Intravacuolar pigmented structures occurred in anthocyanin-producing cultured cells of sweet potato (Ipomoea batatas) were characterized. Formation of the pigmented structures in sweet potato cells was induced by transfer of callus cultured in 2,4-D containing agar medium into 2,4-D free liquid medium under continuous illumination. These structures were found in the vacuoles. The pigmented structures were isolated from the protoplasts by precipitation in 60% (w/w) sucrose after centrifugation. Electron microscopic observations of the anthocyanin-containing cultured cells showed these structures had neither membrane boundary nor internal structures, and were found as strongly osmiophilic globules in vacuoles. Numerous small osmiophilic globules were observed in central vacuoles at the early stage of anthocyanin accumulation, but not found in cytoplasm. Similar pigmented structures in vacuoles were also formed by treatment with neutral red. These observations indicate that these pigmented structure is the high density and insoluble globules highly concentrated with anthocyanin, which was synthesized in cytoplasm and transported to the central vacuoles.
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Nozue et al. (1993) studied this question.