Arabidopsis guard cell chloroplasts import cytosolic ATP for starch turnover and stomatal opening

Mitochondrial respiration is essential for photosynthesis-dependent ATP supply of the plant cytosol

Plants rely on solar energy to synthesize ATP and NADPH for photosynthetic carbon fixation and all cellular need. Mitochondrial respiration is essential in plants, but this may be due to heterotrophic bottlenecks during plant development or because it is also necessary in photosynthetically active cells. In this study, we examined in vivo changes of cytosolic ATP concentration in response to light, employing a biosensing strategy in the moss Physcomitrium patens and revealing increased cytosolic ATP concentration caused by photosynthetic activity. Plants depleted of respiratory Complex I showed decreased cytosolic ATP accumulation, highlighting a critical role of mitochondrial respiration in light-dependent ATP supply of the cytosol. Consistently, targeting mitochondrial ATP production directly, through the construction of mutants deficient in mitochondrial ATPase (complex V), led to drastic growth reduction, despite only minor alterations in photosynthetic electron transport activity. Since P. patens is photoautotrophic throughout its development, we conclude that heterotrophic bottlenecks cannot account for the indispensable role of mitochondrial respiration in plants. Instead, our results support that mitochondrial respiration is essential for ATP provision to the cytosol in photosynthesizing cells. Mitochondrial respiration provides metabolic integration, ensuring supply of cytosolic ATP essential for supporting plant growth and development.

Nano-Fe3O4: Enhancing the tolerance of Elymus nutans to Cd stress through regulating programmed cell death

Cadmium (Cd) poses a significant threat to plant growth and the environment. Nano-Fe3O4 is effective in alleviating Cd stress in plants. Elymus nutans Griseb. is an important fodder crop on the Qinghai-Tibetan Plateau (QTP). However, the potential mechanism by which nano-Fe3O4 alleviates Cd stress in E. nutans is not well understood. E. nutans were subjected to single Cd, single nano-Fe3O4, and co-treatment with nano-Fe3O4 and Cd, and the effects on morphology, Cd uptake, antioxidant enzyme activity, reactive oxygen species (ROS) levels and programmed cell death (PCD) were studied to clarify the regulatory mechanism of nano-Fe3O4. The results showed that Cd stress significantly decreased the germination percentage and biomass of E. nutans. The photosynthetic pigment content decreased significantly under Cd stress. Cd stress also caused oxidative stress and lipid peroxidation, accumulation of excessive ROS, resulting in PCD, but the effect of nano-Fe3O4 was different. Seed germination, seedling growth, and physiological processes were analyzed to elucidate the regulatory role of nano-Fe3O4 nanoparticles in promoting photosynthesis, reducing Cd accumulation, scavenging ROS, and regulating PCD, to promote seed germination and seedling growth in E. nutans. This report provides a scientific basis for improving the tolerance of Elymus to Cd stress by using nano-Fe3O4.

Hydrogen peroxide is required for light-induced stomatal opening across different plant species

Stomatal movement is vital for plants to exchange gases and adaption to terrestrial habitats, which is regulated by environmental and phytohormonal signals. Here, we demonstrate that hydrogen peroxide (H2O2) is required for light-induced stomatal opening. H2O2 accumulates specifically in guard cells even when plants are under unstressed conditions. Reducing H2O2 content through chemical treatments or genetic manipulations results in impaired stomatal opening in response to light. This phenomenon is observed across different plant species, including lycopodium, fern, and monocotyledonous wheat. Additionally, we show that H2O2 induces the nuclear localization of KIN10 protein, the catalytic subunit of plant energy sensor SnRK1. The nuclear-localized KIN10 interacts with and phosphorylates the bZIP transcription factor bZIP30, leading to the formation of a heterodimer between bZIP30 and BRASSINAZOLE-RESISTANT1 (BZR1), the master regulator of brassinosteroid signaling. This heterodimer complex activates the expression of amylase, which enables guard cell starch degradation and promotes stomatal opening. Overall, these findings suggest that H2O2 plays a critical role in light-induced stomatal opening across different plant species.

