Loss of cytosolic glucose-6-phosphate dehydrogenase increases the susceptibility of Arabidopsis thaliana to root-knot nematode infection

Sugar delivery at the tomato root and root galls after Meloidogyne incognita infestation

Root-knot nematodes (RKNs) infect host plants and obtain nutrients such as sugars for their own development. Therefore, inhibiting the nutrient supply to RKNs may be an effective method for alleviating root-knot nematode disease. At present, the pathway by which sucrose is unloaded from the phloem cells to giant cells (GCs) in root galls and which genes related to sugar metabolism and transport play key roles in this process are unclear. In this study, we found that sugars could be unloaded into GCs only from neighboring phloem cells through the apoplastic pathway. With the development of galls, the contents of sucrose, fructose and glucose in the galls and adjacent tissue increased gradually. SUT1, SUT2, SWEET7a, STP10, SUS3 and SPS1 may provide sugar sources for GCs, while STP1, STP2 and STP12 may transport more sugar to phloem parenchyma cells. At the early stage of Meloidogyne incognita infestation, the sucrose content in tomato roots and leaves increased, while the glucose and fructose contents decreased. SWEET7a, SPS1, INV-INH1, INV-INH2, SUS1 and SUS3 likely play key roles in root sugar delivery. These results elucidated the pathway of sugar unloading in tomato galls and provided an important theoretical reference for eliminating the sugar source of RKNs and preventing root-knot nematode disease.

Unveiling plant defense arsenal: metabolic strategies in Brassica oleracea during black rot disease

Alterations in plant metabolism play a key role in the complex plant-pathogen interactions. However, there is still a lack of knowledge about the connection between changes in primary and specialized metabolism and the plant defense against diseases that impact crops. Thus, we aim to study the metabolic reprograming in Brassica oleracea plants upon infection by Xanthomonas campestris pv. campestris (Xcc). To accomplish this, we utilized a combination of untargeted and targeted metabolomics, through UPLC-Q-TOF-MS/MS and 1H-NMR, in two crop lines differing in resistance that were evaluated at two- and four-week intervals following inoculation (T1 and T2, respectively). Besides, to depict the physiological status of the plant during infection, enzymatic activities related to the carbohydrate pathway and oxidative stress were studied. Our results revealed different temporal dynamics in the responses of the susceptible vs. resistant crops lines. Resistant B. oleracea line suppresses carbohydrate metabolism contributing to limit nutrient supplies to the bacterium and prioritizes the induction of defensive compounds such as indolic glucosinolates, salicylic acid, phenylpropanoids and phytoalexins precursors at early infection stages. In contrast, the susceptible line invests in carbohydrate metabolism, including enzymatic activities related to the hexoses turnover, and activates defense signaling related to reactive oxygen species. Thus, each line triggers a different metabolic strategy that will affect how the plant overcomes the disease in terms of resistance and growth. This work provides first insights of a fine-tuned metabolic regulation during Xcc infection in B. oleracea that will contribute to develop new strategies for plant disease management.

Persulfidation maintains cytosolic G6PDs activity through changing tetrameric structure and competing cysteine sulfur oxidation under salt stress in Arabidopsis and tomato

Glucose-6-phosphate dehydrogenases (G6PDs) are essential regulators of cellular redox. Hydrogen sulfide (H2 S) is a small gasotransmitter that improves plant adaptation to stress; however, its role in regulating G6PD oligomerization to resist oxidative stress remains unknown in plants. Persulfidation of cytosolic G6PDs was analyzed by mass spectrometry (MS). The structural change model of AtG6PD6 homooligomer was built by chemical cross-linking coupled with mass spectrometry (CXMS). We isolated AtG6PD6C159A and SlG6PDCC155A transgenic lines to confirm the in vivo function of persulfidated sites with the g6pd5,6 background. Persulfidation occurs at Arabidopsis G6PD6 Cystine (Cys)159 and tomato G6PDC Cys155, leading to alterations of spatial distance between lysine (K)491-K475 from 42.0 Å to 10.3 Å within the G6PD tetramer. The structural alteration occurs in the structural NADP+ binding domain, which governs the stability of G6PD homooligomer. Persulfidation enhances G6PD oligomerization, thereby increasing substrate affinity. Under high salt stress, cytosolic G6PDs activity was inhibited due to oxidative modifications. Persulfidation protects these specific sites and prevents oxidative damage. In summary, H2 S-mediated persulfidation promotes cytosolic G6PD activity by altering homotetrameric structure. The cytosolic G6PD adaptive regulation with two kinds of protein modifications at the atomic and molecular levels is critical for the cellular stress response.

