Abstract
Citrus tristeza is one of the most destructive citrus diseases and is caused by the phloem-restricted Closterovirus, Citrus tristeza virus. Mild strain CTV-B2 does not cause obvious symptoms on indicators whereas severe strain CTV-B6 causes symptoms, including stem pitting, cupping, yellowing, and stiffening of leaves, and vein corking. Our laboratory has previously characterized changes in transcription in sweet orange separately infected with CTV-B2 and CTV-B6. In the present study, transcriptome analysis of Citrus sinensis in response to double infection by CTV-B2 and CTV-B6 was carried out. Four hundred and eleven transcripts were up-regulated and 356 transcripts were down-regulated prior to the onset of symptoms. Repressed genes were overwhelmingly associated with photosynthesis, and carbon and nucleic acid metabolism. Expression of genes related to the glycolytic, oxidative pentose phosphate (OPP), tricarboxylic acid cycle (TCA) pathways, tetrapyrrole synthesis, redox homeostasis, nucleotide metabolism, protein synthesis and post translational protein modification and folding, and cell organization were all reduced. Ribosomal composition was also greatly altered in response to infection by CTV-B2/CTV-B6. Genes that were induced were related to cell wall structure, secondary and hormone metabolism, responses to biotic stress, regulation of transcription, signaling, and secondary metabolism. Transport systems dedicated to metal ions were especially disturbed and ZIPs (Zinc Transporter Precursors) showed different expression patterns in response to co-infection by CTV-B2/CTV-B6 and single infection by CTV-B2. Host plants experienced root decline that may have contributed to Zn, Fe, and other nutrient deficiencies. Though defense responses, such as, strengthening of the cell wall, alteration of hormone metabolism, secondary metabolites, and signaling pathways, were activated, these defense responses did not suppress the spread of the pathogens and the development of symptoms. The mild strain CTV-B2 did not provide a useful level of cross-protection to citrus against the severe strain CTV-B6.
Highlights
Citrus tristeza has historically been one of the most destructive and globally distributed citrus diseases and is still responsible for tremendous economic losses to the citrus industries worldwide (Bennett and Costa, 1949; Moreno et al, 2008)
Ten doubly-inoculated trees became positive for both mild Citrus tristeza virus (CTV)-B2 and severe CTV-B6 based on RT-PCR after 2–4 months (Figure 1)
Sequences homologous to p23 were not found in the mock-inoculated controls, nor when other libraries that contained only CTV-B2 or CTV-B6 were searched for the heterologous p23 sequence
Summary
Citrus tristeza has historically been one of the most destructive and globally distributed citrus diseases and is still responsible for tremendous economic losses to the citrus industries worldwide (Bennett and Costa, 1949; Moreno et al, 2008). The disease is caused by Citrus tristeza virus (CTV), a member of the Closteroviridae, with single-stranded and positive-sense genomic RNA (gRNA) of about ∼19 kb in size (Karasev et al, 1995; Moreno et al, 2008). CTV is phloem-restricted and long-distance spread of the virus is via movement of infected plants or propagation of infected buds. It is transmitted locally by several aphid species in a semi-persistent mode, most notably by Toxoptera citricida (Kirkaldy). Strains of CTV can be classified as mild to severe based on the type and intensity of symptoms caused in different citrus hosts. It is interesting that complete symptoms of Tristeza disease were observed in transgenic Mexican lime plants that expressed the product of CTV ORF 11, an RNA binding protein designated p23 (Ghorbel et al, 2001)
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