Photosynthetic ROS and retrograde signaling pathways

Organellar genome divergence and environmental stress induce transcriptional cytonuclear responses in wheat alloplasmic hybrids

The union of two or more different nuclear genomes with maternally inherited organellar genomes may lead to cytonuclear incompatibilities in plant allopolyploids. These incompatibilities may be reconciled by coevolutionary responses at the genomic and transcriptional levels. To date, the relationship between extent of divergence among parental organellar genomes and cytonuclear coevolutionary responses remains largely unexplored. Here, we studied transcriptional cytonuclear expression in synthetic alloplasmic allohexaploid wheat lines having the same nuclear subgenomic composition (the nuclear genome of Chinese Spring) but with varying cytoplasmic organelles (plasmon donors from B- and D- lineage Triticum/Aegilops species). A positive association between the extent of plastid organellar divergence from naturally occurring organelles in euplasmic Chinese Spring and transcriptional cytonuclear responses was observed. This response was enhanced under stress (highlight) treatment and was determined to be related to differential subgenomic methylation levels. These data comprise a transcriptional dimension of cytonuclear responses to polyploidy and point to a potentially responsible epigenomic regulatory mechanism.

Shrink or expand? Just relax! Bidirectional grana structural dynamics as early light-induced regulator of photosynthesis

Light-induced structural changes in thylakoid membranes have been reported for decades, with conflicting data regarding their shrinkage or expansion during dark-light transitions. Understanding these dynamics is important for both fundamental photosynthesis research and agricultural applications. This research investigated the temporal sequence of thylakoid structural changes during light exposure and their functional significance. We combined high-resolution structural approaches (transmission electron microscopy, confocal microscopy with 3D modeling, and small-angle neutron scattering) with spectroscopic and electrophoretic analyses of the photosynthetic apparatus of Arabidopsis thaliana and Ficus elastica plants. A meta-analysis of published ultrastructural data complemented our experimental approach to resolve existing contradictions. We discovered a three-phase response pattern: initial shrinkage, expansion, and relaxation to dark-state equilibrium. The initial shrinkage specifically regulated the cyclic/linear electron transport ratio, providing rapid photoprotection. We also showed that plants' acclimation to different light regimes modulates the kinetics of this response, with constant-light-grown plants exhibiting faster structural adaptations than those acclimated to glasshouse conditions. This work challenges the traditional binary model of light-induced thylakoid structural dynamics, revealing a sophisticated temporal regulatory mechanism, with the dark-adapted state serving as a relaxed equilibrium. The discovered three-phase response reconciles decades of conflicting observations and reveals how plants achieve rapid photoprotection before engaging longer term adaptive responses.

Lipid metabolism in Cycas panzhihuaensis exposed to combined stress of drought and high temperatures and subsequent recovery

Cycas panzhihuaensis inhabits regions where summer temperatures can exceed 40 °C, and these extreme conditions may intensify with ongoing global warming. However, how this species adapts to such thermal extremes is not well understood. To investigate the responses of C. panzhihuaensis to heat stress, some physiological characteristics along with lipid and fatty acid profiles were analyzed. The results show that heat stress induced soil water loss but did not cause leaf water loss and visible symptoms of leaf damage. However, photoinhibition was induced and heat dissipation was inhibited under the stress. In the recovered plants, both heat dissipation and maximum photochemical efficiency exhibited significant increases compared to the stressed plants but did not return to the control level. Most lipid categories including phospholipids and saccharolipids accumulated significantly following both the stress and subsequent recovery. However, the content of total neutral glycerolipids maintained unchanged after various treatments. The ratio of phosphatidylcholine/phosphatidylethanolamine decreased significantly and the ratios of both digalactosyldiacylglycerol/monogalactosyldiacylglycerol and triacylglycerol/diacylglycerol increased significantly in the stressed plants. Compared to the control plants, the relative content of polyunsaturated fatty acids significantly increased, while that of both saturated and monounsaturated fatty acids significantly declined in both stressed and recovered plants. Under stress conditions, the unsaturation levels of total neutral glycerolipids and their constituent components significantly increased, whereas those of phosphatidylglycerol and total saccharolipids exhibited a marked decrease. In conclusion, C. panzhihuaensis can tolerate extremely high temperatures to some extent which might be associated with the adjustments in lipid composition and unsaturation levels.

