Metabolic responses to potassium availability and waterlogging reshape respiration and carbon use efficiency in oil palm

Nutrient limitations on photosynthesis: from individual to combinational stresses

Liebig's law of the minimum states that increasing photosynthetic productivity on nutrient-impoverished soils depends on addressing the most limiting nutrient. Research has identified the roles of different mineral nutrients in photosynthetic processes. However, diffusional and biochemical regulation of photosynthesis both feature patterns of cumulative effects that jointly determine photosynthetic capacity. More importantly, responses to multiple nutrient stresses are not simply additive and require a comprehensive understanding of how these stresses interact and impact photosynthetic performance. In this review we highlight key macroelements for photosynthesis - nitrogen, phosphorus, potassium, and magnesium - focusing on their unique functions and interactions in regulating carbon fixation under multiple nutrient deficiencies, with the goal of enhancing crop productivity through balanced nutrient applications.

Linking leaf dark respiration to leaf traits and reflectance spectroscopy across diverse forest types

Leaf dark respiration (Rdark), an important yet rarely quantified component of carbon cycling in forest ecosystems, is often simulated from leaf traits such as the maximum carboxylation capacity (Vcmax), leaf mass per area (LMA), nitrogen (N) and phosphorus (P) concentrations, in terrestrial biosphere models. However, the validity of these relationships across forest types remains to be thoroughly assessed. Here, we analyzed Rdark variability and its associations with Vcmax and other leaf traits across three temperate, subtropical and tropical forests in China, evaluating the effectiveness of leaf spectroscopy as a superior monitoring alternative. We found that leaf magnesium and calcium concentrations were more significant in explaining cross-site Rdark than commonly used traits like LMA, N and P concentrations, but univariate trait-Rdark relationships were always weak (r2 ≤ 0.15) and forest-specific. Although multivariate relationships of leaf traits improved the model performance, leaf spectroscopy outperformed trait-Rdark relationships, accurately predicted cross-site Rdark (r2 = 0.65) and pinpointed the factors contributing to Rdark variability. Our findings reveal a few novel traits with greater cross-site scalability regarding Rdark, challenging the use of empirical trait-Rdark relationships in process models and emphasize the potential of leaf spectroscopy as a promising alternative for estimating Rdark, which could ultimately improve process modeling of terrestrial plant respiration.

Regulation of anaplerotic enzymes by melatonin enhances resilience to cadmium toxicity in Vigna radiata (L.) R. Wilczek

Melatonin (Mel) is a tryptophan-derived (N-acetyl-5-methoxytryptamine) molecule. In the present study, role of Mel in the regulation of various anaplerotic enzymes is discussed in relation to N metabolism and H+-ATPase activity in mung bean under Cd stress. The application of Mel to the Cd-stressed mung bean seedlings was remarkable in improving the activity of hexokinase (35.7%), pyruvate kinase (79.2%), phosphoenolpyruvate carboxylase (38.9%) pyruvate dehydrogenase (41.5%), malate dehydrogenase (49.2%), citrate synthase (37.7%), isocitrate dehydrogenase (33.1%), ATP synthase (63.6%), and ATPase (38.6%). Incubation of Cd-stressed seedlings with Mel also improved the activity of nitrate reductase by 89.4%, nitrite reductase by 78.2%, and glutamine synthetase by 35.3% that resulted in higher level of ammonium and their subsequent assimilation to amino acids and proteins. Activation of these enzymes was strongly associated with Mel-induced regulation of H+-ATPase activity that improved K+ retention and N assimilation capacity of the Cd-stressed seedlings of mung bean. The coordinated mechanism of action of tricarboxylic acid (TCA) cycle, N metabolism, and higher K+ levels were helpful in providing protection against detrimental effects of Cd toxicity through improving the defense system and energy level of the plants. However, inclusion of sodium orthovanadate (PM H+-ATPase inhibitor) to the incubation medium reversed the positive effect of Mel and suppressed the performance of plants under Cd-stress. The findings of the study indicate that under Cd stress, the regulatory mechanisms of anaplerotic enzymes and antioxidant defense are mediated by Mel, and this process is facilitated by the retention of K+ induced by H+-ATPase.

Physiological and transcriptomic analyses reveal the molecular mechanism of PsAMT1.2 in salt tolerance

