Overexpression of TaUb2 enhances disease resistance to Pseudomonas syringae pv. tomato DC3000 in tobacco

Ubiquitin (Ub), a 76-amino acid protein, functions as a tag that can be attached to substrates for selected degradation in a Ub/26S proteasome system (UPS). In this paper, effects of the Ub gene TaUb2 from wheat expressed in tobacco plants are reported. Three TaUb2-overexpressing transgenic tobacco lines were used to study the functions of Ub in response to Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000). After infection with Pst DC3000, wild-type (WT) plants displayed more severe disease symptoms than transgenic plants. We observed a smaller stomatal aperture, slower water loss and less leaf wilt in transgenic plants after Pst DC3000 infection compared to WT plants. After Pst DC3000 treatment, electrolyte leakage and malondialdehyde levels were lower and were consistent with the higher antioxidant enzyme activities of superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) that were observed in the transgenic plants compared with the WT plants; however, there was no significant difference in these parameters between the transgenic plants and WT before Pst DC3000 treatment. At the same time, a hypersensitive response (HR) was also observed in transgenic plants when infected by Pst DC3000, which may be induced by a reactive oxygen species (ROS) burst. Real-time RT-PCR analysis revealed that the transcript expressions of some pathogenesis-related genes (PR) in transgenic plants were obviously higher than those in WT plants after Pst DC3000 treatment. These results suggest that Ub is involved in plant disease resistance. Stomatal aperture, antioxidant capacity and ROS-burst-induced disease resistance may be important mechanisms underlying enhanced disease resistance in transgenic tobacco with overexpression of TaUb2.