No abstract

Photoinhibition is caused by an imbalance between the rates of the damage and repair cycle of photosystem II D1 protein in thylakoid membranes. The PSII repair processes include (i) disassembly of damaged PSII-LHCII supercomplexes and PSII core dimers into monomers, (ii) migration of the PSII monomers to the stroma regions of thylakoid membranes, (iii) dephosphorylation of the CP43, D1 and D2 subunits, (iv) degradation of damaged D1 protein, and (v) co-translational insertion of the newly synthesized D1 polypeptide and reassembly of functional PSII complex. Here, we studied the D1 turnover cycle in maize mesophyll and bundle sheath chloroplasts using a protein synthesis inhibitor, lincomycin. In both types of maize chloroplasts, PSII was found as the PSII-LHCII supercomplex, dimer and monomer. The PSII core and the LHCII proteins were phosphorylated in both types of chloroplasts in a light-dependent manner. The rate constants for photoinhibition measured for lincomycin-treated leaves were comparable to those reported for C3 plants, suggesting that the kinetics of the PSII photodamage is similar in C3 and C4 species. During the photoinhibitory treatment the D1 protein was dephosphorylated in both types of chloroplasts but it was rapidly degraded only in the bundle sheath chloroplasts. In mesophyll chloroplasts, PSII monomers accumulated and little degradation of D1 protein was observed. We postulate that the low content of the Deg1 enzyme observed in mesophyll chloroplasts isolated from moderate light grown maize may retard the D1 repair processes in this type of plastids.

We investigated the organization of photosystem II (PSII) in agranal bundle sheath thylakoids from a C 4 plant maize. Using blue native/SDS-PAGE and single particle analysis, we show for the first time that PSII in the bundle sheath (BS) chloroplasts exists in a dimeric form and forms light-harvesting complex II (LHCII)⅐PSII supercomplexes. We also demonstrate that a similar set of photosynthetic membrane complexes exists in mesophyll and agranal BS chloroplasts, including intact LHCI⅐PSI supercomplexes, PSI monomers, PSII core dimers, PSII monomers devoid of CP43, LHCII trimers, LHCII monomers, ATP synthase, and cytochrome b 6 f complex. Fluorescence functional measurements clearly indicate that BS chloroplasts contain PSII complexes that are capable of performing charge separation and are efficiently sensitized by the associated LHCII. We identified a fraction of LHCII present within BS thylakoids that is weakly energetically coupled to the PSII reaction center; however, the majority of BS LHCII is shown to be tightly connected to PSII. Overall, we demonstrate that organization of the photosynthetic apparatus in BS agranal chloroplasts of a model C 4 plant is clearly distinct from that of the stroma lamellae of the C 3 plants. In particular, supramolecular organization of the dimeric LHCII⅐PSII in the BS thylakoids strongly suggests that PSII in the BS agranal membranes may donate electrons to PSI. We propose that the residual PSII activity may supply electrons to poise cyclic electron flow around PSI and prevent PSI overoxidation, which is essential for the CO 2 fixation in BS cells, and hence, may optimize ATP production within this compartment.Oxygenic photosynthesis sustains life on Earth. It couples the formation of molecular oxygen with the biosynthesis of carbohydrates, thus providing the ultimate source of biomass, food, and fossil fuels. In the first step of photosynthesis, the solar energy is captured and converted into the energy-rich molecule ATP and the reducing equivalents (in the form of water-derived protons and electrons) used for the conversion of CO 2 into carbohydrates. The light-driven charge separation is conducted by cooperative interaction of photosystem I (PSI) 3 and photosystem II (PSII), two multimeric chlorophyll-binding protein complexes embedded in the thylakoid membranes of cyanobacteria, algae, and plants. The primary charge separation in the reaction centers of PSII and PSI triggers vectorial electron flow from PSII to PSI via the cytochrome (cyt) b 6 f complex, also present in the thylakoid membranes, resulting in formation of the electrochemical potential gradient across the thylakoid membrane. In this way, linear electron transport powers the activity of ATP synthase to convert ADP to ATP. Both ATP and NADPH produced in the light-driven redox reactions of photosynthesis are subsequently used for fixation and reduction of CO 2 during the photosynthetic dark reactions of the CalvinBenson cycle.Spatial organization of the thylakoid membranes exhibits lateral distribution of the...

The application of a potentiometric electronic tongue for the recognition and classification of plant samples is presented. Homogenates from the leaves of various plants subtypes (maize and guinea grass) were measured with the use of sensor array based on ion-selective and partially selective electrodes. It was found that this device is capable of recognizing with high correctness, the type of plant metabolism (distinguishing between two types of cereals) and the factors affecting nutritional values of plants such as cultivation time and light intensity during growth. The obtained results led to the conclusion that the presented system can be used for the monitoring of crops and estimation of the quality of plants.

Susceptibility of photosystem II complex (PSII) to photoinhibition and degradation of D1 protein has been described in the chloroplasts of C3 plants but so far, the PSII turnover has not been characterised in any C4 plant, which contains two types of chloroplasts differing biochemically and structurally. In maize (Zea mays L. Oleńka), chloroplasts located in mesophyll (M) develop grana, while bundle sheath (BS) chloroplasts are agranal. In this paper, we report the D1 protein phosphorylation, damage and proteolysis in mesophyll as well as in agranal bundle sheath thylakoids of maize plants. Photoinhibitory treatment (1800 μmol photons m–2 s–1) of isolated thylakoids led to donor side inhibition of PSII electron transport and then to damage of reaction centre in both M and BS thylakoids. Rate of D1 degradation rate was faster in BS than in M thylakoids, and the addition of ATP to incubation medium delayed D1 degradation in both types of thylakoids. Furthermore, we demonstrated that the proteases belonging to FtsH and Deg families were present but their amounts significantly differed in M and BS thylakoids. Protease inhibitor studies revealed that serine- and metallo-proteases were involved in degradation of D1 protein. Apparent existence of D1 degradation cycle and the presence of proteolytic enzymes responsible for this process in BS thylakoids confirm that PSII plays an important role in agranal membranes, and when damaged, D1 can be rapidly degraded to enable PSII repair and restoration in these membranes.