In planta imaging of pyridine nucleotides using second-generation fluorescent protein biosensors

Redox changes of pyridine nucleotides in cellular compartments are highly dynamic and their equilibria are under the influence of various reducing and oxidizing reactions. To obtain spatiotemporal data on pyridine nucleotides in living plant cells, typical biochemical approaches require cell destruction. To date, genetically encoded fluorescent biosensors are considered to be the best option to bridge the existing technology gap, as they provide a fast, accurate, and real-time readout. However, the existing pyridine nucleotides genetically encoded fluorescent biosensors are either sensitive to pH change or slow in dissociation rate. Herein, we employed the biosensors which generate readouts that are pH stable for in planta measurement of NADH/NAD+ ratio and NADPH level. We generated transgenic Arabidopsis lines that express these biosensors in plastid stroma and cytosol of whole plants and pollen tubes under the control of CaMV 35S and LAT52 promoters, respectively. These transgenic biosensor lines allow us to monitor real-time dynamic changes in NADH/NAD+ ratio and NADPH level in the plastids and cytosol of various plant tissues, including pollen tubes, root hairs, and mesophyll cells, using a variety of fluorescent instruments. We anticipate that these valuable transgenic lines may allow improvements in plant redox biology studies.

Resource allocation modeling for autonomous prediction of plant cell phenotypes

Predicting the plant cell response in complex environmental conditions is a challenge in plant biology. Here we developed a resource allocation model of cellular and molecular scale for the leaf photosynthetic cell of Arabidopsis thaliana, based on the Resource Balance Analysis (RBA) constraint-based modeling framework. The RBA model contains the metabolic network and the major macromolecular processes involved in the plant cell growth and survival and localized in cellular compartments. We simulated the model for varying environmental conditions of temperature, irradiance, partial pressure of CO2 and O2, and compared RBA predictions to known resource distributions and quantitative phenotypic traits such as the relative growth rate, the C:N ratio, and finally to the empirical characteristics of CO2 fixation given by the well-established Farquhar model. In comparison to other standard constraint-based modeling methods like Flux Balance Analysis, the RBA model makes accurate quantitative predictions without the need for empirical constraints. Altogether, we show that RBA significantly improves the autonomous prediction of plant cell phenotypes in complex environmental conditions, and provides mechanistic links between the genotype and the phenotype of the plant cell.

Calvin cycle and guard cell metabolism impact stomatal function

Stomatal conductance (gs) determines CO2 uptake for photosynthesis (A) and water loss through transpiration, which is essential for evaporative cooling and maintenance of optimal leaf temperature as well as nutrient uptake. Stomata adjust their aperture to maintain an appropriate balance between CO2 uptake and water loss and are therefore critical to overall plant water status and productivity. Although there is considerable knowledge regarding guard cell (GC) osmoregulation (which drives differences in GC volume and therefore stomatal opening and closing), as well as the various signal transduction pathways that enable GCs to sense and respond to different environmental stimuli, little is known about the signals that coordinate mesophyll demands for CO2. Furthermore, chloroplasts are a key feature in GCs of many species, however, their role in stomatal function is unclear and a subject of debate. In this review we explore the current evidence regarding the role of these organelles in stomatal behaviour, including GC electron transport and Calvin-Benson-Bassham (CBB) cycle activity as well as their possible involvement correlating gs and A along with other potential mesophyll signals. We also examine the roles of other GC metabolic processes in stomatal function.

