The fire blight resistance QTL on LG7 of Malus ×robusta 5 is not dependent on the avrRpt2EA 156 S/C amino acid switch

Authors: EMERIEWEN, OFERE - Julius Kuhn Institute; REIM, STEGANIE - Julius Kuhn Institute; RICHTER, KLAUS - Julius Kuhn Institute; WOHNER, THOMAS - Julius Kuhn Institute; FLACHOWSKY, HENRYK - Julius Kuhn Institute; PEIL, ANDREAS - Julius Kuhn Institute; ALDWINCKLE, HERB - Cornell University; Fazio, Gennaro

Submitted to: Journal of Plant Pathology for Germany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2023
Publication Date: 8/7/2023
Citation: Emeriewen, O.F., Reim, S., Richter, K., Wohner, T., Flachowsky, H., Peil, A., Aldwinckle, H., Fazio, G. 2023. The fire blight resistance QTL on LG7 of Malus ×robusta 5 is not dependent on the avrRpt2EA 156 S/C amino acid switch. Journal of Plant Pathology for Germany. https://doi.org/10.1007/s42161-023-01458-2.
DOI: https://doi.org/10.1007/s42161-023-01458-2

Interpretive Summary: One of the sources for genetic resistance to fire blight caused by Erwinia amylovora (a devastating bacterial disease that attacks apples) is found in the wild relative Malus ×robusta 5. This resistance seems to be dependent on the strain of bacteria causing the infection with some strains overcoming such resistance. As such, this crabapple has been mated with susceptible apple cultivars and rootstocks. The progeny was inoculated with several E. amylovora strains in order to study the genetics and mechanism of fire blight resistance and susceptibility. Two genetic factors have been discovered, one on chromosome 3 of apple which seems to be strain specific and another one chromosome 7. Here, we report the detection of the chromosome 7 genetic factor in different ‘Malling 9’ × M. ×robusta 5 population in Germany, and confirm the its independence E. amylovora strains. This finding is important because broad genetic resistance that is not strain specific should be more durable and effective.

Technical Abstract: Malus ×robusta 5, which has been the subject of extensive fire blight resistance research over time, could be highly resistant or susceptible to the fire blight-causative bacterial pathogen, Erwinia amylovora – depending on the strain. As such, this crabapple has been crossed with susceptible apple cultivars and rootstocks, and inoculated with several E. amylovora strains in order to study the genetics and mechanism of its fire blight resistance and susceptibility. A strong resistance QTL, which was first mapped on linkage group 3 (LG3) of M. ×robusta 5 using an F1 progeny derived from a cross with the apple cultivar ‘Idared’ in Germany, was confirmed in two other F1 populations derived from crossing M. ×robusta 5 with the rootstock ‘Malling 9’ in New Zealand, and with ‘Ottawa 3’ in the USA. A second QTL on LG7 was detected in the ‘Idared’ × M. ×robusta 5 population but only with strains that break the LG3 QTL, in contrast to the US population with ‘Ottawa 3’ as another parent, where the LG7 QTL was detected regardless of strain-specificity. This QTL was not identified in the New Zealand population with ‘Malling 9’ as another parent. Here, we report the detection of the LG7 QTL in different ‘Malling 9’ × M. ×robusta 5 population in Germany, and confirm the independence of the LG7 locus to E. amylovora strains.