Explant browning is one of the major problems in the tissue culture process, and polyphenol oxidase(PPO), is the major proteases involved in plant tissue browning. We investigated the effects of polyphenol oxidase on the early stage of browning in explants of the orchid Phalaenopsis. Our results show that PPO activity was significantly higher in explants cultured for 3 d than in the 0 h control. The levels of PPO transcripts and PPO protein were significantly higher in explants cultured for 6 h compared to the 0 h control; these high expression levels were maintained over increasing cultivation time. Cycloheximide(CHX) treatment reduced PPO transcript levels, PPO protein levels, and PPO enzyme activity. High levels of PPO m RNA and PPO protein were detected in the cytoplasm and vascular bundles of Phalaenopsis explants cultured for 6 h compared to explants cultured for 0 h, 24 h, and 3 d. CHX treatment did not significantly affect the distribution of PPO m RNA and PPO protein in explant tissues, but their levels were significantly lower than those of the untreated control.
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