Glutathione-dependent phytohormone responses: teasing apart signaling and antioxidant functions

Glutathione contributes to resistance responses to TMV through a differential modulation of salicylic acid and reactive oxygen species

Systemic acquired resistance (SAR) is induced by pathogens and confers protection against a broad range of pathogens. Several SAR signals have been characterized, but the nature of the other unknown signalling by small metabolites in SAR remains unclear. Glutathione (GSH) has long been implicated in the defence reaction against biotic stress. However, the mechanism that GSH increases plant tolerance against virus infection is not entirely known. Here, a combination of a chemical, virus-induced gene-silencing-based genetics approach, and transgenic technology was undertaken to investigate the role of GSH in plant viral resistance in Nicotiana benthamiana. Tobacco mosaic virus (TMV) infection results in increasing the expression of GSH biosynthesis genes NbECS and NbGS, and GSH content. Silencing of NbECS or NbGS accelerated oxidative damage, increased accumulation of reactive oxygen species (ROS), compromised plant resistance to TMV, and suppressed the salicylic acid (SA)-mediated signalling pathway. Application of GSH or l-2-oxothiazolidine-4-carboxylic acid (a GSH activator) alleviated oxidative damage, decreased accumulation of ROS, elevated plant local and systemic resistance, enhanced the SA-mediated signalling pathway, and increased the expression of ROS scavenging-related genes. However, treatment with buthionine sulfoximine (a GSH inhibitor) accelerated oxidative damage, elevated ROS accumulation, compromised plant systemic resistance, suppressed the SA-mediated signalling pathway, and reduced the expression of ROS-regulating genes. Overexpression of NbECS reduced oxidative damage, decreased accumulation of ROS, increased resistance to TMV, activated the SA-mediated signalling pathway, and increased the expression of the ROS scavenging-related genes. We present molecular evidence suggesting GSH is essential for both local and systemic resistance of N. benthamiana to TMV through a differential modulation of SA and ROS.

Enhanced glutathione content improves lateral root development and grain yield in rice plants

Enhanced glutathione content improves lateral root development by positively regulating the transcripts of root development genes responsive to glutathione treatment, thereby increasing the overall productivity of rice plants. Glutathione is primarily known as a cellular antioxidant molecule, but its role in lateral root development in rice plants has not been elucidated. Here, we have investigated its role in lateral root development of rice Oryza sativa L. Exogenous glutathione (GSH) promoted both the number and length of lateral roots in rice, and the GSH biosynthesis inhibitor buthionine sulfoximine (BSO) significantly reduced these parameters, compared to untreated plants. The inhibition by BSO was reversed with exogenous GSH. Transcript profiling by RNA-seq revealed that expression of the transcription factor genes DREB and ERF and the hormone-related genes AOS, LOX, JAZ, and SAUR were significantly downregulated in the BSO-treated plants and, in contrast, upregulated in plants treated with GSH and with GSH and BSO together. We generated OsGS-overexpressing transgenic plants in which the transgene is controlled by the abiotic-stress-inducible OsRab21 promoter to study the effect of endogenously increased GSH levels. In cold stress, transgenic rice plants enhanced stress tolerance and lateral root development by maintaining redox homeostasis and improving upregulating the expression of transcription factors and hormone-related genes involved in lateral root development. We observed improved root growth of OsGS-overexpressing plants in paddy fields compared to the wild-type controls. These traits may have alleviated transplanting stress during early growth in the field and accounted for the increased productivity. These results provide information and perspectives on the role of GSH in gene expression, lateral root development, and grain yield in rice.

Short-term drought triggers defence mechanisms faster than ABA accumulation in the epiphytic bromeliad Acanthostachys strobilacea

Epiphytic bromeliads might experience drought after a few hours without water, which is especially critical during early life stages. Consequently, juvenile epiphytic bromeliads probably rely on short-term activation of drought tolerance strategies, although the biochemical processes involved are still poorly understood. In this study, we aimed to evaluate the short-term drought response of juvenile plants of the epiphytic bromeliad Acanthostachys strobilacea (Schult. & Schult. f.) Klotzsch. We hypothesized that short-term drought would induce the accumulation of abscisic acid (ABA) and secondary messengers such as reactive oxygen and nitrogen species (ROS and RNS, respectively) before the activation of defence mechanisms. Three-month-old plants were transferred from well-watered to dry substrates and stress markers were assessed at 0, 2, 5, 10, 24, 48, and 72 h. Drought caused a 27.3% decrease in relative water content compared to the well-watered control at 72 h. A nearly 5-fold increment in the ABA content occurred at 72 h of stress, which was about two days after the first detection of peaks in RNS levels and defence mechanisms activation. Indeed, ascorbate peroxidase (EC 1.11.1.11) activities and proline content increased after 10 h, whereas after 24 h a higher catalase (EC 1.11.1.6) activity and osmotic adjustment occurred. Oxidative stress markers and photochemical efficiency of photosystem II indicated no significant damage induced by drought. We concluded that defence mechanisms activation during early drought in juvenile A. strobilacea might be regulated initially by ABA-independent pathways and RNS, while ABA-induced responses are triggered at subsequent stages of stress.

