Vacuolar sucrose cleavage prevents limitation of cytosolic carbohydrate metabolism and stabilizes photosynthesis under abiotic stress

Cytosolic fructose - an underestimated player in the regulation of sucrose biosynthesis

BACKGROUND

Plants must continuously adapt to environmental fluctuations, which significantly influence their photosynthetic performance and overall metabolism. The sucrose cycling system within plant cells plays a critical regulatory role during stress conditions. This study employed a systems biology approach to analyze system stabilities mathematically under various regulatory conditions impacting sucrose cycling dynamics. We investigated the effects of mutations within this cycle, specifically HEXOKINASE1 (Arabidopsis thaliana gin2-1), alongside high-light exposure. Finally, we confirmed the modeling output in vitro by enzyme assays.

RESULTS

The implementation of experimental subcellular metabolite data into a Structural Kinetic Model (SKM) enabled exploration of regulatory responses and system stabilities within a three-compartment model. Within system instabilities, gin2-1 was more instable than its wild type. The gin2-1 mutation particularly was destabilized when fructokinase function was impaired by phosphorylated sugars. Additionally, we confirmed that phosphorylated sugars serve as stronger activators of sucrose-phosphate synthase (SPS) than glucose. Interestingly, models with fructose SPS activation exhibited a similar stability pattern. Consequently, we proposed and confirmed in silico a triple activation of SPS by highly activating phosphorylated sugars and lower activating non-phosphorylated hexoses. Additionally, we biochemically confirmed the previously unknown, but now predicted, activation of SPS by fructose in vitro.

CONCLUSION

In summary, our study highlights the essential role of sucrose cycling in plant cells under stress conditions. The in silico findings reveal that phosphorylated sugars are stronger activators of SPS than glucose and introduce a previously unknown activation mechanism by fructose. These potential activation capacities were confirmed in vitro through SPS enzyme activity assays, underscoring the efficiency of our systems biology approach. Overall, this research provides valuable insights into carbohydrate metabolism regulation and paves the way for future investigations to deepen our understanding of the complexities involved in sucrose cycling and biosynthesis in plants.

Plant cold acclimation and its impact on sensitivity of carbohydrate metabolism

The ability to acclimate to changing environmental conditions is essential for the fitness and survival of plants. Not only are seasonal differences challenging for plants growing in different habitats but, facing climate change, the likelihood of encountering extreme weather events increases. Previous studies of acclimation processes of Arabidopsis thaliana to changes in temperature and light conditions have revealed a multigenic trait comprising and affecting multiple layers of molecular organization. Here, a combination of experimental and computational methods was applied to study the effects of changing light intensities during cold acclimation on the central carbohydrate metabolism of Arabidopsis thaliana leaf tissue. Mathematical modeling, simulation and sensitivity analysis suggested an important role of hexose phosphate balance for stabilization of photosynthetic CO2 fixation. Experimental validation revealed a profound effect of temperature on the sensitivity of carbohydrate metabolism.

Physiological, molecular, and metabolic adaptations of plants to combined salinity and high irradiance stress

Global warming is expected to drive climate change, intensifying extreme weather events and aggravating stress conditions for plants due to the heightened frequency and severity of environmental factors. Among these stresses, the interplay of salinity and high irradiance is particularly critical, as it poses significant threats to crop productivity, food quality, and overall global food security. This review provides a comprehensive analysis of the physiological, molecular, and metabolic responses of various plant species to salinity (S), high irradiance (HL), and their combined stress (S + HL), highlighting the adaptative mechanisms plants employ to mitigate these adverse conditions. This study integrates in silico data, focusing on gene expression profiles and functional classification using Gene Ontology (GO) terms and analysis of transcription factor (TF) families such as MYB, WRKY and bHLH. Alongside gene expression data, we incorporated analyses of growth, development, and metabolism profiles across different species exposed to S, HL and S + HL. The findings point to adaptive mechanisms crucial for resilience, including reconfigurations in gene expression patterns, metabolic pathways and phytohormone profiles, demonstrating their potential in the development of climate-resilient crops. This review offers a framework for further research into multi-stress adaptation strategies. In addition, the importance of advancing crop resilience through these insights, contributing to the development of innovative approaches for sustainable agriculture in a rapidly changing climate, is outlined.

The role of light in regulating plant growth, development and sugar metabolism: a review

Light provides the necessary energy for plant photosynthesis, which allows plants to produce organic matter and energy conversion, during plant growth and development. Light provides material energy to plants as the basis for cell division and differentiation, chlorophyll synthesis, tissue growth and stomatal movement, and light intensity, photoperiod, and light quality play important roles in these processes. There are several regulatory mechanisms involved in sugar metabolism in plants, and light, as one of the regulatory factors, affects cell wall composition, starch granules, sucrose synthesis, and vascular bundle formation. Similarly, sugar species and genes are affected in the context of light-regulated sugar metabolism. We searched the available databases and found that there are fewer relevant reviews. Therefore, this paper provides a summary of the effects of light on plant growth and development and sugar metabolism, further elaborates on the mechanisms of light effects on plants, and provides some new insights for a better understanding of how plant growth is regulated under different light conditions.

Biodegradable Microplastic-Driven Change in Soil pH Affects Soybean Rhizosphere Microbial N Transformation Processes

The potential impacts of biodegradable and nonbiodegradable microplastics (MPs) on rhizosphere microbial nitrogen (N) transformation processes remain ambiguous. Here, we systematically investigated how biodegradable (polybutylene succinate, PBS) MPs and nonbiodegradable (polyethylene, PE) MPs affect microbial N processes by determining rhizosphere soil indicators of typical Glycine max (soybean)-soil (i.e., red and brown soils) systems. Our results show that MPs altered soil pH and dissolved organic carbon in MP/soil type-dependent manners. Notably, soybean growth displayed greater sensitivity to 1% (w/w) PBS MP exposure in red soil than that in brown soil since 1% PBS acidified the red soil and impeded nutrient uptake by plants. In the rhizosphere, 1% PBS negatively impacted microbial community composition and diversity, weakened microbial N processes (mainly denitrification and ammonification), and disrupted rhizosphere metabolism. Overall, it is suggested that biodegradable MPs, compared to nonbiodegradable MPs, can more significantly influence the ecological function of the plant-soil system.

Metabolomics and physio-chemical analyses of mulberry plants leaves response to manganese deficiency and toxicity reveal key metabolites and their pathways in manganese tolerance

Introduction

Manganese (Mn) plays a pivotal role in plant growth and development. Aside aiding in plant growth and development, Mn as heavy metal (HM) can be toxic in soil when applied in excess. Morus alba is an economically significant plant, capable of adapting to a range of environmental conditions and possessing the potential for phytoremediation of contaminated soil by HMs. The mechanism by which M. alba tolerates Mn stresses remains obscure.

Methods

In this study, Mn concentrations comprising sufficiency (0.15 mM), higher regimes (1.5 mM and 3 mM), and deficiency (0 mM and 0.03 mM), were applied to M. alba in pot treatment for 21 days to understand M. alba Mn tolerance. Mn stress effects on the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), intercellular CO2 concentration (Ci), chlorophyll content, plant morphological traits, enzymatic and non-enzymatic parameters were analyzed as well as metabolome signatures via non-targeted LC-MS technique.

Results

Mn deficiency and toxicity decrease plant biomass, Pn, Ci, Gs, Tr, and chlorophyll content. Mn stresses induced a decline in the activities of catalase (CAT) and superoxide dismutase (SOD), while peroxidase (POD) activity, and leaf Mn content, increased. Soluble sugars, soluble proteins, malondialdehyde (MDA) and proline exhibited an elevation in Mn deficiency and toxicity concentrations. Metabolomic analysis indicates that Mn concentrations induced 1031 differentially expressed metabolites (DEMs), particularly amino acids, lipids, carbohydrates, benzene and derivatives and secondary metabolites. The DEMs are significantly enriched in alpha-linolenic acid metabolism, biosynthesis of unsaturated fatty acids, galactose metabolism, pantothenate and CoA biosynthesis, pentose phosphate pathway, carbon metabolism, etc.