Intracellular Mn mobility and differential response to H2O2 accumulation explain the susceptibility of litchi cultivars to dark pericarp disease

Dark pericarp disease (DPD) in litchi is a physiological disease caused by excess manganese (Mn) in pericarp, impacting fruit appearance and marketability and leading to substantial economic loss. The susceptibility of litchi varieties to DPD differs greatly, but the underlying mechanisms remain vague. In this study, we investigated the discrepancies in physiological and biochemical processes in pericarp of two varieties (Feizixiao and Heiye) resistant to DPD and a susceptible cultivar (Guiwei) during fruit development from the same orchard. Pericarp Mn in Guiwei was significantly lower than that in Feizixiao and slightly higher than that in Heiye through fruit growth. Under Mn stress, Feizixiao and Heiye maintained ROS homeostasis, whereas substantial H2O2 accumulated in Guiwei. Reduced anthocyanins and soluble sugars and increased lignin were observed in diseased Guiwei compared to Feizixiao and Heiye. The expression of genes encoding Mn transporters, light-harvesting antenna complex, ROS scavenging proteins and enzymes involved in anthocyanin synthesis was downregulated, whereas that of genes functioning in H2O2 production and lignin synthesis was upregulated in Guiwei, and that of genes involved in glucose metabolism was altered, suggesting that Mn was poorly transported and sequestrated within Guiwei pericarp cells, and excess Mn boosted H2O2 overproduction. The inhibited anthocyanin synthesis, enhanced lignin accumulation and tuned sugar metabolism conferred Guiwei adaptability to Mn stress. Conclusively, the poor Mn intracellular transferability and variations in response to H2O2 accumulation associated with disturbed photosynthetic energy deliver under excess Mn, are collaboratively responsible for the cultivar-dependent DPD vulnerability in litchi.

Nitric Oxide and Melatonin Cross Talk on Photosynthetic Machinery

Nitric oxide (NO) and melatonin (MT) significantly influence photosynthetic processes by modulating redox homeostasis, chlorophyll content, stomatal conductance, and gene expression, particularly under abiotic stress conditions. This review summarizes the intricate crosstalk between NO and melatonin, focusing on their coordinated roles in regulating photosynthetic efficiency. Evidence from various plant species indicates that the application of exogenous NO and melatonin enhances chlorophyll content, photosystem efficiency (particularly PSII), and photosynthetic performance, mitigating stress-induced damage. Molecular analysis demonstrates that both molecules influence key photosynthetic gene modulating photosystems I and II, and Calvin cycle activities. Moreover, NO and melatonin collaboratively regulate stomatal movements through ABA, Ca2⁺, and H2O2 signaling pathways, involving genes such as PMRT1, CIPKs, and OST1. Experimental data from diverse plant species under stress conditions, including drought, salinity, heavy metals, and flooding, highlight their synergistic protective effects. Exploring these mechanisms further may enable practical agricultural strategies involving combined NO and melatonin treatments to improve crop resilience and productivity under increasingly challenging environmental conditions. Future research directions should emphasize unraveling detailed molecular interactions, enabling targeted biotechnological applications in crop improvement programs for enhanced global food security.

Chloroplastic Aspartyl-tRNA Synthetase Is Required for Chloroplast Development, Photosynthesis and Photorespiratory Metabolism

Photorespiration is a complex metabolic process linked to primary plant metabolism and influenced by environmental factors, yet its regulation remains poorly understood. In this study, we identified the asprs3-1 mutant, which displays a photorespiratory phenotype with leaf chlorosis, stunted growth, and diminished photosynthesis under ambient CO2, but normal growth under elevated CO2 conditions. Map-based cloning and genetic complementation identified AspRS3 as the mutant gene, encoding an aspartyl-tRNA synthetase. AspRS3 is localised in both chloroplasts and mitochondria, with the chloroplast being the primary site of its physiological function. The AspRS3 mutation impacts the expression of plastid-encoded and photosynthesis-related genes, leading to decreased levels of chloroplast-encoded proteins such as ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (RBCL) and ferredoxin-dependent glutamate synthase (Fd-GOGAT). Furthermore, we observed an accumulation of photorespiratory intermediates, including glycine and glycerate, and reactive oxygen species (ROS) in asprs3-1. However, under high CO2, the expression of these proteins, the accumulation of photorespiratory intermediates, and ROS levels in asprs3-1 did not significantly differ from those in the wild type. We propose that elevated CO2 mitigates the asprs3-1 phenotype by inhibiting Rubisco oxygenation and photorespiratory metabolism. This study highlights the role of aminoacyl-tRNA synthetases in regulating photorespiration and provides new insights into its metabolic control.