Fuels for ROS signaling in plant immunity

Reactive oxygen species (ROS) signaling has an important role in plant innate immune responses and is primarily mediated by NADPH oxidase, also known as respiratory burst oxidase homologs (RBOHs) in plants. NADPH serves as a fuel for RBOHs and limits the rate or amount of ROS production. Molecular regulation of RBOHs has been extensively studied; however, the source of NADPH for RBOHs has received little attention. Here, we review ROS signaling and the regulation of RBOHs in the plant immune system with a focus on NADPH regulation to achieve ROS homeostasis. We propose an idea to regulate the levels of NADPH as part of a new strategy to control ROS signaling and the corresponding downstream defense responses.

In Silico RNAseq and Biochemical Analyses of Glucose-6-Phosphate Dehydrogenase (G6PDH) from Sweet Pepper Fruits: Involvement of Nitric Oxide (NO) in Ripening and Modulation

Pepper (Capsicum annuum L.) fruit is a horticultural product consumed worldwide which has great nutritional and economic relevance. Besides the phenotypical changes that pepper fruit undergo during ripening, there are many associated modifications at transcriptomic, proteomic, biochemical, and metabolic levels. Nitric oxide (NO) is a recognized signal molecule that can exert regulatory functions in diverse plant processes including fruit ripening, but the relevance of NADPH as a fingerprinting of the crop physiology including ripening has also been proposed. Glucose-6-phosphate dehydrogenase (G6PDH) is the first and rate-limiting enzyme of the oxidative phase of the pentose phosphate pathway (oxiPPP) with the capacity to generate NADPH. Thus far, the available information on G6PDH and other NADPH-generating enzymatic systems in pepper plants, and their expression during the ripening of sweet pepper fruit, is very scarce. Therefore, an analysis at the transcriptomic, molecular and functional levels of the G6PDH system has been accomplished in this work for the first time. Based on a data-mining approach to the pepper genome and fruit transcriptome (RNA-seq), four G6PDH genes were identified in pepper plants and designated CaG6PDH1 to CaG6PDH4, with all of them also being expressed in fruits. While CaG6PDH1 encodes a cytosolic isozyme, the other genes code for plastid isozymes. The time-course expression analysis of these CaG6PDH genes during different fruit ripening stages, including green immature (G), breaking point (BP), and red ripe (R), showed that they were differentially modulated. Thus, while CaG6PDH2 and CaG6PDH4 were upregulated at ripening, CaG6PDH1 was downregulated, and CaG6PDH3 was slightly affected. Exogenous treatment of fruits with NO gas triggered the downregulation of CaG6PDH2, whereas the other genes were positively regulated. In-gel analysis using non-denaturing PAGE of a 50-75% ammonium-sulfate-enriched protein fraction from pepper fruits allowed for identifying two isozymes designated CaG6PDH I and CaG6PDH II, according to their electrophoretic mobility. In order to test the potential modulation of such pepper G6PDH isozymes, in vitro analyses of green pepper fruit samples in the presence of different compounds including NO donors (S-nitrosoglutathione and nitrosocysteine), peroxynitrite (ONOO-), a hydrogen sulfide (H2S) donor (NaHS, sodium hydrosulfide), and reducing agents such as reduced glutathione (GSH) and L-cysteine (L-Cys) were assayed. While peroxynitrite and the reducing compounds provoked a partial inhibition of one or both isoenzymes, NaHS exerted 100% inhibition of the two CaG6PDHs. Taken together these data provide the first data on the modulation of CaG6PDHs at gene and activity levels which occur in pepper fruit during ripening and after NO post-harvest treatment. As a consequence, this phenomenon may influence the NADPH availability for the redox homeostasis of the fruit and balance its active nitro-oxidative metabolism throughout the ripening process.

