2019 ASHS Annual Conference

Snap beans with yellow pods or so called “wax” beans have been known for more than two centuries. They constitute a minor market class of snap beans with some popularity among fresh market growers and processors looking to increase color diversity in their product. The trait is controlled by a single recessive gene designated as y. Being easy to classify visually, y was used to construct the first pre-molecular marker linkage maps in common bean. In early molecular mapping efforts with RFLP markers, y was mapped to the distal end of Pv02 in a wax bean by wild bean cross. Using the BARCBean6K_3 Beadchip, we were able to obtain the physical location of y in two biparental populations (‘Unidor’/‘OSU5630’ and ‘Serin’/‘OSU5630’) and through association mapping in the Bean CAP Snap Bean Diversity Panel (SBDP). The data from these three sources point to a 122 kb region of Pv02 where the gene is located. The interval contains approximately 43 gene models. Using RNA seq data, we identified a pentatricopeptide repeat (PPR) gene that showed relatively high expression in flowers and young and old pods, but not in other tissues. PPR proteins are modular RNA-binding proteins that often control gene expression in both mitochondria and chloroplasts and expression in latter case may produce an albino phenotype. Data from the Bean CAP SBDP revealed that despite their yellow color, wax beans have the lowest levels of carotenoids of all snap beans. Using electron microscopy, we examined the chloroplasts in leaves and pods of wax and normal beans, and found that wax bean pod chloroplasts possess rudimentary thylakoid stacks compared to normal beans. Our hypothesis is that a PPR protein targeted to pod chloroplasts is a defective version of y that interferes with chlorophyll synthesis and carotenoid accumulation. While not so different in flavor and other qualities, our research shows that wax beans are not a useful source of carotenoids.