Ascorbate oxidase: the unexpected involvement of a 'wasteful enzyme' in the symbioses with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi

Revisiting the role of ascorbate oxidase in plant systems

Ascorbic acid (AsA) plays an indispensable role in plants, serving as both an antioxidant and a master regulator of the cellular redox balance. Ascorbate oxidase (AO) is a blue copper oxidase that is responsible for the oxidation of AsA with the concomitant production of water. For many decades, AO was erroneously postulated as an enzyme without any obvious advantage, as it decreases the AsA pool size and thus is expected to weaken plant stress resistance. It was only a decade ago that this perspective shifted towards the fundamental role of AO in orchestrating both AsA and oxygen levels by influencing the overall redox balance in the extracellular matrix. Consistent with its localization in the apoplast, AO is involved in cell expansion, division, resource allocation, and overall plant yield. An increasing number of transgenic studies has demonstrated that AO can also facilitate communication between the surrounding environment and the cell, as its gene expression is highly responsive to factors such as hormonal signaling, oxidative stress, and mechanical injury. This review aims to describe the multiple functions of AO in plant growth, development, and stress resilience, and explore any additional roles the enzyme might have in fruits during the course of ripening.

The ascorbate biosynthesis pathway in plants is known, but there is a way to go with understanding control and functions

Ascorbate (vitamin C) is one of the most abundant primary metabolites in plants. Its complex chemistry enables it to function as an antioxidant, as a free radical scavenger, and as a reductant for iron and copper. Ascorbate biosynthesis occurs via the mannose/l-galactose pathway in green plants, and the evidence for this pathway being the major route is reviewed. Ascorbate accumulation is leaves is responsive to light, reflecting various roles in photoprotection. GDP-l-galactose phosphorylase (GGP) is the first dedicated step in the pathway and is important in controlling ascorbate synthesis. Its expression is determined by a combination of transcription and translation. Translation is controlled by an upstream open reading frame (uORF) which blocks translation of the main GGP-coding sequence, possibly in an ascorbate-dependent manner. GGP associates with a PAS-LOV protein, inhibiting its activity, and dissociation is induced by blue light. While low ascorbate mutants are susceptible to oxidative stress, they grow nearly normally. In contrast, mutants lacking ascorbate do not grow unless rescued by supplementation. Further research should investigate possible basal functions of ascorbate in severely deficient plants involving prevention of iron overoxidation in 2-oxoglutarate-dependent dioxygenases and iron mobilization during seed development and germination.

Functions of elements in soil microorganisms

The soil microbial community fulfils various functions, such as nutrient cycling and carbon (C) sequestration, therefore contributing to maintenance of soil fertility and mitigation of global warming. In this context, a major focus of research has been on C, nitrogen (N) and phosphorus (P) cycling. However, from aquatic and other environments, it is well known that other elements beyond C, N, and P are essential for microbial functioning. Nonetheless, for soil microorganisms this knowledge has not yet been synthesised. To gain a better mechanistic understanding of microbial processes in soil systems, we aimed at summarising the current knowledge on the function of a range of essential or beneficial elements, which may affect the efficiency of microbial processes in soil. This knowledge is discussed in the context of microbial driven nutrient and C cycling. Our findings may support future investigations and data evaluation, where other elements than C, N, and P affect microbial processes.

The ameliorative effects of exogenous inoculation of Piriformospora indica on molecular, biochemical and physiological parameters of Artemisia annua L. under arsenic stress condition

Aim of the current study was to investigate the effect of exogenously inoculated root endophytic fungus, Piriformospora indica, on molecular, biochemical, morphological and physiological parameters of Artemisia annua L. treated with different concentrations (0, 50, 100 and 150 μmol/L) of arsenic (As) stress. As was significantly accumulated in the roots than shoots of P. indica-inoculated plants. As accumulation and immobilization in the roots is directly associated with the successful fungal colonization that restricts most of As as compared to the aerial parts. A total of 4.1, 11.2 and 25.6 mg/kg dry weight of As was accumulated in the roots of inoculated plants supplemented with 50, 100 and 150 μmol/L of As, respectively as shown by atomic absorption spectroscopy. P. indica showed significant tolerance in vitro to As toxicity even at high concentration. Furthermore, flavonoids, artemisinin and overall biomass were significantly increased in inoculated-stressed plants. Superoxide dismutase and peroxidase activities were increased 1.6 and 1.2 fold, respectively under 150 μmol/L stress in P. indica-colonized plants. Similar trend was followed by ascorbate peroxidase, catalase and glutathione reductase. Like that, phenolic acid and phenolic compounds showed a significant increase in colonized plants as compared to their respective control/un-colonize stressed plants. The real-time PCR revealed that transcriptional levels of artemisinin biosynthesis genes, isoprenoids, terpenes, flavonoids biosynthetic pathway genes and signal molecules were prominently enhanced in inoculated stressed plants than un-inoculated stressed plants.

