Photosystem Research Articles

Structure of a diatom photosystem II supercomplex containing a member of Lhcx family and dimeric FCPII.

Diatoms rely on fucoxanthin chlorophyll a/c-binding proteins (FCPs) for their great success in oceans, which have a great diversity in their pigment, protein compositions, and subunit organizations. We report a unique structure of photosystem II (PSII)-FCPII supercomplex from Thalassiosira pseudonana at 2.68-Å resolution by cryo-electron microscopy. FCPIIs within this PSII-FCPII supercomplex exist in dimers and monomers, and a homodimer and a heterodimer were found to bind to a PSII core. The FCPII homodimer is formed by Lhcf7 and associates with PSII through an Lhcx family antenna Lhcx6_1, whereas the heterodimer is formed by Lhcf6 and Lhcf11 and connects to the core together with an Lhcf5 monomer through Lhca2 monomer. An extended pigment network consisting of diatoxanthins, diadinoxanthins, fucoxanthins, and chlorophylls a/c is revealed, which functions in efficient light harvesting, energy transfer, and dissipation. These results provide a structural basis for revealing the energy transfer and dissipation mechanisms and also for the structural diversity of FCP antennas in diatoms.

Toxic effects of lanthanum(III) on photosynthetic performance of rice seedlings: Combined chlorophyll fluorescence, chloroplast structure and thylakoid membrane protein assessment

Rare earth elements (REEs) are emerging as an anticipated pollution in the environment due to their active use in many areas. However, the effects of REEs on the photosynthesis of rice have not been thoroughly explored. Therefore, this study emphasizes how high levels of La(III) affect the thylakoid membrane of rice seedlings, thereby inhibiting photosynthesis and growth. Here, we reported that rice plants treated with La(III) exhibited an increase in La accumulation in the leaves, accompanied by a decrease in chlorophyll content and photosynthetic capacity. La(III) exposure decreased Mg content in leaves, but possibly increased other nutrients including Cu, Mn, and Zn through systemic endocytosis. K-band and L-band appeared in the fluorescence OJIP transients, indicating La(III) stress destroyed the donor and receptor sides of photosystem II (PSII). Numerous reaction centers (RC/CSm) were inactivated by La(III) treatment, which resulted in a reduction in electron transport capacity (decreased ETo/RC and ETo/CSm) and an increase in the dissipation of the excess excitation energy by heat (increased DIo/RC and DIo/CSm). The BN-PAGE analysis of thylakoid membrane protein complexes showed that La(III) induced the degradation of supercomplexes, PSII core, LHCII, PSI core, LHCI, and F1-ATPase binding Cyt b6f complex. Collectively, this study revealed that La(III) causes significant degradation of thylakoid membrane proteins, thereby promoting the decomposition of photosynthetic complexes, ultimately destroying the chloroplast structure and reducing the photosynthetic performance of rice seedlings.

PSI Photoinhibition and Changing CO2 Levels Initiate Retrograde Signals to Modify Nuclear Gene Expression.

Photosystem I (PSI) is a critical component of the photosynthetic machinery in plants. Under conditions of environmental stress, PSI becomes photoinhibited, leading to a redox imbalance in the chloroplast. PSI photoinhibition is caused by an increase in electron pressure within PSI, which damages the iron-sulfur clusters. In this study, we investigated the susceptibility of PSI to photoinhibition in plants at different concentrations of CO2, followed by global gene expression analyses of the differentially treated plants. PSI photoinhibition was induced using a specific illumination protocol that inhibited PSI with minimal effects on PSII. Unexpectedly, the varying CO2 levels combined with the PSI-PI treatment neither increased nor decreased the likelihood of PSI photodamage. All PSI photoinhibition treatments, independent of CO2 levels, upregulated genes generally involved in plant responses to excess iron and downregulated genes involved in iron deficiency. PSI photoinhibition also induced genes encoding photosynthetic proteins that act as electron acceptors from PSI. We propose that PSI photoinhibition causes a release of iron from damaged iron-sulfur clusters, which initiates a retrograde signal from the chloroplast to the nucleus to modify gene expression. In addition, the deprivation of CO2 from the air initiated a signal that induced flavonoid biosynthesis genes, probably via jasmonate production.

