Modulation of tobacco bacterial disease resistance using cytosolic ascorbate peroxidase and Cu,Zn-superoxide dismutase

The effect of over-expression in tobacco plants of cytosolic Cu,Zn-superoxide dismutase (cytsod) and ascorbate peroxidase (cytapx) alone, or in combination, against bacterial wildfire and crown gall diseases, caused by Pseudomonas syringae pv. tabaci and Agrobacterium tumefaciens, respectively, was investigated. Disease tolerance was observed in all the transgenic lines against the two causal agents, with various levels of resistance, with the double transformants (lines 35 and 39) the most resistant against bacterial wild fire. In the case of P. syringae pv. tabaci, disease tolerance and symptom decrease was associated with a lower bacterial population and a higher level of several antioxidant defence enzymes. Transgenic lines also exhibited an enhanced tolerance against A. tumefaciens, with the transgenic line harbouring cytapx (line 51) the most resistant to crown gall disease. However, this was only observed with strain C58 among the three pathogenic strains tested. These results suggest that cytosolic antioxidant defences have a role in increasing tolerance to the oxidative stress caused by some bacterial pathogens, and resistance of these tobacco lines to wildfire disease seems to be independent of tissue necrosis.