Genome-Wide Identification and Expression Analysis of the Ascorbate Oxidase Gene Family in Gossypium hirsutum Reveals the Critical Role of GhAO1A in Delaying Dark-Induced Leaf Senescence

Ascorbate oxidase (AO) plays important roles in plant growth and development. Previously, we reported a cotton AO gene that acts as a positive factor in cell growth. Investigations on Gossypium hirsutum AO (GhAO) family genes and their multiple functions are limited. The present study identified eight GhAO family genes and performed bioinformatic analyses. Expression analyses of the tissue specificity and developmental feature of GhAOs displayed their diverse expression patterns. Interestingly, GhAO1A demonstrated the most rapid significant increase in expression after 1 h of light recovery from the dark. Additionally, the transgenic ao1-1/GhAO1AArabidopsis lines overexpressing GhAO1A in the Arabidopsisao1-1 late-flowering mutant displayed a recovery to the normal phenotype of wild-type plants. Moreover, compared to the ao1-1 mutant, the ao1-1/GhAO1A transgenic Arabidopsis presented delayed leaf senescence that was induced by the dark, indicating increased sensitivity to hydrogen peroxide (H₂O₂) under normal conditions that might be caused by a reduction in ascorbic acid (AsA) and ascorbic acid/dehydroascorbate (AsA/DHA) ratio. The results suggested that GhAOs are functionally diverse in plant development and play a critical role in light responsiveness. Our study serves as a foundation for understanding the AO gene family in cotton and elucidating the regulatory mechanism of GhAO1A in delaying dark-induced leaf senescence.

Effects of Trichoderma seedling treatment with System of Rice Intensification management and with conventional management of transplanted rice

Many benefits of Trichoderma inoculation for improving crop production have been documented, including growth and yield enhancement and the alleviation of biotic and abiotic stresses. However, because rice is usually cultivated under continuous flooding that creates anaerobic soil conditions, this limits the benefits of these beneficial fungi. Cultivating rice with the methods of the System of Rice Intensification (SRI) provides rice plants with a more favorable environment for their colonization by beneficial microbes in the soil because the soil is more aerobic under SRI management and contains more organic matter. This study evaluated the effects of Trichoderma inoculation of rice plants under SRI management compared with transplanted and flooded rice plants, considering also the effects of different means of fertilization and different varieties in rice. Experiments were conducted in 2015 and 2016 under the tropical climate of Nepal's western terai (plains) during both the rainy season (July to November) and the dry season (March to July). The results indicated significantly better performance (P = 0.01) associated with Trichoderma inoculation for both seasons and for both systems of crop management in terms of grain yield and other growth-contributing factors, compared to non-inoculated rice cropping. Relatively higher effects on grain yield were recorded also with organic compared to inorganic fertilization; for unimproved (heirloom) varieties compared with improved varieties; and from SRI vs. conventional flooded crop management. The yield increase with Trichoderma treatments across all trials was 31% higher than in untreated plots (4.9 vs 4.5 mt ha-1). With Trichoderma treatment, yields compared with non-treated plots were 24% higher with organic SRI (6.38 vs 5.13 mt ha-1) and 52% higher with non-organic SRI (6.38 vs 3.53 mt ha-1). With regard to varietal differences, under SRI management Trichoderma inoculation of the improved variety Sukhadhan-3 led to 26% higher yield (6.35 vs 5.04 mt ha-1), and with the heirloom variety Tilkidhan, yield was 41% higher (6.29 vs 4.45 mt ha-1). Economic analysis indicated that expanding the organic cultivation of local landraces under SRI management should be profitable for farmers where such rice has a good market price due to its premium quality and high demand and when SRI enhances yield. These varieties' present low yields can be significantly increased by integrating Trichoderma bio-inoculation with SRI cultural methods. Other recent research has shown that such inoculation can be managed profitably by farmers themselves.

