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Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level. [ABSTRACT FROM AUTHOR]

Alternative electron pathways contribute to regulation of photosynthetic light reactions to adjust to metabolic demands in dynamic environments. The chloroplast NADH dehydrogenase-like (NDH) complex mediates the cyclic electron transport pathway around PSI in different cyanobacteria, algae, and plant species, but it is not fully conserved in all photosynthetic organisms. In order to assess how the physiological role of this complex changed during plant evolution, we isolated Physcomitrella patens lines knocked out for the NDHM gene that encodes a subunit fundamental for the activity of the complex. ndhm knockout mosses indicated high PSI acceptor side limitation upon abrupt changes in illumination. In P. patens , pseudo-cyclic electron transport mediated by flavodiiron proteins (FLVs) was also shown to prevent PSI over-reduction in plants exposed to light fluctuations. flva ndhm double knockout mosses had altered photosynthetic performance and growth defects under fluctuating light compared with the wild type and single knockout mutants. The results showed that while the contribution of NDH to electron transport is minor compared with FLV, NDH still participates in modulating photosynthetic activity, and it is critical to avoid PSI photoinhibition, especially when FLVs are inactive. The functional overlap between NDH- and FLV-dependent electron transport supports PSI activity and prevents its photoinhibition under light variations. [ABSTRACT FROM AUTHOR]

In nature, photosynthetic organisms are exposed to highly dynamic environmental conditions where the excitation energy and electron flow in the photosynthetic apparatus need to be continuously modulated. Fluctuations in incident light are particularly challenging because they drive oversaturation of photosynthesis with consequent oxidative stress and photoinhibition. Plants and algae have evolved several mechanisms to modulate their photosynthetic machinery to cope with light dynamics, such as thermal dissipation of excited chlorophyll states (non-photochemical quenching, NPQ) and regulation of electron transport. The regulatory mechanisms involved in the response to light dynamics have adapted during evolution, and exploring biodiversity is a valuable strategy for expanding our understanding of their biological roles. In this work, we investigated the response to fluctuating light in Nannochloropsis gaditana , a eukaryotic microalga of the phylum Heterokonta originating from a secondary endosymbiotic event. Nannochloropsis gaditana is negatively affected by light fluctuations, leading to large reductions in growth and photosynthetic electron transport. Exposure to light fluctuations specifically damages photosystem I, likely because of the ineffective regulation of electron transport in this species. The role of NPQ, also assessed using a mutant strain specifically depleted of this response, was instead found to be minor, especially in responding to the fastest light fluctuations. [ABSTRACT FROM AUTHOR]

The adaptation to dehydration and rehydration cycles represents a key step in the evolution of photosynthetic organisms and requires the development of mechanisms by which to sense external stimuli and translate them into signaling components. In this study, we used genetically encoded fluorescent sensors to detect specific transient increases in the Ca2+ concentration in the moss Physcomitrella patens upon dehydration and rehydration treatment. Observation of the entire plant in a single time-series acquisition revealed that various cell types exhibited different sensitivities to osmotic stress and that Ca2+ waves originated from the basal part of the gametophore and were directionally propagated towards the top of the plant. Under similar conditions, the vascular plant Arabidopsis thaliana exhibited Ca2+ waves that propagated at a higher speed than those of P. patens. Our results suggest that systemic Ca2+ propagation occurs in plants even in the absence of vascular tissue, even though the rates can be different. [ABSTRACT FROM AUTHOR]

Light is the primary energy source for photosynthetic organisms, but in excess, it can generate reactive oxygen species and lead to cell damage. Plants evolved multiple mechanisms to modulate light use efficiency depending on illumination intensity to thrive in a highly dynamic natural environment. One of the main mechanisms for protection from intense illumination is the dissipation of excess excitation energy as heat, a process called nonphotochemical quenching. In plants, nonphotochemical quenching induction depends on the generation of a pH gradient across thylakoid membranes and on the presence of a protein called PHOTOSYSTEM II SUBUNIT S (PSBS). Here, we generated Physcomitrella patens lines expressing histidinetagged PSBS that were exploited to purify the native protein by affinity chromatography. The mild conditions used in the purification allowed copurifying PSBS with its interactors, which were identified by mass spectrometry analysis to be mainly photosystem II antenna proteins, such as LIGHT-HARVESTING COMPLEX B (LHCB). PSBS interaction with other proteins appears to be promiscuous and not exclusive, although the major proteins copurified with PSBS were components of the LHCII trimers (LHCB3 and LHCBM). These results provide evidence of a physical interaction between specific photosystemII light-harvesting complexes and PSBS in the thylakoids, suggesting that these subunits are major players in heat dissipation of excess energy. [ABSTRACT FROM AUTHOR]

