Enzymatic browning and genome-wide polyphenol oxidase gene identification in three contrasting avocado accessions

This study aims to disentangle avocado enzymatic browning by identifying and analyzing the PPO coding genes. Two avocado accessions (AVO48 and San Miguel) and the Hass cultivar with contrasting browning kinetics and enzyme activity levels were selected for gene characterization. Upon 90 min of light exposure, Hass and San Miguel showed a greater decrease in luminosity retention (closer to 40% of initial luminosity) compared to AVO48 (85% of luminosity). PPO activity in crude extracts was significantly higher (P < 0.05) in San Miguel (696 U μg^-1 protein) than Hass (174 U μg^-1 protein) and AVO48 (46–56 U μg^-1 protein). San Miguel showed a higher V_max K_m^-1 ratio (20.88 min^-1), followed by Hass (14.29 min^-1) and AVO48 (1.64 min^-1), suggesting that San Miguel and Hass have higher substrate affinity. Four PPO coding genes: PamPPO1, PamPPO2, PamPPO3 and PamPPO4 were identified in the Hass genome, all of them containing the main features of plant PPOs, but with specific amino acid combinations in the catalytic pocket of the tyrosinase domain; suggesting that PPO1, PPO2 and PPO4 have monophenolase activity, whereas PPO3, has o-diphenolase activity. The evidence of transcription of PPO3 in fruit of the three genotypes suggests an important role for this gene in avocado pulp browning. PPO2 expression was only found in AVO48. This research provides gene candidates for selective silencing to reduce enzymatic browning.