Soil salinization has become a global problem and high salt concentration in soil negatively affects plant growth. In our previous study, we found that overexpression of PsAMT1.2 from Populus simonii could improve the salt tolerance of poplar, but the physiological and molecular mechanism was not well understood. To explore the regulation pathway of PsAMT1.2 in salt tolerance, we investigated the morphological, physiological and transcriptome differences between the PsAMT1.2 overexpression transgenic poplar and the wild type under salt stress. The PsAMT1.2 overexpression transgenic poplar showed better growth with increased net photosynthetic rate and higher chlorophyll content compared with wild type under salt stress. The overexpression of PsAMT1.2 increased the catalase, superoxide dismutase, peroxidase and ascorbate peroxidase activities, and therefore probably enhanced the reactive oxygen species clearance ability, which also reduced the degree of membrane lipid peroxidation under salt stress. Meanwhile, the PsAMT1.2 overexpression transgenic poplar maintained a relatively high K+/Na+ ratio under salt stress. RNA-seq analysis indicated that PsAMT1.2 might improve plant salt tolerance by regulating pathways related to the photosynthetic system, chloroplast structure, antioxidant activity and anion transport. Among the 1056 differentially expressed genes, genes related to photosystem I and photosystem II were up-regulated and genes related to chloride channel protein-related were down-regulated. The result of the present study would provide new insight into regulation mechanism of PsAMT1.2 in improving salt tolerance of poplar.

Metabolomics-Assisted Breeding in Oil Palm: Potential and Current Perspectives

Oil palm is presently the most important oil-producing crop worldwide in terms of oil production and consumption. However, oil palm cultivation faces important challenges such as adverse climatic conditions, expensive fertilization requirements, and fungal pathogens, including Ganoderma. Intense efforts in oil palm breeding are devoted to improving both oil production yield and resistance to environmental cues. Metabolomics can be of interest because it provides many quantitative traits and metabolic signatures that can be selected for to optimize oil palm performance. Here, we briefly review how metabolomics can help oil palm breeding, and to do so, we give examples of recent metabolomics analyses and provide a roadmap to use metabolomics-assisted breeding.

Leaf day respiration: More than just catabolic CO2 production in the light

Day respiration is a net flux resulting from several CO2‐generating and CO2‐fixing reactions, not only related to catabolism but also to anabolism. We review pieces of evidence that decarboxylating reactions are partly fed by carbon sources disconnected from current photosynthesis and how they reflect various metabolic pathways.

Variation in leaf dark respiration among C3 and C4 grasses is associated with use of different substrates

Measurements of respiratory properties have often been made at a single time point either during daytime using dark-adapted leaves or during nighttime. The influence of the day-night cycle on respiratory metabolism has received less attention but is crucial to understand photosynthesis and photorespiration. Here, we examined how CO2- and O2-based rates of leaf dark respiration (Rdark) differed between midday (after 30-min dark adaptation) and midnight in 8 C3 and C4 grasses. We used these data to calculate the respiratory quotient (RQ; ratio of CO2 release to O2 uptake), and assessed relationships between Rdark and leaf metabolome. Rdark was higher at midday than midnight, especially in C4 species. The day-night difference in Rdark was more evident when expressed on a CO2 than O2 basis, with the RQ being higher at midday than midnight in all species, except in rice (Oryza sativa). Metabolomic analyses showed little correlation of Rdark or RQ with leaf carbohydrates (sucrose, glucose, fructose, or starch) but strong multivariate relationships with other metabolites. The results suggest that rates of Rdark and differences in RQ were determined by several concurrent CO2-producing and O2-consuming metabolic pathways, not only the tricarboxylic acid cycle (organic acids utilization) but also the pentose phosphate pathway, galactose metabolism, and secondary metabolism. As such, Rdark was time-, type- (C3/C4) and species-dependent, due to the use of different substrates.

d-amino acids metabolism reflects the evolutionary origin of higher plants and their adaptation to the environment

d-amino acids are the d stereoisomers of the common l-amino acids found in proteins. Over the past two decades, the occurrence of d-amino acids in plants has been reported and circumstantial evidence for a role in various processes, including interaction with soil microorganisms or interference with cellular signalling, has been provided. However, examples are not numerous and d-amino acids can also be detrimental, some of them inhibiting growth and development. Thus, the persistence of d-amino acid metabolism in plants is rather surprising, and the evolutionary origins of d-amino acid metabolism are currently unclear. Systemic analysis of sequences associated with d-amino acid metabolism enzymes shows that they are not simply inherited from cyanobacterial metabolism. In fact, the history of plant d-amino acid metabolism enzymes likely involves multiple steps, cellular compartments, gene transfers and losses. Regardless of evolutionary steps, enzymes of d-amino acid metabolism, such as d-amino acid transferases or racemases, have been retained by higher plants and have not simply been eliminated, so it is likely that they fulfil important metabolic roles such as serine, folate or plastid peptidoglycan metabolism. We suggest that d-amino acid metabolism may have been critical to support metabolic functions required during the evolution of land plants.

