Photosystem II Repair and Plant Immunity : Lessons Learned from Arabidopsis Mutant Lacking the THYLAKOID LUMEN PROTEIN 18.3

Chloroplasts play an important role in the cellular sensing of abiotic and biotic stress. Signals originating from photosynthetic light reactions, in the form of redox and pH changes, accumulation of reactive oxygen and electrophile species or stromal metabolites are of key importance in chloroplast retrograde signaling. These signals initiate plant acclimation responses to both abiotic and biotic stresses. To reveal the molecular responses activated by rapid fluctuations in growth light intensity, gene expression analysis was performed with Arabidopsis thaliana wild type and the tlp18.3 mutant plants, the latter showing a stunted growth phenotype under fluctuating light conditions (Biochem. J, 406, 415-425). Expression pattern of genes encoding components of the photosynthetic electron transfer chain did not differ between fluctuating and constant light conditions, neither in wild type nor in tlp18.3 plants, and the composition of the thylakoid membrane protein complexes likewise remained unchanged. Nevertheless, the fluctuating light conditions repressed in wild-type plants a broad spectrum of genes involved in immune responses, which likely resulted from shade-avoidance responses and their intermixing with hormonal signaling. On the contrary, in the tlp18.3 mutant plants there was an imperfect repression of defense-related transcripts upon growth under fluctuating light, possibly by signals originating from minor malfunction of the photosystem II (PSII) repair cycle, which directly or indirectly modulated the transcript abundances of genes related to light perception via phytochromes. Consequently, a strong allocation of resources to defense reactions in the tlp18.3 mutant plants presumably results in the stunted growth phenotype under fluctuating light.Peer reviewe

Photosynthetic hydrogen production: Novel protocols, promising engineering approaches and application of semi-synthetic hydrogenases

Photosynthetic production of molecular hydrogen (H-2) by cyanobacteria and green algae is a potential source of renewable energy. These organisms are capable of water biophotolysis by taking advantage of photosynthetic apparatus that links water oxidation at Photosystem II and reduction of protons to H-2 downstream of Photosystem I. Although the process has a theoretical potential to displace fossil fuels, photosynthetic H-2 production in its current state is not yet efficient enough for industrial applications due to a number of physiological, biochemical, and engineering barriers. This article presents a short overview of the metabolic pathways and enzymes involved in H-2 photoproduction in cyanobacteria and green algae and our present understanding of the mechanisms of this process. We also summarize recent advances in engineering photosynthetic cell factories capable of overcoming the major barriers to efficient and sustainable H-2 production

PSB33 protein sustains photosystem II in plant chloroplasts under UV-A light

Plants can quickly and dynamically respond to spectral and intensity variations of the incident light. These responses include activation of developmental processes, morphological changes, and photosynthetic acclimation that ensure optimal energy conversion and minimal photoinhibition. Plant adaptation and acclimation to environmental changes have been extensively studied, but many details surrounding these processes remain elusive. The photosystem II (PSII)-associated protein PSB33 plays a fundamental role in sustaining PSII as well as in the regulation of the light antenna in fluctuating light. We investigated how PSB33 knock-out Arabidopsis plants perform under different light qualities. psb33 plants displayed a reduction of 88% of total fresh weight compared to wild type plants when cultivated at the boundary of UV-A and blue light. The sensitivity towards UV-A light was associated with a lower abundance of PSII proteins, which reduces psb33 plants\u27 capacity for photosynthesis. The UV-A phenotype was found to be linked to altered phytohormone status and changed thylakoid ultrastructure. Our results collectively show that PSB33 is involved in a UV-A light-mediated mechanism to maintain a functional PSII pool in the chloroplast

PSB33 protein sustains Photosystem II in plant chloroplasts under UVA light

Plants can quickly and dynamically respond to spectral and intensity variations of the incident light. These responses include activation of developmental processes, morphological changes, and photosynthetic acclimation that ensure optimal energy conversion and minimal photoinhibition. Plant adaptation and acclimation to environmental changes have been extensively studied, but many details surrounding these processes remain elusive. The Photosystem II (PSII) associated protein PSB33 plays a fundamental role in sustaining PSII as well as in the regulation of the light antenna in fluctuating lights. We investigated how PSB33 knock-out plants perform under different light qualities. psb33 plants displayed 88% lower fresh weight compared to wild type plants when cultivated in the border of UVA-blue light. The sensitivity towards UVA light was associated with a lower abundance of PSII proteins, which reduces psb33 plants´ capacity for photosynthesis. The UVA phenotype was further found to be linked to altered phytohormone status and changed thylakoid ultrastructure. Our results collectively show that PSB33 is involved in a UVA light-mediated mechanism to maintain a functional PSII pool in the chloroplast

Logistics in the life cycle of Photosystem IIlateral movement in the thylakoid membrane and activation of electron transfer