Discussion and conclusion

The upregulation of Galactinol, Myo-inositol, Jasmonic acid, L-aspartic acid, Coproporphyrin I, Trigonelline, Pantothenol, and Pantothenate and their significance in the metabolic pathways makes them Mn stress tolerance metabolites in M. alba. Our findings reveal the fundamental understanding of DEMs in M. alba's response to Mn nutrition and the metabolic mechanisms involved, which may hold potential significance for the advancement of M. alba genetic improvement initiatives and phytoremediation programs.

Overexpression of ZmSUS1 increased drought resistance of maize (Zea mays L.) by regulating sucrose metabolism and soluble sugar content

MAIN CONCLUSION

ZmSUS1 improved drought tolerance of maize by regulating sucrose metabolism and increasing soluble sugar content, and endowing transgenic maize with higher relative water content and photosynthesis levels. Sucrose synthase (SUS), a key enzyme of sugar metabolism, plays an important role in the regulation of carbon partitioning in plant, and affects important agronomic traits and abiotic responses to adversity. However, the function of ZmSUS1 in plant drought tolerance is still unknown. In this study, the expression patterns of ZmSUS1 in different tissues and under drought stress were analyzed in maize (Zea mays L.). It was found that ZmSUS1 was highly expressed during kernel development but also in leaves and roots of maize, and ZmSUS1 was induced by drought stress. Homozygous transgenic maize lines overexpressing ZmSUS1 increased the content and activity of SUS under drought stress and exhibited higher relative water content, proline and abscisic acid content in leaves. Specifically, the net photosynthetic rate and the soluble sugar contents including sucrose, glucose, fructose and SUS decomposition products including UDP-glucose (UDP-G) and ADP-glucose (ADP-G) in transgenic plants were significantly improved after drought stress. RNA-seq analysis showed that overexpressing of ZmSUS1 mainly affected the expression level of carbon metabolism-related genes. Especially the expression level of sucrose metabolism-related genes including sucrose phosphatase gene (SPP), sucrose phosphate synthase gene (SPS) and invertase gene (INV) were significantly up-regulated in transgenic maize. Overall, these results suggested that ZmSUS1 improved drought tolerance by regulating sucrose metabolism and increasing the soluble sugar content, and endowing transgenic maize with higher relative water content and photosynthesis levels, which can serve as a new gene candidate for cultivating drought-resistant maize varieties.

Natural variation of temperature acclimation of Arabidopsis thaliana

Acclimation is a multigenic trait by which plants adjust photosynthesis and metabolism to cope with a changing environment. Here, natural variations of photosynthetic efficiency and acclimation of the central carbohydrate metabolism were analyzed in response to low and elevated temperatures. For this, 18 natural accessions of Arabidopsis thaliana, originating from Cape Verde Islands and Europe, were grown at 22°C before being exposed to 4°C and 34°C for cold and heat acclimation, respectively. Absolute amounts of carbohydrates were quantified together with their subcellular distribution across plastids, cytosol and vacuole. Linear electron transport rates (ETRs) were determined together with the maximum quantum efficiency of photosystem II (Fv/Fm) for all growth conditions and under temperature fluctuation. Under elevated temperature, ETR residuals under increasing photosynthetic photon flux densities significantly correlated with the degree of temperature fluctuation at the original habitat of accessions, indicating a geographical east/west gradient of photosynthetic acclimation capacities. Plastidial sucrose concentrations positively correlated with maximal ETRs under fluctuating temperature, indicating a stabilizing role within the chloroplast. Our findings revealed specific subcellular carbohydrate distributions that contribute differentially to the photosynthetic efficiency of natural Arabidopsis thaliana accessions across a longitudinal gradient. This sheds light on the relevance of subcellular metabolic regulation for photosynthetic performance in a fluctuating environment and supports the physiological interpretation of naturally occurring genetic variation of temperature tolerance and acclimation.

Quantifying the impact of dynamic plant-environment interactions on metabolic regulation

A plant's genome encodes enzymes, transporters and many other proteins which constitute metabolism. Interactions of plants with their environment shape their growth, development and resilience towards adverse conditions. Although genome sequencing technologies and applications have experienced triumphantly rapid development during the last decades, enabling nowadays a fast and cheap sequencing of full genomes, prediction of metabolic phenotypes from genotype × environment interactions remains, at best, very incomplete. The main reasons are a lack of understanding of how different levels of molecular organisation depend on each other, and how they are constituted and expressed within a setup of growth conditions. Phenotypic plasticity, e.g., of the genetic model plant Arabidopsis thaliana, has provided important insights into plant-environment interactions and the resulting genotype x phenotype relationships. Here, we summarize previous and current findings about plant development in a changing environment and how this might be shaped and reflected in metabolism and its regulation. We identify current challenges in the study of plant development and metabolic regulation and provide an outlook of how methodological workflows might support the application of findings made in model systems to crops and their cultivation.

Metabolome plasticity in 241 Arabidopsis thaliana accessions reveals evolutionary cold adaptation processes

Acclimation and adaptation of metabolism to a changing environment are key processes for plant survival and reproductive success. In the present study, 241 natural accessions of Arabidopsis (Arabidopsis thaliana) were grown under two different temperature regimes, 16 °C and 6 °C, and growth parameters were recorded, together with metabolite profiles, to investigate the natural genome × environment effects on metabolome variation. The plasticity of metabolism, which was captured by metabolic distance measures, varied considerably between accessions. Both relative growth rates and metabolic distances were predictable by the underlying natural genetic variation of accessions. Applying machine learning methods, climatic variables of the original growth habitats were tested for their predictive power of natural metabolic variation among accessions. We found specifically habitat temperature during the first quarter of the year to be the best predictor of the plasticity of primary metabolism, indicating habitat temperature as the causal driver of evolutionary cold adaptation processes. Analyses of epigenome- and genome-wide associations revealed accession-specific differential DNA-methylation levels as potentially linked to the metabolome and identified FUMARASE2 as strongly associated with cold adaptation in Arabidopsis accessions. These findings were supported by calculations of the biochemical Jacobian matrix based on variance and covariance of metabolomics data, which revealed that growth under low temperatures most substantially affects the accession-specific plasticity of fumarate and sugar metabolism. Our findings indicate that the plasticity of metabolic regulation is predictable from the genome and epigenome and driven evolutionarily by Arabidopsis growth habitats.

