GENE EXPRESSION IN THE QUARANTINE PEST ERWINIA AMYLOVORA DURING APPLE FLOWER-INFECTION

The bacterial plant disease fire blight is known in Europe since the 1950s, still spreading worldwide and classified as quarantine pest disease in many European countries. Especially in apple or pear orchards, fire blight can result in high economic losses. The main infection path for the fire blight pathogen Erwinia amylovora is through the floral nectaries upon transfer by insects to the flower’s pistil. Although hrp genes, which encode a type III secretion system, were identified as important for the bacterial infection, we know little about their regulation in the early bacteria-plant-interaction. To describe the bacterial infection progress in flowers, the expression of hrpL and hrpA was monitored applying a new qPCR protocol. Single flowers were harvested 6, 24, 48 and 72 h post E. amylovora inoculation from three apple trees ‘Golden Delicious’. Expression of hrpL and hrpA correlated well for single flowers. Interestingly, hrp expression levels between flowers varied, which might reflect different infection efficiencies as observed also in natural infections. In summary, a fast transcriptional induction of hrp expression was observed within 48 hours. Thereafter, hrpL and hrpA expression declined, which correlates well with the onset of transcriptional plant response. This indicates that E. amylovora outcompetes plant defense in time during natural flower infections.