Validation of reference genes for quantifying changes in gene expression in virus-infected tobacco.

To facilitate quantification of gene expression changes in virus-infected tobacco plants, eight housekeeping genes were evaluated for their stability of expression during infection by one of three systemically-infecting viruses (cucumber mosaic virus, potato virus X, potato virus Y) or a hypersensitive-response-inducing virus (tobacco mosaic virus; TMV) limited to the inoculated leaf. Five reference-gene validation programs were used to establish the order of the most stable genes for the systemically-infecting viruses as ribosomal protein L25 > β-Tubulin > Actin, and the least stable genes Ubiquitin-conjugating enzyme (UCE) < PP2A < GAPDH. For local infection by TMV, the most stable genes were EF1α > Cysteine protease > Actin, and the least stable genes were GAPDH < PP2A < UCE. Using two of the most stable and the two least stable validated reference genes, three defense responsive genes were examined to compare their relative changes in gene expression caused by each virus.