Transcriptomic and metabolomic approaches elucidate the systemic response of wheat plants under waterlogging

Extreme weather conditions lead to significant imbalances in crop productivity, which in turn affect food security. Flooding events cause serious problems for many crop species such as wheat. Although metabolic readjustments under flooding are important for plant regeneration, underlying processes remain poorly understood. Here, we investigated the systemic response of wheat to waterlogging using metabolomics and transcriptomics. A 12 d exposure to excess water triggered nutritional imbalances and disruption of metabolite synthesis and translocation, reflected by reductions in plant biomass and growth performance. Metabolic and transcriptomic profiling in roots, xylem sap, and leaves indicated anaerobic fermentation processes as a local response in roots. Differentially expressed genes and ontological categories revealed that carbohydrate metabolism plays an important role in the systemic response. Analysis of the composition of xylem exudates revealed decreased root-to-shoot translocation of nutrients, hormones, and amino acids. Interestingly, among all metabolites measured in xylem exudates, alanine was the most abundant. Immersion of excised leaves derived from waterlogged plants in alanine solution led to increased leaf glucose concentration. Our results suggest an important role of alanine not only as an amino-nitrogen donor but also as a vehicle for carbon skeletons to produce glucose de novo and meet the energy demand during waterlogging.

Transcriptomic responses of oil palm (Elaeis guineensis) stem to waterlogging at plantation in relation to precipitation seasonality

Global warming-induced climate change causes significant agricultural problems by increasing the incidence of drought and flooding events. Waterlogging is an inevitable consequence of these changes but its effects on oil palms have received little attention and are poorly understood. Recent waterlogging studies have focused on oil palm seedlings, with particular emphasis on phenology. However, the transcriptomic waterlogging response of mature oil palms remains elusive in real environments. We therefore investigated transcriptomic changes over time in adult oil palms at plantations over a two-year period with pronounced seasonal variation in precipitation. A significant transcriptional waterlogging response was observed in the oil palm stem core but not in leaf samples when gene expression was correlated with cumulative precipitation over two-day periods. Pathways and processes upregulated or enriched in the stem core response included hypoxia, ethylene signaling, and carbon metabolism. Post-waterlogging recovery in oil palms was found to be associated with responses to heat stress and carotenoid biosynthesis. Nineteen transcription factors (TFs) potentially involved in the waterlogging response of mature oil palms were also identified. These data provide new insights into the transcriptomic responses of planted oil palms to waterlogging and offer valuable guidance on the sensitivity of oil palm plantations to future climate changes.

Exploring the Potential of Multiomics and Other Integrative Approaches for Improving Waterlogging Tolerance in Plants

Soil flooding has emerged as a serious threat to modern agriculture due to the rapid global warming and climate change, resulting in catastrophic crop damage and yield losses. The most detrimental effects of waterlogging in plants are hypoxia, decreased nutrient uptake, photosynthesis inhibition, energy crisis, and microbiome alterations, all of which result in plant death. Although significant advancement has been made in mitigating waterlogging stress, it remains largely enigmatic how plants perceive flood signals and translate them for their adaptive responses at a molecular level. With the advent of multiomics, there has been significant progress in understanding and decoding the intricacy of how plants respond to different stressors which have paved the way towards the development of climate-resistant smart crops. In this review, we have provided the overview of the effect of waterlogging in plants, signaling (calcium, reactive oxygen species, nitric oxide, hormones), and adaptive responses. Secondly, we discussed an insight into past, present, and future prospects of waterlogging tolerance focusing on conventional breeding, transgenic, multiomics, and gene-editing approaches. In addition, we have also highlighted the importance of panomics for developing waterlogging-tolerant cultivars. Furthermore, we have discussed the role of high-throughput phenotyping in the screening of complex waterlogging-tolerant traits. Finally, we addressed the current challenges and future perspectives of waterlogging signal perception and transduction in plants, which warrants future investigation.

Data Science and Plant Metabolomics

The study of plant metabolism is one of the most complex tasks, mainly due to the huge amount and structural diversity of metabolites, as well as the fact that they react to changes in the environment and ultimately influence each other. Metabolic profiling is most often carried out using tools that include mass spectrometry (MS), which is one of the most powerful analytical methods. All this means that even when analyzing a single sample, we can obtain thousands of data. Data science has the potential to revolutionize our understanding of plant metabolism. This review demonstrates that machine learning, network analysis, and statistical modeling are some techniques being used to analyze large quantities of complex data that provide insights into plant development, growth, and how they interact with their environment. These findings could be key to improving crop yields, developing new forms of plant biotechnology, and understanding the relationship between plants and microbes. It is also necessary to consider the constraints that come with data science such as quality and availability of data, model complexity, and the need for deep knowledge of the subject in order to achieve reliable outcomes.

