To better understand plant-pathogen interactions in Huanglongbing disease and develop control strategies we investigated a novel approach known as chemical genomics with Tomato “Psyllid Yellows”, caused by  Candidatus Liberibacter psyllaurous (CLps), as a model of HLB. Chemical genomics involves three key stages starting with designing and performing high-throughput chemical screening, identifying chemicals inducing desired effects and dissecting the genetic targets of the candidate chemical. Our study has been focused on developing a high throughput chemical screen assay using model plants such as tomato and Arabidopsis that can be infected by Candidatus Liberibacter psyllaurous (Clps). The key objective is to identify chemicals that induce plant defense against CLps infection or its transmission via psyllids. We evaluated Arabidopsis thaliana and tomato in different media such as MS sterile media and hydroponic culture, however qPCR results indicated very low and inconsistent numbers of CLps positive plants. We designed a modified detached leaf assay for tomato based on the citrus detached leaf assay (Eldesouky Ammar, USDA-ARS) that has resulted in consistently high (80-85%) number of CLps-positive leaf petioles. We are currently evaluating application of the detached leaf assay in screening chemicals using a tomato CaBP22-GUS transgenic line. This line was developed by Dr. Isgouhi Kaloshian and Dr. Thomas Eulgem (University of California, Riverside) based on a transgenic Arabidopsis reporter line with the  promoter CaBP22^-333 promoter fused to GUS (Knoth et.al. 2009). The Arabidopsis transgenic line has been used successfully in many high-throughput chemical screens to identify chemicals inducing defense responses. In our study, we will test chemical uptake and its effect on the transgenic tomato line using GUS expression by RT-PCR. To develop methods to test responses to candidate chemicals in citrus, we are testing gene expression of sweet orange seedlings following exposure to four chemicals known to induce defense responses in other plants.

Huanglongbing (HLB, citrus greening) associated with ‘Candidatus Liberibacter’ species is a widespread devastating citrus disease not previously reported in California (CA). In March 2012, ‘C. Liberibacter asiaticus’ (CLas) was detected from an Asian citrus psyllid (ACP, Diaphorina citri) sample from Los Angeles, CA at the Jerry Dimitman Laboratory of the Citrus Research Board. Subsequent citrus plant surveys within a 400m area of the CLas-positive ACP sample performed by the California Department of Food and Agriculture identified an infected multi-grafted citrus tree at a residence in Los Angeles, CA. The CLas-positive tree was removed and nucleic acids from different plant tissues (i.e. roots, trunk, stems, and leaves) were extracted and distributed to several federal, state, and university laboratories nationwide for preliminary evaluation.  Labs attempted to identify the species and graft types of the infected citrus, study the genetic characteristics and genome diversity of the detected bacterium, as well as test for other graft-transmissible citrus pathogens (GTCP). Preliminary data suggested that one type of lemon was the original rootstock that received over 20 citrus grafts. CLas DNA population analysis suggested a possible single Asian origin. Preliminary tests indicated the possible presence of other GTCP. Further evaluations on the CA CLas-positive find are ongoing.