Development of polyphenol oxidase activity in the micropylar endosperm of tomato seeds

Polyphenol oxidase (PPO) activity increased markedly in the micropylar region of the endosperm of tomato (Lycopersicon esculentum) seeds after radicle protrusion. Tissue-printing analyses demonstrated that the majority of the activity is localized in the micropylar area. The increase in the activity was due to the increase in the amounts of enzyme. Within the micropylar endosperm region, two PPO isozymes were immunologically detected whose apparent molecular masses were estimated to be approximately 58 and 59 kDa, respectively, by SDS–PAGE. Although PPO activity also developed in the lateral portion of the endosperm, the level of this activity was much lower as compared with that in the micropylar region. Furthermore, the isozyme pattern in the lateral portion differed from that in the micropylar portion. The 58 kDa isozyme that was detected in the latter portion was absent, and only 59 kDa PPO was detectable in the former one. When the endosperm tissues were wounded, an enhancement of the enzyme activity was observed in the wounded region. The wound-induced development of the enzyme activity was associated with the induction of 58 kDa isozyme.