Characterization of three multicopper oxidases in the filamentous fungus Podospora anserina: A new role of an ABR1-like protein in fungal development?

The Podospora anserina genome contains a large family of 15 multicopper oxidases (MCOs), including three genes encoding a FET3-like protein, an ABR1-like protein and an ascorbate oxidase (AO)-like protein. FET3, ABR1 and AO1 are involved in global laccase-like activity since deletion of the relevant genes led to a decrease of activity when laccase substrate (ABTS) was used as substrate. However, contrary to the P. anserina MCO proteins previously characterized, none of these three MCOs seemed to be involved in lignocellulose degradation and in resistance to phenolic compounds and oxidative stress. We showed that the bulk of ferroxidase activity was clearly due to ABR1, and only in minor part to FET3, although ABR1 does not contain all the residues typical of FET3 proteins. Moreover, we showed that ABR1, related to the Aspergillus fumigatus ABR1 protein, was clearly and specifically involved in pigmentation of ascospores. Surprisingly, phenotypes were more severe in mutants lacking both abr1 and ao1. Deletion of the ao1 gene led to an almost total loss of AO activity. No direct involvement of AO1 in fungal developmental process in P. anserina was evidenced, except in a abr1^Δ background. Overall, unlike other previously characterized MCOs, we thus evidence a clear involvement of ABR1 protein in fungal development.

Transcriptome changes induced by arbuscular mycorrhizal fungi in sunflower (Helianthus annuus L.) roots

Arbuscular mycorrhizal (AM) fungi are essential elements of soil fertility, plant nutrition and productivity, facilitating soil mineral nutrient uptake. Helianthus annuus is a non-model, widely cultivated species. Here we used an RNA-seq approach for evaluating gene expression variation at early and late stages of mycorrhizal establishment in sunflower roots colonized by the arbuscular fungus Rhizoglomus irregulare. mRNA was isolated from roots of plantlets at 4 and 16 days after inoculation with the fungus. cDNA libraries were built and sequenced with Illumina technology. Differential expression analysis was performed between control and inoculated plants. Overall 726 differentially expressed genes (DEGs) between inoculated and control plants were retrieved. The number of up-regulated DEGs greatly exceeded the number of down-regulated DEGs and this difference increased in later stages of colonization. Several DEGs were specifically involved in known mycorrhizal processes, such as membrane transport, cell wall shaping, and other. We also found previously unidentified mycorrhizal-induced transcripts. The most important DEGs were carefully described in order to hypothesize their roles in AM symbiosis. Our data add a valuable contribution for deciphering biological processes related to beneficial fungi and plant symbiosis, adding an Asteraceae, non-model species for future comparative functional genomics studies.

Comparative transcriptome analysis of nodules of two Mesorhizobium-chickpea associations with differential symbiotic efficiency under phosphate deficiency

Phosphate (Pi) deficiency is known to be a major limitation for symbiotic nitrogen fixation (SNF), and hence legume crop productivity globally. However, very little information is available on the adaptive mechanisms, particularly in the important legume crop chickpea (Cicer arietinum L.), which enable nodules to respond to low-Pi availability. Thus, to elucidate these mechanisms in chickpea nodules at molecular level, we used an RNA sequencing approach to investigate transcriptomes of the nodules in Mesorhizobium mediterraneum SWRI9-(MmSWRI9)-chickpea and M. ciceri CP-31-(McCP-31)-chickpea associations under Pi-sufficient and Pi-deficient conditions, of which the McCP-31-chickpea association has a better SNF capacity than the MmSWRI9-chickpea association during Pi starvation. Our investigation revealed that more genes showed altered expression patterns in MmSWRI9-induced nodules than in McCP-31-induced nodules (540 vs. 225) under Pi deficiency, suggesting that the Pi-starvation-more-sensitive MmSWRI9-induced nodules required expression change in a larger number of genes to cope with low-Pi stress than the Pi-starvation-less-sensitive McCP-31-induced nodules. The functional classification of differentially expressed genes (DEGs) was examined to gain an understanding of how chickpea nodules respond to Pi starvation, caused by soil Pi deficiency. As a result, more DEGs involved in nodulation, detoxification, nutrient/ion transport, transcriptional factors, key metabolic pathways, Pi remobilization and signalling were found in Pi-starved MmSWRI9-induced nodules than in Pi-starved McCP-31-induced nodules. Our findings have enabled the identification of molecular processes that play important roles in the acclimation of nodules to Pi deficiency, ultimately leading to the development of Pi-efficient chickpea symbiotic associations suitable for Pi-deficient soils.