A Chloroplast-Localized Glucose-6-Phosphate Dehydrogenase Positively Regulates Stripe Rust Resistance in Wheat

Glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme of the pentose phosphate pathway (PPP), plays a pivotal role in plant stress responses. However, the function and mechanism of G6PDHs in crop plants challenged by fungal pathogens remain poorly understood. In this study, a wheat G6DPH gene responding to infection by Puccinia striiformis f. sp. tritici (Pst), designated TaG6PDH2, was cloned and functionally identified. TaG6PDH2 expression was significantly upregulated in wheat leaves inoculated with Pst or treated with abiotic stress factors. Heterologous mutant complementation and enzymatic properties indicate that TaG6PDH2 encodes a G6PDH protein. The transient expression of TaG6PDH2 in Nicotiana benthamiana leaves and wheat protoplasts revealed that TaG6PDH2 is a chloroplast-targeting protein. Silencing TaG6PDH2 via the barley stripe mosaic virus (BSMV)-induced gene silencing (VIGS) system led to compromised wheat resistance to the Pst avirulent pathotype CYR23, which is implicated in weakened H₂O₂ accumulation and cell death. In addition, TaG6PDH2 was confirmed to interact with the wheat glutaredoxin TaGrxS4. These results demonstrate that TaG6PDH2 endows wheat with increased resistance to stripe rust by regulating reactive oxygen species (ROS) production.

Glucose-6-Phosphate Dehydrogenases: The Hidden Players of Plant Physiology

Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes a metabolic hub between glycolysis and the pentose phosphate pathway (PPP), which is the oxidation of glucose-6-phosphate (G6P) to 6-phosphogluconolactone concomitantly with the production of nicotinamide adenine dinucleotide phosphate (NADPH), a reducing power. It is considered to be the rate-limiting step that governs carbon flow through the oxidative pentose phosphate pathway (OPPP). The OPPP is the main supplier of reductant (NADPH) for several "reducing" biosynthetic reactions. Although it is involved in multiple physiological processes, current knowledge on its exact role and regulation is still piecemeal. The present review provides a concise and comprehensive picture of the diversity of plant G6PDHs and their role in seed germination, nitrogen assimilation, plant branching, and plant response to abiotic stress. This work will help define future research directions to improve our knowledge of G6PDHs in plant physiology and to integrate this hidden player in plant performance.

Cytosolic glucose-6-phosphate dehydrogenases play a pivotal role in Arabidopsis seed development

Embryo development is essential for seed yield and post-germination growth. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in oxidative pentose phosphate pathway (OPPP), is widely involved in plant development and stress tolerance by providing nicotinamide adenine dinucleotide phosphate (NADPH). In this study, the double mutant (g6pd5/6), overexpression line (G6PD5/6^OE) and complementation line (g6pd5/6^Comp) of cytosolic glucose-6-phosphate dehydrogenases (Cyt-G6PD) were used to investigate Cyt-G6PD roles in embryo development of Arabidopsis. The results showed that the germination rate of g6pd5/6 seeds was delayed in comparison with that of Col-0; moreover, 11.5% of g6pd5/6 seeds did not germinate. The dysfunction of Cyt-G6PD resulted in decreased fresh weight and primary root length of g6pd5/6 seedlings. The height and silique length of g6pd5/6 plants were also decreased. Moreover, the abortion rate of siliques and seeds of g6pd5/6 plants were increased compared with those of Col-0, G6PD5/6^OE and g6pd5/6^Comp lines. However, the dysfunction of Cyt-G6PD did not affect pollen activity; but in g6pd5/6, the embryo development was partially delayed or inhibited. The contents of fatty acids and storage proteins, two main storage materials in Arabidopsis seeds, were decreased in g6pd5/6 seeds. Exogenous application of fatty acids (C18:2; C18:3) alleviated the delayed germination of g6pd5/6 seeds. RT-qPCR results further demonstrated that the early embryo development genes were down-regulated in g6pd5/6. Taken together, Cyt-G6PD plays a pivotal role in plant seed development by regulating the transcriptions of early embryo development genes and the accumulation of storage materials (especially fatty acids).

