LIGHT- AND OXIDATIVE STRESS IN PLANTS AND CYANOBACTERIA

Environmental factors which impose adverse effects on photosynthetic organisms include light, drought, heavy metals and oxidative agents. By using a multidisciplinary approach, we investigate the mechanisms by which abiotic stress factors and inducers of reactive oxygen species damage plants and cyanobacteria. We also study the defense and acclimation responses which are induced by these stimuli.

Electron transport in the Photosystem II complex of photosynthesis

Photosystem II (PSII) of the photosynthetic apparatus is a multisubunit pigment-protein complex in the thylakoid membrane, which performs light-induced oxidation of water and reduction of plastoquinone. We study the mechanisms of charge separation and charge recombination by thermoluminescence and chlorophyll fluorescence measurements. Our results show that the dominating charge recombination route is the indirect non-radiative pathway via the primary radical pair (P₆₈₀⁺Phe^-). By using site-directed mutants we have shown the participation of several amino acid residues (D1-Ala344, CP43-Glu354) of the D1 protein in the ligation of the Mn cluster of water oxidation.

Damage of the photosynthetic apparatus by visible and ultraviolet light

Visible light is not only the basic driving force of photosynthesis, but also an important damaging factor. We have demonstrated that photodamage involves the ³P₆₈₀⁺Phe-. ³P₆₈₀ process via the main route of charge recombination, which leads to the production of highly damaging singlet oxygen through the interaction of ³P₆₈₀ and molecular O₂. We have also shown that the efficiency of ¹O₂ formation is influenced by the redox potential of Phe, which plays an important role in protection against photodamage.

The flux of UV-B (290-320 nm) radiation is increasing at the surface of Earth due to the recent depletion of the stratospheric ozone layer, which is potentially harmful for all forms of life. In plant cells, an important target of UV-B is the photosynthetic apparatus. We have demonstrated that the primary site of damage by UV-B (and UV-A) radiation is the Mn cluster of the water-oxidizing complex in PSII. This effect is correlated with direct UV- absorption by Mn(III) and Mn(IV) ions. UV-stress induced damage of PSII can be repaired via de-novo synthesis of the D1 and D2 reaction center subunits. We have also shown that this repair process involves the FtsH protease, cyclic nucleotides, especially cGMP, as well as the HSP16 heat shock protein.

Stress-induced defense and acclimation responses in cyanobacteria

We investigate the molecular background of various stress responses (high light, UV radiation, heavy metals, oxidative agents, oxygen availability etc.). We have studied the light-dependent regulation of the psbA genes, encoding the D1 reaction center protein, in several cyanobacteria using quantitative RT PCR and have shown differential responses within the psbA gene family. Our recent interest includes the mechanism of heavy-metal tolerance and heavy-metal induced expression of Synechocystis 6803 genes. We have studied gene expression patterns in response to Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Cr³⁺, Cr⁶⁺, As³⁺, As⁵⁺, and constructed bioluminescent Synechocystis 6803 strains, which specifically respond to Co²⁺, Ni²⁺ or Zn²⁺.

Complex plant stress monitoring

Quantitative assessment of plant responses to various stress conditions can be achieved by combined application of various remote sensing methods (digital photography, thermal imaging, chlorophyll fluorescence imaging) together with highly controlled growth environment. Based on these principles we are developing a complex plant stress diagnostic system, which is used for quantitative testing of drought tolerance as well as leaf rust infection in different wheat varieties.

Detection of active oxygen species in plant systems

Reactive oxygen species (ROS) are associated with stress responses acting as primary elicitors, propagators of oxidative damage, by-products as well as signalling molecules. The involvement of ROS in plant stress is generally concluded from detecting products of oxidative damage. In collaboration with University of Pécs (PTE, Hungary), we have developed direct detection techniques based on fluorescence quenching of reporter molecules upon combining with ROS.
A combination of two imaging techniques: one based on detecting spatial and intensity distribution of the ROS sensors and another based on variable Chl-a fluorescence provides valuable information on light-driven ROS generation and its detrimental effects on the investigation of the role of ROS production in leaves.

Quantitative assessment of plant responses to various stress conditions can be achieved by combined application of various remote sensing methods (digital photography, thermal imaging, chlorophyll fluorescence imaging) together with highly controlled growth environment. Based on these principles we are developing a complex plant stress diagnostic system, which is used for quantitative testing of drought tolerance as well as leaf rust infection in different wheat varieties.