2013 Annual Report
2) Identify molecular and biological mechanisms of citrus HLB and control of the disease
Based on the genome sequence of Candidatus Liberibacter asiaticus (Las) that we obtained, we designed new primers and probes that target various regions of the bacterial genome. Using these newly developed primers and probes along with standardized protocols, we revealed the genetic diversity of Las bacteria collected from Florida and China. Eight different types (designated as A to H) of Las were identified based on their prophage sequence variations. Sequence analysis of these 8 types of sequences revealed the variations in this region may result from the frequent recombination and reassortment. Typing results using primers specific to the different types of sequences indicated that Type A and B were two major groups that located in FP1 and FP2, respectively. Las-infected periwinkle and dodder contained all types of Las populations; while psyllids only contained type A, B, C and E, and very low titer to none of D, F, G, and H. Las-infected citrus plants contained various types of Las populations, and only the type D population was associated with huanglongbing (HLB) symptoms: high titer of D with typical blotchy mottle, extreme low to none of D with vein yellowing or other atypical HLB symptoms. Similar results were obtained from global samples, indicating the dramatic variations of these prophage regions among the isolates both from Florida and China. These intraspecies variations derived from the prophage activities may be important for the bacterial adaptation to their host plants and insects.