12-Oxophytodienoic Acid Reductase 3 (OPR3) Functions as NADPH-Dependent α,β-Ketoalkene Reductase in Detoxification and Monodehydroascorbate Reductase in Redox Homeostasis

Improved pokeweed genome assembly and early gene expression changes in response to jasmonic acid

BACKGROUND

Jasmonic acid (JA) is a phytohormone involved in regulating responses to biotic and abiotic stress. Although the JA pathway is well characterized in model plants such as Arabidopsis thaliana, less is known about many non-model plants. Phytolacca americana (pokeweed) is native to eastern North Americana and is resilient to environmental stress. The goal of this study was to produce a publicly available pokeweed genome assembly and annotations and use this resource to determine how early response to JA changes gene expression, with particular focus on genes involved in defense.

RESULTS

We assembled the pokeweed genome de novo from approximately 30 Gb of PacBio Hifi long reads and achieved an NG50 of ~ 13.2 Mb and a minimum 93.9% complete BUSCO score for gene annotations. With this reference, we investigated the early changes in pokeweed gene expression following JA treatment. Approximately 5,100 genes were differentially expressed during the 0-6 h time course with almost equal number of genes with increased and decreased transcript levels. Cluster and gene ontology analyses indicated the downregulation of genes associated with photosynthesis and upregulation of genes involved in hormone signaling and defense. We identified orthologues of key transcription factors and constructed the first JA gene response network integrated with our transcriptomic data from orthologues of Arabidopsis genes. We discovered that pokeweed did not use leaf senescence as a means of reallocating resources during stress; rather, most secondary metabolite synthesis genes were constitutively expressed, suggesting that pokeweed directs its resources for survival over the long term. In addition, pokeweed synthesizes several RNA N-glycosylases hypothesized to function in defense, each with unique expression profiles in response to JA.

CONCLUSIONS

Our investigation of the early response of pokeweed to JA illustrates patterns of gene expression involved in defence and stress tolerance. Pokeweed provides insight into the defense mechanisms of plants beyond those observed in research models and crops, and further study may yield novel approaches to improving the resilience of plants to environmental changes. Our assembled pokeweed genome is the first within the taxonomic family Phytolaccaceae to be publicly available for continued research.

Glutathione: a key modulator of plant defence and metabolism through multiple mechanisms

Redox reactions are fundamental to energy conversion in living cells, and also determine and tune responses to the environment. Within this context, the tripeptide glutathione plays numerous roles. As an important antioxidant, glutathione confers redox stability on the cell and also acts as an interface between signalling pathways and metabolic reactions that fuel growth and development. It also contributes to the assembly of cell components, biosynthesis of sulfur-containing metabolites, inactivation of potentially deleterious compounds, and control of hormonal signalling intensity. The multiplicity of these roles probably explains why glutathione status has been implicated in influencing plant responses to many different conditions. In particular, there is now a considerable body of evidence showing that glutathione is a crucial player in governing the outcome of biotic stresses. This review provides an overview of glutathione synthesis, transport, degradation, and redox turnover in plants. It examines the expression of genes associated with these processes during pathogen challenge and related conditions, and considers the diversity of mechanisms by which glutathione can influence protein function and gene expression.

Evolutionary insights into strategy shifts for the safe and effective accumulation of ascorbate in plants

Plants accumulate high concentrations of ascorbate, commonly in their leaves, as a redox buffer. While ascorbate levels have increased during plant evolution, the mechanisms behind this phenomenon are unclear. Moreover, has the increase in ascorbate concentration been achieved without imposing any detrimental effects on the plants? In this review, we focus on potential transitions in two regulatory mechanisms related to ascorbate biosynthesis and the availability of cellular dehydroascorbate (DHA) during plant evolution. The first transition might be that the trigger for the transcriptional induction of VTC2, which encodes the rate-limiting enzyme in ascorbate biosynthesis, has shifted from oxidative stress (in green algae) to light/photosynthesis (in land plants), probably enabling the continuous accumulation of ascorbate under illumination. This could serve as a preventive system against the unpredictable occurrence of oxidative stress. The second transition might be that DHA-degrading enzymes, which protect cells from the highly reactive DHA in green algae and mosses, have been lost in ferns or flowering plants. Instead, flowering plants may have increased glutathione concentrations to reinforce the DHA reduction capacity, possibly allowing ascorbate accumulation and avoiding the toxicity of DHA. These potential transitions may have contributed to strategies for plants' safe and effective accumulation of ascorbate.

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.

Effects of Date Palm Waste Compost Application on Root Proteome Changes of Barley (Hordeum vulgare L.)

