The diversity of substrates for plant respiration and how to optimize their use

Carbon Dynamics Under Drought and Recovery in Grapevine's Leaves

Drought stress reduces leaf net assimilation (AN) and phloem export, but the equilibrium between the two is unknown. Consequently, the leaf carbon balance and the primary use of the leaf nonstructural carbohydrates (NSC) under water deficit are unclear. Also, we do not know how quickly leaves can replenish their NSC storage and resume export after rehydration. Hence, we dried grapevines to either zero AN, leaf turgor loss, or complete wilting while following the leaf carbon dynamics. The vines ceased growth and minimized carbon export under drought, conserving the leaves NSC until AN zeroed. Subsequently, the leaves slowly depleted their NSC storage. However, the NSC depletion rate in the leaves was too slow to support the leaf's energetic requirements, potentially transforming the leaves into carbon sinks. Even under extreme drought (-2 MPa), the leaves had substantial NSC reserves (38% of the controls). After rehydration, all surviving leaves recovered their NSC storage within a week, and even leaves that were later shed had functional phloem export in the week after rehydration. The study reveals the leaf carbon relations under drought, highlighting the preference of the leaf to conserve its NSC storage rather than utilize it.

Non-canonical plant metabolism

Metabolism is essential for plant growth and has become a major target for crop improvement by enhancing nutrient use efficiency. Metabolic engineering is also the basis for producing high-value plant products such as pharmaceuticals, biofuels and industrial biochemicals. An inherent problem for such engineering endeavours is the tendency to view metabolism as a series of distinct metabolic pathways-glycolysis, the tricarboxylic acid cycle, the Calvin-Benson cycle and so on. While these canonical pathways may represent a dominant or frequently occurring flux mode, systematic analyses of metabolism via computational modelling have emphasized the inherent flexibility of the metabolic network to carry flux distributions that are distinct from the canonical pathways. Recent experimental estimates of metabolic network fluxes using 13C-labelling approaches have revealed numerous instances in which non-canonical pathways occur under different conditions and in different tissues. In this Review, we bring these non-canonical pathways to the fore, summarizing the evidence for their occurrence and the context in which they operate. We also emphasize the importance of non-canonical pathways for metabolic engineering. We argue that the introduction of a high-flux pathway to a desired metabolic product will, by necessity, require non-canonical supporting fluxes in central metabolism to provide the necessary carbon skeletons, energy and reducing power. We illustrate this using the overproduction of isoprenoids and fatty acids as case studies.

Importance of the leaf respiratory quotient

Rates of leaf respiratory CO2-release (RCO2) are important for terrestrial biosphere models that estimate carbon exchange between plants and the atmosphere. Hitherto, models of RCO2 have primarily been based on considerations of respiratory energy demand (particularly ATP) for maintenance and growth purposes. Respiratory ATP synthesis is closely tied to the rate of respiratory O2-uptake (RO2), with relative engagement of the alternative oxidase influencing the ATP:O ratio. However, the extent to which respiratory ATP synthesis is coupled to leaf RCO2 depends on the respiratory quotient (RQ, mol CO2 efflux per unit mol O2 uptake), with models predicting leaf RCO2 assuming that the RQ is at unity. Here, we show systematic inter-specific, temporal and temperature-dependent variation in leaf RQ, with values of RQ ranging from 0.51 to 2.2, challenging model assumptions on the RQ. We discuss possible mechanisms underlying the variation in leaf RQ, potential ways forward in terms of new measurement protocols, and perspectives for modelled RCO2. Our analyses highlight a range of outstanding research questions that need to be answered before we can mechanistically model leaf RCO2 at various scales.

Cytoprotective role of pyruvate in mitigating abiotic stress response in Arabidopsis thaliana

Pyruvate is a central metabolite in cellular respiration and metabolism. It can neutralize reactive oxygen species (ROS), safeguard mitochondrial membrane potential, and regulate gene expression under oxidative stress. However, its role in abiotic stress tolerance in plants needs to be explored. Therefore, the current study investigated the role of pyruvate and its metabolism in response to different abiotic stresses in the model plant Arabidopsis thaliana. We retrieved transcript profiling data for pyruvate metabolism and transportation genes (D-LDH, AlaAT, PK, MPC, PDC, PDH, NAD-ME) from public databases. The study's findings indicate that these genes' expression is regulated in response to different abiotic stresses. Moreover, the promoter region of these genes contained multiple cis-acting elements like ABRE, ARE, P-box, and MBS, which are associated with plants' abiotic stress response. Furthermore, colorimetric analysis showed higher pyruvate content under different abiotic stresses. Therefore, exogenous pyruvate treatment was given before and after different abiotic stresses, which could combat the toxicity of pro-oxidant molecules by pyruvate intake. The semiquantitative RT-PCR analysis revealed that exogenous pyruvate treatment enhances the expression of important transcription factors WRKY2, GH3.3, DREB2A, and bZIP1, and stress-responsive genes e.g., APX1, ERD5, ADC2, and HSP70 in addition to abiotic stresses. Moreover, Arabidopsis plants pre-treated with pyruvate before oxidative stress showed less RBOHD expression. Additionally, pyruvate's cytoprotective role was compared to other well-known antioxidants such as NAC, Trolox, and GSH. Finally, untargeted GC-MS/MS analysis of abiotic stress-treated Arabidopsis plants showed a higher metabolite level of β-hydroxy-pyruvic acid, indicating the crucial role of pyruvate during abiotic stress.