Mycorrhizal Fungal Effects on Plant Growth, Osmolytes, and CsHsp70s and CsPIPs Expression in Leaves of Cucumber under a Short-Term Heat Stress

Arbuscular mycorrhizal (AM) fungi enhance plant stress tolerance, but it is unclear whether AM fungi affect heat tolerance in cucumbers. This study aimed to analyze how an AM fungus, Diversispora versiformis, affected growth, chlorophyll, five osmolytes, and plasma membrane intrinsic proteins (PIPs) and heat shock protein 70 (Hsp70) gene expression in cucumber leaves after a short-term (80 h) heat stress. Heat treatment significantly reduced root AM fungal colonization rate (0.26 folds). Heat treatment also distinctly suppressed plant height, stem diameter, and biomass, whereas AM fungal inoculation improved these growth variables as well as the chlorophyll index, with the benefit being more obvious under heat than under no-heat stress conditions. Heat treatment triggered differential changes in osmolytes (sucrose, fructose, and betaine) of inoculated and uninoculated cucumbers, whereas inoculation with AM fungus significantly raised leaf sucrose, fructose, glucose, betaine, and proline levels when compared to non-AM fungal inoculation. Heat treatment increased the expression of two (CsPIP1;6 and CsPIP2;1) of eight CsPIPs in inoculated and uninoculated plants, whereas AM fungal inoculation up-regulated the expression of CsPIP1;6, CsPIP2;1, and CsPIP2;6 under heat stress conditions. Hsp70s expressed differently in inoculated and uninoculated plants under heat versus no-heat stress, with 6 of 11 CsHsp70s down-regulated in inoculated plants. Under heat stress conditions, AM fungus only up-regulated CsHsp70-8 expression in 11 Hsp70s, while another eight CsHsp70s were down-regulated. Heat treatment and AM fungal inoculation both increased the expression of CsHsp70-8 and CsPIP1;6. It was concluded that AM fungus-inoculated cucumbers have high levels of growth, chlorophyll, and osmolytes under heat stress and do not require high CsPIPs and CsHsp70s expression to tolerate a short-term heat treatment.

Early leaf responses of cell physiological and sensor-based signatures reflect susceptibility of wheat seedlings to infection by leaf rust

Leaf rust caused by Puccinia triticina Erikss. can have devastating effects on wheat (Triticum aestivum L.), causing severe economic losses. This comprehensive study serves to facilitate our understanding of the impact of carbohydrate and antioxidant metabolism in association with sensor-based phenotyping and leaf rust stress responses in wheat seedlings. After 24 h of inoculation (hai) very susceptible variety to leaf rust (Ficko) increased cell-wall invertase (cwInv; EC 3.2.1.26), compared to other varieties that significantly increased cwInv later. This could mean that the Ficko variety cannot defend itself from leaf rust infections once symptoms have started to develop. Also, Ficko had significantly decreased amounts of cytoplasmic invertase (cytInv; EC 3.2.1.26) at 8 hai. The downregulation of cytInv in susceptible plants may facilitate the maintenance of elevated apoplastic sucrose availability favoring the pathogen. The significant role of vacuolar invertase (vacInv; EC 3.2.1.26) in moderately resistant varieties was recorded. Also, a significant decrease of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and UDP-glucose pyrophosphorylase (UGPase; EC 2.7.7.9) in moderately resistant varieties might restrict normal development of leaf rust due to reduced sugar. During plant-pathogen interaction, when the invader spreads systemically throughout the plant, the main role of ascorbate peroxidase (APX; EC 1.11.1.11) activity in one moderately resistant variety (Olimpija) and catalase (CAT; EC 1.11.1.6) activity in another moderately resistant variety (Alka) is to protect the plant against oxidative damage in the early stages of infection. Non-invasive phenotyping with a sensor-based technique could be used as a rapid method for pre-symptomatic determination of wheat leaf rust resistance or susceptibility.

The impact of boron nutrient supply in mulberry (Morus alba) response to metabolomics, enzyme activities, and physiological parameters

Boron (B) is essential for normal and healthy plant growth. Therefore, Boron stress is a common abiotic stress that limits plant growth and productivity. However, how mulberry copes with boron stress remains unclear. In this study, seedlings of the Morus alba cultivar, Yu-711, were treated with five different concentrations of boric acid (H₃BO₃), including deficient (0 and 0.02 mM), sufficient (0.1 mM) and toxic (0.5 and 1 mM) levels. Physiological parameters, enzymatic activities and non-targeted liquid chromatography-mass spectrometry (LC-MS) technique were employed to evaluate the effects of boron stress on the net photosynthetic rate (Pn), chlorophyll content, stomatal conductance (Gs), transpiration rate (Tr), intercellular CO₂ concentration (Ci) and metabolome signatures. Physiological analysis revealed that Boron deficiency and toxicity induced a decline in P_n, C_i, G_s, T_r, and chlorophyll content. Also, enzymatic activities, including catalase (CAT) and superoxide dismutase (SOD), decreased, while POD activity increased in response to Boron stress. Osmotic substances such as soluble sugars, soluble proteins, and proline (PRO) presented elevated levels under all Boron concentrations. Metabolome analysis indicated that differential metabolites, including amino acids, secondary metabolites, carbohydrates, and lipids, played a key role in Yu-711's response to Boron stress. These metabolites were mainly involved in amino acid metabolism, biosynthesis of other secondary metabolites, lipid metabolism, metabolism of cofactors and vitamins, and metabolism of other amino acids pathways. Our findings reveal the various metabolites pathways in mulberry response to boron nutrient supply and may serve as fundamental knowledge in breeding resistance mulberry plants, so that it can cope with climate changes.

Limitation of sucrose biosynthesis shapes carbon partitioning during plant cold acclimation

Cold acclimation is a multigenic process by which many plant species increase their freezing tolerance. Stabilization of photosynthesis and carbohydrate metabolism plays a crucial role in cold acclimation. To study regulation of primary and secondary metabolism during cold acclimation of Arabidopsis thaliana, metabolic mutants with deficiencies in either starch or flavonoid metabolism were exposed to 4°C. Photosynthesis was determined together with amounts of carbohydrates, anthocyanins, organic acids and enzyme activities of the central carbohydrate metabolism. Starch deficiency was found to significantly delay soluble sugar accumulation during cold acclimation, while starch overaccumulation did not affect accumulation dynamics but resulted in lower total amounts of \sucrose and glucose. Anthocyanin amounts were lowered in both starch deficient and overaccumulating mutants. Vice versa, flavonoid deficiency did not result in a changed starch amount, which suggested a unidirectional signalling link between starch and flavonoid metabolism. Mathematical modelling of carbon metabolism indicated kinetics of sucrose biosynthesis to be limiting for carbon partitioning in leaf tissue during cold exposure. Together with cold-induced dynamics of citrate, fumarate and malate amounts, this provided evidence for a central role of sucrose phosphate synthase activity in carbon partitioning between biosynthetic and dissimilatory pathways which stabilizes photosynthesis and metabolism at low temperature.

Light Drives and Temperature Modulates: Variation of Phenolic Compounds Profile in Relation to Photosynthesis in Spring Barley

Accumulation and metabolic profile of phenolic compounds (PheCs; serving as UV-screening pigments and antioxidants) as well as carbon fixation rate (A_n) and plant growth are sensitive to irradiance and temperature. Since these factors are naturally co-acting in the environment, it is worthy to study the combined effects of these environmental factors to assess their possible physiological consequences. We investigated how low and high irradiance in combination with different temperatures modify the metabolic profile of PheCs and expression of genes involved in the antioxidative enzyme and PheCs biosynthesis, in relation to photosynthetic activity and availability of non-structural carbohydrates (NSC) in spring barley seedlings. High irradiance positively affected A_n, NSC, PheCs content, and antioxidant activity (AOX). High temperature led to decreased A_n, NSC, and increased dark respiration, whilst low temperature was accompanied by reduction of UV-A shielding but increase of PheCs content and AOX. Besides that, irradiance and temperature caused changes in the metabolic profile of PheCs, particularly alteration in homoorientin/isovitexin derivatives ratio, possibly related to demands on AOX-based protection. Moreover, we also observed changes in the ratio of sinapoyl-/feruloyl- acylated flavonoids, the function of which is not yet known. The data also strongly suggested that the NSC content may support the PheCs production.