Function of iron-stress-induced protein A in cyanobacterial cells with monomeric and trimeric photosystem I.

The acclimation of cyanobacteria to iron deficiency is crucial for their survival in natural environments. In response to iron deficiency, many cyanobacterial species induce the production of a pigment-protein complex called iron-stress-induced protein A (IsiA). IsiA proteins associate with photosystem I (PSI) and can function as light-harvesting antennas or dissipate excess energy. They may also serve as chlorophyll storage during iron limitation. In this study, we examined the functional role of IsiA in cells of Synechocystis sp. PCC 6803 grown under iron limitation conditions by measuring the cellular IsiA content and its capability to transfer energy to PSI. We specifically tested the effect of the oligomeric state of PSI by comparing wild-type (WT) Synechocystis sp. PCC 6803 with mutants lacking specific subunits of PSI, namely PsaL/PsaI (PSI subunits XI/VIII) and PsaF/PsaJ (PSI subunits III/IX). Time-resolved fluorescence spectroscopy revealed that IsiA formed functional PSI3-IsiA18 supercomplexes, wherein IsiA effectively transfers energy to PSI on a timescale of 10 ps at room temperature-measured in isolated complexes and in vivo-confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a notable fraction (40%) remained unconnected to PSI, supporting the notion of a dual functional role of IsiA. Cells with monomeric PSI under iron deficiency contained, on average, only 3 to 4 IsiA complexes bound to PSI. These results show that IsiA can transfer energy to trimeric and monomeric PSI but to varying degrees and that the acclimatory production of IsiA under iron stress is controlled by its ability to perform its light-harvesting function.

Identification of key responsive leaf physiochemical traits for ozone sensitivity in 16 Italian cultivars of Triticum durum subjected to peak concentrations

AbstractPlants of 16 Italian cultivars of durum wheat at the tillering stage were exposed to a single pulse of ozone (O3, 200 ppb, 5 h) to reveal the mechanisms explaining their O3 tolerance/sensitivity by considering some leaf physiochemical traits [e.g., stomatal conductance, photosystem II (PSII) efficiency, antioxidant enzyme activity, and phytohormone synthesis]. At the end of the recovery period (48 h from the beginning of the exposure), all cultivars showed visible foliar injury in the form of widespread chlorosis which developed in bifacial ivory necrotic lesions scattered among the leaf veins of completely expanded leaves. Mongibello leaves showed not only the highest severity of visible injury (McKinney index = 62%), but also a reduction of PSII performance (Fv/Fm ratio: −22% compared with controls) together with an activation of the dissipation of the excess excitation energy as heat (qNP: +35%). The photosynthetic impairment observed throughout the whole experiment, was likely due to stomatal limitations (gs: −36% as average) and indicated that this cultivar was very sensitive to a pulse of O3. Similar results were obtained for Latino and Meridiano. Conversely, Iride, Orobel, and Portorico showed the lowest severity of visible injury (McKinney index around 13%) suggesting that they could be considered tolerant to a pulse of O3. These cultivars showed none or only transient changes due to O3 in terms of photosynthetic activity (e.g., A values), while the slight decrease of Fv/Fm ratio did not determine the photoinhibition of the PSII activity, since mechanisms for dissipating excess excitation energy were being activated. Ozone treatment induced a significant reduction of CO2 assimilation rate (−34%) in Orobel and this is not attributable to stomatal and non‐stomatal limitations. The remaining cultivars were considered moderately sensitive to O3, with McKinney index in the range of 20–43%. The measured biochemical traits are not decisive in determining the O3 sensitivity. In particular, the activity of phenylalanine ammonia lyase increased only in seven cultivars; among them, only in Latino, Vetrodur, and Simeto, this enhancement reflected in an accumulation of phenolic compounds suggesting potential higher protection against reactive oxygen species. This work highlights the relevance of identifying O3‐sensitive cultivars, in the optic to develop sustainable strategies to mitigate the impact of peak O3 episodes on wheat production.