Glycine max (L.) Merr. (Soybean) metabolome responses to potassium availability

Potassium (K⁺) has vital physiological and metabolic functions in plants and its availability can impact tolerance to biotic and abiotic stress conditions. Limited studies have investigated the effect of K⁺ fertilization on soybean metabolism. Using integrated omics, ionomics and metabolomics, we investigated the field-grown Glycine max (soybean) response, after four K⁺ soil fertilization rates. Soybean leaf and pod tissue (valves and immature seeds) extracts were analysed by ultra-performance liquid chromatography coupled to high-resolution mass spectrometry (UPLC-HRMS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Multivariate analyses (PCA-X&Y e O2PLS-DA) showed that 51 compounds of 19 metabolic pathways were regulated in response to K⁺ availability. Under very low potassium availability, soybean plants accumulated of Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺, and B in young and old leaves. Potassium fertilization upregulated carbohydrate, galactolipid, and flavonol glycoside biosynthesis in leaves and pod valves, while K⁺ deficient pod tissues showed increasing amino acids, oligosaccharides, benzoic acid derivatives, and isoflavones contents. Severely K⁺ deficient soils elicited isoflavones, coumestans, pterocarpans, and soyasaponins in trifoliate leaves, likely associated to oxidative and photodynamic stress status. Additionally, results demonstrate that L-asparagine content is higher in potassium deficient tissues, suggesting this compound as a biomarker of K⁺ deficiency in soybean plants. These results demonstrate that potassium soil fertilization did not linearly contribute to changes in specialised constitutive metabolites of soybean. Altogether, this work provides a reference for improving the understanding of soybean metabolism as dependent on K⁺ availability.

Exact mass GC-MS analysis: Protocol, database, advantages and application to plant metabolic profiling

Plant metabolomics has been used widely in plant physiology, in particular to analyse metabolic responses to environmental parameters. Derivatization (via trimethylsilylation and methoximation) followed by GC-MS metabolic profiling is a major technique to quantify low molecular weight, common metabolites of primary carbon, sulphur and nitrogen metabolism. There are now excellent opportunities for new generation analyses, using high resolution, exact mass GC-MS spectrometers that are progressively becoming relatively cheap. However, exact mass GC-MS analyses for routine metabolic profiling are not common, since there is no dedicated available database. Also, exact mass GC-MS is usually dedicated to structural resolution of targeted secondary metabolites. Here, we present a curated database for exact mass metabolic profiling (made of 336 analytes, 1064 characteristic exact mass fragments) focused on molecules of primary metabolism. We show advantages of exact mass analyses, in particular to resolve isotopic patterns, localise S-containing metabolites, and avoid identification errors when analytes have common nominal mass peaks in their spectrum. We provide a practical example using leaves of different Arabidopsis ecotypes and show how exact mass GC-MS analysis can be applied to plant samples and identify metabolic profiles.

Mycorrhiza-mediated potassium transport in Medicago truncatula can be evaluated by using rubidium as a proxy

Arbuscular mycorrhizal (AM) fungi considerably improve plant nutrient acquisition, particularly phosphorus and nitrogen. Despite the physiological importance of potassium (K⁺) in plants, there is increasing interest in the mycorrhizal contribution to plant K⁺ nutrition. Yet, methods to track K⁺ transport are often costly and limiting evaluation opportunities. Rubidium (Rb⁺) is known to be transported through same pathways as K⁺. As such our research efforts attempt to evaluate if Rb⁺ could serve as a viable proxy for evaluating K⁺ transport in AM symbiosis. Therefore, we examined the transport of K⁺ in Medicago truncatula colonized by the AM fungus Rhizophagus irregularis isolate 09 having access to various concentrations of Rb⁺ in custom-made two-compartment systems. Plant biomass, fungal root colonization, and shoot nutrient concentrations were recorded under sufficient and limited K⁺ regimes. We report that AM plants displayed higher shoot Rb⁺ and K⁺ concentrations and a greater K⁺:Na⁺ ratio relative to non-colonized plants in both sufficient and limited K⁺ conditions. Consequently, our results indicate that Rb⁺ can be used as a proxy to assess the movement of K⁺ in AM symbiosis, and suggest the existence of a mycorrhizal uptake pathway for K⁺ nutrition in M. truncatula.

Reprogramming of Plant Central Metabolism in Response to Abiotic Stresses: A Metabolomics View

Abiotic stresses rewire plant central metabolism to maintain metabolic and energy homeostasis. Metabolites involved in the plant central metabolic network serve as a hub for regulating carbon and energy metabolism under various stress conditions. In this review, we introduce recent metabolomics techniques used to investigate the dynamics of metabolic responses to abiotic stresses and analyze the trend of publications in this field. We provide an updated overview of the changing patterns in central metabolic pathways related to the metabolic responses to common stresses, including flooding, drought, cold, heat, and salinity. We extensively review the common and unique metabolic changes in central metabolism in response to major abiotic stresses. Finally, we discuss the challenges and some emerging insights in the future application of metabolomics to study plant responses to abiotic stresses.