Integrative regulatory mechanisms of stomatal movements under changing climate

Global climate change-caused drought stress, high temperatures and other extreme weather profoundly impact plant growth and development, restricting sustainable crop production. To cope with various environmental stimuli, plants can optimize the opening and closing of stomata to balance CO2 uptake for photosynthesis and water loss from leaves. Guard cells perceive and integrate various signals to adjust stomatal pores through turgor pressure regulation. Molecular mechanisms and signaling networks underlying the stomatal movements in response to environmental stresses have been extensively studied and elucidated. This review focuses on the molecular mechanisms of stomatal movements mediated by abscisic acid, light, CO2 , reactive oxygen species, pathogens, temperature, and other phytohormones. We discussed the significance of elucidating the integrative mechanisms that regulate stomatal movements in helping design smart crops with enhanced water use efficiency and resilience in a climate-changing world.

Integrated physiological and transcriptomic analyzes reveal the duality of TiO2 nanoparticles on alfalfa (Medicago sativa L.)

Alfalfa (Medicago sativa L.) is a feed crop due to its rich nutrition and high productivity. The utilization of titanium oxide nanoparticles (TiO2 NPs) brings benefits to agricultural production but also has potential hazards. To investigate the duality and related mechanism of TiO2 NPs on alfalfa, its different doses including 0, 50, 100, 200, 500, and 1000 mg L- 1 (CK, Ti-50, Ti-100, Ti-200, Ti-500, and Ti-1000) were sprayed on leaves. The results showed that greater doses of TiO2 NPs (500 and 1000 mg L-1) negatively affected the physiological parameters, including morphology, biomass, leaf ultrastructure, stomata, photosynthesis, pigments, and antioxidant ability. However, 100 mg L-1 TiO2 NPs revealed an optimal positive effect; compared with the CK, it dramatically increased plant height, fresh weight, and dry weight by 22%, 21%, and 41%, respectively. Additionally, TiO2 NPs at low doses significantly protected leaf tissue, promoted stomatal opening, and enhanced the antioxidant system; while higher doses had phytotoxicity. Hence, TiO2 NPs are dose-dependent on alfalfa. The transcriptomic analysis identified 4625 and 2121 differentially expressed genes (DEGs) in the comparison of CK vs. Ti-100 and CK vs. Ti-500, respectively. They were mainly enriched in photosynthesis, chlorophyll metabolism, and energy metabolism. Notably, TiO2 NPs-induced phytotoxicity on photosynthetic parameters happened concurrently with the alterations of the genes involved in the porphyrin and chlorophyll metabolism and carbon fixation in photosynthetic organisms in the KEGG analysis. Similarly, it affected the efficiency of alfalfa energy transformation processes, including pyruvate metabolism and chlorophyll synthesis. Several key related genes in these pathways were validated. Therefore, TiO2 NPs have positive and toxic effects by regulating morphology, leaf ultrastructure, stomata, photosynthesis, redox homeostasis, and genes related to key pathways. It is significant to understand the duality of TiO2 NPs and cultivate varieties resistant to nanomaterial pollution.

Arabidopsis Sucrose Synthase 3 (SUS3) regulates starch accumulation in guard cells at the end of day

Starch in the stomatal guard cells is largely synthesized using carbon precursors originating from sugars imported from the leaf mesophyll. Such heterotrophic nature of guard cell starch synthesis prompted us to investigate the role of cytosolic sucrose synthases (SUS) in this pathway. Out of the six members of the Arabidopsis SUS gene family, SUS3 was the most highly expressed isoform in guard cells. The Arabidopsis sus3 mutant displayed changes in guard cell starch contents comparable to the Wild Type (WT) up until 6 h into the day. After this time point, sus3 guard cells surprisingly started to accumulate starch at very high rates, reaching the end of the day with significantly more starch than WT. Based on the phenotype of the sus3 mutant, we suggest that in guard cells, SUS3 is involved in the regulation of carbon fluxes towards starch synthesis during the second half of the day. SUS3 may be part of a previously predicted guard cell futile cycle of metabolic reactions, in which sucrose is re-synthesized from UDP-glucose to avoid excessive starch synthesis toward the end of the day. This is in contrast to typical storage organs, in which cytosolic SUS is required to produce ADP-glucose for starch synthesis.