Photo-Activating Biomimetic Polyoxomolybdate for Boosting Oxygen Evolution in Neutral Electrolytes.

Inspired by the metal-oxo cluster structural feature and charge separation behaviour of the oxygen evolving center (OEC) in photosystem II (PS-II) under photoirradiation, a new crystalline photochromic polyoxomolybdate, MV2 [β-Mo8 O26 ] (1, MV=methyl viologen cation), is designed as a biomimetic oxygen evolution reaction (OER) catalyst in neutral electrolytes. After photoinduced electron transfer (PIET) with colour change from colourless to grey, it remains in an ultra-stable charge-separated state over a year under ambient conditions. The observed overpotential at 10 mA ⋅ cm-2 and Tafel slope decrease by 49 mV and 62.8 mV ⋅ dec-1 after coloration, respectively. The outstanding OER performance of the coloured state in neutral electrolytes even outperforms the commercial RuO2 benchmark. Experimental and theoretical studies show that oxygen holes within polyanions after irradiation serve as sites for enhancing direct O-O coupling, thus effectively promoting OER. This is the first successful application of electron-transfer photochromism to realize OER activity gain.

Photo‐Activating Biomimetic Polyoxomolybdate for Boosting Oxygen Evolution in Neutral Electrolytes

AbstractInspired by the metal‐oxo cluster structural feature and charge separation behaviour of the oxygen evolving center (OEC) in photosystem II (PS‐II) under photoirradiation, a new crystalline photochromic polyoxomolybdate, MV2[β‐Mo8O26] (1, MV=methyl viologen cation), is designed as a biomimetic oxygen evolution reaction (OER) catalyst in neutral electrolytes. After photoinduced electron transfer (PIET) with colour change from colourless to grey, it remains in an ultra‐stable charge‐separated state over a year under ambient conditions. The observed overpotential at 10 mA ⋅ cm−2 and Tafel slope decrease by 49 mV and 62.8 mV ⋅ dec−1 after coloration, respectively. The outstanding OER performance of the coloured state in neutral electrolytes even outperforms the commercial RuO2 benchmark. Experimental and theoretical studies show that oxygen holes within polyanions after irradiation serve as sites for enhancing direct O−O coupling, thus effectively promoting OER. This is the first successful application of electron‐transfer photochromism to realize OER activity gain.

The cyanobacterial FtsH4 protease controls accumulation of protein factors involved in the biogenesis of photosystem I

Membrane-bound FtsH proteases are universally present in prokaryotes and in mitochondria and chloroplasts of eukaryotic cells. These metalloproteases are often critical for viability and play both protease and chaperone roles to maintain cellular homeostasis. In contrast to most bacteria bearing a single ftsH gene, cyanobacteria typically possess four FtsH proteases (FtsH1–4) forming heteromeric (FtsH1/3 and FtsH2/3) and homomeric (FtsH4) complexes. The functions and substrate repertoire of each complex are however poorly understood. To identify substrates of the FtsH4 protease complex we established a trapping assay in the cyanobacterium Synechocystis PCC 6803 utilizing a proteolytically inactivated trapFtsH4-His. Around 40 proteins were specifically enriched in trapFtsH4 pulldown when compared with the active FtsH4. As the list of putative FtsH4 substrates contained Ycf4 and Ycf37 assembly factors of Photosystem I (PSI), its core PsaB subunit and the IsiA chlorophyll-binding protein that associates with PSI during iron stress, we focused on these PSI-related proteins. Therefore, we analysed their degradation by FtsH4 in vivo in Synechocystis mutants and in vitro using purified substrates. The data confirmed that FtsH4 degrades Ycf4, Ycf37, IsiA, and also the individual PsaA and PsaB subunits in the unassembled state but not when assembled within the PSI complexes. A possible role of FtsH4 in the PSI life-cycle is discussed.

