Production and diffusion of chloroplastic H2O2 and its implication to signalling.

Hydrogen peroxide (H(2)O(2)) is recognized as an important signalling molecule. There are two important aspects to this function: H(2)O(2) production and its diffusion to its sites of action. The production of H(2)O(2) by photosynthetic electron transport and its ability to diffuse through the chloroplast envelope membranes has been investigated using spin trapping electron paramagnetic resonance spectroscopy and H(2)O(2)-sensitive fluorescence dyes.

The production and scavenging of reactive oxygen species in the plastoquinone pool of chloroplast thylakoid membranes.

Reactive oxygen species (ROS) resulting from oxygen reduction, superoxide anion radical O2(*-) and hydrogen peroxide H(2)O(2) are very significant in the cell metabolism of aerobic organisms. They can be destructive and lead to apoptosis and they can also serve as signal molecules. In the light, chloroplasts are known to be one of the main sources of ROS in plants.

Putative function of cytochrome b559 as a plastoquinol oxidase.

The function of cytochrome b559 (cyt b559) in photosystem II (PSII) was studied in a tobacco mutant in which the conserved phenylalanine at position 26 in the beta-subunit was changed to serine. Young leaves of the mutant showed no significant difference in chloroplast ultra structure or in the amount and activity of PSII, while in mature leaves the size of the grana stacks and the amount of PSII were significantly reduced. Mature leaves of the mutant showed a higher susceptibility to photoinhibition and a higher production of singlet oxygen, as shown by spin trapping electron paramagnetic resonance (EPR) spectroscopy.

Kinetics of the plastoquinone pool oxidation following illumination Oxygen incorporation into photosynthetic electron transport chain.

The oxidation of the PQ-pool after illumination with 50 or 500 micromol quantam(-2)s(-1) was measured in isolated thylakoids as the increase in DeltaA(263), i.e., as the appearance of PQ.

Oxygen reduction in chloroplast thylakoids results in production of hydrogen peroxide inside the membrane.

Hydrogen peroxide production in isolated pea thylakoids was studied in the presence of cytochrome c to prevent disproportionation of superoxide radicals outside of the thylakoid membranes. The comparison of cytochrome c reduction with accompanying oxygen uptake revealed that hydrogen peroxide was produced within the thylakoid. The proportion of electrons from water oxidation participating in this hydrogen peroxide production increased with increasing light intensity, and at a light intensity of 630 micromol quanta m(-2) s(-1) it reached 60% of all electrons entering the electron transport chain.

Photosystem I is not solely responsible for oxygen reduction in isolated thylakoids.

It was found that the contribution of segments of photosynthetic electron transport chain (PETC) besides Photosystem I (PSI) to oxygen reduction increased with increase in light intensity, and at high intensities achieved 50% at pH 5.0, and was higher than 60% at pH 6.5 and pH 7.