Physiological and transcriptomic responses of Lanzhou Lily (Lilium davidii, var. unicolor) to cold stress

Low temperature induces changes in plants at physiological and molecular levels, thus affecting growth and development. The Lanzhou lily (Lilium davidii, var. unicolor) is an important medicinal plant with high economic value. However, the molecular mechanisms underlying its photosynthetic and antioxidation responses to low temperature still remain poorly understood. This study subjected the Lanzhou lily to the two temperatures of 20°C (control) and 4°C (low temperature) for 24 h. Physiological parameters related to membrane integrity, photosynthesis, antioxidant system, and differentially expressed genes were investigated. Compared with control, low temperature increased the relative electrical conductivity by 43.2%, while it decreased net photosynthesis rate, ratio of variable to maximal fluorescence, and catalase activity by 47.3%, 10.1%, and 11.1%, respectively. In addition, low temperature significantly increased the content of soluble protein, soluble sugar, and proline, as well as the activity of superoxide dismutase and peroxidase. Comparative transcriptome profiling showed that a total of 238,109 differentially expressed genes were detected. Among these, 3,566 were significantly upregulated while 2,982 were significantly downregulated in response to low temperature. Gene Ontology enrichment analysis indicated that in response to low temperature, the mostly significantly enriched differentially expressed genes were mainly involved in phosphorylation, membrane and protein kinase activity, as well as photosynthesis, light harvesting, light reaction, and alpha,alpha-trehalose-phosphate synthase activity. Kyoto Encyclopedia of Genes and Genomes enrichment analysis also indicated that the most significantly enriched pathways involved ribosome biogenesis in eukaryotes, phenylalanine metabolism, circadian rhythm, porphyrin and chlorophyll metabolism, photosynthesis of antenna proteins, photosynthesis, and carbon fixation in photosynthetic organisms. Moreover, the expression patterns of 10 randomly selected differentially expressed genes confirmed the RNA-Seq results. These results expand the understanding of the physiological and molecular mechanisms underlying the response of the Lanzhou lily to low temperature stress.

Fertilizer Rate-Associated Increase in Foliar Jasmonate Burst Observed in Wounded Arabidopsis thaliana Leaves is Attenuated at eCO2

The predicted future increase in tropospheric carbon dioxide (CO2) levels will have major effects on C3 plants and their interactions with other organisms in the biosphere. In response to attack by chewing arthropod herbivores or nectrotrophic pathogens, many plants mount a rapid and intense increase in jasmonate-related phytohormones that results in a robust defense response; however, previous studies have shown that C3 plants grown at elevated CO2 may have lower induced jasmonate levels, particularly in well nitrate-fertilized plants. Given the relationship between atmospheric CO2, photorespiration, cellular reductant and redox status, nitrogen assimilation and phytohormones, we compared wound-induced responses of the C3 plant Arabidopsis thaliana. These plants were fertilized at two different rates (1 or 10 mM) with nitrate or ammonium and grown at ambient or elevated CO2. In response to artificial wounding, an increase in cellular oxidative status leads to a strong increase in jasmonate phytohormones. At ambient CO2, increased oxidative state of nitrate-fertilized plants leads to a robust 7-iso-jasmonyl-L-isoleucine increase; however, the strong fertilizer rate-associated increase is alleviated in plants grown at elevated CO2. As well, the changes in ascorbate in response to wounding and wound-induced salicylic acid levels may also contribute to the suppression of the jasmonate burst. Understanding the mechanism underlying the attenuation of the jasmonate burst at elevated CO2 has important implications for fertilization strategies under future predicted climatic conditions.

H2O2 mediates ALA-induced glutathione and ascorbate accumulation in the perception and resistance to oxidative stress in Solanum lycopersicum at low temperatures

BACKGROUND

Low temperature is a crucial factor influencing plant growth and development. The chlorophyll precursor, 5-aminolevulinic acid (ALA) is widely used to improve plant cold tolerance. However, the interaction between H₂O₂ and cellular redox signaling involved in ALA-induced resistance to low temperature stress in plants remains largely unknown. Here, the roles of ALA in perceiving and regulating low temperature-induced oxidative stress in tomato plants, together with the roles of H₂O₂ and cellular redox states, were characterized.

