Control of the appearance of ascorbate peroxidase (EC 1.11.1.11) in mustard seedling cotyledons by phytochrome and photooxidative treatments.

In photosynthetic cells the plastidic ascorbate-glutathione pathway is considered the major sequence involved in the elimination of active oxygen species. Ascorbate peroxidase (APO; EC 1.11.1.11) is an essential constituent of this pathway. In the present paper control of the appearance of APO was studied in the cotyledons of mustard (Sinapis alba L.) seedlings with the following results: (i) Two isoforms of APO (APO I, APO II) could be separated by anion-exchange chromatography; APO I is a plastidic protein, while APO II is extraplastidic, very probably cytosolic. (ii) The appearance of APO is regulated by light via phytochrome. This control is observed with both isoforms. Moreover, a strong positive control over APO II appearance (very probably over APO II synthesis) is exerted by photooxidative treatment of the plastids. (iii) Additional synthesis of extraplastidic APO II is induced by a signal created by intraplastidic pigment-photosensitized oxidative stress. The response is obligatorily oxygen-dependent and abolished by quenchers of singlet oxygen such as α-tocopherol and p-benzoquinone. (iv) A short-term (4 h) photooxidative treatment suffices to saturate the signal. Signal transduction cannot be abolished or diminished by replacing the plants in non-photooxidizing conditions. Several observations indicate that control of APO synthesis by active oxygen is not an experimental artifact but a natural phenomenon.