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The transcriptome of roots of sweet orange tree with symptoms of citrus blight

John Hartung: USDA ARS MPPL

<div>Citrus blight is a very important progressive decline disease of commercial citrus in Florida. Diagnosis is made by demonstrating physical blockage of xylem cells that prevents the movement of water through affected portions of the trunk. The etiology of the disease has not been established, although the disease can be transmitted by root grafts, suggesting a viral etiology. We used a direct water uptake assay in citrus trunks to identify symptomatic trees from 4 commercial groves in Florida. Scaffold roots were taken from trees that failed to take up water and total RNA was extracted from phloem-enriched root tissues. Seven RNA extracts were used as templates for 100 bp paired-end RNASeq with the Illumina system. The overall pattern of transcription in the seven libraries was very similar and about 200 transcripts (6%) were up or down regulated in all seven libraries. These transcripts were limited to pathways that included modifications to cell walls, minor carbohydrate metabolism, lipid metabolism, secondary metabolism (notably terpenes) and mitochondrial electron transport. An analysis of gene ontology showed large alterations to metabolic and cellular processes, biological regulation and cellular component organization. Trehalose metabolism was altered, consistent with a defense response to dehydration or other cellular stress. Identification of specific metabolic pathways altered by citrus blight disease may lead to improved management of the disease.</div>