José A. Hernández Cortés, Senior Researcher (CEBAS-CSIC, Biotechnology and Fruit Trees Group, Murcia)
The effect of over-expression of cytosolic Cu,Zn-superoxide dismutase (cytsod) and ascorbate peroxidase (cytapx) alone, or in combination, in tobacco plants, increased the resistance to bacterial wild fire, caused by Pseudomonas syringae pv. Tabaci.
In the non-transformed controls, the inoculated areas with P. syringae pv tabaci became initially chlorotic at 2 days post-inoculation (dpi), then subsequently water soaked and necrotic. At 6 dpi, the necrotic areas extended to the border of the leaves and finally the necrotic area appeared surrounded by a chlorotic yellow area. In transgenic lines harboring cytsod (line 17) or cytapx (line 51) necrotic areas were confined within the infiltrated sites, mimicking symptoms of hypersensitive reaction (HR) observed during an incompatible plant-pathogen interaction. In contrast, in the transgenic lines harboring both cytsod and cytapx genes (lines 35 and 39), infected leaf tissue presented only a chlorotic area and localized necrosis occurred only occasionally (Fig. 1).
In addition, the symptoms observations correlated with reduced bacterial growth, mainly in lines showing fewer symptoms (Lines 35 and 39). The reduced necrotic lesions observed in the transgenic lines are parallel with increased antioxidant enzyme levels (SOD, APX, Catalase, and Glutathione Reductase). In this sense, the balance between SOD (O2.--scavenger and H2O2-producer enzyme) and H2O2-scavenger enzymes (APX, Catalase and Peroxidases) is crucial to control ROS accumulation and the triggering of cell death (Fig. 2). The observed resistance of transgenic lines to bacterial wild fire seemed to be independent of tissue necrosis, as observed in other plant-pathogens interactions (Kiraly et al. 1999; 2008). These authors suggested that the induction of antioxidant enzymes might reduce plant cell necrosis after infection (Kiraly et al., 2002; 2008).
The results of this work showed that the overexpression of CuZn-SOD and APX in the cytosol of tobacco plants enhances resistance against P. syringae pv tabaci, highlighting the importance of the cytosolic antioxidant machinery in the response to biotic stress, similar to the results showing the importance of this cell compartment in the response to abiotic stress, such as salinity or drought stress (Hernández et al., 2000; Faize et al., 2011).
For more information, please consult:
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