Due to its unique ability to split water, Photosystem II (PSII) is easily accessible to oxidative damage. Photoinhibited PSII centres diffuse laterally from the grana core region of the thylakoid membrane to the stroma lamellae in order to allow replacement of damaged proteins and cofactors. The 'new born' PSII centres in this region are characterized by the absence of the water splitting capacity and very poor ability to bind the secondary quinone acceptor, QB. After the repair process PSII has to regain the water splitting capacity. This requires a set of well-defined electron transfer reactions leading to assembly of the Mn-cluster. In order to minimize the danger of photoinhibition during these earlier stages of photoactivation of PSII, auxiliary donors to the primary donor P680+, such as redox active tyrosine on D2 protein, YD, and cytochrome b559 become involved in the electron transport reactions by providing necessary electrons. Cytochrome b559 may also serve as an electron acceptor to QA if elevated light intensities occur during the photoactivation process. These reactions lead to activation of QB binding, and finally to the assembly of the Mn-cluster. All these electron transport events occur simultaneously with the lateral movement of PSII centres back to the appressed regions of the grana core, where the pool of the most active PSII is situated

Partial inhibition of the inter-photosystem electron transfer at cytochrome b6f complex promotes periodic surges of hydrogen evolution in Chlamydomonas reinhardtii

Periodic surges of H2 evolution were observed in the wild-type strain CC-5325 of the green unicellular alga Chlamydomonas reinhardtii in the presence of the electron transport inhibitors dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB, 3.5 μM) and 2,4-dinitrophenylether and iodonitrothymol (DNP-INT, 0.6 μM). Addition of DBMIB partly inhibited the electron transfer from Cytochrome b6f complex to Photosystem I, over-reduced the plastoquinone pool, gradually inhibited photosystem II and created anoxic conditions in cells. During 30 days of anaerobic incubation, continues H2 photoproduction with a minimum rate of 1 ml/L of culture per day was accompanied with additional outbursts of H2 evolution. The first noticeable peak of H2 evolution was observed on day 6 of incubation, with maximum rate of 23 ml of H2 per L of culture per day. It was repeated on day 9 and day 22 with the 2 and 4 times lower rates respectively. Addition of DNP-NT showed similar effect by inducing the H2 photoproduction for 15 days, albeit at much lower rates. Contribution of the direct and indirect pathways to the H2 production is shown by fluorescence, thermoluminescence and electron paramagnetic resonance spectroscopy. It is proposed that photosynthetic electron transport in combination with photorespiration, chlororespiration and starch accumulation can switch on and off between photosynthetic, H2 producing and survival modes of cell metabolism. Controlled switching between these modes could potentially maintain the long lasting photosynthetic H2 production in the wild-type of Chlamydomonas

Formation of tyrosine radicals in photosystem II under far-red illumination

Photosystem II (PS II) contains two redox-active tyrosine residues on the donor side at symmetrical positions to the primary donor, P680. TyrZ, part of the water-oxidizing complex, is a preferential fast electron donor while TyrD is a slow auxiliary donor to P680 +. We used PS II membranes from spinach which were depleted of the water oxidation complex (Mn-depleted PS II) to study electron donation from both tyrosines by time-resolved EPR spectroscopy under visible and far-red continuous light and laser flash illumination. Our results show that under both illumination regimes, oxidation of TyrD occurs via equilibrium with TyrZ • at pH 4.7 and 6.3. At pH 8.5 direct TyrD oxidation by P680 + occurs in the majority of the PS II centers. Under continuous far-red light illumination these reactions were less effective but still possible. Different photochemical steps were considered to explain the far-red light-induced electron donation from tyrosines and localization of the primary electron hole (P680 +) on the ChlD1 in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested

Fractionation of the thylakoid membranes from tobacco. A tentative isolation of ‘end membrane’ and purified ‘stroma lamellae’ membranes

Thylakoids isolated from tobacco were fragmented by sonication and the vesicles so obtained were separated by partitioning in aqueous polymer two-phase systems. By this procedure, grana vesicles were separated from stroma exposed membrane vesicles. The latter vesicles could be further fractionated by countercurrent distribution, with dextran-polyethylene glycol phase systems, and divided into two main populations, tentatively named 'stroma lamellae' and 'end membrane'. Both these vesicle preparations have high chlorophyll a/b ratio, high photosystem (PS) I and low PS II content, suggesting their origin from stroma exposed regions of the thylakoid. The two vesicle populations have been compared with respect to biochemical composition and photosynthetic activity. The 'end membrane' has a higher chlorophyll a/b ratio (5.7 vs. 4.7), higher P700 content (4.7 vs. 3.3 mmol/mol of chlorophyll). The 'end membrane' has the lowest PS II content, the ratio PS I/PS II being more than 10, as shown by EPR measurements. The PS II in both fractions is of the β-type. The decay of fluorescence is different for the two populations, the 'stroma lamellae' showing a very slow decay even in the presence of K3Fe(CN)6 as an acceptor. The two vesicle populations have very different surface properties: the end membranes prefer the upper phase much more than the stroma lamellae, a fact which was utilized for their separation. Arguments are presented which support the suggestion that the two vesicle populations originate from the grana end membranes and the stroma lamellae, respectively