Growth inhibition of Pontederia crassipes to imidazolinones herbicides-group exposure

ALS-inhibiting imidazolinone herbicides are widely used for selective weed control in Clearfield® cropping systems. However, their physicochemical properties promote dispersion into adjacent aquatic environments, posing risks to non-target organisms such as aquatic macrophytes. This study aimed to elucidate the toxicological effects of the commercial formulation Kifix® (a mixture of imazapyr and imazapic) on Pontederia crassipes, with emphasis on its biochemical and physiological responses. Two experiments were conducted using herbicide concentrations ranging from 0.2-1.0 mg L-1, alongside untreated controls. Multiple parameters were evaluated in leaves and roots at 7 and 14 days after application, including visual symptoms, chlorophyll index, growth parameters, chlorophyll a fluorescence, gas exchange, epidermal anatomy, reactive oxygen species, lipid peroxidation, electrolyte leakage, antioxidant enzyme activity, glycolate oxidase, glutathione S-transferase, and acetolactate synthase activity, as well as carbohydrate, amino acid, and protein content. Upon exposure, mature leaves exhibited photochemical impairment, compromising carbon assimilation and photorespiration, and leading to carbohydrate accumulation. Stomatal aperture and conductance were also negatively affected. Oxidative stress responses and antioxidant enzyme activity changed in both leaves and roots. Notably, acetolactate synthase activity increased in treated plants, while protein and amino acid contents remained unchanged. Overall, Kifix® significantly impaired P. crassipes, particularly by inhibiting the development of new tissues-such as leaves and plantlets essential for reproduction and spread-while also triggering physiological and biochemical disturbances in mature tissues.

Dual Localization and Functional Divergence of V-ATPase Subunit A: Nuclear Shuttling Mediates Distinct Roles in Dark- and MeJA-Induced Leaf Senescence

Retrograde signaling regulates plant senescence, but the role of vacuoles in this process remains unclear. Here, we demonstrate that rice vacuolar H+-ATPase subunit A (OsVHA-A) localizes to both the cytoplasm and nucleus. Sucrose treatment increased OsVHA-A expression and nuclear accumulation, while darkness reduced it. Methyl jasmonate (MeJA) initially promoted OsVHA-A nuclear translocation but decreased it upon prolonged exposure. Downregulation of OsVHA-A expression accelerated MeJA-induced rice leaf senescence but delayed darkness-induced senescence. MeJA treatment also significantly upregulated the expression of OsMYC2 and OsMAPK6 in OsVHA-A-RNAi plants compared to wild-type plants. Moreover, OsVHA-A downregulation notably increased the level of expression of genes associated with sugar signaling and transport under dark conditions. Immunoprecipitation-mass spectrometry and molecular docking analyses identified interactions between OsVHA-A and OsTPR1, OsMed14, sucrose transporters, and enzymes involved in sucrose metabolism. The binding of OsVHA-A with OsTPR1 and OsSUS1 was confirmed by BiFC. These findings highlight the multifunctional role of OsVHA-A in coordinating organelles and nuclear signaling, providing new insights and potential strategies for manipulating senescence to improve rice yield and quality.