Retrograde signaling in plants: A critical review focusing on the GUN pathway and beyond

Plastids communicate their developmental and physiological status to the nucleus via retrograde signaling, allowing nuclear gene expression to be adjusted appropriately. Signaling during plastid biogenesis and responses of mature chloroplasts to environmental changes are designated "biogenic" and "operational" controls, respectively. A prominent example of the investigation of biogenic signaling is the screen for gun (genomes uncoupled) mutants. Although the first five gun mutants were identified 30 years ago, the functions of GUN proteins in retrograde signaling remain controversial, and that of GUN1 is hotly disputed. Here, we provide background information and critically discuss recently proposed concepts that address GUN-related signaling and some novel gun mutants. Moreover, considering heme as a candidate in retrograde signaling, we revisit the spatial organization of heme biosynthesis and export from plastids. Although this review focuses on GUN pathways, we also highlight recent progress in the identification and elucidation of chloroplast-derived signals that regulate the acclimation response in green algae and plants. Here, stress-induced accumulation of unfolded/misassembled chloroplast proteins evokes a chloroplast-specific unfolded protein response, which leads to changes in the expression levels of nucleus-encoded chaperones and proteases to restore plastid protein homeostasis. We also address the importance of chloroplast-derived signals for activation of flavonoid biosynthesis leading to production of anthocyanins during stress acclimation through sucrose non-fermenting 1-related protein kinase 1. Finally, a framework for identification and quantification of intercompartmental signaling cascades at the proteomic and metabolomic levels is provided, and we discuss future directions of dissection of organelle-nucleus communication.

Fructan oligosaccharide priming alters apoplastic sugar dynamics and improves resistance against Botrytis cinerea in chicory

Carbohydrates such as fructans can be involved in priming or defence stimulation, and hence potentially provide new strategies for crop protection against biotic stress. Chicory (Cichorium intybus) is a model plant for fructan research and is a crop with many known health benefits. Using the chicory-Botrytis cinerea pathosystem, we tested the effectiveness of fructan-induced immunity, focussing on different plant and microbial fructans. Sugar dynamics were followed after priming and subsequent pathogen infection. Our results indicated that many higher plants might detect extracellular levan oligosaccharides (LOS) of microbial origin, while chicory also detects extracellular small inulin-type fructooligosaccharides (FOS) of endogenous origin, thus differing from the findings of previous fructan priming studies. No clear positive effects were observed for inulin or mixed-type fructans. An elicitor-specific burst of reactive oxygen species was observed for sulfated LOS, while FOS and LOS both behaved as genuine priming agents. In addition, a direct antifungal effect was observed for sulfated LOS. Intriguingly, LOS priming led to a temporary increase in apoplastic sugar concentrations, mainly glucose, which could trigger downstream responses. Total sugar and starch contents in total extracts of LOS-primed leaves were higher after leaf detachment, indicating they could maintain their metabolic activity. Our results indicate the importance of balancing intra- and extracellular sugar levels (osmotic balance) in the context of 'sweet immunity' pathways.

Advances in Plant Metabolomics and Its Applications in Stress and Single-Cell Biology

In the past two decades, the post-genomic era envisaged high-throughput technologies, resulting in more species with available genome sequences. In-depth multi-omics approaches have evolved to integrate cellular processes at various levels into a systems biology knowledge base. Metabolomics plays a crucial role in molecular networking to bridge the gaps between genotypes and phenotypes. However, the greater complexity of metabolites with diverse chemical and physical properties has limited the advances in plant metabolomics. For several years, applications of liquid/gas chromatography (LC/GC)-mass spectrometry (MS) and nuclear magnetic resonance (NMR) have been constantly developed. Recently, ion mobility spectrometry (IMS)-MS has shown utility in resolving isomeric and isobaric metabolites. Both MS and NMR combined metabolomics significantly increased the identification and quantification of metabolites in an untargeted and targeted manner. Thus, hyphenated metabolomics tools will narrow the gap between the number of metabolite features and the identified metabolites. Metabolites change in response to environmental conditions, including biotic and abiotic stress factors. The spatial distribution of metabolites across different organs, tissues, cells and cellular compartments is a trending research area in metabolomics. Herein, we review recent technological advancements in metabolomics and their applications in understanding plant stress biology and different levels of spatial organization. In addition, we discuss the opportunities and challenges in multiple stress interactions, multi-omics, and single-cell metabolomics.

An integrated analysis of the rice transcriptome and lipidome reveals lipid metabolism plays a central role in rice cold tolerance

BACKGROUND

Rice (Oryza sativa L.) is one of the most widely grown food crops, and its yield and quality are particularly important for a warm-saturated diet. Cold stress restricts rice growth, development, and yield; however, the specific mechanism of cold tolerance in rice remains unknown.

RESULTS

The analysis of leaf physiological and photosynthetic characteristics showed that the two rice varieties were significantly affected by cold stress, but the cold-tolerant variety KY131 had more stable physiological characteristics, maintaining relatively good photosynthetic capacity. To better explore the transcriptional regulation mechanism and biological basis of rice response to cold stress, a comprehensive analysis of the rice transcriptome and lipidome under low temperature and control temperature conditions was carried out. The transcriptomic analysis revealed that lipid metabolism, including membrane lipid and fatty acid metabolism, may be an important factor in rice cold tolerance, and 397 lipid metabolism related genes have been identified. Lipidomics data confirmed the importance of membrane lipid remodeling and fatty acid unsaturation for rice adaptation to cold stress. This indicates that the changes in the fluidity and integrity of the photosynthetic membrane under cold stress lead to the reduction of photosynthetic capacity, which could be relieved by increased levels of monogalactosyldiacylglycerol that mainly caused by markedly increased expression of levels of 1,2-diacylglycerol 3-beta-galactosyltransferase (MGD). The upregulation of phosphatidate phosphatase (PAP2) inhibited the excessive accumulation of phosphatidate (PA) to produce more phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG), thereby preventing of membrane phase transition under cold stress. In addition, fatty acid β-oxidation is worth further study in rice cold tolerance. Finally, we constructed a metabolic model for the regulatory mechanism of cold tolerance in rice, in which the advanced lipid metabolism system plays a central role.

CONCLUSIONS

Lipidome analysis showed that membrane lipid composition and unsaturation were significantly affected, especially phospholipids and galactolipids. Our study provides new information to further understand the response of rice to cold stress.

Predicting plant growth response under fluctuating temperature by carbon balance modelling

Quantification of system dynamics is a central aim of mathematical modelling in biology. Defining experimentally supported functional relationships between molecular entities by mathematical terms enables the application of computational routines to simulate and analyse the underlying molecular system. In many fields of natural sciences and engineering, trigonometric functions are applied to describe oscillatory processes. As biochemical oscillations occur in many aspects of biochemistry and biophysics, Fourier analysis of metabolic functions promises to quantify, describe and analyse metabolism and its reaction towards environmental fluctuations. Here, Fourier polynomials were developed from experimental time-series data and combined with block diagram simulation of plant metabolism to study heat shock response of photosynthetic CO₂ assimilation and carbohydrate metabolism in Arabidopsis thaliana. Simulations predicted a stabilising effect of reduced sucrose biosynthesis capacity and increased capacity of starch biosynthesis on carbon assimilation under transient heat stress. Model predictions were experimentally validated by quantifying plant growth under such stress conditions. In conclusion, this suggests that Fourier polynomials represent a predictive mathematical approach to study dynamic plant-environment interactions.

