Chemical and morphological expression of the B and Asp seedcoat genes in Phaseolus vulgaris

Three common bean (Phaseolus vulgaris L.) seedcoat color (or glossiness) ge notypes, differing from each other by a single substitution at a seedcoat l ocus, were analyzed for presence and concentration of three anthocyanins: d elphinidin 3-O-glucoside, petunidin 3-O-glucoside, and malvidin 3-O-glucosi de. The three anthocyanins were present in Florida common bean breeding lin e 5-593 (P C J G B V Asp), matte black (P C J G B V nsp), and dark brown vi olet (P C J G b V Asp), but the amounts varied greatly depending on the gen otype, Dark brown violet had 19% of the total anthocyanin content when comp ared to 5-593, whereas matte black had amounts intermediate between the two other genotypes, The B gene acts to regulate the production of precursors of anthocyanins in the seedcoat color pathway above the level of dihydrokae mpferol formation, perhaps at the chalcone synthase or chalcone isomerase s teps in the biosynthetic pathway. We hypothesize that B regulates simultane ously the flavonoid (color) and isoflavonoid (resistance) pathways. The I g ene for resistance to bean common mosaic virus (BCMV) is known to be linked closely to B, It is therefore hypothesized that the I gene function may be to respond to BCMV infection by dramatically increasing (over a low consti tuitive level) production in the 5-dehydroxy isoflavonoid pathway, which le ads to synthesis of the major phytoalexin, phaseollin, for resistance to BC MV, Alternatively, the B and I genes may be allelic. The Asp gene affects s eedcoat glossiness by means of a structural change to the seedcoat, We demo nstrate that Asp in the recessive condition (asp/asp) changes the size and shape of the palisade cells of the seedcoat epidermis, making them signific antly smaller than either 5-593 or dark brown violet, Asp, therefore, limit s the amounts of anthocyanins in the seedcoat by reducing the size of palis ade cells.