Comparative Proteomic Analysis of Grapevine Rootstock in Response to Waterlogging Stress

Waterlogging severely affects global agricultural production. Clarifying the regulatory mechanism of grapevine in response to waterlogging stress will help to improve the waterlogging tolerance of grapevine. In the present study, the physiological and proteomic responses of SO4 grapevine rootstock to different waterlogging tolerances were comparatively assayed. The results showed that the activities of SOD and POD first increased and then decreased, while the change trend of CAT and APX activities was the opposite. In addition, the MDA and H₂O₂ contents increased after waterlogging treatment, but the chlorophyll a and chlorophyll b contents decreased. A total of 5,578 grapevine proteins were identified by the use of the tandem mass tag (TMT) labeling technique. Among them, 214 (103 and 111 whose expression was upregulated and downregulated, respectively), 314 (129 and 185 whose expression was upregulated and downregulated, respectively), and 529 (248 and 281 whose expression was upregulated and downregulated, respectively) differentially expressed proteins (DEPs) were identified in T0d vs. T10d, T10d vs. T20d, and T0d vs. T20d comparison groups, respectively. Enrichment analysis showed that these DEPs were mainly involved in glutathione metabolism, carbon fixation, amino sugar and nucleotide sugar metabolism, biosynthesis of amino acids, photosynthesis, carbon metabolism, starch, and sucrose metabolism, galactose metabolism, protein processing and ribosomes. To further verify the proteomic data, the expression of corresponding genes that encode eight DEPs was confirmed by quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). The results of this study presented an important step toward understanding the resistance mechanisms of grapevine in response to waterlogging stress at the proteome level.

Potassium dependency of enzymes in plant primary metabolism

Potassium is a macroelement essential to many aspects of plant life, such as photosynthesis, phloem transport or cellular electrochemistry. Many enzymes in animals or microbes are known to be stimulated or activated by potassium (K⁺ ions). Several plant enzymes are also strictly K⁺-dependent, and this can be critical when plants are under K deficiency and thus intracellular K⁺ concentration is low. Although metabolic effects of low K conditions have been documented, there is presently no review focusing on roles of K⁺ for enzyme catalysis or activation in plants. In this mini-review, we compile the current knowledge on K⁺-requirement of plant enzymes and take advantage of structural data to present biochemical roles of K⁺. This information is instrumental to explain direct effects of low K⁺ content on metabolism and this is illustrated with recent metabolomics data.

K Deprivation Modulates the Primary Metabolites and Increases Putrescine Concentration in Brassica napus

Potassium (K) plays a crucial role in plant growth and development and is involved in different physiological and biochemical functions in plants. Brassica napus needs higher amount of nutrients like nitrogen (N), K, phosphorus (P), sulfur (S), and boron (B) than cereal crops. Previous studies in B. napus are mainly focused on the role of N and S or combined deficiencies. Hence, little is known about the response of B. napus to K deficiency. Here, a physiological, biochemical, and molecular analysis led us to investigate the response of hydroponically grown B. napus plants to K deficiency. The results showed that B. napus was highly sensitive to the lack of K. The lower uptake and translocation of K induced BnaHAK5 expression and significantly declined the growth of B. napus after 14 days of K starvation. The lower availability of K was associated with a decrease in the concentration of both S and N and modulated the genes involved in their uptake and transport. In addition, the lack of K induced an increase in Ca2+ and Mg2+ concentration which led partially to the accumulation of positive charge. Moreover, a decrease in the level of arginine as a positively charged amino acid was observed which was correlated with a substantial increase in the polyamine, putrescine (Put). Furthermore, K deficiency induced the expression of BnaNCED3 as a key gene in abscisic acid (ABA) biosynthetic pathway which was associated with an increase in the levels of ABA. Our findings provided a better understanding of the response of B. napus to K starvation and will be useful for considering the importance of K nutrition in this crop.

Metabolomics-Based Evaluation of Crop Quality Changes as a Consequence of Climate Change

Fruit composition determines the fruit quality and, consequently, consumer acceptance. As fruit quality can be modified by environmental conditions, it will be impacted by future alterations produced by global warming. Therefore, agricultural activities will be influenced by the changes in climatological conditions in cultivable areas, which could have a high socioeconomic impact if fruit production and quality decline. Currently, different stresses are being applied to several cultivated species to evaluate their impact on fruit metabolism and plant performance. With the use of metabolomic tools, these changes can be precisely measured, allowing us to determine changes in the patterns of individual compounds. As these changes depend on both the stress severity and the specific species involved and even on the specific cultivar, individual analysis must be conducted. To date, the most-studied crops have mainly been crops that are widely cultivated and have a high socioeconomic impact. In the near future, with the development of these metabolomic strategies, their implementation will be extended to other species, which will allow the adaptation of cultivation conditions and the development of varieties with high adaptability to climatological changes.

Involvement of salicylic acid in the response to potassium deficiency revealed by metabolomics

Potassium (K) deficiency has consequences not only on cellular ion balance and transmembrane potential but also on metabolism. In fact, several enzymes are K-dependent including enzymes in catabolism, causing an alteration in glycolysis and respiration. In addition, K deficiency is associated with the induction of specific pathways and accumulation of metabolic biomarkers, such as putrescine. However, such drastic changes are usually observed when K deficiency is established. Here, we carried out a kinetic analysis with metabolomics to elucidate early metabolic events when nutrient conditions change from K-sufficiency to K-deficiency in Arabidopsis rosettes from both wild type and mutants affected in both K absorption and low-K signalling (hak5 akt1 cipk23). Our results show that mutants have a metabolomics pattern similar to K-deficient wild-type, showing a constitutive metabolic response to low K. In addition, shifting to low K conditions induces (i) changes in sugar metabolism and (ii) an accumulation of salicylic acid metabolites before the appearance of biomarkers of K deficiency (putrescine and aconitate), and such an accumulation is more pronounced in mutants. Our results suggest that early events in the response to low K conditions involve salicylic acid metabolism.