Stomata variation in the process of polyploidization in Chinese chive (Allium tuberosum)

BACKGROUND

Stomatal variation, including guard cell (GC) density, size and chloroplast number, is often used to differentiate polyploids from diploids. However, few works have focused on stomatal variation with respect to polyploidization, especially for consecutively different ploidy levels within a plant species. For example, Allium tuberosum, which is mainly a tetraploid (2n = 4x = 32), is also found at other ploidy levels which have not been widely studied yet.

RESULTS

We recently found cultivars with different ploidy levels, including those that are diploid (2n = 2x = 16), triploid (2n = 3x = 24), pseudopentaploid (2n = 34-42, mostly 40) and pseudohexaploid (2n = 44-50, mostly 48). GCs were evaluated for their density, size (length and width) and chloroplast number. There was no correspondence between ploidy level and stomatal density, in which anisopolyploids (approximately 57 and 53 stomata/mm2 in triploid and pseudopentaploid, respectively) had a higher stomatal density than isopolyploids (approximately 36, 43, and 44 stomata/mm2 in diploid, tetraploid and pseudohexaploid, respectively). There was a positive relationship between ploidy level and GC chloroplast number (approximately 44, 45, 51, 72 and 90 in diploid to pseudohexaploid, respectively). GC length and width also increased with ploidy level. However, the length increased approximately 1.22 times faster than the width during polyploidization.

CONCLUSIONS

This study shows that GC size increased with increasing DNA content, but the rate of increase differed between length and width. In the process of polyploidization, plants evolved longer and narrower stomata with more chloroplasts in the GCs.

Graphene nanoparticles improve alfalfa (Medicago sativa L.) growth through multiple metabolic pathways under salinity-stressed environment

Graphene, one of the emerging carbon nanomaterials, has many advantages and applications. Salinity stress seriously affects ecology and agroforestry worldwide. The effects of graphene on alfalfa under salinity stress were investigated. The results indicated that graphene promoted alfalfa growth under non-salinity stress but caused some degree of damage to root cells and leaf parameters. Graphene used in salinity stress had a positive effect on growth parameters, chlorophyll, photosynthetic gas parameters, stomatal opening, ion balance, osmotic homeostasis, cell membrane integrity and antioxidant system, while it decreased Na+, lipid peroxidation and reactive oxygen species levels. Correlation analysis revealed that most of the parameters were significantly correlated; and principal component analysis indicated that the first two dimensions (78.1% and 4.1%) explained 82.2% of the total variability, and the majority of them exceeded the average contribution. Additionally, Gene Ontology functional enrichment analysis and Kyoto Encyclopedia of Genes and Genomes signaling pathway enrichment analysis showed that there were numerous differentially expressed genes and pathways to regulate alfalfa responding to salinity stress. Taken together, the findings reveal that graphene does not enter the plant, but improves the properties and adsorption of soil to enhance salt tolerance and seedling growth of alfalfa through morphological, physiological, biochemical, and transcriptomic aspects. Furthermore, this study provides a reference for the application of graphene to improve soil environment and agricultural production.

Unveiling the dark side of guard cell metabolism

Evidence suggests that guard cells have higher rate of phosphoenolpyruvate carboxylase (PEPc)-mediated dark CO2 assimilation than mesophyll cells. However, it is unknown which metabolic pathways are activated following dark CO2 assimilation in guard cells. Furthermore, it remains unclear how the metabolic fluxes throughout the tricarboxylic acid (TCA) cycle and associated pathways are regulated in illuminated guard cells. Here we carried out a13C-HCO3 labelling experiment in tobacco guard cells harvested under continuous dark or during the dark-to-light transition to elucidate principles of metabolic dynamics downstream of CO2 assimilation. Most metabolic changes were similar between dark-exposed and illuminated guard cells. However, illumination altered the metabolic network structure of guard cells and increased the 13C-enrichment in sugars and metabolites associated to the TCA cycle. Sucrose was labelled in the dark, but light exposure increased the 13C-labelling and leads to more drastic reductions in the content of this metabolite. Fumarate was strongly labelled under both dark and light conditions, while illumination increased the 13C-enrichment in pyruvate, succinate and glutamate. Only one 13C was incorporated into malate and citrate in either dark or light conditions. Our results indicate that several metabolic pathways are redirected following PEPc-mediated CO2 assimilation in the dark, including gluconeogenesis and the TCA cycle. We further showed that the PEPc-mediated CO2 assimilation provides carbons for gluconeogenesis, the TCA cycle and glutamate synthesis and that previously stored malate and citrate are used to underpin the specific metabolic requirements of illuminated guard cells.