A quantum trajectory picture of single photon absorption and energy transport in photosystem II.

We use quantum trajectory theory to study the dynamics of the first step in photosynthesis for a single photon interacting with photosystem II (PSII). By considering individual trajectories we are able to look beyond the ensemble average dynamics to compute the PSII system evolution conditioned upon individual photon counting measurements. Measurements of the transmitted photon beam strongly affects the system state, since detection of an outgoing photon confirms that the PSII must be in the electronic ground state, while a null measurement implies it is in an excited electronic state. We show that under ideal conditions, observing the null result transforms a state with a low excited state population to a state with nearly all population contained in the excited states. We study the PSII dynamics conditioned on such photon counting for both a pure excitonic model of PSII and a more realistic model with exciton-phonon coupling to a dissipative phononic environment. In the absence of such coupling, we show that the measured fluorescence rates show oscillations constituting a photon-counting witness of excitonic coherence. Excitonic coupling to the phonon environment has a strong effect on the observed rates of fluorescence, damping the oscillations. Addition of non-radiative decay and incoherent transitions to radical pair states in the reaction center to the phononic model allows extraction of a quantum efficiency of 92.5% from the long-time evolution, consistent with bulk experimental measurements.

Effects of elevated temperature and copper exposure on the physiological state of the coral Galaxea fascicularis

The co-occurrence of elevated seawater temperature and local stressors (heavy metal contamination) affects the ecophysiology of phototrophic species, and represents a risk to the environmental quality of coral reefs. Therefore, we investigated the effects of both Cu alone and Cu in combination with elevated temperature (ET) on the physiology of the coral Galaxea fascicularis, and measured the parameters related to the photo-physiology and oxidative state. G.fascicularis is one of the dominant coral species in the South China Sea which exhibits strong adaptability to environmental stress. We exposed the common coral species G.fascicularis to a series of environmentally relevant concentrations of Cu at 29 °C (normal temperature, NT) and 32 °C (elevated temperature, ET) for 96 h. Single polyps were used in the experiments, which reduced individual variability when compared to the coral colonies. The results suggested that: i) Cu or ET had significant negative effects on the actual operating ability of photosystem Ⅱ (PSII), but not on the maximal chlorophyll fluorescence in darkness (Fv/Fm). ii) Symbiodiniaceae density was significantly reduced by high Cu concentrations, for Cu-NT and Cu-ET, a high concentration of Cu (40 μg/L) significantly impacted Symbiodiniaceae density, causing a 75.4% and 81.0% decrease, respectively. iii) the content of malondialdehyde (MDA) in coral tissues increased significantly under Cu-ET. iv) a certain range of copper concentration (25–30 μg/L) increased the pigment content of the Symbiodiniacea. Our results indicated that the combined stressors of Cu and ET made the coral tissue sloughed, caused the coral tissue damaged by lipid oxidation, reduced the photosynthetic capacity of the Symbiodiniacea, and led to the excretion of Symbiodiniacea.

Exploring the tropical acclimation of European and American hop cultivars (Humulus lupulus L.): Focus on physiology, productivity, and chemical composition