Interplay Between ROS and Hormones in Plant Defense Against Pathogens

Reactive oxygen species (ROS) are toxic by-products of aerobic cellular metabolism. However, ROS conduct multiple functions, and specific ROS sources can have beneficial or detrimental effects on plant health. This review explores the complex dynamics of ROS in plant defense mechanisms, focusing on their involvement in basal resistance, hypersensitive response (HR), and systemic acquired resistance (SAR). ROS, including superoxide anion (O2-), singlet oxygen (1O2), hydroxyl radicals (OH), and hydrogen peroxide (H2O2), are generated through various enzymatic pathways. They may serve to inhibit pathogen growth while also activating defense-related gene expression as signaling molecules. Oxidative damage in cells is mainly attributed to excess ROS production. ROS produce metabolic intermediates that are involved in various signaling pathways. The oxidative burst triggered by pathogen recognition initiates hyper-resistance (HR), a localized programmed cell death restricting pathogen spread. Additionally, ROS facilitate the establishment of SAR by inducing systemic signaling networks that enhance resistance across the plant. The interplay between ROS and phytohormones such as jasmonic acid (JA), salicylic acid (SA), and ethylene (ET) further complicates this regulatory framework, underscoring the importance of ROS in orchestrating both local and systemic defense responses. Grasping these mechanisms is essential for creating strategies that enhance plant resilience to biotic stresses.

Elucidating the interaction and toxicity of cadmium and cerium on the growth of maize seedlings: Insights from morpho-physiological and biochemical analysis

The exploitation of rare earth elements (REEs) is often accompanied by heavy metal contamination. However, our understanding regarding the growth responses of plants to the co-existence of REEs and heavy metals (HMs), remains limited. In this study, cerium (Ce) and cadmium (Cd) were selected as representatives of REEs and HMs to investigate their interactive effects on maize growth through multiple model analyses. The results revealed that both Cd and Ce induce oxidative injuries by increasing reactive oxygen species (ROS) content in a dose-dependent manner. Ce can enhance chlorophyll content while reducing leaf yellowing induced by Cd. The addition of 10 and 100 mg· L-1 Ce significantly increased the Chla content in 50 μM Cd sets by 52.2 % and 50.2 % compared to Cd50Ce0 treatment, respectively. Evaluation of the physiological and biochemical effect level index (PBELI) showed that the primary interaction mode of Cd and Ce was antagonism. The co-existence of Cd (50 μM) and Ce (100 mg· L-1) poses a higher ecological risk than Ce alone. These results demonstrated that combined exposure to Cd and Ce exhibited diverse effects in mitigating the inhibition of maize growth, thereby improving our understanding of phytotoxicity resulting from metal mixtures in the environment.

The chloroplast-located HKT transporter plays an important role in fertilization and development in Physcomitrium patens

Cell survival depends on the maintenance of cell homeostasis that involves all the biochemical, genomic and transport processes that take place in all the organelles within a eukaryote cell. In particular, ion homeostasis is required to regulate the membrane potential and solute transport across all membranes, any alteration in these parameters will reflect in the malfunctioning of any organelle, and consequently, in the development of the organism. In plant cells, sodium transporters play a central role in keeping the concentrations of this cation across all membranes under physiological conditions to prevent its toxic effects. HKT transporters are a family of membrane proteins exclusively present in plants, with some homologs present in prokaryotes. HKT transporters have been associated to salt tolerance in plants, retrieving any leak of the cation into the xylem, or removing it from aerial parts, including the flowers, to be transported to the roots along the phloem. This function has been assigned as most of the HKT transporters are located at the plasma membrane. Here, we report the localization of the HKT from Physcomitrium patens to the thylakoid membrane, reminiscent of the prokaryote origin of these family of transporters. Mutation of PpHKT leads to several alterations in the phenotype of the organism, including the lack of sporophyte formation, and changes in expression of many genes. These alterations suggest that the breakdown in chloroplast ion homeostasis triggers a signalling cascade to the nucleus to communicate its status, being important for the moss to complete its life cycle.

Reactive oxygen species-mediated signal transduction and utilization strategies in microalgae

Reactive oxygen species (ROS) are crucial in stress perception, the integration of environmental signals, and the activation of downstream response networks. This review emphasizes ROS-mediated signaling pathways in microalgae and presents an overview of strategies for leveraging ROS. Eight distinct signaling pathways mediated by ROS in microalgae have been summarized, including the calcium signaling pathway, the target of rapamycin signaling pathway, the mitogen-activated protein kinase signaling pathway, the cyclic adenosine monophosphate/protein kinase A signaling pathway, the ubiquitin/protease pathway, the ROS-regulated transcription factors and enzymes, the endoplasmic reticulum stress, and the retrograde ROS signaling. Moreover, this review outlines three strategies for utilizing ROS: two-stage cultivation, combined stress with phytohormones, and strain engineering. The physicochemical properties of various ROS, together with their redox reactions with downstream targets, have been elucidated to reveal the role of ROS in signal transduction processes while delineating the ROS-mediated signal transduction network within microalgae.

