Stelinski, L. L. | Search bioRxiv

bioRxiv2023

Partial elimination of Wolbachia in the Asian citrus psyllid, Diaphorina citri, increases phytopathogen acquisition and decreases fitness.

Wolbachia is a maternally inherited intracellular bacterium that infects a wide range of arthropods. Wolbachia can have a significant impact on host biology and development, often due to its effects on reproduction. We investigated Wolbachia infection in the Asian citrus psyllid, Diaphorina citri, which transmits Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening disease. D. citri are naturally infected with Wolbachia; therefore, investigating Wolbachia-mediated effects on D. citri fitness and CLas transmission required artificial elimination of this endosymbiont with application of doxycycline. Doxycycline treatment of psyllids reduced Wolbachia infection by approximately 60% in both male and female D. citri; however, this reduction varied between generations of treated psyllids. Psyllids treated with doxycycline exhibited higher CLas acquisition as both adults and nymphs as compared with negative controls. In addition, doxycycline-treated psyllids exhibited decreased fitness as measured by reduced egg and nymph production as well as adult emergence as compared with controls lines where Wolbachia was not manipulated. Our results indicate that Wolbachia benefits D. citri by improving fitness and potentially competes with CLas by interfering with phytopathogen acquisition. Targeted manipulation of endosymbionts in this phytopathogen vector may yield disease management tools. Importance of the studyThis study provides insights into the critical role of Wolbachia in the Asian citrus psyllid, Diaphorina citri, a known vector of the presumed causal agent of citrus greening disease. Our data indicate a beneficial role of Wolbachia in D. citri, whereby the bacteria greatly enhance overall fitness and affect sex ratio of the host but interfere with acquisition of the phytopathogen, Candidatus Liberibacter asiaticus (CLas). By artificially eliminating Wolbachia from psyllids, the study confirms its endosymbiotic role and opens potential avenues for development of disease control methods. Specifically, our data suggest that targeted manipulation of insect endosymbionts like Wolbachia may potentially contribute new tools for management of this economically devastating and globally widespread disease of citrus.

microbiology↗

bioRxiv2020

Salicylic acid mediated immune response of Citrus Sinensis to varying frequencies of herbivory and pathogen inoculation

Plant immunity against pathogens and pests is comprised of complex mechanisms orchestrated by signaling pathways regulated by plant hormones [Salicylic acid (SA) and Jasmonic acid (JA)]. Investigations of plant immune response to phytopathogens and phloem-feeders have revealed that SA plays a critical role in reprogramming of the activity and/or localization of transcriptional regulators via post-translational modifications. We explored the contributing effects of herbivory by a phytopathogen vector [Asian citrus psyllid, Diaphorina citri] and pathogen [Candidatus Liberibacter asiaticus (CLas)] infection on response of sweet orange [Citrus sinensis (L.) Osbeck] using manipulative treatments designed to mimic the types of infestations/infections that citrus growers experience when cultivating citrus in the face of Huanglongbing (HLB) disease. A one-time (7 d) inoculation access period with CLas-infected vectors caused SA-associated upregulation of PR-1, stimulating defense response after a long period of infection without herbivory (270 and 330 days). In contrast, while repeated (monthly) pulses of 7 d psyllid feeding injury stimulated immunity in CLas-infected citrus by increasing [SA] in leaves initially (up to 120 d), long-term (270 and 330 days) repeated herbivory caused [SA] to decrease coincident with upregulation of genes associated with SA metabolism (BMST and DMR6). Similarly, transcriptional responses and metabolite (SA and its analytes) accumulation in citrus exposed to a continuously reproducing population of D. citri exhibited a transitory upregulation of genes associated with SA signaling at 120 days and a posterior downregulation after long-term psyllid (adults and nymphs) feeding (270 and 330 days). Herbivory played an important role in regulation of SA accumulation in mature leaves of C. sinensis, whether or not those trees were coincidentally infected with CLas. Our results indicate that prevention of feeding injury inflicted by D. citri from the tritrophic interaction may allow citrus plants to better cope with the consequences of CLas infection, highlighting the importance of vector suppression as a component of managing this cosmopolitan disease. Author SummaryWe explored tritrophic interactions among an insect vector (Diaphorina citri) - phytopathogen (Candidatus Liberibacter asiaticus) - and cultivated fruit crop [sweet orange, Citrus sinensis (L) Osbeck]. Transcriptional and metabolic responses of plants were analyzed over an extended time-course of disease progression after various frequencies of herbivore feeding and durations of pathogen infection using manipulative treatments designed to mimic the types of infestations/infections that citrus growers experience when cultivating citrus in the presence of the devastating citrus disease, huanglongbing. We found that in the absence of coincident psyllid feeding damage, citrus trees could mount a defense response against the pathogen by activating the salicylic acid (SA) pathway. Repeated, monthly pulses of herbivory led to pronounced stimulation of SA transcription that was coincident with diminished pathogen titers in plants. Although insect injury initially activated SA-dependent defense responses, continuous and/or long-term ([≥] 270 d) herbivory shut down PR-1-dependent defense responses against the pathogen. Our results provide a mechanism explaining how vector suppression contributes to maintaining health of cultivated citrus in areas where huanglongbing is endemic. Our results also point to specific gene targets that may yield novel genotypes expressing tolerance against CLas after appropriate manipulations.

plant biology↗