Rethinking polyphenol oxidases in wheat: beyond the "low-PPO is always better" paradigm

Polyphenol oxidase (PPO) activity is a key determinant of wheat quality, influencing enzymatic browning of end products while also contributing to biochemical defence against (a)biotic stresses. In this review, we present a comprehensive structural characterization of more than 25 distinct alleles across the Ppo-1 and Ppo-2 gene families, revealing extensive allelic diversity between bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum ssp. durum (Desf.) Husn.). Comparative analyses across the A, B, and D genomes reveal substantial structural polymorphisms that contribute to functional diversification of Ppo genes. Our synthesis indicates that this broad allelic repertoire provides a genetic toolkit for fine-tuning PPO activity across tissues and phenological stages. This regulation is important for environmental adaptation while maintaining high processing quality standards. Emerging evidence challenges the prevailing assumption that "low-PPO is always better." Higher PPO activity levels in certain cultivars are often associated with more robust defence responses, potentially providing greater protection against pathogens and environmental stressors. However this relationship remains largely correlative and requires functional validation. These observations suggest that excessive suppression of PPO activity may inadvertently weaken plant resilience. Overall, the data support a paradigm shift toward strategic modulation of PPO activity, balancing the maintenance of plant defence across diverse agro-climatic environments with the preservation of end-use quality in wheat-derived products.