Allelic variations of functional markers for polyphenol oxidase (PPO) genes in Indian bread wheat (Triticum aestivum L.) cultivars

Abstract

Polyphenol oxidase (PPO) activity causes undesirable browning and discolouration of products manufactured from bread wheat (Triticum aestivumL.) during processing or storage. PPO is a copper-containing metalloprotein which catalyses hydroxylation of o-monophenols to o-diphenols and oxidation of o-diphenols to o-quinones. Auto-oxidation and polymerization of quinones with amino acid group of cellular proteins results in dark and brown discolouration of products made from bread wheat grains (Anderson and Morris 2001). The darkening phenomena of such products may reduce the quality of products and thus affect consumer acceptance. Flour protein content has a negative association with flour PPO activity (Park et al. 1997) presumably because of reactivity of phenolic side groups (Demeke et al. 2001). In wheat, PPO genes belong to a multi-gene family and are classified into two clusters: kernel and nonkernel, based on gene expression sites (Anderson et al. 2006; Jukanti et al. 2006). Recently, Massa et al. (2007) reported 21 distinct PPO sequences in kernel-type genes in wheat and wild relatives. Many studies have implied that PPO activity is mainly conditioned by the genes located on homoeologous group 2 chromosomes in wheat and wild relatives (Demeke et al. 2001; Watanabe et al. 2004, 2006; Sun et al. 2005; He et al. 2007, 2009; Raman et al. 2007). The high activity PPO alleles on chromosomes 2AL and 2DL are most thoroughly studied and were first reported by Wrigley and McIntosh (1975). A relatively low PPO activity was also found to be associated with chromosomes 2B, 3D and 6B (Demeke et al. 2001; Fuerst et al. 2008). Functional Sequence tagged site (STS) markers for PPO genes on chromosomes 2A and 2D have been developed, based on DNA sequences in GenBank (Sun et al. 2005; He et al. 2007;