Identification of QTLs associated with metribuzin tolerance in field pea (Pisum sativum L.)

Competition with weeds is a major constraint to production of field pea in Australia, exacerbated by limited herbicide control options. Metribuzin is considered to be a safe herbicide, but may be phytotoxic to both weeds and target crop. A preliminary glasshouse-based assay was used to identify the optimal concentration required for discrimination between tolerant and sensitive field pea genotypes as 10 ppm metribuzin. This dosage was subsequently used to screen the Kaspa 9 PBA Oura recombinant inbred line genetic mapping population of 185 individuals for tolerance to metribuzin in three individual controlled environment assays. After two weeks of metribuzin treatment, plants were assessed on the basis of both a numerical score for symptoms such as chlorosis and necrosis, and plant damage as a percentage of necrosis. The two phenotypic parameters showed a high level of correlation (r = 0.85-0.97). The locations and magnitudes of effect of quantitative trait loci (QTLs) were determined for metribuzin tolerance (based on both symptom score and plant damage) as well as several related morphological traits. Analysis of all characters detected a single genomic region located on linkage group (LG) Ps IV (LOD scores 3.5-5.7), accounting for proportions of phenotypic variance for plant symptom score and percentage of plant necrosis varying from 12 to 21%. Genetic markers based on genic sequences that closely flank the metribuzin tolerance QTL are suitable for implementation in field pea breeding programs. In addition, comparative genomics between field pea and Medicago truncatula identified a cytochrome P450 monooxygenase gene in the vicinity of the QTL, potentially involved in nontarget-site metabolism-based herbicide tolerance.