Photochemical regulation of microcystin synthesis and release in cyanobacteria Microcystis aeruginosa by triplet state dissolved organic matter

The increasing frequency of cyanobacterial blooms, particularly those induced by Microcystis aeruginosa (M. aeruginosa), poses severe economic, ecological and health challenges due to the production of microcystins (MCs). Environmental parameters such as light and nutrient availability influence MCs production, while the role of dissolved organic matter (DOM) photochemical processes in regulating these remains unclear. This study investigates the effects of light-induced triplet dissolved organic matter (3DOM⁎), derived from various DOM origins, on the photosynthesis, MC synthesis and release by M. aeruginosa. Using DOM photochemical experiments and spectroscopic analysis for humic acid, fulvic acid and extracellular organic matter, the regulatory mechanisms underlying gene expression related to MCs production (mcyD and mcyH) using real-time quantitative PCR was elucidated. Our findings indicate that 3DOM⁎ induces an oxidative stress in M. aeruginosa, reducing photosynthetic efficiency and modulating gene expression, thereby regulating MCs synthesis and release. Moreover, the optical properties of DOM from various sources exhibited distinct differential impacts on M. aeruginosa, highlighting the complex influence of DOM photochemistry on aquatic ecosystems. This research offers novel insights into the effects of DOM photochemical processes on MCs regulation and proposes strategies for managing cyanobacterial blooms and mitigating MCs contamination, contributing to significantly improved water quality management.

Morphological, Physiological, and Molecular Responses to Heat Stress in Brassicaceae

Food security is threatened by global warming, which also affects agricultural output. Various components of cells perceive elevated temperatures. Different signaling pathways in plants distinguish between the two types of temperature increases, mild warm temperatures and extremely hot temperatures. Given the rising global temperatures, heat stress has become a major abiotic challenge, affecting the growth and development of various crops and significantly reducing productivity. Brassica napus, the second-largest source of vegetable oil worldwide, faces drastic reductions in seed yield and quality under heat stress. This review summarizes recent research on the genetic and physiological impact of heat stress in the Brassicaceae family, as well as in model plants Arabidopsis and rice. Several studies show that extreme temperature fluctuations during crucial growth stages negatively affect plants, leading to impaired growth and reduced seed production. The review discusses the mechanisms of heat stress adaptation and the key regulatory genes involved. It also explores the emerging understanding of epigenetic modifications during heat stress. While such studies are limited in B. napus, contrasting trends in gene expression have been observed across different species and cultivars, suggesting these genes play a complex role in heat stress tolerance. Key knowledge gaps are identified regarding the impact of heat stress during the growth stages of B. napus. In-depth studies of these stages are still needed. The profound understanding of heat stress response mechanisms in tissue-specific models are crucial in advancing our knowledge of thermo-tolerance regulation in B. napus and supporting future breeding efforts for heat-tolerant crops.

EXECUTER1 and singlet oxygen signaling: A reassessment of nuclear activity

Chloroplasts are recognized as environmental sensors, capable of translating environmental fluctuations into diverse signals to communicate with the nucleus. Among the reactive oxygen species produced in chloroplasts, singlet oxygen (1O2) has been extensively studied due to its dual roles, encompassing both damage and signaling activities, and the availability of conditional mutants overproducing 1O2 in chloroplasts. In particular, investigating the Arabidopsis (Arabidopsis thaliana) mutant known as fluorescent (flu) has led to the discovery of EXECUTER1 (EX1), a plastid 1O2 sensor residing in the grana margin of the thylakoid membrane. 1O2-triggered EX1 degradation is critical for the induction of 1O2-responsive nuclear genes (SOrNGs). However, a recent study showed that EX1 relocates from chloroplasts to the nucleus upon 1O2 release, where it interacts with WRKY18 and WRKY40 (WRKY18/40) transcription factors to regulate SOrNG expression. In this study, we challenge this assertion. Our confocal microscopy analysis and subcellular fractionation assays demonstrate that EX1 does not accumulate in the nucleus. While EX1 appears in nuclear fractions, subsequent thermolysin treatment assays indicate that it adheres to the outer nuclear region rather than localizing inside the nucleus. Furthermore, luciferase complementation imaging and yeast 2-hybrid assays reveal that EX1 does not interact with nuclear WRKY18/40. Consequently, our study refines the current model of 1O2 signaling by ruling out the nuclear relocation of intact EX1 as a means of communication between the chloroplast and nucleus.