Potassium modulates central carbon metabolism to participate in regulating CO2 transport and assimilation in Brassica napus leaves

Potassium (K) regulates plant metabolism and enhances plant's ability to adapt to adversity. However, under different K deficiency stress, the net photosynthetic rate (A_n) was reduced, influenced by CO₂ conductance or biochemical capacities. The interplay between metabolome and photosynthetic characteristics under K deficiency stress was analyzed to explore the mechanisms by which K regulates photosynthetic capacity. With increasing K deficiency stress, dominations limiting A_n varied from CO₂ conductance to biochemical limitations. Multivariate analyses indicated that organic acids, amino acids and sedoheptulose-7-bisphosphate were significantly related to A_n, CO₂ conductance and carboxylation rate. Under moderate K deficiency, organic acids were up-regulated. Acidification of subcellular compartments reduced sedoheptulose-1,7-bisphosphatase activity, inducing downregulation of sedoheptulose-7-bisphosphate and hindrance of ribulose bisphosphate regeneration. Moreover, increased CO₂ shortage with increasing K deficiency induced a shift of increased citric acid to amino acid synthesis, causing excessive accumulation of amino acids. In addition, the reduced serine level indicated impaired photorespiration. These two changes triggered more serious reduction in photosynthetic capacity. The intimate, changes in photosynthetic capacities were tightly coupled with shifts in central C metabolism, which provides insights into the methods used to enhance A_n and plant's adaptability to abiotic stresses, through the regulation of C metabolites using molecular technology.

Plant low-K responses are partly due to Ca prevalence and the low-K biomarker putrescine does not protect from Ca side effects but acts as a metabolic regulator

Potassium (K) deficiency is a rather common situation that impacts negatively on biomass, photosynthesis and N assimilation, making K fertilization often unavoidable. Effects of K deficiency have been investigated for several decades and recently progress has been made in identifying metabolomics signatures thereby offering potential to monitor the K status of crops in the field. However, effects of low K conditions could also be due to the antagonism with other nutrients like calcium (Ca) and the well-known biomarker of K deficiency, putrescine, could be a response to Ca/K imbalance rather than K deficiency per se. To sort this out, we carried out experiments in sunflower grown at either low or high K, at high or low Ca, with or without putrescine added to the nutrient solution. Using metabolomics and proteomics analysis, we show that a significant part of the low K response, such as lower photosynthesis and N assimilation, is due to calcium and can be suppressed by low Ca conditions. Putrescine addition tends to restore photosynthesis and N assimilation but unlike low Ca does not suppress but aggravates the impact of low K conditions on catabolism, including the typical fall-over in pyruvate kinase. We conclude that (a) the effects of K deficiency on key metabolic processes can be partly alleviated by the use of low Ca and not only by K fertilization and (b) in addition to its role as a metabolite, putrescine participates in acclimation to low K via the regulation of the content in enzymes involved in carbon primary metabolism.

The effect of potassium fertilization on the metabolite profile of tomato fruit (Solanum lycopersicum L.)

The macronutrient potassium (K) has vital physiological functions in plants and its availability can strongly impact quality of crops like tomato. The impact of K nutrition on conventional tomato fruit quality parameters has been described several times, but detailed investigations on the effect of K supply on the fruit metabolite profile are still rare. To fill this gap, we investigated the influence of K fertilization on the metabolite profile of tomato fruits. For this purpose, an outdoor pot experiment with three different cocktail tomato cultivars was performed. A fertilization regimen with five K levels was applied, ranging from deficiency to sufficient supply. Fruit samples were analyzed by untargeted GC×GC-MS to cover the primary metabolite profile as well as some secondary metabolites. As verified using ICP-OES, fruit K content was highly proportional to the supplied amount of K. At the metabolite profile level, the most prominent and cultivar-independent effect of increased K fertilization was the rise of tricarboxylic acid (TCA) cycle intermediates. Further effects were more cultivar-specific, for example an increase of the mobile nitrogen pool (e.g. amines like putrescine and amides like asparagine), changes in the profile of minor sugars (especially disaccharides) as well as higher levels of some secondary metabolites. Pronounced response patterns were mainly observed in the cultivars Primavera and Yellow Submarine that were recently characterized as higher yielding, demanding a stronger consideration of cultivar differences in future studies.