Proteomic analysis was performed to investigate the differentially abundant proteins (DAPs) in barley roots during the tillering stage. Bioinformatic tools were used to interpret the biological function, the pathway analysis and the visualisation of the network amongst the identified proteins. A total of 72 DAPs (33 upregulated and 39 downregulated) among a total of 2580 proteins were identified in response to compost treatment, suggesting multiple pathways of primary and secondary metabolism, such as carbohydrates and energy metabolism, phenylpropanoid pathway, glycolysis pathway, protein synthesis and degradation, redox homeostasis, RNA processing, stress response, cytoskeleton organisation, and phytohormone metabolic pathways. The expression of DAPs was further validated by qRT-PCR. The effects on barley plant development, such as the promotion of root growth and biomass increase, were associated with a change in energy metabolism and protein synthesis. The activation of enzymes involved in redox homeostasis and the regulation of stress response proteins suggest a protective effect of compost, consequently improving barley growth and stress acclimation through the reduction of the environmental impact of productive agriculture. Overall, these results may facilitate a better understanding of the molecular mechanism of compost-promoted plant growth and provide valuable information for the identification of critical genes/proteins in barley as potential targets of compost.

OPDAylation of Thiols of the Redox Regulatory Network In Vitro

cis-(+)-12-Oxophytodienoic acid (OPDA) is a reactive oxylipin produced by catalytic oxygenation of polyunsaturated α-linolenic acid (18:3 (ω − 3)) in the chloroplast. Apart from its function as precursor for jasmonic acid synthesis, OPDA serves as a signaling molecule and regulator on its own, namely by tuning enzyme activities and altering expression of OPDA-responsive genes. A possible reaction mechanism is the covalent binding of OPDA to thiols via the addition to the C=C double bond of its α,β-unsaturated carbonyl group in the cyclopentenone ring. The reactivity allows for covalent modification of accessible cysteinyl thiols in proteins. This work investigated the reaction of OPDA with selected chloroplast and cytosolic thioredoxins (TRX) and glutaredoxins (GRX) of Arabidopsis thaliana. OPDA reacted with TRX and GRX as detected by decreased m-PEG maleimide binding, consumption of OPDA, reduced ability for insulin reduction and inability to activate glyceraldehyde-3-phosphate dehydrogenase and regenerate glutathione peroxidase (GPXL8), and with lower efficiency, peroxiredoxin IIB (PRXIIB). OPDAylation of certain protein thiols occurs quickly and efficiently in vitro and is a potent post-translational modification in a stressful environment.

The jasmonate biosynthesis Gene OsOPR7 can mitigate salinity induced mitochondrial oxidative stress

Salinity poses a serious threat to global agriculture and human food security. A better understanding of plant adaptation to salt stress is, therefore, mandatory. In the non-photosynthetic cells of the root, salinity perturbs oxidative balance in mitochondria, leading to cell death. In parallel, plastids accumulate the jasmonate precursor cis (+)12-Oxo-Phyto-Dienoic Acid (OPDA) that is then translocated to peroxisomes and has been identified as promoting factor for salt-induced cell death as well. In the current study, we probed for a potential interaction between these three organelles that are primarily dealing with oxidative metabolism. We made use of two tools: (i) Rice OPDA Reductase 7 (OsOPR7), an enzyme localised in peroxisomes converting OPDA into the precursors of the stress hormone JA-Ile. (ii) A Trojan Peptoid, Plant PeptoQ, which can specifically target to mitochondria and scavenge excessive superoxide accumulating in response to salt stress. We show that overexpression of OsOPR7 as GFP fusion in tobacco (Nicotiana tabacum L. cv. Bright Yellow 2, BY-2) cells, as well as a pretreatment with Plant PeptoQ can mitigate salt stress with respect to numerous aspects including proliferation, expansion, ionic balance, redox homeostasis, and mortality. This mitigation correlates with a more robust oxidative balance, evident from a higher activity of superoxide dismutase (SOD), lower levels of superoxide and lipid peroxidation damage, and a conspicuous and specific upregulation of mitochondrial SOD transcripts. Although both, Plant PeptoQ and ectopic OsOPR7, were acting in parallel and mostly additive, there are two specific differences: (i) OsOPR7 is strictly localised to the peroxisomes, while Plant PeptoQ found in mitochondria. (ii) Plant PeptoQ activates transcripts of NAC, a factor involved in retrograde signalling from mitochondria to the nucleus, while these transcripts are suppressed significantly in the cells overexpressing OsOPR7. The fact that overexpression of a peroxisomal enzyme shifting the jasmonate pathway from the cell-death signal OPDA towards JA-Ile, a hormone linked with salt adaptation, is accompanied by more robust redox homeostasis in a different organelle, the mitochondrion, indicates that cross-talk between peroxisome and mitochondrion is a crucial factor for efficient adaptation to salt stress.