Temperature-driven changes in membrane fluidity differentially impact FILAMENTATION TEMPERATURE-SENSITIVE H2-mediated photosystem II repair

The Arabidopsis (Arabidopsis thaliana) yellow variegated2 (var2) mutant, lacking functional FILAMENTATION TEMPERATURE-SENSITIVE H2 (FtsH2), an ATP-dependent zinc metalloprotease, is a powerful tool for studying the photosystem II (PSII) repair process in plants. FtsH2, forming hetero-hexamers with FtsH1, FtsH5, and FtsH8, plays an indispensable role in PSII proteostasis. Although abiotic stresses like cold and heat increase chloroplast reactive oxygen species (ROS) and PSII damage, var2 mutants behave like wild-type plants under heat stress but collapse under cold stress. Our study on transgenic var2 lines expressing FtsH2 variants, defective in either substrate extraction or proteolysis, reveals that cold stress causes an increase in membrane viscosity, demanding more substrate extraction power than proteolysis by FtsH2. Overexpression of FtsH2 lacking substrate extraction activity does not rescue the cold-sensitive phenotype, while overexpression of FtsH2 lacking protease activity does in var2, with other FtsH isomers present. This indicates that FtsH2's substrate extraction activity is indispensable under cold stress when membranes become more viscous. As temperatures rise and membrane fluidity increases, substrate extraction activity from other isomers suffices, explaining the var2 mutant's heat stress resilience. These findings underscore the direct effect of membrane fluidity on the functionality of the thylakoid FtsH complex under stress. Future research should explore how membrane fluidity impacts proteostasis, potentially uncovering strategies to modulate thermosensitivity.

Effects of Drought Stress at the Booting Stage on Leaf Physiological Characteristics and Yield of Rice

Drought stress is a major environmental constraint that limits rice (Oryza sativa L.) production worldwide. In this study, we investigated the effects of drought stress at the booting stage on rice leaf physiological characteristics and yield. The results showed that drought stress would lead to a significant decrease in chlorophyll content and photosynthesis in rice leaves, which would affect rice yield. Three different rice varieties were used in this study, namely Hanyou73 (HY73), Huanghuazhan (HHZ), and IRAT109. Under drought stress, the chlorophyll content of all cultivars decreased significantly: 11.1% and 32.2% decreases in chlorophyll a and chlorophyll b in HHZ cultivars, 14.1% and 28.5% decreases in IRAT109 cultivars, and 22.9% and 18.6% decreases in HY73 cultivars, respectively. In addition, drought stress also led to a significant decrease in leaf water potential, a significant increase in antioxidant enzyme activity, and an increase in malondialdehyde (MDA) content, suggesting that rice activated a defense mechanism to cope with drought-induced oxidative stress. This study also found that drought stress significantly reduced the net photosynthetic rate and stomatal conductance of rice, which, in turn, affected the yield of rice. Under drought stress, the yield of the HHZ cultivars decreased most significantly, reaching 30.2%, while the yields of IRAT109 and HY73 cultivars decreased by 13.0% and 18.2%, respectively. The analysis of yield composition showed that the number of grains per panicle, seed-setting rate, and 1000-grain weight were the key factors affecting yield formation. A correlation analysis showed that there was a significant positive correlation between yield and net photosynthetic rate, stomatal conductance, chla/chlb ratio, Rubisco activity, and Fv/Fm, but there was a negative correlation with MDA and non-photochemical quenching (NPQ). In summary, the effects of drought stress on rice yield are multifaceted, involving changes in multiple agronomic traits. The results highlight the importance of selecting and nurturing rice varieties with a high drought tolerance, which should have efficient antioxidant systems and high photosynthetic efficiency. Future research should focus on the genetic mechanisms of these physiological responses in order to develop molecular markers to assist in the breeding of drought-tolerant rice varieties.