Hop (Humulus lupulus L.) cones are widely used by the beer industry to provide flavor, aroma, and bitterness due to the presence of α-acids, β-acids, and volatile oil components. Although the brewing sector plays an important role in Brazil’s economy, hops are entirely imported. In this study, we aimed to investigate the physiological acclimation potential, productivity, and chemical composition of four hop cultivars in a warm tropical region of Brazil. For this purpose, three North American-bred hop cultivars (‘Cascade’, ‘Chinook’ and ‘Triple Pearl’) and one European hop cultivar (‘Saaz’) that grew under tropical ambient conditions for two consecutive years were used. ‘Saaz’ and ‘Triple Pearl’ showed higher mortality and smaller photosynthetic rates, while ‘Cascade’ and ‘Chinook’ cultivars showed the best physiological and growth performance under a tropical-type climate. With a few exceptions, maximum quantum yield of photosystem II (PSII), chlorophyll content, and antioxidant enzymes activity were very similar between these two cultivars. A higher dry mass of cones was observed in ‘Chinook’ and ‘Cascade’, however, with a higher variability within each cultivar. Both cultivars increased the production of the dry mass of cones in the second year. The average concentration of bitter acids in ‘Cascade’ and ‘Chinook’ was below the expected values, but there was a considerable increase in the volatile oil content between the first and second year of cultivation. We concluded that 'Cascade' and 'Chinook', American cultivars, showed better development and lower mortality compared to 'Triple Pearl' (American cultivar) and 'Saaz' (European cultivar) under tropical climate conditions in Brazil. Although the concentration of bitter acids was lower than expected in the first two years, the concentration may increase with plant maturity.

Physiological, transcriptional and metabolomic evidence for arbuscular mycorrhizal fungi and Lactobacillus plantarum in peanut resistance to salinity stress

AbstractArbuscular mycorrhizal fungi (AMF) and Lactobacillus plantarum (LP) play pivotal roles in plant salinity resistance; however, difficulties are still exist in ascertaining their synergistic effects in counteracting legume soil salinity. Here, two peanut cultivars (salt‐tolerant and salt‐sensitive) were subjected to salinity stress, and the alleviation effects of combined microbial agent (CMA, inoculation with AMF + application with LP) on peanut salinity tolerance have been comprehensively characterized. CMA significantly enhanced the biomass production, leaf relative water content, increased the net photosynthetic rate, the maximal photochemical efficiency of photosystem II (PSII) and strengthened the antioxidant system, while dramatically decreased the reactive oxygen species (ROS) accumulation, lipid peroxidation and relative electrolyte conductivity under salinity conditions. Moreover, transcriptional and metabolomic evidence advocated that a subset of stress‐responsive pathways involved in plant growth (e.g. sucrose and starch), photosystem, antioxidant response, signal transduction (e.g. phytohormone and MAPK), osmotic homeostasis (e.g. total soluble sugar and amino acids) and root metabolism (e.g. asparagine and phenylpropanoid) have been regulated by CMA. Taken together, the physiological, transcriptional and metabolomic results indicate that CMA could induce peanut salinity tolerance through increasing plant growth performance, maintaining photosynthetic apparatus integrity, enhancing antioxidant system and regulating root metabolism. This study provides a promising CMA product and would be important for deepening the knowledge of the mechanisms regarding bacterial–fungal interactions.

Effects of maize and peanut co-ridge intercropping on crop photosynthetic characteristics and intercropping advantages

