title: Candidatus Liberibacter americanus induces significant reprogramming of the transcriptome of the susceptible citrus genotype.
abstract: BACKGROUND
  Citrus huanglongbing (HLB) disease is caused by endogenous, phloem-restricted, Gram negative, uncultured bacteria named Candidatus Liberibacter africanus (CaLaf), Ca. L. asiaticus (CaLas), and Ca. L. americanus (CaLam), depending on the continent where the bacteria were first detected. The Asian citrus psyllid vector, Diaphorina citri, transmits CaLas and CaLam and both Liberibacter species are present in Brazil. Several studies of the transcriptional response of citrus plants manifesting HLB symptoms have been reported, but only for CaLas infection. This study evaluated the transcriptional reprogramming of a susceptible genotype of sweet orange challenged with CaLam, using a customized 385K microarray containing approximately 32,000 unigene transcripts. We analyzed global changes in gene expression of CaLam-infected leaves of sweet orange during the symptomatic stage of infection and compared the results with previously published microarray studies that used CaLas-infected plants. Twenty candidate genes were selected to validate the expression profiles in symptomatic and asymptomatic PCR-positive leaves infected with CaLas or CaLam.
  
  
  RESULTS
  The microarray analysis identified 633 differentially expressed genes during the symptomatic stage of CaLam infection. Among them, 418 (66%) were upregulated and 215 (34%) were down regulated. Five hundred and fourteen genes (81%) were orthologs of genes from Arabidopsis thaliana. Gene set enrichment analysis (GSEA) revealed that several of the transcripts encoded transporters associated with the endomembrane system, especially zinc transport. Among the most biologically relevant gene transcripts in GSEA were those related to signaling, metabolism and/or stimulus to hormones, genes responding to stress and pathogenesis, biosynthesis of secondary metabolites, oxidative stress and transcription factors belonging to different families. Real time PCR of 20 candidate genes validated the expression pattern of some genes in symptomatic and asymptomatic leaves infected with CaLam or CaLas.
  
  
  CONCLUSIONS
  Many gene transcripts and biological processes are significantly altered upon CaLam infection. Some of them had been identified in response to CaLas infection, while others had not been previously reported. These data will be useful for selecting target genes for genetic engineering to control HLB.
authors.affiliation: Centro de Citricultura Sylvio Moreira, Instituto Agronômico de Campinas, Cordeirópolis, São Paulo, Brazil.
authors.forename: Valéria
authors.initials: V
authors.lastname: Mafra
authors.forename: Polyana K
authors.initials: PK
authors.lastname: Martins
authors.forename: Carolina S
authors.initials: CS
authors.lastname: Francisco
authors.forename: Marcelo
authors.initials: M
authors.lastname: Ribeiro-Alves
authors.forename: Juliana
authors.initials: J
authors.lastname: Freitas-Astúa
authors.forename: Marcos A
authors.initials: MA
authors.lastname: Machado
mesh_terms: D050260:Carbohydrate Metabolism / Q000235:genetics; D032084:Citrus sinensis / Q000235:genetics / Q000378:metabolism* / Q000382:microbiology; D046228:Microarray Analysis; D018384:Oxidative Stress / Q000235:genetics; D052585:Phloem / Q000235:genetics / Q000378:metabolism; D010935:Plant Diseases / Q000235:genetics* / Q000382:microbiology; D018515:Plant Leaves / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D012230:Rhizobiaceae / Q000235:genetics / Q000472:pathogenicity* / Q000502:physiology; D014157:Transcription Factors / Q000235:genetics; D059467:Transcriptome
chemical_list: D014157:Transcription Factors