RESULTS

Low concentrations (10-25 mg·L^- 1) of ALA enhanced low temperature-induced oxidative stress tolerance of tomato seedlings. The most effective concentration was 25 mg·L^- 1, which markedly increased the ratio of reduced glutathione and ascorbate (GSH and AsA), and enhanced the activities of superoxide dismutase, catalase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. Furthermore, gene expression of respiratory burst oxidase homolog1 and H₂O₂ content were upregulated with ALA treatment under normal conditions. Treatment with exogenous H₂O₂, GSH, and AsA also induced plant tolerance to oxidative stress at low temperatures, while inhibition of GSH and AsA syntheses significantly decreased H₂O₂-induced oxidative stress tolerance. Meanwhile, scavenging or inhibition of H₂O₂ production weakened, but did not eliminate, GSH- or AsA- induced tomato plant tolerance to oxidative stress at low temperatures.

CONCLUSIONS

Appropriate concentrations of ALA alleviated the low temperature-induced oxidative stress in tomato plants via an antioxidant system. The most effective concentration was 25 mg·L^- 1. The results showed that H₂O₂ induced by exogenous ALA under normal conditions is crucial and may be the initial step for perception and signaling transmission, which then improves the ratio of GSH and AsA. GSH and AsA may then interact with H₂O₂ signaling, resulting in enhanced antioxidant capacity in tomato plants at low temperatures.

Cross-tolerance to biotic and abiotic stresses in plants: a focus on resistance to aphid infestation

Plants co-evolved with an enormous variety of microbial pathogens and insect herbivores under daily and seasonal variations in abiotic environmental conditions. Hence, plant cells display a high capacity to respond to diverse stresses through a flexible and finely balanced response network that involves components such as reduction-oxidation (redox) signalling pathways, stress hormones and growth regulators, as well as calcium and protein kinase cascades. Biotic and abiotic stress responses use common signals, pathways and triggers leading to cross-tolerance phenomena, whereby exposure to one type of stress can activate plant responses that facilitate tolerance to several different types of stress. While the acclimation mechanisms and adaptive responses that facilitate responses to single biotic and abiotic stresses have been extensively characterized, relatively little information is available on the dynamic aspects of combined biotic/abiotic stress response. In this review, we consider how the abiotic environment influences plant responses to attack by phloem-feeding aphids. Unravelling the signalling cascades that underpin cross-tolerance to biotic and abiotic stresses will allow the identification of new targets for increasing environmental resilience in crops.

Decreased glutathione reductase2 leads to early leaf senescence in Arabidopsis

Glutathione reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) to reduced glutathione (GSH) and participates in the ascorbate-glutathione cycle, which scavenges H2 O2 . Here, we report that chloroplastic/mitochondrial GR2 is an important regulator of leaf senescence. Seed development of the homozygous gr2 knockout mutant was blocked at the globular stage. Therefore, to investigate the function of GR2 in leaf senescence, we generated transgenic Arabidopsis plants with decreased GR2 using RNAi. The GR2 RNAi plants displayed early onset of age-dependent and dark- and H2 O2 -induced leaf senescence, which was accompanied by the induction of the senescence-related marker genes SAG12 and SAG13. Furthermore, transcriptome analysis revealed that genes related to leaf senescence, oxidative stress, and phytohormone pathways were upregulated directly before senescence in RNAi plants. In addition, H2 O2 accumulated to higher levels in RNAi plants than in wild-type plants and the levels of H2 O2 peaked in RNAi plants directly before the early onset of leaf senescence. RNAi plants showed a greater decrease in GSH/GSSG levels than wild-type plants during leaf development. Our results suggest that GR2 plays an important role in leaf senescence by modulating H2 O2 and glutathione signaling in Arabidopsis.

Low glutathione regulates gene expression and the redox potentials of the nucleus and cytosol in Arabidopsis thaliana

Reduced glutathione (GSH) is considered to exert a strong influence on cellular redox homeostasis and to regulate gene expression, but these processes remain poorly characterized. Severe GSH depletion specifically inhibited root meristem development, while low root GSH levels decreased lateral root densities. The redox potential of the nucleus and cytosol of Arabidopsis thaliana roots determined using roGFP probes was between -300 and -320 mV. Growth in the presence of the GSH-synthesis inhibitor buthionine sulfoximine (BSO) increased the nuclear and cytosolic redox potentials to approximately -260 mV. GSH-responsive genes including transcription factors (SPATULA, MYB15, MYB75), proteins involved in cell division, redox regulation (glutaredoxinS17, thioredoxins, ACHT5 and TH8) and auxin signalling (HECATE), were identified in the GSH-deficient root meristemless 1-1 (rml1-1) mutant, and in other GSH-synthesis mutants (rax1-1, cad2-1, pad2-1) as well as in the wild type following the addition of BSO. Inhibition of auxin transport had no effect on organ GSH levels, but exogenous auxin decreased the root GSH pool. We conclude that GSH depletion significantly increases the redox potentials of the nucleus and cytosol, and causes arrest of the cell cycle in roots but not shoots, with accompanying transcript changes linked to altered hormone responses, but not oxidative stress.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Enhancing crop resilience to combined abiotic and biotic stress through the dissection of physiological and molecular crosstalk

