The effector protein AvrRpt2EA from Erwinia amylovora induces salicylic acid dependent response in fire blight susceptible apples

Abstract

Fire blight disease, caused by the bacterium Erwinia amylovora, has developed to an economical important disease in cultivation of pome fruits in many regions of the world. The bacterial cysteine protease AvrRpt2EA was identified as a central molecule in the host-pathogen interaction and it is important for pathogen recognition in the fire blight resistant crabapple Malus ×robusta 5. However, little is known about its role as virulence factor in susceptible apples. To investigate its function in planta, transgenic lines of the fire blight-susceptible cultivar ‘Pinova’ were generated, which contain an plant-optimized version of AvrRpt2EA driven by a heat shock-inducible promoter. After induced expression of AvrRpt2EA, the transgenic lines showed symptoms similar to natural fire blight infections, such as shoot necrosis and browning of older leaves. Furthermore, an increase of the expression of the PR-1 gene was shown, which was used as molecular marker for salicylic acid (SA) dependent systemic acquired resistance (SAR). Additional analysis reveal that the levels of SA and its derivatives were increased after AvrRpt2EA expression, too, with diverse kinetics in leaves of different ages. In contrast, no induction of the expression level of VSP2 paralogs was found, which were used as marker genes for the activation of the jasmonic acid (JA)-dependent defense pathway. This was also confirmed by metabolic profiling of JA and its derivatives. In conclusion, the results of this study show that AvrRpt2EA alone acts as virulence factor causing fire blight disease symptoms in susceptible apple plants and induces the formation of SA and SA-dependent SAR.