A glucocorticoid-inducible gene expression system can cause growth defects in tobacco

We find that an expression system widely used to chemically induce transgenes of interest in tobacco (Nicotiana tabacum Petit Havana SR1) can cause severe growth defects in this species. This gene expression system has been shown to cause non-specific effects (including growth retardation) in other plant species, but has until now been largely accepted to be a relatively problem-free system for use in tobacco. The expression system is based on the ability of the glucocorticoid dexamethasone (DEX) to activate a non-plant chimeric transcription factor (GVG), which then activates expression of a transgene of interest. The aberrant growth phenotype only manifests itself after DEX application and only occurs in plants in which the constitutive levels of GVG expression are higher than average. We found that similar to 30% of all transgenic plants produced showed some level of growth retardation under our standard growth conditions. However, by modulating irradiance levels following DEX application, we also showed that the manifestation and severity of the aberrant phenotype is highly dependent upon growth conditions, highlighting that such conditions are a critical parameter to consider during all stages of using this gene expression system. We also identified an increase in ACC oxidase gene expression as an early, sensitive and robust molecular marker for the aberrant phenotype. This molecular marker should be valuable to investigators wishing to readily identify transgenic plants in which GVG expression levels are beyond a threshold that begins to produce non-specific effects of the gene expression system under a defined set of growth conditions.