Phenotypic and genetic analyses of yellow spot malady in lettuce

Authors: PENG, HUI - University Of California; Zhao, Rebecca; SMITH, RICHARD - University Of California; Simko, Ivan

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2022
Publication Date: 8/23/2022
Citation: Peng, H., Zhao, R.B., Smith, R., Simko, I. 2022. Phenotypic and genetic analyses of yellow spot malady in lettuce. Scientia Horticulturae. 305. Article 111389. https://doi.org/10.1016/j.scienta.2022.111389.
DOI: https://doi.org/10.1016/j.scienta.2022.111389

Interpretive Summary: In recent years, an unknown malady was observed on lettuce plants grown both in a greenhouse and field. The major symptoms are manifested as individual or linked yellow spots (YS) that usually appear on an upper part of the adaxial surface of leaves when plants near the harvest maturity (unpublished data by Smith, Peng, and Simko). The intensity of yellow spots progresses from light yellow to more intensive yellow that eventually turns into leaf necrosis. When YS occurs at low severity, it may have only a negligible effect on yield or the product quality. However, the malady may cause more substantial problems at extremally high severity when the symptoms extend to the core thus affecting the production of whole heads, lettuce hearts, and fresh-cut lettuce (unpublished data by Peng and Simko). The YS affected area displayed decreased chlorophyll fluorescence and photosynthesis that were not caused by the effect fungi, bacteria, viruses, insects, or foliar nematodes. Among 31 tested chemical elements, four macronutrients (P, Ca, Mg, S) and eight microminerals (B, Cd, Cu, Li, Mn, Ni, Si, and Sn) had significantly changed concentration in the YS affected leaves. The trait exhibited a relatively high broad-sense inheritability (H2 = 47.9% ~ 68.4%). The genome-wide association study (GWAS) involving 479 accessions revealed the effect of eight loci. A total of 162 genes were predicted in the identified chromosomal regions, with four oxidation-reduction (redox) catalyzers being potentially the most promising candidate genes involved in the regulation of this physiological disorder.

Technical Abstract: Lettuce (Lactuca sativa L.) is an important leafy vegetable crop grown worldwide. An unknown malady that causes Yellow Spot (YS) blemishes on leaves has recently appeared in commercial fields in California, potentially reducing lettuce yield and/or quality. YS occurred on all morphological types of cultivated lettuce regardless of their color, with the highest (1.08) and lowest (0.09) rate found in stem and leaf lettuce, respectively. The spots ranging in diameter from 0.5 to 3.1 mm (depending on genotype) were mostly observed on the adaxial leaf surface of mature or nearly mature plants. YS symptoms began as light-yellow circular spots that later became more intensely yellow and eventually turned necrotic. The YS affected area displayed decreased chlorophyll fluorescence and photosynthesis that were not caused by the effect fungi, bacteria, viruses, insects, or foliar nematodes. Among 31 tested chemical elements, four macronutrients (P, Ca, Mg, S) and eight microminerals (B, Cd, Cu, Li, Mn, Ni, Si, and Sn) had significantly changed concentration in the YS affected leaves. The trait exhibited a relatively high broad-sense inheritability (H2 = 47.9% ~ 68.4%) in four experiments conducted at two locations in three years. The genome-wide association study (GWAS) involving 479 accessions revealed the effect of eight loci (qYS2.1, qYS3.1, qYS3.2, qYS4.1, qYS5.1, qYS5.2, qYS6.1, and qYS7.1) defined by 10 significant SNP markers. A total of 162 genes were predicted in the identified chromosomal regions, with four oxidation-reduction (redox) catalyzers being potentially the most promising candidate genes involved in the regulation of this physiological disorder.