A vacuolar hexose transport is required for xylem development in the inflorescence stem

In Angiosperms, the development of the vascular system is controlled by a complex network of transcription factors. However, how nutrient availability in the vascular cells affects their development remains to be addressed. At the cellular level, cytosolic sugar availability is regulated mainly by sugar exchanges at the tonoplast through active and/or facilitated transport. In Arabidopsis (Arabidopsis thaliana), among the genes encoding tonoplastic transporters, SUGAR WILL EVENTUALLY BE EXPORTED TRANSPORTER 16 (SWEET16) and SWEET17 expression has been previously detected in the vascular system. Here, using a reverse genetics approach, we propose that sugar exchanges at the tonoplast, regulated by SWEET16, are important for xylem cell division as revealed in particular by the decreased number of xylem cells in the swt16 mutant and the accumulation of SWEET16 at the procambium-xylem boundary. In addition, we demonstrate that transport of hexoses mediated by SWEET16 and/or SWEET17 is required to sustain the formation of the xylem secondary cell wall. This result is in line with a defect in the xylem cell wall composition as measured by Fourier-transformed infrared spectroscopy in the swt16swt17 double mutant and by upregulation of several genes involved in secondary cell wall synthesis. Our work therefore supports a model in which xylem development partially depends on the exchange of hexoses at the tonoplast of xylem-forming cells.

Transcriptional regulation of cell growth and reprogramming of systemic response in wheat (Triticum turgidum subsp. durum) seedlings by Bacillus paralicheniformis TRQ65

Bacillus paralicheniformis TRQ65 reprograms the gene expression patterns associated with systemic response to potentially facilitate its colonization and stimulate cell growth and plant biomass. Plant growth-promoting rhizobacteria (PGPR) carry out numerous mechanisms that enhance growth in seedlings, such as nutrient solubilization, phytohormone production, biocontrol activity, and regulation of induced systemic resistance (ISR) and acquired systemic resistance (ASR). Bacillus paralicheniformis TRQ65 is a biological and plant growth-promoting bacterium isolated from wheat (Triticum turgidum subsp. durum) rhizosphere. In this study, we performed a transcriptomic analysis of wheat seedlings inoculated with the native rhizobacterium Bacillus paralicheniformis TRQ65 (1 × 10⁷ cells∙g ^-1 of soil) at early development stages (GS15). A morphometrical assay was carried out to confirm growth promotion and after the cultivation period, TRQ65 was re-isolated to define inoculum persistence. Inoculated seedlings showed a significant (P < 0.05) increase in shoot length (93.48%) and dry weight in both shoot (117.02%) and root (48.33%) tissues; also, the strain persisted in the soil at 1.4 × 10⁷ UFC∙g^-1 of soil. A total of 228 differentially expressed genes (DEGs) (FDR < 0.05 and |log2 fold change|≥ 1.3) were observed in response to TRQ65 inoculation, of which 185 were down-regulated and 43 were up-regulated. The transcriptional patterns were characterized by the regulation of multidimensional cell growth (ROS, Ca⁺² channel, and NADPH oxidases activity), suppression of defense mechanism (PR proteins, PDFs, ROS, transcription factors), induction of central stimuli receptors (RALF, WAK, MAPK), carbohydrate metabolism (invertase activity) and phytohormone-related transport (ABCG transporter and AAAP). These results suggest that B. paralicheniformis TRQ65 is a promising bioinoculant agent for increasing wheat growth and development by reprogramming ISR and ASR simultaneously, suppressing defense mechanisms and inducing central stimuli response.

Temperature-induced dynamics of plant carbohydrate metabolism

Carbohydrates are direct products of photosynthetic CO₂ assimilation. Within a changing temperature regime, both photosynthesis and carbohydrate metabolism need tight regulation to prevent irreversible damage of plant tissue and to sustain energy metabolism, growth and development. Due to climate change, plants are and will be exposed to both long-term and short-term temperature changes with increasing amplitude. Particularly sudden fluctuations, which might comprise a large temperature amplitude from low to high temperature, pose a challenge for plants from the cellular to the ecosystem level. A detailed understanding of fundamental regulatory processes, which link photosynthesis and carbohydrate metabolism under such fluctuating environmental conditions, is essential for an estimate of climate change consequences. Further, understanding these processes is important for biotechnological application, breeding and engineering. Environmental light and temperature regimes are sensed by a molecular network that comprises photoreceptors and molecular components of the circadian clock. Photosynthetic efficiency and plant productivity then critically depend on enzymatic regulation and regulatory circuits connecting plant cells with their environment and re-stabilising photosynthetic efficiency and carbohydrate metabolism after temperature-induced deflection. This review summarises and integrates current knowledge about re-stabilisation of photosynthesis and carbohydrate metabolism after perturbation by changing temperature (heat and cold).

Metabolic flux analysis of the non-transitory starch tradeoff for lipid production in mature tobacco leaves

The metabolic plasticity of tobacco leaves has been demonstrated via the generation of transgenic plants that can accumulate over 30% dry weight as triacylglycerols. In investigating the changes in carbon partitioning in these high lipid-producing (HLP) leaves, foliar lipids accumulated stepwise over development. Interestingly, non-transient starch was observed to accumulate with plant age in WT but not HLP leaves, with a drop in foliar starch concurrent with an increase in lipid content. The metabolic carbon tradeoff between starch and lipid was studied using 13CO2-labeling experiments and isotopically nonstationary metabolic flux analysis, not previously applied to the mature leaves of a crop. Fatty acid synthesis was investigated through assessment of acyl-acyl carrier proteins using a recently derived quantification method that was extended to accommodate isotopic labeling. Analysis of labeling patterns and flux modeling indicated the continued production of unlabeled starch, sucrose cycling, and a significant contribution of NADP-malic enzyme to plastidic pyruvate production for the production of lipids in HLP leaves, with the latter verified by enzyme activity assays. The results suggest an inherent capacity for a developmentally regulated carbon sink in tobacco leaves and may in part explain the uniquely successful leaf lipid engineering efforts in this crop.

Metabolic regulation of subcellular sucrose cleavage inferred from quantitative analysis of metabolic functions

Quantitative analysis of experimental metabolic data is frequently challenged by non-intuitive, complex patterns which emerge from regulatory networks. The complex output of metabolic regulation can be summarised by metabolic functions which comprise information about dynamics of metabolite concentrations. In a system of ordinary differential equations, metabolic functions reflect the sum of biochemical reactions which affect a metabolite concentration, and their integration over time reveals metabolite concentrations. Further, derivatives of metabolic functions provide essential information about system dynamics and elasticities. Here, invertase-driven sucrose hydrolysis was simulated in kinetic models on a cellular and subcellular level. Both Jacobian and Hessian matrices of metabolic functions were derived for quantitative analysis of kinetic regulation of sucrose metabolism. Model simulations suggest that transport of sucrose into the vacuole represents a central regulatory element in plant metabolism during cold acclimation which preserves control of metabolic functions and limits feedback-inhibition of cytosolic invertases by elevated hexose concentrations.

Seasonal changes in carbohydrates and water content predict dynamics of frost hardiness in various temperate tree species

Predicting tree frost tolerance is critical to select adapted species according to both the current and predicted future climate. The relative change in water to carbohydrate ratio is a relevant trait to predict frost acclimation in branches from many tree species. The objective of this study is to demonstrate the interspecific genericity of this approach across nine tree species. In the studied angiosperm species, frost hardiness dynamics were best correlated to a decrease in water content at the early stage of acclimation (summer and early autumn). Subsequently, frost hardiness dynamics were more tightly correlated to soluble carbohydrate contents until spring growth resumption. Based on different model formalisms, we predicted frost hardiness at different clade levels (angiosperms, family, genus and species) with high to moderate accuracy (1.5-6.0 °C root mean squared error (RMSE)) and robustness (2.8-6.1 °C prediction RMSE). The TOT model, taking all soluble carbohydrate and polyols into account, was more effective and adapted for large scale studies aiming to explore frost hardiness across a wide range of species. The ISC model taking the individual contribution of each soluble carbohydrate molecule into account was more efficient at finer scale such as family or species. The ISC model performance also suggests that the role of solutes cannot be reduced to a 'bulk' osmotic effect as could be computed if all of them were located in a single, common, compartment. This study provides sets of parameters to predict frost hardiness in a wide range of species, and clues for targeting specific carbohydrate molecules to improve frost hardiness.