The receptor-like cytosolic kinase RIPK activates NADP-malic enzyme 2 to generate NADPH for fueling ROS production

Reactive oxygen species (ROS) production is a conserved immune response in Arabidopsis primarily mediated by respiratory burst oxidase homolog D (RBOHD), a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase associated with the plasma membrane. A rapid increase in NADPH is necessary to fuel RBOHD proteins and thus maintain ROS production. However, the molecular mechanism by which NADPH is generated to fuel RBOHD remains unclear. In this study, we isolated a new mutant allele of FLAGELLIN-INSENSITIVE 4 (FIN4), which encodes the first enzyme in de novo NAD biosynthesis. fin4 mutants show reduced NADPH levels and impaired ROS production. However, FIN4 and other genes involved in NAD- and NADPH-generating pathways are not highly upregulated upon elicitor treatment, raising a possibility that a cytosolic NADP-linked dehydrogenase might be post-transcriptionally activated to maintain the NADPH supply close to RBOHD. To verify this possibility, we isolated the proteins associated with RPM1-INDUCED PROTEIN KINASE (RIPK), a receptor-like cytoplasmic kinase that regulates broad-spectrum ROS signaling in plant immunity, and identified NADP-malic enzyme 2 (NADP-ME2), an NADPH-generating enzyme. Compared with wild-type plants, nadp-me2 mutants display decreased NADP-ME activity, lower NADPH levels, and reduced ROS production in response to immune elicitors. Furthermore, we found that RIPK can directly phosphorylate NADP-ME2 and enhance its activity in vitro. The phosphorylation of the NADP-ME2 S371 residue contributes to ROS production upon immune elicitor treatment and susceptibility to the necrotrophic bacterium Pectobacterium carotovorum. Collectively, our study suggests that RIPK phosphorylates and activates NADP-ME2 to rapidly increase cytosolic NADPH, thus fueling RBOHD to sustain ROS production in plant immunity.

Glucose-6-phosphate dehydrogenase and abscisic acid mediate programmed cell death induced by aluminum toxicity in soybean root tips

Programmed cell death (PCD) induced by aluminum (Al) is considered an important reason of Al phytotoxicity. However, the underlying mechanism of how Al induces PCD remains largely unknown in plants. The roles of glucose-6-phosphate dehydrogenase (G6PDH) and abscisic acid (ABA) in regulating Al-induced PCD were investigated in soybean roots. Al treatment increased G6PDH activity, while inhibition of G6PDH activity alleviated PCD occurrence and reactive oxygen species (ROS) accumulation under Al stress. Overexpression of cytosolic G6PDH1 enhanced G6PDH activity, thus promoting ROS production and cell death under Al exposure. Inhibition of NADPH oxidase activity mitigated ROS generation and cell death under Al stress. Further investigation demonstrated that G6PDH positively regulated the activity of NADPH oxidase under Al treatment using pharmacological and transgenic approach. Furthermore, Al stress increased ABA production, while inhibition of ABA biosynthesis alleviated PCD occurrence and ROS accumulation under Al stress. Interestingly, ABA upregulated G6PDH1 expression and G6PDH activity under Al stress. These results suggest that G6PDH mediates Al-induced PCD occurrence through the activation of NADPH oxidase-dependent ROS production, and ABA acts upstream of G6PDH in this process. This study will provide novel clues for the improvement of Al phytotoxicity in acidic soils.

