Ultrastructural Localization of Polyphenoloxidase Activity in Leaves of Healthy and Diseased Waterhyacinth

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Abstract
The ultrastructural localization of polyphenol oxidase (PPO) was studied in healthy and Acremonium zonatum-infected water hyacinth leaves. The desposition of an electron-dense reaction product in leaf tissue incubated in D-3,4-dihydroxyphenylalanine (DOPA) was a positive test for PPO activity. In healthy leaves, reaction product was observed in the thylakoid spaces and fret channels of chloroplasts in only 3 types of cells; vascular parenchyma, bundle sheath and phenol-storing cells. Sections from diseased leaves had reaction product in all cells containing chloroplasts. This observation was consistent throughout the leaf whether in areas immediately surrounding a lesion or in green tissue up to 5 cm distant. PPO activity was not observed in any organelle other than chloroplasts in either healthy or diseased leaves. The increase of this enzyme activity after infection is highly suggestive of an active role for PPO during pathogenesis.