Data Management and Modeling in Plant Biology

The study of plant-environment interactions is a multidisciplinary research field. With the emergence of quantitative large-scale and high-throughput techniques, amount and dimensionality of experimental data have strongly increased. Appropriate strategies for data storage, management, and evaluation are needed to make efficient use of experimental findings. Computational approaches of data mining are essential for deriving statistical trends and signatures contained in data matrices. Although, current biology is challenged by high data dimensionality in general, this is particularly true for plant biology. Plants as sessile organisms have to cope with environmental fluctuations. This typically results in strong dynamics of metabolite and protein concentrations which are often challenging to quantify. Summarizing experimental output results in complex data arrays, which need computational statistics and numerical methods for building quantitative models. Experimental findings need to be combined by computational models to gain a mechanistic understanding of plant metabolism. For this, bioinformatics and mathematics need to be combined with experimental setups in physiology, biochemistry, and molecular biology. This review presents and discusses concepts at the interface of experiment and computation, which are likely to shape current and future plant biology. Finally, this interface is discussed with regard to its capabilities and limitations to develop a quantitative model of plant-environment interactions.

Functional Characterization of a Cucumber (Cucumis sativus L.) Vacuolar Invertase, CsVI1, Involved in Hexose Accumulation and Response to Low Temperature Stress

Cucumber (Cucumis sativus L.), an important vegetable plant species, is susceptible to low temperature stress especially during the seedling stage. Vacuolar invertase (VI) plays important roles in plant responses to abiotic stress. However, the molecular and biochemical mechanisms of VI function in cucumber, have not yet been completely understood and VI responses to low temperature stress and it functions in cold tolerance in cucumber seedlings are also in need of exploration. The present study found that hexose accumulation in the roots of cucumber seedlings under low temperature stress is closely related to the observed enhancement of invertase activity. Our genome-wide search for the vacuolar invertase (VI) genes in cucumber identified the candidate VI-encoding gene CsVI1. Expression profiling of CsVI1 showed that it was mainly expressed in the young roots of cucumber seedlings. In addition, transcriptional analysis indicated that CsVI1 expression could respond to low temperature stress. Recombinant CsVI1 proteins purified from Pichia pastoris and Nicotiana benthamiana leaves could hydrolyze sucrose into hexoses. Further, overexpression of CsVI1 in cucumber plants could increase their hexose contents and improve their low temperature tolerance. Lastly, a putative cucumber invertase inhibitor was found could form a complex with CsVI1. In summary, these results confirmed that CsVI1 functions as an acid invertase involved in hexose accumulation and responds to low temperature stress in cucumber seedlings.

Photoprotection and optimization of sucrose usage contribute to faster recovery of photosynthesis after water deficit at high temperatures in wheat

Plants are increasingly exposed to events of elevated temperature and water deficit, which threaten crop productivity. Understanding the ability to rapidly recover from abiotic stress, restoring carbon assimilation and biomass production, is important to unravel crop climate resilience. This study compared the photosynthetic performance of two Triticum aestivum L. cultivars, Sokoll and Paragon, adapted to the climate of Mexico and UK, respectively, exposed to 1-week water deficit and high temperatures, in isolation or combination. Measurements included photosynthetic assimilation rate, stomatal conductance, in vitro activities of Rubisco (EC 4.1.1.39) and invertase (INV, EC 3.2.1.26), antioxidant capacity and chlorophyll a fluorescence. In both genotypes, under elevated temperatures and water deficit (WD38°C), the photosynthetic limitations were mainly due to stomatal restrictions and to a decrease in the electron transport rate. Chlorophyll a fluorescence parameters clearly indicate differences between the two genotypes in the photoprotection when subjected to WD38°C and showed faster recovery of Paragon after stress relief. The activity of the cytosolic invertase (CytINV) under these stress conditions was strongly related to the fast photosynthesis recovery of Paragon. Taken together, the results suggest that optimal sucrose export/utilization and increased photoprotection of the electron transport machinery are important components to limit yield fluctuations due to water shortage and elevated temperatures.

Acclimation in plants - the Green Hub consortium

Acclimation is the capacity to adapt to environmental changes within the lifetime of an individual. This ability allows plants to cope with the continuous variation in ambient conditions to which they are exposed as sessile organisms. Because environmental changes and extremes are becoming even more pronounced due to the current period of climate change, enhancing the efficacy of plant acclimation is a promising strategy for mitigating the consequences of global warming on crop yields. At the cellular level, the chloroplast plays a central role in many acclimation responses, acting both as a sensor of environmental change and as a target of cellular acclimation responses. In this Perspective article, we outline the activities of the Green Hub consortium funded by the German Science Foundation. The main aim of this research collaboration is to understand and strategically modify the cellular networks that mediate plant acclimation to adverse environments, employing Arabidopsis, tobacco (Nicotiana tabacum) and Chlamydomonas as model organisms. These efforts will contribute to 'smart breeding' methods designed to create crop plants with improved acclimation properties. To this end, the model oilseed crop Camelina sativa is being used to test modulators of acclimation for their potential to enhance crop yield under adverse environmental conditions. Here we highlight the current state of research on the role of gene expression, metabolism and signalling in acclimation, with a focus on chloroplast-related processes. In addition, further approaches to uncovering acclimation mechanisms derived from systems and computational biology, as well as adaptive laboratory evolution with photosynthetic microbes, are highlighted.

Restriction of cytosolic sucrose hydrolysis profoundly alters development, metabolism, and gene expression in Arabidopsis roots

Plant roots depend on sucrose imported from leaves as the substrate for metabolism and growth. Sucrose and hexoses derived from it are also signalling molecules that modulate growth and development, but the importance for signalling of endogenous changes in sugar levels is poorly understood. We report that reduced activity of cytosolic invertase, which converts sucrose to hexoses, leads to pronounced metabolic, growth, and developmental defects in roots of Arabidopsis (Arabidopsis thaliana) seedlings. In addition to altered sugar and downstream metabolite levels, roots of cinv1 cinv2 mutants have reduced elongation rates, cell and meristem size, abnormal meristematic cell division patterns, and altered expression of thousands of genes of diverse functions. Provision of exogenous glucose to mutant roots repairs relatively few of the defects. The extensive transcriptional differences between mutant and wild-type roots have hallmarks of both high sucrose and low hexose signalling. We conclude that the mutant phenotype reflects both low carbon availability for metabolism and growth and complex sugar signals derived from elevated sucrose and depressed hexose levels in the cytosol of mutant roots. Such reciprocal changes in endogenous sucrose and hexose levels potentially provide rich information about sugar status that translates into flexible adjustments of growth and development.

Impaired chloroplast positioning affects photosynthetic capacity and regulation of the central carbohydrate metabolism during cold acclimation

Photosynthesis and carbohydrate metabolism of higher plants need to be tightly regulated to prevent tissue damage during environmental changes. The intracellular position of chloroplasts changes due to a changing light regime. Chloroplast avoidance and accumulation response under high and low light, respectively, are well known phenomena, and deficiency of chloroplast movement has been shown to result in photodamage and reduced biomass accumulation. Yet, effects of chloroplast positioning on underlying metabolic regulation are less well understood. Here, we analysed photosynthesis together with metabolites and enzyme activities of the central carbohydrate metabolism during cold acclimation of the chloroplast unusual positioning 1 (chup1) mutant of Arabidopsis thaliana. We compared cold acclimation under ambient and low light and found that maximum quantum yield of PSII was significantly lower in chup1 than in Col-0 under both conditions. Our findings indicated that net CO₂ assimilation in chup1 is rather limited by biochemistry than by photochemistry. Further, cold-induced dynamics of sucrose phosphate synthase differed significantly between both genotypes. Together with a reduced rate of sucrose cycling derived from kinetic model simulations our study provides evidence for a central role of chloroplast positioning for photosynthetic and metabolic acclimation to low temperature.