Alternative electron sinks in chloroplasts and mitochondria of halophytes as a safety valve for controlling ROS production during salinity

Electron flow through the electron transport chain (ETC) is essential for oxidative phosphorylation in mitochondria and photosynthesis in chloroplasts. Electron fluxes depend on environmental parameters, e.g., ionic and osmotic conditions and endogenous factors, and this may cause severe imbalances. Plants have evolved alternative sinks to balance the reductive load on the electron transport chains in order to avoid overreduction, generation of reactive oxygen species (ROS), and to cope with environmental stresses. These sinks act primarily as valves for electron drainage and secondarily as regulators of tolerance-related metabolism, utilizing the excess reductive energy. High salinity is an environmental stressor that stimulates the generation of ROS and oxidative stress, which affects growth and development by disrupting the redox homeostasis of plants. While glycophytic plants are sensitive to high salinity, halophytic plants tolerate, grow, and reproduce at high salinity. Various studies have examined the ETC systems of glycophytic plants, however, information about the state and regulation of ETCs in halophytes under non-saline and saline conditions is scarce. This review focuses on alternative electron sinks in chloroplasts and mitochondria of halophytic plants. In cases where information on halophytes is lacking, we examined the available knowledge on the relationship between alternative sinks and gradual salinity resilience of glycophytes. To this end, transcriptional responses of involved components of photosynthetic and respiratory ETCs were compared between the glycophyte Arabidopsis thaliana and the halophyte Schrenkiella parvula, and the time-courses of these transcripts were examined in A. thaliana. The observed regulatory patterns are discussed in the context of reactive molecular species formation in halophytes and glycophytes.

Low Nitrogen Input Mitigates Quantitative but Not Qualitative Reconfiguration of Leaf Primary Metabolism in Brassica napus L. Subjected to Drought and Rehydration

In the context of climate change and the reduction of mineral nitrogen (N) inputs applied to the field, winter oilseed rape (WOSR) will have to cope with low-N conditions combined with water limitation periods. Since these stresses can significantly reduce seed yield and seed quality, maintaining WOSR productivity under a wide range of growth conditions represents a major goal for crop improvement. N metabolism plays a pivotal role during the metabolic acclimation to drought in Brassica species by supporting the accumulation of osmoprotective compounds and the source-to-sink remobilization of nutrients. Thus, N deficiency could have detrimental effects on the acclimation of WOSR to drought. Here, we took advantage of a previously established experiment to evaluate the metabolic acclimation of WOSR during 14 days of drought, followed by 8 days of rehydration under high- or low-N fertilization regimes. For this purpose, we selected three leaf ranks exhibiting contrasted sink/source status to perform absolute quantification of plant central metabolites. Besides the well-described accumulation of proline, we observed contrasted accumulations of some "respiratory" amino acids (branched-chain amino acids, lysineand tyrosine) in response to drought under high- and low-N conditions. Drought also induced an increase in sucrose content in sink leaves combined with a decrease in source leaves. N deficiency strongly decreased the levels of major amino acids and subsequently the metabolic response to drought. The drought-rehydration sequence identified proline, phenylalanine, and tryptophan as valuable metabolic indicators of WOSR water status for sink leaves. The results were discussed with respect to the metabolic origin of sucrose and some amino acids in sink leaves and the impact of drought on source-to-sink remobilization processes depending on N nutrition status. Overall, this study identified major metabolic signatures reflecting a similar response of oilseed rape to drought under low- and high-N conditions.

Biomolecular interaction of purified recombinant Arabidopsis thaliana's alternative oxidase 1A with TCA cycle metabolites: Biophysical and molecular docking studies

In higher plants, the mitochondrial alternative oxidase (AOX) pathway plays an essential role in maintaining the TCA cycle/cellular carbon and energy balance under various physiological and stress conditions. Though the activation of AOX pathway upon exogenous addition of α-ketoacids/TCA cycle metabolites [pyruvate, α-ketoglutarate (α-KG), oxaloacetic acid (OAA), succinate and malic acid] to isolated mitochondria is known, the molecular mechanism of interaction of these metabolites with AOX protein is limited. The present study is designed to understand the biomolecular interaction of pure recombinant Arabidopsis thaliana AOX1A with TCA cycle metabolites under in vitro conditions using various biophysical and molecular docking studies. The binding of α-KG, fumaric acid and OAA to rAtAOX1A caused conformational change in the microenvironment of tryptophan residues as evidenced by red shift in the synchronous fluorescence spectra (∆λ = 60 nm). Besides, a decrease in conventional fluorescence emission spectra, tyrosine specific synchronous fluorescence spectra (∆λ = 15 nm) and α-helical content of CD spectra revealed the conformation changes in rAtAOX1A structure associated with binding of various TCA cycle metabolites. Further, surface plasmon resonance (SPR) and microscale thermophoresis (MST) studies revealed the binding affinity, while docking studies identified binding pocket residues, respectively, for these metabolites on rAtAOX1A.