Gene expression profile of the developing endosperm in durum wheat provides insight into starch biosynthesis

BACKGROUND

Durum wheat (Triticum turgidum subsp. durum) is widely grown for pasta production, and more recently, is gaining additional interest due to its resilience to warm, dry climates and its use as an experimental model for wheat research. Like in bread wheat, the starch and protein accumulated in the endosperm during grain development are the primary contributors to the calorific value of durum grains.

RESULTS

To enable further research into endosperm development and storage reserve synthesis, we generated a high-quality transcriptomics dataset from developing endosperms of variety Kronos, to complement the extensive mutant resources available for this variety. Endosperms were dissected from grains harvested at eight timepoints during grain development (6 to 30 days post anthesis (dpa)), then RNA sequencing was used to profile the transcriptome at each stage. The largest changes in gene expression profile were observed between the earlier timepoints, prior to 15 dpa. We detected a total of 29,925 genes that were significantly differentially expressed between at least two timepoints, and clustering analysis revealed nine distinct expression patterns. We demonstrate the potential of our dataset to provide new insights into key processes that occur during endosperm development, using starch metabolism as an example.

CONCLUSION

We provide a valuable resource for studying endosperm development in this increasingly important crop species.

Persistence of algal toxicity induced by polystyrene nanoplastics at environmentally relevant concentrations

Polystyrene (PS) often found in the ocean is one of the most commonly used plastic polymers in the world and can exist in different particle sizes. In particular, PS degrades relatively faster and widely accumulates at the nanoscale. Therefore, the penetration is strong and it is easy to enter the body and cause adverse effects. However, the persistence or recovery of their toxicity remains largely unclear. Here, we designed two subexperiments (exposure and recovery experiments) and investigated the persistence of the toxicity of polystyrene (PS) NPs at a wide concentration range (0.01-10 mg/L) to diatoms (Phaeodactylum tricornutum). PS-NPs significantly inhibited algal growth and clearly wrinkled the surfaces of cells, membrane permeability was significantly increased, and the steady-state state of cell redox and mitochondrial membrane potential was disturbed. However, in the recovery experiment, the increased membrane permeability was observed to persist, but the induced oxidative damage was reversible, and the absorbed NPs could be excreted. Integrated omics techniques (metabolomics and transcriptomics) revealed that PS-NPs significantly disrupts cell metabolism, including disturbances in fatty acid biosynthesis and enhanced biosynthesis of phenylalanine, tyrosine, and tryptophan. Inhibition of fatty acid, amino acid, energy and carbohydrate metabolism and disturbance of the antioxidant system contribute to the persistence of toxicity. These findings highlight the phenomena and mechanisms of the persistence of phytotoxicity and are critical to the accurate assessment of NPs.

