Levels of polyphenol oxidase activity in leaves of Milicia seedlings is indicative of the resistance to Phytolyma lata-induced gall disease

Phytolyma lata-induced galling has remained one of the foremost challenges limiting the regeneration of Milicia species in nurseries and degraded forests in West Africa. Although planting of Milicia progenies resistant to Phytolyma attack has been identified as an opportunity for long-term control of the pest, the absence of biomarkers for early detection of resistance has limited the selection of resistant seedlings for planting. We compared phenolic metabolites and oxidative enzymes in resistant and susceptible Milicia progenies with the aim of identifying potential biomarkers for resistance to Phytolyma-induced galling in seedlings. Levels of total phenolics and ortho-dihydroxyphenolics did not significantly differ between the susceptible and resistant saplings or among seedlings. Similarly, peroxidase activity did not differ among sapling progenies or among seedlings. Thus, no clearly defined role could be ascribed to phenolics and peroxidase in resistance of Milicia species to P. lata-induced galling. However, polyphenol oxidase (PPO) activity was over two-fold higher in the resistant than the tolerant or susceptible progenies, suggesting that higher levels of PPO activity corresponded with an increased level of resistance to galling. With further validation, PPO activity in leaves of Milicia seedlings can be used as a biomarker of resistance to Phytolyma-induced galling.

Automated summary

Phytolyma lata-induced galling is a significant challenge for the regeneration of Milicia species in West Africa. Planting resistant progenies has been identified as a long-term control strategy, but the lack of biomarkers for early detection of resistance has been a limiting factor. The study suggests that polyphenol oxidase (PPO) activity in leaves of Milicia seedlings can serve as a biomarker for resistance to Phytolyma-induced galling.