The paper describes the genome sequence, gene annotation, and transcriptional profiling of the microalga Nannochloropsis gaditana B-31. The study identifies possible targets of genetic modification that could improve the yield of the strain for biodiesel production. The genome resource is available online at www.nannochloropsis.org with multiple tools for information retrieval.Nannochloropsis is rapidly emerging as a model organism for the study of biofuel production in microalgae. Here, we report a high-quality genomic assembly of Nannochloropsis gaditana, consisting of large contigs, up to 500 kbp long, and scaffolds that in most cases span the entire length of the chromosomes. We identified 10646 complete genes and characterized possible alternative transcripts. The annotation of the predicted genes and the analysis of cellular processes revealed traits relevant for the genetic improvement of this organism such as genes involved in DNA recombination, RNA silencing, and cell wall synthesis. We also analyzed the modification of the transcriptional profile in nitrogen deficiency—a condition known to stimulate lipid accumulation. While the content of lipids increased, we did not detect major changes in expression of the genes involved in their biosynthesis. At the same time, we observed a very significant down-regulation of mitochondrial gene expression, suggesting that part of the Acetyl-CoA and NAD(P)H, normally oxidized through the mitochondrial respiration, would be made available for fatty acids synthesis, increasing the flux through the lipid biosynthetic pathway. Finally, we released an information resource of the genomic data of N. gaditana, available online at www.nannochloropsis.org. [ABSTRACT FROM PUBLISHER]

Nonphotochemical quenching (NPQ) dissipates excess energy to protect the photosynthetic apparatus from excess light. The moss Physcomitrella patens exhibits strong NPQ by both algal-type light-harvesting complex stress-related (LHCSR)–dependent and plant-type S subunit of Photosystem II (PSBS)-dependent mechanisms. In this work, we studied the dependence of NPQ reactions on zeaxanthin, which is synthesized under light stress by violaxanthin deepoxidase (VDE) from preexisting violaxanthin. We produced vde knockout (KO) plants and showed they underwent a dramatic reduction in thermal dissipation ability and enhanced photoinhibition in excess light conditions. Multiple mutants (vde lhcsr   KO and vde psbs   KO) showed that zeaxanthin had a major influence on LHCSR-dependent NPQ, in contrast with previous reports in Chlamydomonas reinhardtii. The PSBS-dependent component of quenching was less dependent on zeaxanthin, despite the near-complete violaxanthin to zeaxanthin exchange in LHC proteins. Consistent with this, we provide biochemical evidence that native LHCSR protein binds zeaxanthin upon excess light stress. These findings suggest that zeaxanthin played an important role in the adaptation of modern plants to the enhanced levels of oxygen and excess light intensity of land environments. [ABSTRACT FROM AUTHOR]

Plants adjust their photosynthetic activity to changing light conditions. A central regulation of photosynthesis depends on the xanthophyll cycle, in which the carotenoid violaxanthin is converted into zeaxanthin in strong light, thus activating the dissipation of the excess absorbed energy as heat and the scavenging of reactive oxygen species. Violaxanthin deepoxidase (VDE), the enzyme responsible for zeaxanthin synthesis, is activated by the acidification of the thylakoid lumen when photosynthetic electron transport exceeds the capacity of assimilatory reactions: at neutral pH, VDE is a soluble and inactive enzyme, whereas at acidic pH, it attaches to the thylakoid membrane where it binds its violaxanthin substrate. VDE also uses ascorbate as a cosubstrate with a pH-dependent Km that may reflect a preference for ascorbic acid. We determined the structures of the central lipocalin domain of VDE (VDEcd) at acidic and neutral pH. At neutral pH, VDEcd is monomeric with its active site occluded within a lipocalin barrel. Upon acidification, the barrel opens up and the enzyme appears as a dimer. A channel linking the two active sites of the dimer can harbor the entire carotenoid substrate and thus may permit the parallel deepoxidation of the two violaxanthin β-ionone rings, making VDE an elegant example of the adaptation of an asymmetric enzyme to its symmetric substrate. [ABSTRACT FROM AUTHOR]