A salt-stress-regulator from the Poplar R2R3 MYB family integrates the regulation of lateral root emergence and ABA signaling to mediate salt stress tolerance in Arabidopsis

The roles of most MYB transcription factors (TFs) in the poplar remain unclear. Here, we demonstrate that PtrSSR1, a salt-stress-regulator in the Populus trichocarpa R2R3 MYB gene family, mediates the tolerance of transgenic Arabidopsis plants to salt stress. The transcripts of PtrSSR1 could be induced by salt stress rapidly in poplar. Subcellular localization and yeast assays indicated that PtrSSR1 encoded a nuclear protein with transactivation activity. The Arabidopsis transformants overexpressing PtrSSR1 clearly displayed lateral root emergence (LRE) inhibition compared with wild-type (Wt) under normal conditions; while upon NaCl treatment, the transformants showed improved tolerance, and the LRs emerged faster from salt-induced inhibition. A strong correlation could exist between the LRE mediated by PtrSSR1 and abscisic acid (ABA), mainly because the transformants displayed more sensitivity to exogenous ABA during both seed germination and LRE, and had a distinctly increased level of endogenous ABA. Furthermore, several ABA- and salt-related genes, such as NCED3, ABI1 and CBL1, were up-regulated. Thus, our results suggest that elevation in the endogenous ABA content bring alteration of plant LR development, and that the poplar R2R3 MYB TF PtrSSR1 vitally improve salt stress tolerance by integrating the regulation of LRE and ABA signaling in Arabidopsis.

Tomato SlGGP-LIKE gene participates in plant responses to chilling stress and pathogenic infection

Plants are always exposed to abiotic and biotic stresses which can adversely affect their growth and development. As an important antioxidant, AsA plays a vital role in plant defence against damage caused by stresses. In this study, we cloned a tomato GDP-_L-galactose phosphorylase-like (SlGGP-LIKE) gene and investigated its role in resistance to abiotic and biotic stresses by using antisense transgenic (AS) tomato lines. The AsA content in AS plants was lower than that in WT plants. Under chilling stress, the growth of AS plants was inhibited significantly, and they yielded higher levels of ROS, REC and MDA but demonstrated weaker APX activity than that shown by WT plants. Additionally, the declined values of Pn, Fv/Fm, oxidisable P700, and D1 protein content of PSII in AS lines were significant. Furthermore, the effect on xanthophyll cycle of AS plants was more severe than that on WT plants, and the ratio of zeaxanthin (Z)/(V + A + Z) and (Z + 0.5 A)/(V + A + Z) in AS lines was lower than that in WT plants. In spite of chilling stress, under Pseudomonas syringae pv.tomato (Pst) DC3000 strain infection, AS plants showed lesser bacterial cell growth and dead cells than those shown by WT plants. This finding indicated that AS plants demonstrated stronger resistance against pathogenic infection. Results suggest that SlGGP-LIKE gene played an important role in plant defence against chilling stress and pathogenic infection.

Large-scale transcriptome comparison of sunflower genes responsive to Verticillium dahliae

BACKGROUND

Sunflower Verticillium wilt (SVW) is a vascular disease caused by root infection with Verticillium dahliae (V. dahlia). It is a serious threat to the yield and quality of sunflower. However, chemical and agronomic measures for controlling this disease are not effective. The selection of more resistant genotypes is a desirable strategy to reduce contamination. A deeper knowledge of the molecular mechanisms and genetic basis underlying sunflower Verticillium wilt is necessary to accelerate breeding progress.