Potassium deficiency reconfigures sugar export and induces catecholamine accumulation in oil palm leaves

Metabolic effects of potassium (K) deficiency have been described for nearly 70 years but specific effects of low K availability on sugar composition, sugar export rate and its relationship with other leaf metabolites are not very well documented. Having such pieces of information is nevertheless essential to identify metabolic signatures to monitor K fertilization. This is particularly true in oil-producing crop species such as oil palm (Elaeis guineensis), which is strongly K-demanding and involves high sugar dependence for fruit formation because of low carbon use efficiency in lipid synthesis. Here, we used metabolic analyses, measured sugar export rates with ¹³C isotopic labeling and examined the effects of K availability on both leaflet and rachis sugar metabolism in oil palm seedlings. We show that low K leads to a modification of sugar composition mostly in rachis and decreased sucrose and hexose export rates from leaflets. As a result, leaflets contained more starch and induced alternative pathways such as raffinose synthesis, although metabolites of the raffinose pathway remained quantitatively minor. The alteration of glycolysis by low K was compensated for by an increase in alternative sugar phosphate utilization by tyrosine metabolism, resulting in considerable amounts of tyramine and dopamine.

Metabolic Responses to Waterlogging Differ between Roots and Shoots and Reflect Phloem Transport Alteration in Medicago truncatula

Root oxygen deficiency that is induced by flooding (waterlogging) is a common situation in many agricultural areas, causing considerable loss in yield and productivity. Physiological and metabolic acclimation to hypoxia has mostly been studied on roots or whole seedlings under full submergence. The metabolic difference between shoots and roots during waterlogging, and how roots and shoots communicate in such a situation is much less known. In particular, the metabolic acclimation in shoots and how this, in turn, impacts on roots metabolism is not well documented. Here, we monitored changes in the metabolome of roots and shoots of barrel clover (Medicago truncatula), growth, and gas-exchange, and analyzed phloem sap exudate composition. Roots exhibited a typical response to hypoxia, such as γ-aminobutyrate and alanine accumulation, as well as a strong decline in raffinose, sucrose, hexoses, and pentoses. Leaves exhibited a strong increase in starch, sugars, sugar derivatives, and phenolics (tyrosine, tryptophan, phenylalanine, benzoate, ferulate), suggesting an inhibition of sugar export and their alternative utilization by aromatic compounds production via pentose phosphates and phosphoenolpyruvate. Accordingly, there was an enrichment in sugars and a decline in organic acids in phloem sap exudates under waterlogging. Mass-balance calculations further suggest an increased imbalance between loading by shoots and unloading by roots under waterlogging. Taken as a whole, our results are consistent with the inhibition of sugar import by waterlogged roots, leading to an increase in phloem sugar pool, which, in turn, exert negative feedback on sugar metabolism and utilization in shoots.

Potassium and Magnesium Mediate the Light and CO2 Photosynthetic Responses of Grapevines

Potassium (K) and magnesium (Mg) deficiency are common stresses that can impact on grape yield and quality, but their effects on photosynthesis have received little attention. Understanding the diffusional and biochemical limitations to photosynthetic constraints will help to guide improvements in cultural practices. Accordingly, the photosynthetic response of Vitis vinifera cvs. Shiraz and Chardonnay to K or Mg deficiency was assessed under hydroponic conditions using miniature low-nutrient-reserve vines. Photosynthesis was at least partly reduced by a decline in stomatal conductance. Light and CO₂-saturated photosynthesis, maximum rate of ribulose 1.5 bisphospate (RuBP) carboxylation (V_cmax) and maximum rate of electron transport (J_max) all decreased under K and Mg deficiency. Likewise, chlorophyll fluorescence and electron transport were lower under both nutrient deficiencies while dark respiration increased. K deficiency drastically reduced shoot biomass in both cultivars, while root biomass was greatly reduced under both Mg and K deficiency. Taken together, these results indicate that the decrease in biomass was likely due to both stomatal and biochemical limitations in photosynthesis. Optimising photosynthesis through adequate nutrition will thus support increases in biomass with carry-on positive effects on crop yields.

Seed Germination in Oil Palm (Elaeis guineensis Jacq.): A Review of Metabolic Pathways and Control Mechanisms

Oil palm is an oil-producing crop of major importance at the global scale. Oil palm mesocarp lipids are used for myriads industrial applications, and market demand has been growing for decades. In addition, oil palm seeds are oleaginous, and the oil extracted therefrom can be used for several purposes, from food to cosmetics. As such, there is a huge need in oil palm seeds to maintain the global cohort of more than 2 billion trees. However, oil palm seed germination is a rather difficult process, not only to break dormancy, but also because it is long and often reaches lower-than-expected germination rates. Surprisingly, despite the crucial importance of germination for oil palm plantation management, our knowledge is still rather limited, in particular about germinating oil palm seed metabolism. The present review incorporates different pieces of information that have been obtained in the past few years, in oil palm and in other palm species, in order to provide an overview of germination metabolism and its control. Further insights can also be gained from other oleaginous model plants, such as Arabidopsis or canola, however, palm seeds have peculiarities that must be accounted for, to gain a better understanding of germinating seed metabolism.

