Integrative path modeling and QTL mapping identify maturity, stem strength, and cell wall composition driving lettuce resistance to Sclerotinia minor

Authors: Simko, Ivan; MAMO, BULLO ERENA - University Of California; CANTU, SHANE - Michigan State University; PENG, HUI - University Of California; GRUBE SIDEMAN, REBECCA - University Of New Hampshire; Hayes, Ryan; SUBBARAO, KRISHNA - University Of California

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2025
Publication Date: 6/5/2025
Citation: Simko, I., Mamo, B., Cantu, S.L., Peng, H., Grube Sideman, R., Hayes, R.J., Subbarao, K.V. 2025. Integrative path modeling and QTL mapping identify maturity, stem strength, and cell wall composition driving lettuce resistance to Sclerotinia minor. Scientific Reports. 15. Article 19824. https://doi.org/10.1038/s41598-025-03775-1.
DOI: https://doi.org/10.1038/s41598-025-03775-1

Interpretive Summary: Lettuce is vulnerable to a disease called ‘lettuce drop’, which causes plants to collapse rapidly and is a major issue for growers. This disease, caused by a soil-borne fungus, is difficult to control and often returns year after year. While some lettuce plants show partial resistance, fully resistant varieties are not yet available. This study explored how certain traits, like the timing of plant maturity, stem firmness, and the composition of plant cell walls, affect lettuce resistance to this disease. By analyzing these traits in different lettuce cultivars, it was found that plants that matured earlier and had stronger stems were more resistant to lettuce drop. Four key genetic regions tied to resistance and stem strength were identified. Importantly, the study revealed that resistance could be enhanced in plants that do not mature too early, opening new possibilities for breeding lettuce with improved resistance to the disease while maintaining desired maturity levels.

Technical Abstract: Lettuce (Lactuca sativa) is highly vulnerable to Sclerotinia minor, the pathogen causing lettuce drop. Breeding for resistance is the most effective control strategy; however, full resistance has not been achieved, and current resistant sources are often linked with undesirable traits, such as early bolting. This study aimed to unravel the genetic basis of partial resistance to S. minor and its relationship with plant maturity (bolting), stem mechanical strength (SMS), and cell wall composition (CWC) using a recombinant inbred line (RIL) population derived from a cross between the susceptible iceberg cv. ‘Salinas’ and the resistant oil-seed accession PI 251246. Field evaluations indicated that resistance was linked to earlier bolting, stronger stems, and higher pentose content. Path analysis demonstrated that earlier-maturing plants exhibited increased resistance through enhanced SMS and modified CWC, particularly with higher xylose and lower arabinose levels. Further analysis indicated a significant relationship between syringyl lignin content and resistance, especially in plants with varying bolting responses. Three key quantitative trait loci (QTLs) on linkage groups (LG) 2, 6, and 7 were consistently associated with resistance, bolting, and SMS. Importantly, residual QTL analysis revealed that the resistance locus on LG7 acted independently of maturity, suggesting a distinct resistance mechanism. Callose synthase emerged as a key candidate gene within the LG7 resistance QTL, located near - but distinct from - genes associated with plant maturity and flowering. These findings provide valuable insights into decoupling resistance from early bolting, suggesting a pathway for breeding lettuce cultivars with improved disease resistance and delayed bolting.