Breeding and Characterization of Hexaploid Wheats with Nil Levels of Grain Polyphenol Oxidase.

Polyphenol oxidase (PPO) is a ubiquitous enzyme in plants, responsible for many browning reactions and reduction of food product quality.  In common (bread) wheat, PPO occurs in the external layers of grain, often is carried into flour via milling, and can be responsible for the discoloration of wheat food products, especially Asian-style noodles.  Previous reports have indicated durum (tetraploid) wheats have low or nil levels of grain PPO, while common wheat PPO levels generally are high, with a few notable exceptions having low levels.  Low levels of grain PPO are highly desirable in hard white wheat market classes.  A survey of white wheats from the USDA-ARS-NSGC, based on color reaction of grain in tyrosine solutions, resulted in the discovery of hexaploid accessions with nil to low levels of grain PPO.  Two such accessions, PI 117635 and PI 134049 (Seaspray), were intermated, and PPO levels determined in 57 randomly selected progeny lines grown in three environments, Yuma, AZ in 2006, and Corvallis, OR in 2007 and 2008.  PPO levels, recorded as optical densities (OD) of product in an L-dopa based assay, ranged in the progeny lines from 0.075 to 0.275.  PPO levels of the durum cultivar Ben, averaged 0.092.  The parental lines, PI 117635 and PI 134049 averaged 0.125 and 0.340, respectively.  Grain PPO levels of the 57 progeny lines were highly heritable, as determined by regression analysis of parent-progeny relationships.  DNA marker analysis demonstrated the nil PPO trait was related to the presence of a previously reported “low” allele at the Ppo-A1 locus, but was independent of allelic status at the Ppo-D1 locus.  We conclude conventional breeding approaches can develop hexaploid wheats with PPO levels as low as those of durum wheats.