To clarify the photosynthetic mechanism contributing to the enhancement of intercropping advantages through co-ridge intercropping of maize and peanut, we conducted a field randomized block experiment under two phosphorus levels of 0(P0) and 180 kg P2O5·hm-2(P180) with flat intercropping of maize and peanut (FIC) as the control. We analyzed the effects of co-ridge intercropping of maize and peanut (RIC) and groove-ridge intercropping of maize and peanut (GIC) on crop leaf area index (LAI), SPAD values, CO2 carboxylation ability, photosystems coordination (ΦPSⅠ/PSⅡ), and intercropping advantage of yield. The results showed that RIC significantly increased SPAD value at the silking stage of intercropping maize, and significantly improved the apparent quantum yield of photosynthesis (AQY), maximum electron transfer rate (Jmax), maximum rate of Rubisco carboxylation (Vc,max), net photosynthetic rate at the CO2 saturation (Amax) and ΦPSⅠ/PSⅡ of intercropping maize compared with those of FIC and GIC at silking stage and milking stage, but reduced the ratio of variable fluorescence Fk to amplitude Fj-Fo(Wk) and the ratio of variable fluorescence Fj to amplitude Fp-Fo(Vj) of the functional leaf photosystem Ⅱ (PSⅡ) at the milking stage of maize. There were no significant differences in these parameters between FIC and GIC. Compared with FIC, both RIC and GIC increased LAI of intercropping peanut at late growth stage and SPAD value at pod setting stage, significantly improved Vc,max, Amax, and ΦPSⅠ/PSⅡ, and reduced Wk and Vj values of intercropping peanut functional leaves at pod expanding stage. The difference in these parameters between RIC and GIC were not significant. The land equivalent ratio and intercropping advantages of RIC were higher than those of FIC and GIC. Phosphorus application could further promote Vc,max, Jmax, Amax and ΦPSⅠ/PSⅡ of intercropping maize and peanut, and significantly improve yield advantages of intercropping. The findings indicated that co-ridge intercropping could enhance CO2 carboxylation and fixation by improving photosynthetic electron transport and pho-tosystems coordination, improve the photosynthetic rate of functional leaves of maize and peanut, thus increase crop yield and intercropping advantages.

Photoinactivation vs repair of photosystem II as target of thermal stress in epipelic and epipsammic microphytobenthos communities.

Microphytobenthos (MPB) inhabiting intertidal flats are exposed to large and sudden changes in temperature, often simultaneously with exposure to direct sunlight. These conditions are expected to negatively impact photosynthesis by exacerbating the photoinhibition under high light. This study addressed the photoinhibitory effects of short-term exposure to cold (5°C) and moderate heat (35°C) on MPB dominated by motile epipelic (EPL) and immotile epipsammic (EPM) diatom species, by evaluating the seasonal variation of photoinactivation and repair of photosystem II (PSII). The susceptibility to PSII photoinactivation and the counteracting repair capacity were measured by the constant rates kPI and kREC, respectively. The photoacclimation state was characterized by hysteresis light-response curves (HLC) of the relative electron transport rate, rETR, and of the nonphotochemical quenching index Y(NPQ). Under non-stress conditions (20°C), kREC was on average almost 10x higher than the corresponding kPI (20.4 vs 2.70 × 10-4 s-1, respectively), indicating the operation of efficient repair mechanisms. Overall, the exposure to low and high temperatures affected both PSII photoinactivation and repair but causing smaller impacts in the former than in the latter. Also, cold stress caused larger effects on repair (decrease of kREC) than on photoinactivation (increase of kPI), but heat stress affected similarly the two processes. These effects varied seasonally, suggesting a role of thermal acclimation, as heat stress had stronger effects in cold-acclimated samples and cold stress resulted in stronger effects in heat-acclimated samples. The changes in kPI and kREC occurred despite the high light-acclimated phenotype found all year round, indicating that these processes vary independently from the photoacclimation state. The results also showed that photoprotection processes, as measured by energy-dependent non-photochemical index qE, appear to have an important role, both by preventing PSII photoinactivation and by alleviating the impacts on PSII repair under acute thermal stress.

FZL, a dynamin-like protein localized to curved grana edges, is required for efficient photosynthetic electron transfer in Arabidopsis.

Photosynthetic electron transfer and its regulation processes take place on thylakoid membranes, and the thylakoid of vascular plants exhibits particularly intricate structure consisting of stacked grana and flat stroma lamellae. It is known that several membrane remodeling proteins contribute to maintain the thylakoid structure, and one putative example is FUZZY ONION LIKE (FZL). In this study, we re-evaluated the controversial function of FZL in thylakoid membrane remodeling and in photosynthesis. We investigated the sub-membrane localization of FZL and found that it is enriched on curved grana edges of thylakoid membranes, consistent with the previously proposed model that FZL mediates fusion of grana and stroma lamellae at the interfaces. The mature fzl thylakoid morphology characterized with the staggered and less connected grana seems to agree with this model as well. In the photosynthetic analysis, the fzl knockout mutants in Arabidopsis displayed reduced electron flow, likely resulting in higher oxidative levels of Photosystem I (PSI) and smaller proton motive force (pmf). However, nonphotochemical quenching (NPQ) of chlorophyll fluorescence was excessively enhanced considering the pmf levels in fzl, and we found that introducing kea3-1 mutation, lowering pH in thylakoid lumen, synergistically reinforced the photosynthetic disorder in the fzl mutant background. We also showed that state transitions normally occurred in fzl, and that they were not involved in the photosynthetic disorders in fzl. We discuss the possible mechanisms by which the altered thylakoid morphology in fzl leads to the photosynthetic modifications.

Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2

Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when TyrD● is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash when TyrD is not oxidized. The additional feature was "W-shaped" with troughs at 434nm and 446nm. This feature was absent when TyrD was reduced, but was present (i) when TyrD was physically absent (and replaced by phenylalanine) or (ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434nm and 446nm, seemed to require the native structural environment around the reduced TyrD and its H bonding partners to be present. We found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in the W-shaped difference spectrum. However, the use of a mutant of the PD2 axial His ligand, the D2-His197Ala, shows that the PD2 environment seems involved in the formation of "W-shaped" signal.

Response of sugarcane plants with modified cytokinin homeostasis under water deficit conditions

Cytokinins (CK) are one of the main phytohormones involved in plant growth and development and play a role in plant adaptation to stress. In this study, the response to water deficit stress was investigated in sugarcane (Saccharum spp. hybrids) plants after exogenous application of CK and partial silencing of endogenous cytokinin oxidase/dehydrogenase (CKX) gene expression in transgenic plants. The CKX enzyme is responsible for CK breakdown in plant cells. A foliar drench of N6-benzyladenin on leaves (100 μM) resulted in plants that survived more than 10 days of water deficit stress compared with the untreated control plants, which displayed senescence and leaf wilting after 10 days without water. Furthermore, after 10 days without water, the treated plants maintained a high efficiency of the photosystem II (PSII) and displayed higher levels of chlorophyll and relative water content, and after 20 days without water, high catalase activity and proline content when compared with the untreated controls. In addition, sugarcane calli were transformed with a recombinant vector containing a 237 bp sugarcane ShCKX target sequence which included a conserved CK binding domain cloned in sense and antisense orientations. Partial silencing of CKX expression in the transgenic plants resulted in significantly lower salicylic acid levels after 21 days of deficit stress compared with untransformed controls. Stressed transgenic plants accumulated less H2O2, more antioxidants (superoxide dismutase and catalase) and proline when compared with the wild type (WT) plants. Moreover, the transgenic plants were able to retain high levels of chlorophyll and most maintained the efficiency of PSII after watering was stopped. The transgenic genotype with the highest level of CKX silencing displayed delayed leaf senescence and maintained high leaf water content under water deficit stress compared with the WT plants. Both the exogenous application of CK and the partial silencing of CKX expression enabled the plants to resprout after re-watering following the stress period. The results indicate that the manipulation of CK levels, either by increasing CK exogenously or deceasing their break down by partial silencing of the CKX gene, in sugarcane positively affects the response to water deficit stress through a variety of physiological mechanisms.

Impact of varying light spectral compositions on photosynthesis, morphology, chloroplast ultrastructure, and expression of light-responsive genes in Marchantia polymorpha