Vacuolar sucrose homeostasis is critical for plant development, seed properties, and night-time survival in Arabidopsis

Most cellular sucrose is present in the cytosol and vacuoles of plant cells; however, little is known about the effect of this sucrose compartmentation on plant properties. Here, we examined the effects of altered intracellular sucrose compartmentation in Arabidopsis thaliana leaves by heterologously expressing the sugar beet (Beta vulgaris) vacuolar sucrose loader BvTST2.1 and by generating lines with reduced vacuolar invertase activity (amiR vi1-2). Heterologous expression of BvTST2.1 led to increased monosaccharide levels in leaves, whereas sucrose levels remained constant, indicating that vacuolar invertase activity in mesophyll vacuoles exceeds sucrose uptake. This notion was supported by analysis of tobacco (Nicotiana benthamiana) leaves transiently expressing BvTST2.1 and the invertase inhibitor NbVIF. However, sucrose levels were strongly elevated in leaf extracts from amiR vi1-2 lines, and experiments confirmed that sucrose accumulated in the corresponding vacuoles. The amiR vi1-2 lines exhibited impaired early development and reduced seed weight. When germinated in the dark, amiR vi1-2 seedlings were less able to convert sucrose into monosaccharides than the wild type. Cold temperatures strongly down-regulated both VI genes, but the amiR vi1-2 lines showed normal frost tolerance. These observations indicate that increased vacuolar sucrose levels fully compensate for the effects of low monosaccharide concentrations on frost tolerance.

Cold acclimation has a differential effect on leaf vascular bundle structure and carbon export rates in natural Arabidopsis accessions originating from southern and northern Europe

Acclimation to low but non-freezing temperature represents an ecologically important process for Arabidopsis thaliana but also for many other plant species from temperate regions. Cold acclimation comprises and affects numerous molecular and physiological processes and the maintenance of sugar supply of sink tissue by photosynthetically active source tissue is essential for plant survival. Here, changes in vascular bundle (VB) structure at the leaf petiole were analysed together with sucrose exudation rates before and after cold acclimation. Six natural Arabidopsis accessions originating from southern and northern Europe were compared. Photosynthetic efficiency, that is, maximum and effective quantum yield of photosystem II, revealed a significant effect of environmental condition. Only for northern accessions was a highly significant negative correlation observed between leaf sucrose exudation rates, xylem, and petiole cross-sectional areas. Furthermore, only for northern accessions was a significant increase of VB and leaf petiole cross-sectional area observed during cold acclimation. In contrast, variance of cross-sectional areas of cold acclimated southern accessions was strongly reduced compared to control plants, while mean areas remained similar under both conditions. In summary, these findings suggest that natural Arabidopsis accessions from northern Europe significantly adjust sink strength and leaf VB structure to maintain plant growth and photosynthesis under low temperature.

CsSWEET1a and CsSWEET17 Mediate Growth and Freezing Tolerance by Promoting Sugar Transport across the Plasma Membrane

Sugars Will Eventually be Exported Transporters (SWEETs) are important in plant biological processes. Expression levels of CsSWEET1a and CsSWEET17 are induced by cold acclimation (CA) and cold stress in Camellia sinensis. Here, we found that CsSWEET17 was alternatively spliced, and its exclusion (Ex) transcript was associated with the CA process. Both plasma membrane-localized CsSWEET1a and CsSWEET17 transport hexoses, but cytoplasm-localized CsSWEET17-Ex does not. These results indicate that alternative splicing may be involved in regulating the function of SWEET transporters in response to low temperature in plants. The extra C-terminal of CsSWEET17, which is not found in the tonoplast fructose transporter AtSWEET17, did not affect its plasma membrane localization but promoted its sugar transport activities. The overexpression (OE) of CsSWEET1a and CsSWEET17 genes resulted in an increased sugar uptake in Arabidopsis, affecting plant germination and growth. The leaf and seed sizes of the CsSWEET17-OE lines were significantly larger than those of the wild type. Moreover, the OE of CsSWEET1a and CsSWEET17 significantly reduced the relative electrolyte leakage levels under freezing stress. Compared with the wild type, the expression of AtCWINV genes was suppressed in both CsSWEET1a-OE and CsSWEET17-OE lines, indicating the alteration in sugar contents in the cell walls of the OE lines. Furthermore, the interaction between CsSWEET1a and CsSWEET17 was confirmed using yeast two-hybrid and bimolecular fluorescence complementation assays. We showed that CsSWEET1a and CsSWEET17 form homo-/heterodimers in the plasma membrane and mediate the partitioning of sugars between the cytoplasm and the apoplast, thereby regulating plant growth and freezing tolerance.

Indications for a Central Role of Hexokinase Activity in Natural Variation of Heat Acclimation in Arabidopsis thaliana

Diurnal and seasonal changes of abiotic environmental factors shape plant performance and distribution. Changes of growth temperature and light intensity may vary significantly on a diurnal, but also on a weekly or seasonal scale. Hence, acclimation to a changing temperature and light regime is essential for plant survival and propagation. In the present study, we analyzed photosynthetic CO₂ assimilation and metabolic regulation of the central carbohydrate metabolism in two natural accessions of Arabidopsis thaliana that originate from north western Russia and south Italy during exposure to heat and a combination of heat and high light. Our findings indicate that it is hardly possible to predict photosynthetic capacities under combined stress from single stress experiments. Further, capacities of hexose phosphorylation were found to be significantly lower in the Italian than in the Russian accession, which could explain an inverted sucrose-to-hexose ratio. Together with the finding of significantly stronger accumulation of anthocyanins under heat/high light, these observations indicate a central role of hexokinase activity in the stabilization of photosynthesis and carbohydrate metabolism during environmental changes.

Hot drought reduces the effects of elevated CO2 on tree water-use efficiency and carbon metabolism

Trees are increasingly exposed to hot droughts due to CO₂ -induced climate change. However, the direct role of [CO₂ ] in altering tree physiological responses to drought and heat stress remains ambiguous. Pinus halepensis (Aleppo pine) trees were grown from seed under ambient (421 ppm) or elevated (867 ppm) [CO₂ ]. The 1.5-yr-old trees, either well watered or drought treated for 1 month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10 d period. Continuous whole-tree shoot and root gas-exchange measurements were supplemented by primary metabolite analysis. Elevated [CO₂ ] reduced tree water loss, reflected in lower stomatal conductance, resulting in a higher water-use efficiency throughout amplifying heat stress. Net carbon uptake declined strongly, driven by increases in respiration peaking earlier in the well-watered (31-32°C) than drought (33-34°C) treatments unaffected by growth [CO₂ ]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO₂ ] was subtle and mainly reflected in enhanced root protein stability. The impact of elevated [CO₂ ] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO₂ ] cannot counterbalance the impacts of hot drought extremes in Aleppo pine.

