Genetic determinants of lettuce resistance to drop caused by Sclerotinia minor identified through genome-wide association mapping frequently co-locate with loci regulating anthocyanin content

Authors: Simko, Ivan; Sthapit Kandel, Jinita; PENG, HUI - University Of California; Zhao, Rebecca; SUBBARAO, KRISHNA - University Of California

Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2023
Publication Date: 8/7/2023
Citation: Simko, I., Sthapit Kandel, J., Peng, H., Zhao, R.B., Subbarao, K.V. 2023. Genetic determinants of lettuce resistance to drop caused by Sclerotinia minor identified through genome-wide association mapping frequently co-locate with loci regulating anthocyanin content. Theoretical and Applied Genetics. https://doi.org/10.1007/s00122-023-04421-y.
DOI: https://doi.org/10.1007/s00122-023-04421-y

Interpretive Summary: Lettuce is the most economically important leafy vegetable grown primarily in moderate climates around the world. Extensive, year-round cultivation of lettuce in the Salinas Valley of California gives rise to a high disease pressure by many pathogens, including Sclerotinia minor (Jagger), a soilborne fungus causing lettuce drop disease. Lettuce drop, also known as Sclerotinia rot occurs in both greenhouse and field lettuce production systems worldwide especially in cool, moist regions. The fungus produces survival structures called sclerotia that remain viable in the soil environment for at least seven years. We conducted five experiments over four years with a diverse set of nearly 500 lettuce accessions to evaluate their response to the pathogen in an artificially infested field. The lowest disease incidence (DI) was observed in cultivars Eruption, Infantry, and Annapolis (median DI of 12.1 - 17.5%), while the highest DI was recorded for cultivars Reine des Glaces, Wayahead, and line FL. 43007 (median DI of 81.0 - 95.2%). Overall, significantly lower DI was observed in red and dark red accessions compared to those with those with reduced content of red pigments known as anthocyanins . The frequent co-location of resistance loci with those for red leaf color, together with the lower DI observed in red and dark red accessions implies a causal relationship between anthocyanin content and resistance to S. minor. Resistance loci associated with anthocyanin content can be used to develop lettuce with a relatively high partial resistance and red color, while those not associated with anthocyanins can be used to develop partially resistant cultivars of green color.

Technical Abstract: Lettuce (Lactuca sativa L.), one of the most economically important vegetables grown primarily in moderate climates around the world, is susceptible to many diseases including lettuce drop caused by the soilborne fungus Sclerotinia minor. Complete resistance to S. minor has not been identified in cultivated lettuce nor its wild relatives. We conducted five experiments over four years with a diverse set of nearly 500 lettuce accessions to evaluate their response to the pathogen in an artificially infested field. The lowest disease incidence (DI) was observed in cultivars Eruption, Infantry, and Annapolis (median DI of 12.1 - 17.5%), while the highest DI was recorded for cultivars Reine des Glaces, Wayahead, and line FL. 43007 (median DI of 81.0 - 95.2%). Overall, significantly lower DI was observed in red and dark red accessions compared to those with a lower anthocyanin content. Genome-wide association mapping identified 17 QTLs for resistance to S. minor, 12 for the presence of red leaf color or its variations caused by the anthocyanin content, and one for the green color intensity. Eight of the QTLs for disease resistance and red color co-located. The frequent co-location of QTLs, together with the lower DI observed in red and dark red accessions implies a causal relationship between anthocyanin content and resistance to S. minor. Resistance QTLs associated with anthocyanin content can be used to develop lettuce with a relatively high partial resistance and red color, while those not associated with anthocyanins can be used to develop partially resistant cultivars of green color.