing (HLB, or citrus greening disease) affects all citrus varieties world-wide. In the USA, Asia, and South America the causal agent is “CandidatusLiberibacter asiaticus” (CLas), a phloem-limited, uncultured, alphaproteobacterium. The hemipteran insect vector,Diaphorina citri(Asian citrus psyllid) acquires and transmitsCLas in a circulative, propagative manner. In addition toCLas,D. citrihosts multiple symbiotic bacteria includingWolbachia(wDi). InD. citri, wDi has been sequenced and studied but specific roles inD. citribiology are unknown. Using well established quantitative PCR methods we measuredCLas titer inD. citricollected from four groves in central Florida with distinct HLB management strategies and tested whetherCLas and wDi titer were correlated in a sub-set of these insects. Grove site had the largest effect onCLas titer. Sex had no effect onCLas titer, while higher wDi titer was correlated withCLas-uninfected insects. Our results suggest that more directed follow-up research is necessary and important to clarify whether field management tactics influenceCLas titer inD. citriand to better understand gene-by-environment interactions amongD. citri, wDi andCLas. Now that millions of trees in Florida have been treated with injectable formulations of oxytetracycline, which is likely to decrease bacterial populations inD. citri, this study may represent the last biologically meaningful snapshot of grove-level vector-pathogen ecology in the state during the HLB epidemic.

AbstractThe Asian citrus psyllid,Diaphorina citri, is a vector of ‘CandidatusLiberibacter asiaticus’ (CLas), a gram-negative, obligate biotroph whose infection inCitrusspecies is associated with citrus greening disease, or Huanglongbing (HLB). Strategies to blockCLas transmission byD. citriremain the best way to prevent the spread of the disease into new citrus growing regions. However, identifying control strategies to block HLB transmission poses significant challenges, such as the discovery and delivery of antimicrobial compounds targeting the bacterium and overcoming consumer hesitancy towards accepting the treatment. Here, we computationally identified and tested a series of 20-mer nodule-specific cysteine-rich peptides (NCRs) derived from the Mediterranean legume,Medicago truncatulaGaertn. (barrelclover) to identify those peptides that could effectively prevent or reduceCLas infection in citrus leaves and/or preventCLas acquisition by the bacterium’s insect vector,D. citri. A set of NCR peptides were tested in a screening pipeline involving three distinct assays: a bacterial culture assay, aCLas-infected excised citrus leaf assay, and aCLas-infected nymph acquisition assay that includedD. citrinymphs, the only stage ofD. citri’s life-cycle that can acquireCLas leading to the development of vector competent adult insects. We demonstrate that a subset ofM. truncatula-derived NCRs inhibit bothCLas growth in citrus leaves andCLas acquisition byD. citrifromCLas-infected leaves. These findings reveal NCR peptides as a new class and source of biopesticide molecules to controlCLas for the prevention and/or treatment of HLB.