The Function of Ascorbate Oxidase in Tobacco

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Abstract
The function of the apoplastic enzyme ascorbate oxidase (AO) was investigated in tobacco (Nicotiana tabacum). The abundance of AO mRNA was up-regulated by light. Cytosolic ascorbate peroxidase (APX1) transcripts were also highest in the light. In contrast, l-galactono-γ-lactone dehydrogenase, stromal APX, and thylakoid APX transcripts remained constant over the day/night cycle. Salicylic acid inhibited growth, increased expression of the pathogenesis-related protein (PR) 1a, and decreased AO transcript abundance. In contrast, the application of auxin enhanced growth and increased AO and PR 1a gene expression. Therefore, AO transcript abundance varied in a manner similar to hormone-mediated changes in plant growth. To study the effects of modified AO expression on growth, transformed tobacco plants expressing AO in the sense and antisense orientations were generated. The resultant large changes in apoplastic AO activity in the transformed tobacco plants had little effect on whole leaf ascorbate (AA) content, but they had dramatic effects on apoplastic AA levels. Enhanced AO activity oxidized the apoplastic AA pool, whereas decreased AO activity increased the amount of AA compared with dehydroascorbate. A relationship was observed between AO activity and plant height and biomass. Native AO transcript levels were no longer subject to light/dark regulation in AO sense and antisense plants. Taken together, these data show that there is an interaction between hormone, redox, and light signals at the level of the apoplast via modulation of ion of AA content.