D6 protein kinase in root xylem benefiting resistance to Fusarium reveals infection and defense mechanisms in tung trees

Fusarium oxysporum, a global soil-borne pathogen, causes severe disease in various cultivated plants. The mechanism underlying infection and resistance remains largely elusive. Vernicia fordii, known as the tung tree, suffers from disease caused by F. oxysporum f. sp. fordiis (Fof-1), while its sister species V. montana displays high resistance to Fof-1. To investigate the process of infection and resistance ability, we demonstrated that Fof-1 can penetrate the epidermis of root hairs and then centripetally invade the cortex and phloem in both species. Furthermore, Fof-1 spread upwards through the root xylem in susceptible V. fordii trees, whereas it failed to infect the root xylem in resistant V. montana trees. We found that D6 PROTEIN KINASE LIKE 2 (VmD6PKL2) was specifically expressed in the lateral root xylem and was induced after Fof-1 infection in resistant trees. Transgenic analysis in Arabidopsis and tomato revealed that VmD6PKL2 significantly enhanced resistance in both species, whereas the d6pkl2 mutant displayed reduced resistance against Fof-1. Additionally, VmD6PKL2 was identified to interact directly with synaptotagmin (VmSYT3), which is specifically expressed in the root xylem and mediates the negative regulation responding to Fof-1. Our data suggested that VmD6PKL2 could act as a resistance gene against Fof-1 through suppression of VmSYT3-mediated negative regulation in the lateral root xylem of the resistant species. These findings provide novel insight into Fusarium wilt resistance in plants.

Label-Free Proteomic Analysis of Smoke-Drying and Shade-Drying Processes of Postharvest Rhubarb: A Comparative Study

Postharvest processing plays a very important role in improving the quality of traditional Chinese medicine. According to previous studies, smoke-drying could significantly promote the accumulation of the bioactive components and pharmacological activities of rhubarb, but so far, the molecular mechanism has not been studied yet. In this research, to study the molecular mechanisms of postharvest processing for rhubarb during shade-drying and smoke-drying, label-free proteomic analyses were conducted. In total, 1,927 differentially abundant proteins (DAPs) were identified from rhubarb samples treated by different drying methods. These DAPs were mainly involved in response and defense, signal transduction, starch, carbohydrate and energy metabolism, and anthraquinone and phenolic acid biosynthesis. Smoke-drying significantly enhanced the expression of proteins involved in these metabolic pathways. Accordingly, the molecular mechanism of the accumulation of effective ingredients of rhubarb was clarified, which provided a novel insight into the biosynthesis of active ingredients that occur during the rhubarb dry process.

Nitric oxide and hydrogen sulfide modulate the NADPH-generating enzymatic system in higher plants

Nitric oxide (NO) and hydrogen sulfide (H2S) are two key molecules in plant cells that participate, directly or indirectly, as regulators of protein functions through derived post-translational modifications, mainly tyrosine nitration, S-nitrosation, and persulfidation. These post-translational modifications allow the participation of both NO and H2S signal molecules in a wide range of cellular processes either physiological or under stressful circumstances. NADPH participates in cellular redox status and it is a key cofactor necessary for cell growth and development. It is involved in significant biochemical routes such as fatty acid, carotenoid and proline biosynthesis, and the shikimate pathway, as well as in cellular detoxification processes including the ascorbate-glutathione cycle, the NADPH-dependent thioredoxin reductase (NTR), or the superoxide-generating NADPH oxidase. Plant cells have diverse mechanisms to generate NADPH by a group of NADP-dependent oxidoreductases including ferredoxin-NADP reductase (FNR), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH), NADP-dependent malic enzyme (NADP-ME), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and both enzymes of the oxidative pentose phosphate pathway, designated as glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). These enzymes consist of different isozymes located in diverse subcellular compartments (chloroplasts, cytosol, mitochondria, and peroxisomes) which contribute to the NAPDH cellular pool. We provide a comprehensive overview of how post-translational modifications promoted by NO (tyrosine nitration and S-nitrosation), H2S (persulfidation), and glutathione (glutathionylation), affect the cellular redox status through regulation of the NADP-dependent dehydrogenases.

