Quantitative Trait Loci (QTL) and Candidate Gene Identification for the Non-Darkening Seed Coat Trait in Dry Beans (Phaseolus vulgaris L.)
Seed coat colour is one of the main determinants of seed quality in commercial market classes of dry bean (P. vulgaris L.), including pinto bean. The beige background of the conventional pinto bean seed coat turns brown with age through a process known as postharvest seed coat darkening (PSCD). Beans with darkened seed coat are discounted in the market because they are perceived to be old, hard-to-cook beans. Seed coat darkening (SCD) is attributed to oxidation of proanthocyanidins (PAs) in the seed coat. Breeding for beans with reduced risk of PSCD has been given more attention in recent years because they are appealing to consumers and command higher prices in the market. To achieve this objective, it is important to understand the genetic control of the SCD trait and its relationship with other traits. J is a Mendelian genetic locus known to be responsible for SCD trait in dry beans in which the recessive allele produces a non-darkening (ND) seed coat phenotype. The main objectives of this thesis were to identify quantitative trait loci (QTL) and a gene associated with PSCD in dry beans and understand its function in promoting SCD. A mapping population consisting of 128 F5 recombinant inbred lines (RILs) derived from a cross between a ND cranberry-like bean ‘Wit-rood boontje’ and a slow-darkening pinto bean ‘1533-15’ was genotyped with an Illumina BARCBEAN6K_3 BeadChip and F5:6 seeds were characterized for seed coat phenotype. A major QTL was identified on chromosome Pv10 which explained 48.1% of the phenotypic variation for seed coat darkening. Amplicon sequencing of 21 candidate genes underlying the QTL revealed a single nucleotide deletion (c.703delG) in the candidate gene Phvul.010G130600 in ND RILs. In silico analysis indicated that Phvul.010G130600 encodes a protein with strong amino acid sequence identity (70%) to a R2R3-MYB-type transcription factor MtPAR, which regulates PA biosynthesis in Medicago truncatula seed coat tissue. The ND trait was correlated with reduced grain yield; however, it is possible to develop high yielding lines. In comparison with the darkening genotypes, the ND genotypes were perceived as sweeter, and had softer and smoother texture and shorter cooking times.