RESULTS

An RNA-Seq approach was used to perform global transcriptome profiling on the roots of resistant (S18) and susceptible (P77) sunflower genotypes infected with V. dahlia. Different pairwise transcriptome comparisons were examined over a time course (6, 12 and 24 h, and 2, 3, 5 and 10 d post inoculation). In RD, SD and D datasets, 1231 genes were associated with SVW resistance in a genotype-common transcriptional pattern. Moreover, 759 and 511 genes were directly related to SVW resistance in the resistant and susceptible genotypes, respectively, in a genotype-specific transcriptional pattern. Most of the genes were demonstrated to participate in plant defense responses; these genes included peroxidase (POD), glutathione peroxidase, aquaporin PIP, chitinase, L-ascorbate oxidase, and LRR receptors. For the up-regulated genotype-specific differentially expressed genes (DEGs) in the resistant genotype, higher average fold-changes were observed in the resistant genotype compared to those in the susceptible genotype. An inverse effect was observed in the down-regulated genotype-specific DEGs in the resistant genotype. KEGG analyses showed that 98, 112 and 52 genes were classified into plant hormone signal transduction, plant-pathogen interaction and flavonoid biosynthesis categories, respectively. Many of these genes, such as CNGC, RBOH, FLS2, JAZ, MYC2 NPR1 and TGA, regulate crucial points in defense-related pathway and may contribute to V. dahliae resistance in sunflower.

CONCLUSIONS

The transcriptome profiling results provided a clearer understanding of the transcripts associated with the crosstalk between sunflower and V. dahliae. The results identified several differentially expressed unigenes involved in the hyper sensitive response (HR) and the salicylic acid (SA)/jasmonic acid (JA)-mediated signal transduction pathway for resistance against V. dahliae. These results are useful for screening resistant sunflower genotypes.

Transcript Profiling Reveals the Presence of Abiotic Stress and Developmental Stage Specific Ascorbate Oxidase Genes in Plants

Abiotic stress and climate change is the major concern for plant growth and crop yield. Abiotic stresses lead to enhanced accumulation of reactive oxygen species (ROS) consequently resulting in cellular damage and major losses in crop yield. One of the major scavengers of ROS is ascorbate (AA) which acts as first line of defense against external oxidants. An enzyme named ascorbate oxidase (AAO) is known to oxidize AA and deleteriously affect the plant system in response to stress. Genome-wide analysis of AAO gene family has led to the identification of five, three, seven, four, and six AAO genes in Oryza sativa, Arabidopsis, Glycine max, Zea mays, and Sorghum bicolor genomes, respectively. Expression profiling of these genes was carried out in response to various abiotic stresses and during various stages of vegetative and reproductive development using publicly available microarray database. Expression analysis in Oryza sativa revealed tissue specific expression of AAO genes wherein few members were exclusively expressed in either root or shoot. These genes were found to be regulated by both developmental cues as well as diverse stress conditions. The qRT-PCR analysis in response to salinity and drought stress in rice shoots revealed OsAAO2 to be the most stress responsive gene. On the other hand, OsAAO3 and OsAAO4 genes showed enhanced expression in roots under salinity/drought stresses. This study provides lead about important stress responsive AAO genes in various crop plants, which could be used to engineer climate resilient crop plants.

Movement Protein of Cucumber Mosaic Virus Associates with Apoplastic Ascorbate Oxidase

Plant viral movement proteins facilitate virion movement mainly through interaction with a number of factors from the host. We report the association of a cell wall localized ascorbate oxidase (CsAO4) from Cucumis sativus with the movement protein (MP) of Cucumber mosaic virus (CMV). This was identified first in a yeast two-hybrid screen and validated by in vivo pull down and bimolecular fluorescence complementation (BiFC) assays. The BiFC assay showed localization of the bimolecular complexes of these proteins around the cell wall periphery as punctate spots. The expression of CsAO4 was induced during the initial infection period (up to 72 h) in CMV infected Nicotiana benthamiana plants. To functionally validate its role in viral spread, we analyzed the virus accumulation in CsAO4 overexpressing Arabidopsis thaliana and transiently silenced N. benthamiana plants (through a Tobacco rattle virus vector). Overexpression had no evident effect on virus accumulation in upper non-inoculated leaves of transgenic lines in comparison to WT plants at 7 days post inoculation (dpi). However, knockdown resulted in reduced CMV accumulation in systemic (non-inoculated) leaves of NbΔAO-pTRV2 silenced plants as compared to TRV inoculated control plants at 5 dpi (up to 1.3 fold difference). In addition, functional validation supported the importance of AO in plant development. These findings suggest that AO and viral MP interaction helps in early viral movement; however, it had no major effect on viral accumulation after 7 dpi. This study suggests that initial induction of expression of AO on virus infection and its association with viral MP helps both towards targeting of the MP to the apoplast and disrupting formation of functional AO dimers for spread of virus to nearby cells, reducing the redox defense of the plant during initial stages of infection.