Overexpression of a Cytosolic 6-Phosphogluconate Dehydrogenase Gene Enhances the Resistance of Rice to Nilaparvata lugens

The pentose phosphate pathway (PPP) plays an important role in plant growth and development, and plant responses to biotic and abiotic stresses. Yet, whether the PPP regulates plant defenses against herbivorous insects remains unclear. In this study, we cloned a rice cytosolic 6-phosphogluconate dehydrogenase gene, Os6PGDH1, which encodes the key enzyme catalyzing the third step in the reaction involving the oxidative phase of the PPP, and explored its role in rice defenses induced by brown planthopper (BPH) Nilaparvata lugens. Levels of Os6PGDH1 transcripts were detected in all five examined tissues, with the highest in outer leaf sheaths and lowest in inner leaf sheaths. Os6PGDH1 expression was strongly induced by mechanical wounding, infestation of gravid BPH females, and jasmonic acid (JA) treatment. Overexpressing Os6PGDH1 (oe6PGDH) decreased the height of rice plants and the mass of the aboveground part of plants, but slightly increased the length of plant roots. In addition, the overexpression of Os6PGDH1 enhanced levels of BPH-induced JA, jasmonoyl-isoleucine (JA-Ile), and H₂O₂, but decreased BPH-induced levels of ethylene. Bioassays revealed that gravid BPH females preferred to feed and lay eggs on wild-type (WT) plants over oe6PGDH plants; moreover, the hatching rate of BPH eggs raised on oe6PGDH plants and the fecundity of BPH females fed on these were significantly lower than the eggs and the females raised and fed on WT plants. Taken together, these results indicate that Os6PGDH1 plays a pivotal role not only in rice growth but also in the resistance of rice to BPH by modulating JA, ethylene, and H₂O₂ pathways.

Trichoderma harzianum Improves Defense Against Fusarium oxysporum by Regulating ROS and RNS Metabolism, Redox Balance, and Energy Flow in Cucumber Roots

Plant survival in the terrestrial ecosystem is influenced by both beneficial and harmful microbes. Trichoderma spp. are a group of filamentous fungi that promote plant growth and resistance to harmful microbes. Previously, we showed that the genus Trichoderma could effectively suppress Fusarium wilt in cucumber. However, the mechanisms that underlie the effects of the genus Trichoderma on plant defense have not been fully substantiated. Two essential metabolic pathways, such as the ascorbate (AsA)-glutathione (GSH) cycle and the oxidative pentose phosphate pathway (OPPP), have been shown to participate in plant tolerance to biotic stressors; nevertheless, the involvement of these pathways in Trichoderma-induced enhanced defense remains elusive. Here, we show that Trichoderma harzianum could alleviate oxidative and nitrostative stress by minimizing reactive oxygen species (ROS; hydrogen peroxide and superoxide) and reactive nitrogen species (nitric oxide [NO]) accumulation, respectively, under Fusarium oxysporum infection in cucumber roots. The genus Trichoderma enhanced antioxidant potential to counterbalance the overproduced ROS and attenuated the transcript and activity of NO synthase and nitrate reductase. The genus Trichoderma also stimulated S-nitrosylated glutathione reductase activity and reduced S-nitrosothiol and S-nitrosylated glutathione content. Furthermore, the genus Trichoderma enhanced AsA and GSH concentrations and activated their biosynthetic enzymes, γ-GCS and l-galactono-1,4-lactone dehydrogenase. Interestingly, the genus Trichoderma alleviated Fusarium-inhibited activity of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, enzymes involved in the OPPP. Such positive regulation of the key enzymes indicates the adequate maintenance of the AsA-GSH pathway and the OPPP, which potentially contributed to improve redox balance, energy flow, and defense response. Our study advances the current knowledge of Trichoderma-induced enhanced defense against F. oxysporum in cucumber.