Auxin-dependent gene expression of ascorbate oxidase and cell-elongation in plants.

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The biological function of ascorbate oxidase (AAO) has not been clarified yet, although AAO has been suggested to be involved in cell growth. We investigated AAO expression during non-synchronous, synchronous and elongation cultures of tobacco BY-2 cells. In non-synchronous culture, AAO mRNA was abundant in logarithmic growth phase. In synchronous division culture, AAO mRNA was detected in all phases but the levels were quite low in G1 phase. In elongation culture, the levels of AAO mRNA increased during elongation of cells. AAO activity in the culture medium, as well as ascorbate and dehydroascorbate contents in the cells, also increased during the elongation. We propose that AAO expression and production of dehydroascorbate is under the control of cell cycle, and that AAO may function as an ascorbate oxidizer apoplastically in the process of cell elongation. When pumpkin AAO cDNA was introduced into tobacco BY-2 (Nicotiana tabacum L. cv. Bright Yellow No. 2) cultured cells by Agrobacterium-mediated transformation, the transgenic cells expressed and secreted the recombinant pumpkin AAO into the culture medium. These transgenic cells showed no morphological difference from wild-type cells. However, in the presence of applied hormones protoplasts prepared from the transgenic cells elongated more rapidly than those of wild-type cells. We propose that AAO may play a key role in the regulation of cell expansion perhaps by controlling transport processes through plasma membrane, but not by affecting the cell wall.