Ethylene insensitive mutation improves Arabidopsis plant tolerance to NO2 exposure

Ethylene signaling was addressed, for the first time, in plant responses to nitrogen dioxide (NO₂) by comparatively analyzing the performance of Arabidopsis ethylene insensitive 2 (ein2-1) with wild-type (WT) plants. Following NO₂ fumigation, severe leaf wilting and chlorosis occurred in WT plants, but much less symptoms were observed in ein2-1. The activities of superoxide dismutase (SOD), peroxidase (PRX) and catalase (CAT) were 39%, 92%, and 11% higher, respectively, in ein2-1 than in WT following NO₂ exposure. Although glutathione contents and the ratio of its reduced form (GSH) to oxidized form (GSSG) were decreased by NO₂, an obviously alleviated degree was detected in ein2-1 relative to WT. Correspondingly, the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and electrolyte leakage were 25%, 24%, and 29% lower, respectively, in ein2-1 than in WT. The difference of oxidative stress between two tested genotypes was also revealed by the leaf staining regarding the production and distribution of H₂O₂, superoxide anion (O₂˙-), and cell death. The genes involved in antioxidation or oxidation-reduction processes mostly presented a stronger expression in ein2-1 than in WT under NO₂ stress. The photosynthesis-related parameters including chlorophyll and soluble sugar contents, net photosynthetic rate (Pn), and ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity and gene expression, and chlorophyll fluorescence parameters were affected, generally, to a lesser degree in ein2-1 than in WT following NO₂ fumigation. The enzymatic activities and gene expressions of invertases mostly displayed a higher level in ein2-1 relative to WT following NO₂ fumigation. For example, the activities of cytoplasmic, cell wall and vacuolar invertases were 76%, 26%, and 26% higher, respectively, in ein2-1 than in WT. Together, these data suggest that ethylene signal insensitivity efficiently improves plant tolerance to NO₂ exposure, and the possible mechanisms might be correlated with leaf antioxidative defense, photosynthesis-related processes, and sucrose metabolisms.

Mathematical Modeling of Plant Metabolism in a Changing Temperature Regime

Changes in environmental temperature regimes significantly affect plant growth, development and reproduction. Within a multigenic process termed acclimation, many plant species of the temperate region are able to adjust their metabolism to low and high temperature. Temperature-induced metabolic reprogramming is a nonlinear process affecting numerous enzyme kinetic reactions and pathways. The analysis of metabolic reprogramming during temperature acclimation is essentially supported by mathematical modeling which enables the study of nonlinear enzyme kinetics in context of metabolic networks and pathway regulation. This chapter introduces mathematical modeling of plant metabolism during a dynamic environmental temperature regime. A focus is laid on kinetic modeling and thermodynamic constraints.

An Advanced Lipid Metabolism System Revealed by Transcriptomic and Lipidomic Analyses Plays a Central Role in Peanut Cold Tolerance

Cold stress restricts peanut (Arachis hypogaea L.) growth, development, and yield. However, the specific mechanism of cold tolerance in peanut remains unknown. Here, the comparative physiological, transcriptomic, and lipidomic analyses of cold tolerant variety NH5 and cold sensitive variety FH18 at different time points of cold stress were conducted to fill this gap. Transcriptomic analysis revealed lipid metabolism including membrane lipid and fatty acid metabolism may be a significant contributor in peanut cold tolerance, and 59 cold-tolerant genes involved in lipid metabolism were identified. Lipidomic data corroborated the importance of membrane lipid remodeling and fatty acid unsaturation. It indicated that photosynthetic damage, resulted from the alteration in fluidity and integrity of photosynthetic membranes under cold stress, were mainly caused by markedly decreased monogalactosyldiacylglycerol (MGDG) levels and could be relieved by increased digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) levels. The upregulation of phosphatidate phosphatase (PAP1) and phosphatidate cytidylyltransferase (CDS1) inhibited the excessive accumulation of PA, thus may prevent the peroxidation of membrane lipids. In addition, fatty acid elongation and fatty acid β-oxidation were also worth further studied in peanut cold tolerance. Finally, we constructed a metabolic model for the regulatory mechanism of peanut cold tolerance, in which the advanced lipid metabolism system plays a central role. This study lays the foundation for deeply analyzing the molecular mechanism and realizing the genetic improvement of peanut cold tolerance.

Resolving subcellular plant metabolism

Plant cells are characterized by a high degree of compartmentalization and a diverse proteome and metabolome. Only a very limited number of studies has addressed combined subcellular proteomics and metabolomics which strongly limits biochemical and physiological interpretation of large-scale 'omics data. Our study presents a methodological combination of nonaqueous fractionation, shotgun proteomics, enzyme activities and metabolomics to reveal subcellular diurnal dynamics of plant metabolism. Subcellular marker protein sets were identified and enzymatically validated to resolve metabolism in a four-compartment model comprising chloroplasts, cytosol, vacuole and mitochondria. These marker sets are now available for future studies that aim to monitor subcellular metabolome and proteome dynamics. Comparing subcellular dynamics in wild type plants and HXK1-deficient gin2-1 mutants revealed a strong impact of HXK1 activity on metabolome dynamics in multiple compartments. Glucose accumulation in the cytosol of gin2-1 was accompanied by diminished vacuolar glucose levels. Subcellular dynamics of pyruvate, succinate and fumarate amounts were significantly affected in gin2-1 and coincided with differential mitochondrial proteome dynamics. Lowered mitochondrial glycine and serine amounts in gin2-1 together with reduced abundance of photorespiratory proteins indicated an effect of the gin2-1 mutation on photorespiratory capacity. Our findings highlight the necessity to resolve plant metabolism to a subcellular level to provide a causal relationship between metabolites, proteins and metabolic pathway regulation.

Dynamics of Plant Metabolism during Cold Acclimation

Plants have evolved strategies to tightly regulate metabolism during acclimation to a changing environment. Low temperature significantly constrains distribution, growth and yield of many temperate plant species. Exposing plants to low but non-freezing temperature induces a multigenic processes termed cold acclimation, which eventually results in an increased freezing tolerance. Cold acclimation comprises reprogramming of the transcriptome, proteome and metabolome and affects communication and signaling between subcellular organelles. Carbohydrates play a central role in this metabolic reprogramming. This review summarizes current knowledge about the role of carbohydrate metabolism in plant cold acclimation with a focus on subcellular metabolic reprogramming, its thermodynamic constraints under low temperature and mathematical modelling of metabolism.

Functional Characterization of Invertase Inhibitors PtC/VIF1 and 2 Revealed Their Involvements in the Defense Response to Fungal Pathogen in Populus trichocarpa

In higher plants, cell wall invertase (CWI) and vacuolar invertase (VI) were considered to be essential coordinators in carbohydrate partitioning, sink strength determination, and stress responses. An increasing body of evidence revealed that the tight regulation of CWI and VI substantially depends on the post-translational mechanisms, which were mediated by small proteinaceous inhibitors (C/VIFs, Inhibitor of β-Fructosidases). As yet, the extensive survey of the molecular basis and biochemical property of C/VIFs remains largely unknown in black cottonwood (Populus trichocarpa Torr. & A. Gray), a model species of woody plants. In the present work, we have initiated a systematic review of the genomic structures, phylogenies, cis-regulatory elements, and conserved motifs as well as the tissue-specific expression, resulting in the identification of 39 genes encoding C/VIF in poplar genome. We characterized two putative invertase inhibitors PtC/VIF1 and 2, showing predominant transcript levels in the roots and highly divergent responses to the selected stress cues including fusarium wilt, drought, ABA, wound, and senescence. In silico prediction of the signal peptide hinted us that they both likely had the apoplastic targets. Based on the experimental visualization via the transient and stable transformation assays, we confirmed that PtC/VIF1 and 2 indeed secreted to the extracellular compartments. Further validation of their recombinant enzymes revealed that they displayed the potent inhibitory affinities on the extracted CWI, supporting the patterns that act as the typical apoplastic invertase inhibitors. To our knowledge, it is the first report on molecular characterization of the functional C/VIF proteins in poplar. Our results indicate that PtC/VIF1 and 2 may exert essential roles in defense- and stress-related responses. Moreover, novel findings of the up- and downregulated C/VIF genes and functional enzyme activities enable us to further unravel the molecular mechanisms in the promotion of woody plant performance and adapted-biotic stress, underlying the homeostatic control of sugar in the apoplast.