What is the role of putrescine accumulated under potassium deficiency?

Biomarker metabolites are of increasing interest in crops since they open avenues for precision agriculture, whereby nutritional needs and stresses can be monitored optimally. Putrescine has the potential to be a useful biomarker to reveal potassium (K⁺ ) deficiency. In fact, although this diamine has also been observed to increase during other stresses such as drought, cold or heavy metals, respective changes are comparably low. Due to its multifaceted biochemical properties, several roles for putrescine under K⁺ deficiency have been suggested, such as cation balance, antioxidant, reactive oxygen species mediated signalling, osmolyte or pH regulator. However, the specific association of putrescine build-up with low K⁺ availability in plants remains poorly understood, and possible regulatory roles must be consistent with putrescine concentration found in plant tissues. We hypothesize that the massive increase of putrescine upon K⁺ starvation plays an adaptive role. A distinction of putrescine function from that of other polyamines (spermine, spermidine) may be based either on its specificity or (which is probably more relevant under K⁺ deficiency) on a very high attainable concentration of putrescine, which far exceeds those for spermidine and spermine. putrescine and its catabolites appear to possess a strong potential in controlling cellular K⁺ and Ca²⁺ , and mitochondria and chloroplasts bioenergetics under K⁺ stress.

δ15 N values in plants are determined by both nitrate assimilation and circulation

Nitrogen (N) assimilation is associated with ¹⁴ N/¹⁵ N fractionation such that plant tissues are generally ¹⁵ N-depleted compared to source nitrate. In addition to nitrate concentration, the δ¹⁵ N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ¹⁵ N values in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ¹⁵ N in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging. There were strong interorgan δ¹⁵ N differences for contrasting situations between the two species, and a high ¹⁵ N-enrichment in root nitrate. Modelling shows that this ¹⁵ N-enrichment can be explained by nitrate circulation and compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 30‰ difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference. Our study thus suggests that plant δ¹⁵ N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of δ¹⁵ N of circulating phloem nitrate.

Potassium Alleviates Post-anthesis Photosynthetic Reductions in Winter Wheat Caused by Waterlogging at the Stem Elongation Stage

Waterlogging occurs frequently at the stem elongation stage of wheat in southern China, decreasing post-anthesis photosynthetic rates and constraining grain filling. This phenomenon, and the mitigating effect of nutrient application, should be investigated as it could lead to improved agronomic guidelines. We exposed pot-cultured wheat plants at the stem elongation stage to waterlogging treatment in combination with two rates of potassium (K) application. Waterlogging treatment resulted in grain yield losses, which we attributed to a reduction in the 1,000-grain weight caused by an early decline in the net photosynthetic rate (Pn) post-anthesis. These decreases were offset by increasing K application. Stomatal conductance (G _s) and the intercellular CO₂ concentration (C _i) decreased in the period 7-21 days after anthesis (DAA), and these reductions were exacerbated by waterlogging. However, in the period 21-28 DAA, G _s and C _i increased, while Pn decreased continuously, suggesting that non-stomatal factors constrained photosynthesis. On DAA 21, Pn was reduced by waterlogging, but photochemical efficiency (Φ _PSII ) remained unchanged, indicating a reduction in the dissipation of energy captured by photosystem II (PSII) through the CO₂ assimilation pathway. This reduction in energy dissipation increased the risk of photodamage, as shown by early reductions in Φ _PSII in waterlogged plants on DAA 28. However, increased K application promoted root growth and nutrient status under waterlogging, thereby improving photosynthesis post-anthesis. In conclusion, the decrease in Pn caused by waterlogging was attributable to stomatal closure during early senescence; during later senescence, a reduction in CO₂ assimilation accounted for the reduced Pn and elevated the risk of photodamage. However, K application mitigated waterlogging-accelerated photosynthetic reductions and reduced yield losses.

Effects of Potassium Fertilization on Oil Palm Fruit Metabolism and Mesocarp Lipid Accumulation

Potassium fertilization is commonly practiced in oil palm (Elaeis guineensis) plantations to increase yield. However, its effects on fruit oil content and composition are not well documented. Here, we conducted bunch, metabolomics, and oil composition analyses in two contrasting crosses (Deli × La Mé and Deli × Yangambi) grown under different K fertilization conditions. K availability impacted bunch oil content, resulting in lower water content and higher oil proportion in fruit mesocarp, in Deli × La Mé only, thus showing differential responses of crosses to K. Oil composition at maturity did not significantly change under low K conditions despite clear alterations in fruit metabolism associated with lipid production during maturation, demonstrating the resilience of oil biosynthetic metabolism. However, the analysis of variance in oil content (across K treatments and crosses) demonstrates that sugar availability, lipid synthesis rates, and metabolic recycling are all important in determining the oil content.