The OPR gene family in watermelon: Genome-wide identification and expression profiling under hormone treatments and root-knot nematode infection

The enzyme 12-oxo-phytodienoic acid reductase (OPR) is important in the jasmonic acid (JA) biosynthesis pathway and thus plays a vital role in plant defence. However, systematic and comprehensive analyses of OPR genes in watermelon and their roles in defence responses are extremely limited. The physicochemical properties, phylogenetic tree, gene structure and cis-acting elements of watermelon OPR genes were analysed using bioinformatics, and qRT-PCR and RNA-Seq were applied to assay expression of OPR genes in watermelon. A total of five OPR family genes were identified in watermelon, which were unevenly distributed across the four chromosomes. Phylogenetic analysis assigned OPR members from different plant species to five subfamilies (OPRI-OPRV). The motif compositions of OPR members were relatively conserved. Expression analysis using qRT-PCR revealed that ClOPR genes, except for ClOPR5, were highly expressed in the flower and fruit. RNA-seq analysis showed that the ClOPR genes had different expression patterns during flesh and rind development. Furthermore, the ClOPR genes, particularly ClOPR2 and ClOPR4, were significantly upregulated by exogenous JA, salicylic acid (SA) and ethylene (ET) treatments. In addition, red light induced expression of ClOPR2 and ClOPR4 in leaves and roots of root-knot nematode (RKN)-infected watermelon plants, suggesting their involvement in red light-induced defence against RKN. These results provide a theoretical basis for elucidating the diverse functions of OPR family genes in watermelon.

Transcriptome Analysis of Pre-Storage 1-MCP and High CO2-Treated 'Madoka' Peach Fruit Explains the Reduction in Chilling Injury and Improvement of Storage Period by Delaying Ripening

Cold storage of peach fruit at low temperatures may induce chilling injury (CI). Pre-storage 1-MCP and high CO₂ treatments were reported among the methods to ameliorate CI and reduce softening of peach fruit. However, molecular data indicating the changes associated with pre-storage 1-MCP and high CO₂ treatments during cold storage of peach fruit are insufficient. In this study, a comparative analysis of the difference in gene expression and physico-chemical properties of fruit at commercial harvest vs. stored fruit for 12 days at 0 °C (cold-stored (CS), pre-storage 1-MCP+CS, and pre-storage high CO₂+CS) were used to evaluate the variation among treatments. Several genes were differentially expressed in 1-MCP+CS- and CO₂+CS-treated fruits as compared to CS. Moreover, the physico-chemical and sensory data indicated that 1-MCP+CS and CO₂+CS suppressed CI and delayed ripening than the CS, which could lead to a longer storage period. We also identified the list of genes that were expressed commonly and exclusively in the fruit treated by 1-MCP+CS and CO₂+CS and compared them to the fruit quality parameters. An attempt was also made to identify and categorize genes related to softening, physiological changes, and other ripening-related changes. Furthermore, the transcript levels of 12 selected representative genes from the differentially expressed genes (DEGs) in the transcriptome analysis were confirmed via quantitative real-time PCR (qRT-PCR). These results add information on the molecular mechanisms of the pre-storage treatments during cold storage of peach fruit. Understanding the genetic response of susceptible cultivars such as ‘Madoka’ to CI-reducing pre-storage treatments would help breeders release CI-resistant cultivars and could help postharvest technologists to develop more CI-reducing technologies.

Recent Developments in Enzymatic Antioxidant Defence Mechanism in Plants with Special Reference to Abiotic Stress

The stationary life of plants has led to the evolution of a complex gridded antioxidant defence system constituting numerous enzymatic components, playing a crucial role in overcoming various stress conditions. Mainly, these plant enzymes are superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), which work as part of the antioxidant defence system. These enzymes together form a complex set of mechanisms to minimise, buffer, and scavenge the reactive oxygen species (ROS) efficiently. The present review is aimed at articulating the current understanding of each of these enzymatic components, with special attention on the role of each enzyme in response to the various environmental, especially abiotic stresses, their molecular characterisation, and reaction mechanisms. The role of the enzymatic defence system for plant health and development, their significance, and cross-talk mechanisms are discussed in detail. Additionally, the application of antioxidant enzymes in developing stress-tolerant transgenic plants are also discussed.