Brassinosteroids mediate moderate soil-drying to alleviate spikelet degeneration under high temperature during meiosis of rice

This study tested the hypothesis that brassinosteroids (BRs) mediate moderate soil-drying (MD) to alleviate spikelet degeneration under high temperature (HT) stress during meiosis of rice (Oryza sativa L.). A rice cultivar was pot-grown and subjected to normal temperature (NT) and HT treatments during meiosis, and two irrigation regimes including well-watered (WW) and MD were imposed to the plants simultaneously. The MD effectively alleviated the spikelet degeneration and yield loss under HT stress mainly via improving root activity and canopy and panicle traits including higher photosynthetic capacity, tricarboxylic acid cycle activity, and antioxidant capacity than WW. These parameters were regulated by BRs levels in plants. The decrease in BRs levels at HT was due mainly to the enhanced BRs decomposition, and the MD could rescue the BRs deficiency at HT via enhancing BRs biosynthesis and impeding decomposition. The connection between BRs and HT was verified by using rice BRs-deficient mutants, transgenic rice lines, and chemical regulators. Similar results were obtained in the open-air field experiment. The results suggest that BRs can mediate the MD to alleviate spikelet degeneration under HT stress during meiosis mainly via enhancing root activity, canopy traits, and young panicle traits of rice.

Mitochondrial H2S donor AP39 induces stomatal closure by modulating guard cell mitochondrial activity

Hydrogen sulfide (H2S) is a gaseous signaling molecule involved in numerous physiological processes in plants, including gas exchange with the environment through the regulation of stomatal pore width. Guard cells (GCs) are pairs of specialized epidermal cells that delimit stomatal pores and have a higher mitochondrial density and metabolic activity than their neighboring cells. However, there is no clear evidence on the role of mitochondrial activity in stomatal closure induction. In this work, we showed that the mitochondrial-targeted H2S donor AP39 induces stomatal closure in a dose-dependent manner. Experiments using inhibitors of the mitochondrial electron transport chain (mETC) or insertional mutants in cytochrome c (CYTc) indicated that the activity of mitochondrial CYTc and/or complex IV are required for AP39-dependent stomatal closure. By using fluorescent probes and genetically encoded biosensors we reported that AP39 hyperpolarized the mitochondrial inner potential (Δψm) and increased cytosolic ATP, cytosolic hydrogen peroxide levels, and oxidation of the glutathione pool in GCs. These findings showed that mitochondrial-targeted H2S donors induce stomatal closure, modulate guard cell mETC activity, the cytosolic energetic and oxidative status, pointing to an interplay between mitochondrial H2S, mitochondrial activity, and stomatal closure.

Bioenergetics of pollen tube growth in Arabidopsis thaliana revealed by ratiometric genetically encoded biosensors

Pollen tube is the fastest-growing plant cell. Its polarized growth process consumes a tremendous amount of energy, which involves coordinated energy fluxes between plastids, the cytosol, and mitochondria. However, how the pollen tube obtains energy and what the biological roles of pollen plastids are in this process remain obscure. To investigate this energy-demanding process, we developed second-generation ratiometric biosensors for pyridine nucleotides which are pH insensitive between pH 7.0 to pH 8.5. By monitoring dynamic changes in ATP and NADPH concentrations and the NADH/NAD⁺ ratio at the subcellular level in Arabidopsis (Arabidopsis thaliana) pollen tubes, we delineate the energy metabolism that underpins pollen tube growth and illustrate how pollen plastids obtain ATP, NADPH, NADH, and acetyl-CoA for fatty acid biosynthesis. We also show that fermentation and pyruvate dehydrogenase bypass are not essential for pollen tube growth in Arabidopsis, in contrast to other plant species like tobacco and lily.

The green light gap: a window of opportunity for optogenetic control of stomatal movement

Green plants are equipped with photoreceptors that are capable of sensing radiation in the ultraviolet-to-blue and the red-to-far-red parts of the light spectrum. However, plant cells are not particularly sensitive to green light (GL), and light which lies within this part of the spectrum does not efficiently trigger the opening of stomatal pores. Here, we discuss the current knowledge of stomatal responses to light, which are either provoked via photosynthetically active radiation or by specific blue light (BL) signaling pathways. The limited impact of GL on stomatal movements provides a unique option to use this light quality to control optogenetic tools. Recently, several of these tools have been optimized for use in plant biological research, either to control gene expression, or to provoke ion fluxes. Initial studies with the BL-activated potassium channel BLINK1 showed that this tool can speed up stomatal movements. Moreover, the GL-sensitive anion channel GtACR1 can induce stomatal closure, even at conditions that provoke stomatal opening in wild-type plants. Given that crop plants in controlled-environment agriculture and horticulture are often cultivated with artificial light sources (i.e. a combination of blue and red light from light-emitting diodes), GL signals can be used as a remote-control signal that controls stomatal transpiration and water consumption.

