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Specific In Planta Recognition of Two GKLR Proteins of the Downy Mildew Bremia lactucae Revealed in a Large Effector Screen in Lettuce

November 2013 , Volume 26 , Number  11
Pages  1,259 - 1,270

Joost H. M. Stassen,1 Erik den Boer,2 Pim W. J. Vergeer,1 Annemiek Andel,1 Ursula Ellendorff,3 Koen Pelgrom,2 Mathieu Pel,4 Johan Schut,3 Olaf Zonneveld,5 Marieke J. W. Jeuken,2 and Guido Van den Ackerveken1

1Plant-Microbe Interactions, Department of Biology, Utrecht University, Utrecht, 3584 CH, The Netherlands; 2Laboratory of Plant Breeding, Wageningen University, Wageningen, 6700 AJ, The Netherlands; 3Rijk Zwaan, De Lier, 2678 ZG, The Netherlands; 4Enza zaden, Enkhuizen, 1600 AA, The Netherlands; 5Syngenta, Enkhuizen, 1601 BK, The Netherlands

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Accepted 12 July 2013.

Breeding lettuce (Lactuca sativa) for resistance to the downy mildew pathogen Bremia lactucae is mainly achieved by introgression of dominant downy mildew resistance (Dm) genes. New Bremia races quickly render Dm genes ineffective, possibly by mutation of recognized host-translocated effectors or by suppression of effector-triggered immunity. We have previously identified 34 potential RXLR(-like) effector proteins of B. lactucae that were here tested for specific recognition within a collection of 129 B. lactucae-resistant Lactuca lines. Two effectors triggered a hypersensitive response: BLG01 in 52 lines, predominantly L. saligna, and BLG03 in two L. sativa lines containing Dm2 resistance. The N-terminal sequences of BLG01 and BLG03, containing the signal peptide and GKLR variant of the RXLR translocation motif, are not required for in planta recognition but function in effector delivery. The locus responsible for BLG01 recognition maps to the bottom of lettuce chromosome 9, whereas recognition of BLG03 maps in the RGC2 cluster on chromosome 2. Lactuca lines that recognize the BLG effectors are not resistant to Bremia isolate Bl:24 that expresses both BLG genes, suggesting that Bl:24 can suppress the triggered immune responses. In contrast, lettuce segregants displaying Dm2-mediated resistance to Bremia isolate Bl:5 are responsive to BLG03, suggesting that BLG03 is a candidate Avr2 protein.

© 2013 The American Phytopathological Society