Effects of Agrobacterium tumefaciens-Mediated Transformation with the Hygromycin Resistance Gene Hpt on the Tobacco Transcriptome

Positive selectable markers are genes that encode a detectable trait and allow the growth of transformed tissue. They are broadly used in transgenic technologies as part of the cassette delivered to the plant cell. Because of low transformation frequencies, they are necessary to promote regeneration from transformed over untransformed cells. Although the intended purpose of a selectable marker gene is to enable selection and recovery of transgenic tissue, unintended consequences may also occur due to pleiotropic and/or position effects as well as the mode of DNA delivery. Thus, an understanding of the unintended effects becomes relevant, as they risk being mistaken for phenotypes associated with the co-transformed transgenes being studied. The knowledge gained from this study will enable the development of strategies to minimize those effects. To date, no studies have addressed the unintended effects of expressing the hygromycin B resistance gene (hpt). We studied the transcriptome of transgenic tobacco plants, carrying the hpt gene, generated with Agrobacterium-mediated transformation.  Transgenic hpt plants do not have any observable phenotype other than hygromycin resistance.  However, compared to a wild-type control, over three hundred genes are modulated.  The largest number of down-regulated genes control photosynthesis and cell-wall metabolism, whereas a number of transcription factors, protein degradation and vesicular trafficking genes are up-regulated. Twelve genes known to be related to drug metabolism and detoxification are also up-regulated. This study provides the first description of the inadverdent effects that a transgenic hpt gene, delivered by Agrobacterium tumefaciens, has on the tobacco transcriptome.