We investigated the function of chlorophyll a/b binding antenna proteins Chlorophyll Protein 26 (CP26) and CP24 in light harvesting and regulation of photosynthesis by isolating Arabidopsis thaliana knockout lines that completely lacked one or both of these proteins. All three mutant lines had a decreased efficiency of energy transfer from numeric light-harvesting to the reaction center of photosystem II (PSII) due to the physical disconnection of LHCII from PSII and complex II (LHCII) formation of PSII reactions center depleted domains in grana partitions. Photosynthesis was affected in plants lacking CP24 but not in plants lacking CP26: the former mutant had decreased electron transport rates, a lower θpH gradient across the grana membranes, reduced capacity for nonphotochemical quenching, and limited growth. Furthermore, the PSII particles of these plants were organized in unusual two-dimensional arrays in the grana membranes. Surprisingly, overall electron transport, nonphotochemical quenching, and growth of the double mutant were restored to wild type. Fluorescence induction kinetics and electron transport measurements at selected steps of the photosynthetic chain suggested that limitation in electron transport was due to restricted electron transport between QA and QB, which retards plastoquinone diffusion. We conclude that CP24 absence alters PSII: organization and consequently limits plastoquinone diffusion. [ABSTRACT FROM AUTHOR]

The lutein-5,6-epoxide (Lx) cycle operates in some plants between lutein (L) and its monoepoxide, Lx. Whereas recent studies have established the photoprotective roles of the analogous violaxanthin cycle, physiological functions of the Lx cycle are still unknown. In this article, we investigated the operation of the Lx cycle in light-harvesting antenna complexes (Lhcs) of Inga sapindoides Willd, a tropical tree legume accumulating substantial Lx in shade leaves, to identify the xanthophyll-binding sites involved in short- and long-term responses of the Lx cycle and to analyze the effects on light-harvesting efficiency. In shade leaves, Lx was converted into L upon light exposure, which then replaced Lx in the peripheral V1 site in trimeric Lhcs and the internal L2 site in both monomeric and trimeric Lhcs, leading to xanthophyll composition resembling sun-type Lhcs. Similar to the violaxanthin cycle, the Lx cycle was operating in both photosystems, yet the light-induced Lx → L conversion was not reversible overnight. Interestingly, the experiments using recombinant Lhcb5 reconstituted with different Lx and/or L levels showed that reconstitution with Lx results in a significantly higher fluorescence yield due to higher energy transfer efficiencies among chlorophyll (Chl) a molecules, as well as from xanthophylls to Chl a. Furthermore, the spectroscopic analyses of photosystem I-LHCI from I. sapindoides revealed prominent red-most Chl forms, having the lowest energy level thus far reported for higher plants, along with reduced energy transfer efficiency from antenna pigments to Chl a. These results are discussed in the context of photoacclimation and shade adaptation. [ABSTRACT FROM AUTHOR]

The kinetics of response to strong light have been examined in deeply shaded leaves of the tropical tree legume (Inga sp.) which have extraordinarily high levels of the α-xanthophyll lutein-epoxide that are co-located in pigment–protein complexes of the photosynthetic apparatus with the β-xanthophyll violaxanthin. As in other species, rapidly reversible photoprotection (measured as non-photochemical chlorophyll fluorescence quenching) is initiated within the time frame of sun-flecks (minutes), before detectable conversion of violaxanthin to antheraxanthin or zeaxanthin. Photoprotection is stabilized within hours of exposure to strong light by simultaneously engaging the reversible violaxanthin cycle and a slowly reversible conversion of lutein-epoxide to lutein. It is proposed that this lutein ‘locks in’ a primary mechanism of photoprotection during photoacclimation in this species, converting efficient light-harvesting antennae of the shade plant into potential excitation dissipating centres. It is hypothesized that lutein occupies sites L2 and V1 in light-harvesting chlorophyll protein complexes of photosystem II, facilitating enhanced photoprotection through the superior singlet and/or triplet chlorophyll quenching capacity of lutein. [ABSTRACT FROM PUBLISHER]