UC Riverside

Huanglongbing (HLB) is a bacterial disease associated with Candidatus Liberibacter asiaticus (CLas), Candidatus Liberibacter americanus, and Candidatus Liberibacter africanus. These bacterial species are transmitted by hemipteran psyllids. For many years, this disease has caused substantial damage to citrus around the world and in recent years, it has significantly impacted citrus production in the U.S.A. The lack of effective control and curative measures against psyllids and the pathogen are the major factors contributing to HLB-related damages. In addition to these factors, limitations such as longer generation time, limited genomics resources and regulatory restrictions hinder rapid research on HLB. Herein, I report a novel model plant-pathosystem of Arabidopsis thaliana and Candidatus Liberibacter psyllaurous (CLps) that has potential to facilitate rapid development of control strategies for HLB disease. Based on 98% identity in 16S rDNA sequence and other genome sequence similarities, CLps is considered as a close relative to CLas. The CLps pathogen can infect solanaceous plants such as tomato, potato and pepper. To our knowledge, this is the first study demonstrating Liberibacter infection in Arabidopsis. The CLps pathogen can be transmitted by psyllid Bactericera cockerelli (BC) into Arabidopsis. Over time an increase in bacterial titer was evident in 19 genetically diverse ecotypes and defense-compromised lines of Arabidopsis. Similar to CLas, CLps systemically infects Arabidopsis. I identified several similarities between the phenotypic and transcriptional changes in CLps infected Col-0 and NahG (a salicylic acid deficient Col-0) lines and the responses in CLas infected citrus. Some unique changes such as development-related transcriptional changes associated with floral transition and a novel phenotype of excessive growth of small size axillary/cauline leaves were observed only in CLps infected Arabidopsis. A foliar spray technique for testing candidate defense-inducing chemicals with Arabidopsis/BC/CLps system and detached leaf assay with tomato/BC/ CLps system were evaluated for their effects on Liberibacter infection. The timeline of experimental assays with Arabidopsis-CLps is considerably shorter than that for HLB-citrus related assays reported previously. Our model plant-pathosystem can be used as a resource in HLB-related preliminary studies to more rapidly understand the effect of (or manipulate) potential genes and defense inducing chemicals on Liberibacter infection.