Cell wall remodeling in mycorrhizal symbiosis: a way towards biotrophism

Cell walls are deeply involved in the molecular talk between partners during plant and microbe interactions, and their role in mycorrhizae, i.e., the widespread symbiotic associations established between plant roots and soil fungi, has been investigated extensively. All mycorrhizal interactions achieve full symbiotic functionality through the development of an extensive contact surface between the plant and fungal cells, where signals and nutrients are exchanged. The exchange of molecules between the fungal and the plant cytoplasm takes place both through their plasma membranes and their cell walls; a functional compartment, known as the symbiotic interface, is thus defined. Among all the symbiotic interfaces, the complex intracellular interface of arbuscular mycorrhizal (AM) symbiosis has received a great deal of attention since its first description. Here, in fact, the host plasma membrane invaginates and proliferates around all the developing intracellular fungal structures, and cell wall material is laid down between this membrane and the fungal cell surface. By contrast, in ectomycorrhizae (ECM), where the fungus grows outside and between the root cells, plant and fungal cell walls are always in direct contact and form the interface between the two partners. The organization and composition of cell walls within the interface compartment is a topic that has attracted widespread attention, both in ecto- and endomycorrhizae. The aim of this review is to provide a general overview of the current knowledge on this topic by integrating morphological observations, which have illustrated cell wall features during mycorrhizal interactions, with the current data produced by genomic and transcriptomic approaches.

Altered apoplastic ascorbate redox state in tobacco plants via ascorbate oxidase overexpression results in delayed dark-induced senescence in detached leaves

Ascorbate oxidase (AO) is an apoplastic enzyme that uses oxygen to catalyse the oxidation of ascorbate (AA) to dehydroascorbate (DHA) via the unstable radical monodehydroascorbate (MDHA). Here, we report that transgenic tobacco plants (Nicotiana tabacum L. cv. Xanthi) with an in vivo lowered apoplastic AA redox state through increased AO expression demonstrate signs of delayed dark-induced senescence compared with wild-type plants, as shown by chlorophyll loss assay. In situ localization of hydrogen peroxide (H2O2) suggests that, although transgenic plants have higher constitutive levels of H2O2 under normal growth conditions, imposed dark-induced senescence results in smaller induction levels of H2O2, an observation which correlates with increased antioxidant enzyme activities and an induction in the expression of AA recycling genes compared with that in wild-type plants. Our current findings, combined with previous studies which showed the contribution of AO in the regulation of AA redox state, suggest that the reduction in AA redox state in the leaf apoplast of these transgenic plants results in an increase in the endogenous levels of H2O2, which provides a form of 'acquired tolerance' to oxidative stress imposed by dark-induced senescence.

Ascorbate oxidase is the potential conductor of a symphony of signaling pathways

The functional role of ascorbate oxidase (AO; EC 1.10.3.3) has never been fully explained so far, due to the difficulties in understanding the presence of an enzyme specifically oxidizing ascorbate with no obvious advantage, and the apparent disadvantage of lowering plant stress resistance as a consequence of ascorbate consumption. Here we suggest a complete change of perspective, by proposing an essential role of AO as a modulator of both ascorbate and oxygen content, with relevant implications related to signaling. By affecting the overall redox state, AO is actually involved in redox regulation in the extracellular matrix. In addition, AO can contribute to creating a hypoxic microenvironment, especially relevant in the maintenance of meristem identity and the establishment of mutualistic plant-microbe interactions. We also hypothesize the possible involvement of AO in the activation of a signaling cascade analogous to the mechanism of prolyl hydroxylases/Hypoxia Inducible Factors in animals.