Marchantia polymorpha is a convenient model for studying light of different spectral compositions on various physiological and biochemical processes because its photoreceptor system is vastly simplified. The influence of red light (RL, 660 nm), far-red light (FRL, 730 nm), blue light (BL, 450 nm), and green light (GL, 525 nm) compared to white light (high-pressure sodium light (HPSL), white LEDs (WL 450 + 580 nm) and white fluorescent light (WFL) on photosynthetic and transpiration rates, photosystem II (PSII) activity, photomorphogenesis, and the expression of light and hormonal signaling genes was studied. The ultrastructure of the chloroplasts in different tissues of the gametophyte M. polymorpha was examined. FRL led to the formation of agranal chloroplasts (in the epidermis and the chlorenchyma) with a high starch content (in the parenchyma), which led to a reduced intensity of photosynthesis. BL increased the transcription of genes for the biosynthesis of secondary metabolites - chalcone synthase (CHS), cellulose synthase (CELL), and L-ascorbate peroxidase (APOX3), which is consistent with the increased activity of low-molecular weight antioxidants. FRL increased the expression of phytochrome apoprotein (PHY) and cytokinin oxidase (CYTox) genes, but the expression of the phytochrome interacting factor (PIF) gene decreased, which was accompanied by a significant change in gametophyte morphology. Analysis of crosstalk gene expression, and changes in morphology and photosynthetic activity was carried out.

OsPML2, a chloroplast envelope localized transporter is involved in manganese homeostasis in rice

Manganese (Mn), a vital element, plays crucial roles in various biochemical and physiological processes by serving as an essential cofactor for numerous enzymes and acting as a catalytically active metal within biological clusters. In this study, we investigate the role of PHOTOSYNTHESIS-AFFECTED MUTANT 71-LIKE 2 (OsPML2), a member of the UNCHARACTERIZED PROTEIN FAMILY 0016 (UPF0016) family, in regulating Mn homeostasis in rice. OsPML2 was highly expressed in young leaves, ovaries, and stigmas. Cross sections from young leaves revealed that OsPML2 was mainly expressed in the phloem region and mesophyll cells. Furthermore, heterologous expression of OsPML2 restored the growth of Mn uptake-defective yeast strain Δsmf1 under Mn-limited conditions. Subcellular localization analysis demonstrated that OsPML2 was specifically localized in the chloroplast envelope. Knockdown of OsPML2 resulted in reduced chloroplast Mn content, significantly affecting plant growth under Mn deficiency. Furthermore, analysis of isolated thylakoid membranes using blue native gels indicated a compromised accumulation of photosystem II (PSII) complexes in OsPML2 knockdown lines. Additionally, grain yield, grain length, and width were significantly reduced in OsPML2 knockdown plants. Collectively, our findings provide insights into the transport function of OsPML2, which facilitates Mn transport from the cytosol to chloroplast stroma and influences the accumulation of PSII complexes in rice.

The Influence of Sodium Salt on Growth, Photosynthesis, Na+/K+ Homeostasis and Osmotic Adjustment of Atriplex canescens under Drought Stress

Atriplex canescens is widely cultivated as drought and salt-tolerant fodder in arid regions of Northwest China, which is used for photoremediation of degraded land and soil and water conservation. To explore the growth performance of A. canescens when exposed to drought and salt stress, seedlings were treated with a range of drought stress (WC1: 75 ± 3.6%, WC2: 49 ± 2.9% and WC3: 27 ± 2.5% of soil water content) and the corresponding drought stress with additional sodium salt supplementation (NaCl:Na2SO4 = 1:1 with the total concentration of Na+ set to 150 mM). The findings of this paper indicated that moderate sodium salt could stimulate the growth of A. canescens and effectively alleviate the deleterious impact of drought stress by increasing the turgor potential (ψt) and relative water content (RWC) and decreasing the leaf water osmotic potential (ψs). Furthermore, the photosynthetic capacity was improved and the negative effects of drought stress on photosystem II (PSII) were mitigated. The extra 150 mM sodium salt also markedly increased the contribution of Na+ to ψs and the contribution of betaine to ψs. In summary, these results indicate that A. canescens can adapt to drought stress by accumulating enough Na+ for osmotic adjustment (OA). Additionally, this paper is aimed to provide a fundamental basis for the utilization and cultivation of A. canescens as a favored pasture crop in the Qaidam basin, thus increasing the ecological and environmental benefits for arid regions worldwide.