ИСТИНА

In the absence of available electron acceptors, or in competition with them, molecular oxygen can accept electrons from the photosynthetic electron transport chain (PETC) in the Mehler reaction, thus generating reactive oxygen species (ROS) in chloroplast. According to the thermodynamic characteristics and a number of experimental data, O2 photoreduction may take place in multiple sites of PETC, especially under stress conditions. One can indicate two main sites of the Mehler reaction in PETC: the O2 photoreduction with the H2O2 formation on the Photosystem (PS) II acceptor side and in the plastoquinone (PQ) pool in the classical Mehler reaction; and the superoxide generation in PS I locality − the part of the Mehler-peroxidase reaction and the water-water cycle − with H2O as a final product. It is suggested that site-specific increasing or decreasing of the O2 photoreduction play important role in photosynthetic acclimation and adaptation under environmental stresses. PS I associated superoxide production by stromal electron carriers generally alter redox state of the ascorbate-glutatione pools, thus mediating redox signaling. Whereas superoxide and H2O2 overproduction near Fe-containing proteins in PS I could be accompanied with the hydroxyl radicals formation in a Fenton-type reaction and lead to the leaf senescence and apoptosis. The quinones-mediated O2 photoreduction at the PS II acceptor side and in the PQ pool can be suggested as a source of H2O2 that directly trigger a variety of signal cascades under stress conditions. A number of kinetic models of photosynthetic electron transport were already developed, however few of them account for the multiple-sited O2 photoreduction and mechanisms that may regulate ROS formation in PETC so far. In the present work, an attempt is made to create a model which includes the main features of our current knowledge about mechanisms of the regulatory processes that control O2 photoreduction in various sites of the PETC under environmental stresses. The theoretical considerations concerning the mechanism that controls the Mehler reaction in chloroplasts is presented here. The model can be a useful tool for further studies of the above problem and may be applied to describe the experimental data on the O2 photoreduction in different sites in PETC under environmental stress.