γ-Aminobutyric acid may directly or indirectly regulate Arabidopsis ALMT9

The mechanism by which GABA regulates stomatal pore aperture and anion transport activity of ALUMINUM-ACTIVATED MALATE TRANSPORTER 9 is debated.

Exogenous melatonin promotes the growth of alfalfa (Medicago sativa L.) under NaCl stress through multiple pathways

Salinity is one of the most common factors affecting alfalfa (Medicago sativa L.), and NaCl is one of the main factors of salinity stress which can cause heavy losses in agricultural production in the world. The application of exogenous melatonin (MT) plays a major role in numerous plants against various stress environments. The effects of exogenous MT on the NaCl tolerance of alfalfa treated with the control, 100 µmol L^-1 MT, 150 mmol L^-1 NaCl, or 150 mmol L^-1 NaCl+ 100 µmol L^-1 MT were investigated. The results showed that MT increased growth parameters, inhibited chlorophyll degradation and promoted photosynthetic gas exchange parameters (photosynthetic rate, conductance to H₂O, and transpiration rate) and stomatal opening under NaCl stress. Osmotic regulation substances such as soluble sugar, proline and glycine betaine were the highest in the NaCl treatment and the second in the NaCl+MT treatment. Nitrogen, phosphorus, potassium, calcium and magnesium were reduced and sodium was increased by NaCl, whereas these levels were reversed by the NaCl+MT treatment. MT inhibited cell membrane imperfection, lipid peroxidation and reactive oxygen species (ROS) accumulation caused by NaCl stress. MT up-regulated the gene expression and activity of antioxidant enzymes and increased the content of antioxidant non-enzyme substances to scavenge excessive ROS in NaCl-treated plants. In addition, all indicators interacted with each other to a certain extent and could be grouped according to the relative values. All variables were divided into PC 1 (89.2 %) and PC 2 (4 %). They were clustered into two categories with opposite effects, and most of them were significant variables. Hence, these findings reveal that exogenous MT alleviates the inhibitory effects of NaCl stress on photosynthesis, stomata opening, osmotic adjustment, ion balance and redox homeostasis, enhancing tolerance and growth of alfalfa. Furthermore, it suggests that MT could be implemented to improve the NaCl tolerance of alfalfa.

Starch biosynthesis in guard cells has features of both autotrophic and heterotrophic tissues

The pathway of starch synthesis in guard cells (GCs), despite the crucial role starch plays in stomatal movements, is not well understood. Here, we characterized starch dynamics in GCs of Arabidopsis (Arabidopsis thaliana) mutants lacking enzymes of the phosphoglucose isomerase-phosphoglucose mutase-ADP-glucose pyrophosphorylase starch synthesis pathway in leaf mesophyll chloroplasts or sugar transporters at the plastid membrane, such as glucose-6-phosphate/phosphate translocators, which are active in heterotrophic tissues. We demonstrate that GCs have metabolic features of both photoautotrophic and heterotrophic cells. GCs make starch using different carbon precursors depending on the time of day, which can originate both from GC photosynthesis and/or sugars imported from the leaf mesophyll. Furthermore, we unravel the major enzymes involved in GC starch synthesis and demonstrate that they act in a temporal manner according to the fluctuations of stomatal aperture, which is unique for GCs. Our work substantially enhances our knowledge on GC starch metabolism and uncovers targets for manipulating GC starch dynamics to improve stomatal behavior, directly affecting plant productivity.