title: Quantitative proteomics identifies 38 proteins that are differentially expressed in cucumber in response to cucumber green mottle mosaic virus infection.
abstract: BACKGROUND
  Since it was first reported in 1935, Cucumber green mottle mosaic virus (CGMMV) has become a serious pathogen in a range of cucurbit crops. The virus is generally transmitted by propagation materials, and to date no effective chemical or cultural methods of control have been developed to combat its spread. The current study presents a preliminary analysis of the pathogenic mechanisms from the perspective of protein expression levels in an infected cucumber host, with the objective of elucidating the infection process and potential strategies to reduce both the economic and yield losses associated with CGMMV.
  
  
  METHODS
  Isobaric tags for relative and absolute quantitation (iTRAQ) technology coupled with liquid chromatography-tandem mass spectrometric (LC-MS/MS) were used to identify the differentially expressed proteins in cucumber plants infected with CGMMV compared with mock-inoculated plants. The functions of the proteins were deduced by functional annotation and their involvement in metabolic processes explored by KEGG pathway analysis to identify their interactions during CGMMV infection, while their in vivo expression was further verified by qPCR.
  
  
  RESULTS
  Infection by CGMMV altered both the expression level and absolute quantity of 38 proteins (fold change >0.6) in cucumber hosts. Of these, 23 were found to be up-regulated, while 15 were down-regulated. Gene ontology (GO) analysis revealed that 22 of the proteins had a combined function and were associated with molecular function (MF), biological process (BP) and cellular component (CC). Several other proteins had a dual function with 1, 7, and 2 proteins being associated with BP/CC, BP/MF, CC/MF, respectively. The remaining 3 proteins were only involved in MF. In addition, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified 18 proteins that were involved in 13 separate metabolic pathways. These pathways were subsequently merged to generate three network diagrams illustrating the interactions between the different pathways, while qPCR was used to track the changes in expression levels of the proteins identified at 3 time points during CGMMV infection. Taken together these results greatly expand our understanding of the relationships between CGMMV and cucumber hosts.
  
  
  CONCLUSIONS
  The results of the study indicate that CGMMV infection significantly changes the physiology of cucumbers, affecting the expression levels of individual proteins as well as entire metabolic pathways. The bioinformatic analysis also identified several pathogenesis-related (PR) proteins that could be useful in the development of disease-resistant plants.
authors.affiliation: Department of Plant Pathology, China Agricultural University, Beijing, 100193, PR China. huawei.liu@ars.usda.gov.|Beijing Engineering Research Centre of Seed and Plant Health (BERC-SPH), Beijing Key Laboratory of Seed Disease Testing and Control (BKL-SDTC), Beijing, 100193, PR China. huawei.liu@ars.usda.gov.|Molecular Plant Pathology Laboratory, USDA-ARS, Beltsville, MD, 20705, USA. huawei.liu@ars.usda.gov.
authors.forename: Hua-Wei
authors.initials: HW
authors.lastname: Liu
authors.affiliation: Department of Plant Pathology, China Agricultural University, Beijing, 100193, PR China. lcq19880305@126.com.|Beijing Engineering Research Centre of Seed and Plant Health (BERC-SPH), Beijing Key Laboratory of Seed Disease Testing and Control (BKL-SDTC), Beijing, 100193, PR China. lcq19880305@126.com.
authors.forename: Chao-Qiong
authors.initials: CQ
authors.lastname: Liang
authors.affiliation: Department of Plant Pathology, China Agricultural University, Beijing, 100193, PR China. pengfeiliu@cau.edu.cn.|Beijing Engineering Research Centre of Seed and Plant Health (BERC-SPH), Beijing Key Laboratory of Seed Disease Testing and Control (BKL-SDTC), Beijing, 100193, PR China. pengfeiliu@cau.edu.cn.
authors.forename: Peng-Fei
authors.initials: PF
authors.lastname: Liu
authors.affiliation: Department of Plant Pathology, China Agricultural University, Beijing, 100193, PR China. luolaixin@cau.edu.cn.|Beijing Engineering Research Centre of Seed and Plant Health (BERC-SPH), Beijing Key Laboratory of Seed Disease Testing and Control (BKL-SDTC), Beijing, 100193, PR China. luolaixin@cau.edu.cn.
authors.forename: Lai-Xin
authors.initials: LX
authors.lastname: Luo
authors.affiliation: Department of Plant Pathology, China Agricultural University, Beijing, 100193, PR China. lijq231@cau.edu.cn.|Beijing Engineering Research Centre of Seed and Plant Health (BERC-SPH), Beijing Key Laboratory of Seed Disease Testing and Control (BKL-SDTC), Beijing, 100193, PR China. lijq231@cau.edu.cn.
authors.forename: Jian-Qiang
authors.initials: JQ
authors.lastname: Li
mesh_terms: D002853:Chromatography, Liquid; D018553:Cucumis sativus / Q000502:physiology / Q000821:virology*; D054884:Host-Pathogen Interactions; D010935:Plant Diseases / Q000821:virology*; D010940:Plant Proteins / Q000032:analysis*; D020543:Proteome / Q000032:analysis*; D040901:Proteomics; D013312:Stress, Physiological; D053719:Tandem Mass Spectrometry; D017898:Tobamovirus / Q000254:growth & development*
chemical_list: D010940:Plant Proteins; D020543:Proteome