Plants growing in their natural habitats are often challenged simultaneously by multiple stress factors, both abiotic and biotic. Research has so far been limited to responses to individual stresses, and understanding of adaptation to combinatorial stress is limited, but indicative of non-additive interactions. Omics data analysis and functional characterization of individual genes has revealed a convergence of signaling pathways for abiotic and biotic stress adaptation. Taking into account that most data originate from imposition of individual stress factors, this review summarizes these findings in a physiological context, following the pathogenesis timeline and highlighting potential differential interactions occurring between abiotic and biotic stress signaling across the different cellular compartments and at the whole plant level. Potential effects of abiotic stress on resistance components such as extracellular receptor proteins, R-genes and systemic acquired resistance will be elaborated, as well as crosstalk at the levels of hormone, reactive oxygen species, and redox signaling. Breeding targets and strategies are proposed focusing on either manipulation and deployment of individual common regulators such as transcription factors or pyramiding of non- (negatively) interacting components such as R-genes with abiotic stress resistance genes. We propose that dissection of broad spectrum stress tolerance conferred by priming chemicals may provide an insight on stress cross regulation and additional candidate genes for improving crop performance under combined stress. Validation of the proposed strategies in lab and field experiments is a first step toward the goal of achieving tolerance to combinatorial stress in crops.

Analysis of cytosolic isocitrate dehydrogenase and glutathione reductase 1 in photoperiod-influenced responses to ozone using Arabidopsis knockout mutants

Oxidative stress caused by ozone (O3 ) affects plant development, but the roles of specific redox-homeostatic enzymes in O3 responses are still unclear. While growth day length may affect oxidative stress outcomes, the potential influence of day length context on equal-time exposures to O3 is not known. In Arabidopsis Col-0, day length affected the outcome of O3 exposure. In short-days (SD), few lesions were elicited by treatments that caused extensive lesions in long days (LD). Lesion formation was not associated with significant perturbation of glutathione, ascorbate, NADP(H) or NAD(H). To investigate roles of two genes potentially underpinning this redox stability, O3 responses of mutants for cytosolic NADP-isocitrate dehydrogenase (icdh) and glutathione reductase 1 (gr1) were analysed. Loss of ICDH function did not affect O3 -induced lesions, but slightly increased glutathione oxidation, induction of other cytosolic NADPH-producing enzymes and pathogenesis-related gene 1 (PR1). In gr1, O3 -triggered lesions, salicylic acid accumulation, and induction of PR1 were all decreased relative to Col-0 despite enhanced accumulation of glutathione. Thus, even at identical irradiance and equal-time exposures, day length strongly influences phenotypes triggered by oxidants of atmospheric origin, while in addition to its antioxidant function, the GR-glutathione system seems to play novel signalling roles during O3 exposure.

Regulating the redox gatekeeper: vacuolar sequestration puts glutathione disulfide in its place

The case is made for the potential importance of compartmentalization in redox signaling with new data on the transporters that may be involved.

Analysis of knockout mutants suggests that Arabidopsis NADP-MALIC ENZYME2 does not play an essential role in responses to oxidative stress of intracellular or extracellular origin

NADPH is a pivotal molecule in oxidative stress, during which it is potentially produced by several cytosolic NADP-linked dehydrogenases. This study investigated the response and functional importance of the major leaf cytosolic NADP-malic enzyme in Arabidopsis (NADP-ME2) during oxidative stress. Data from both microarray and targeted quantitative PCR analyses showed that NADP-ME2 transcripts accumulated in response to ozone or in mutants undergoing intracellular oxidative stress. To test the functional importance of this response, loss-of-function nadp-me2 mutants were obtained and the effects of oxidative stress of intracellular and extracellular origin were tested. Despite much decreased leaf NADP-ME activity, nadp-me2 showed a wild-type phenotype when exposed to ozone. Introduction of the nadp-me2 mutations into the catalase-deficient cat2 background did not alter growth inhibition or lesions triggered by intracellular oxidative stress. Similarly, loss of NADP-ME2 function had little effect on cat2-triggered changes in glutathione or NADPH. While single nadp-me2 mutations produced slight effects on basal resistance to one type of bacteria, they did not affect resistance induced by the cat2 mutation. Taken together, the results suggest that, although NADP-ME2 induction is part of the response to oxidative stress, the enzyme is not an essential determinant of the outcome of such stress.