Comparison of Necrotrophic Behaviour and Electrolyte Leakage of Resistant and Tolerant Pear (Pyrus communis L.) Cultivars to Mutant Strains of Causal Agent of Fire Blight Disease under In Vitro Conditions

Abstract

Response of pear cultivars to causal agent of fire blight diseases is expressed by both resistance and tolerance, however the difference of the two is not clear. To better understand the mechanisms of resistance and tolerance of pear cultivars to the disease, the necrotrophic behaviour and electrolyte leakage of resistant (Dargazi) and tolerant (Harrow Sweet) cultivars was compared with susceptible cultivar (Barttlet) after inoculation by wild type and mutant strains (hrpN-, dspA/E- and hrpW-) of the bacterium. The appearance of symptoms in reaction to wild type strains occurred after three days in susceptible and tolerant cultivars, and in seven days in resistant cultivar. However, in resistant cultivar, Dargazi, disease progress slowed down. In all cultivars, one to two days before necrosis, electrolyte leakage increased to ~70%, and with the completion of necrosis it reached to 100%. The mutant strain hrpW- did not affect incidence of necrosis and electrolyte leakage, indicating no effect of this protein on resistance mechanisms. Using strain hrpN- increased the necrosis and electrolyte leakage in resistant cultivar and delayed these indices in tolerant cultivar in comparison with susceptible cultivar. Also, dspA/E- strain accelerated necrosis and electrolyte leakage in resistant cultivar in comparison with tolerant cultivar, and had no symptoms on susceptible cultivar. Considering the dual role of HrpN in pathogencity and stimulation of host defense system, the results indicated key role of HrpN protein on acquired defense system (ADS) in relation to the pathogenic role in the resistant cultivar (Dargazi) and its inverse relationship in tolerant cultivar (Harrow Sweet). This dual behaviour can be used as a tool for deeper understanding of the fire blight resistance mechanisms in pear, and also as an index for distinctness of resistant and tolerant pear cultivars.