DISTRIBUTION AND VARIABILITY OF CEREAL POLYPHENOL OXIDASE(S) GENE FAMILIES

Abstract

Polyphenol oxidases (PPOs) are copper-binding enzymes that oxidize polyphenols to quinones. PPOs are ubiquitous in plants, but, knowledge on their evolution and diversity in cereals is explored to a limited extent. This study reports their distribution and diversity in maize, sorghum, barley and millet. We have identified additional PPO proteins in the four crops than those previously reported. In all, 27 PPO were studied and an overall sequence identity across the species ranged between 30-99%. In addition to several variants of ‘HxxYC’ motif, a novel motif (HRxYxxFxER) that begins with third conserved histidine residue of ‘Copper A’ is reported. Another tri-peptide motif, ‘AGS’ was found to be 100% conserved. Twin-tyrosine (YxY) motif was substituted by ‘FTY’ or ‘YRF’ motif in four millet and one barley sequences. Among the 27 PPOs, 7 proteins were predicted to be synthesized via secretory pathway. PROSITE Scan analysis identified several domains including Zinc finger C2H2-type, Immunoglobulin [Ig]-like, and ‘TAT’ signal. Phylogenetic analysis revealed two major clades and also indicated a non-species specific diversification of the cereal PPOs investigated. Overall, our study analysed the diversity among the four cereal PPOs, it is observed that their number and distribution is consistent with their implications in different roles.