The control of root growth by reactive oxygen species in Salix nigra Marsh. seedlings

Phytotoxicity, Morphological, and Metabolic Effects of the Sesquiterpenoid Nerolidol on Arabidopsis thaliana Seedling Roots

Natural herbicides that are based on allelopathy of compounds, can offer effective alternatives to chemical herbicides towards sustainable agricultural practices. Nerolidol, a sesquiterpenoid alcohol synthesized by many plant families, was shown to be the most effective allelopathic compound in a preliminary screening performed with several other sesquiterpenoids. In the present study, Arabidopsis thaliana seedlings were treated for 14 d with various cis-nerolidol concentrations (0, 50, 100, 200, 400, and 800 µM) to investigate its effects on root growth and morphology. To probe the underlying changes in root metabolome, we conducted untargeted gas chromatography mass spectrometry (GC-MS) based metabolomics to find out the specificity or multi-target action of this sesquiterpenoid alcohol. Oxidative stress (measured as levels of H₂O₂ and malondialdehyde (MDA) by-product) and antioxidant enzyme activities, i.e., superoxide dismutase (SOD) and catalase (CAT) were also evaluated in the roots. Nerolidol showed an IC₅₀ (120 µM), which can be considered low for natural products. Nerolidol caused alterations in root morphology, brought changes in auxin balance, induced changes in sugar, amino acid, and carboxylic acid profiles, and increased the levels of H₂O₂ and MDA in root tissues in a dose-dependent manner. Several metabolomic-scale changes induced by nerolidol support the multi-target action of nerolidol, which is a positive feature for a botanical herbicide. Though it warrants further mechanistic investigation, nerolidol is a promising compound for developing a new natural herbicide.

Metabolic rearrangements in imbibed maize (Zea mays L) embryos in the presence of oxidative stressors

Cadmium (Cd) is a metal known to generate oxidative stress in plants and may be particularly harmful during germination. Herein, the growth and metabolic rearrangements of maize embryo axes subjected during the imbibition stage to Cd ions and other two well-known oxidative stressors, methyl viologen (MV) and hydrogen peroxide (H₂O₂), were assessed for 48 h. Similar decreases in embryo's length were detected for all stressed axes up to 48 h of imbibition. By this time, treated embryos revealed greater accumulation of reactive oxygen species (ROS) and increased levels of carbonylated and ubiquitinated proteins. The proteolytic activities were intensely enhanced in the treated axes, particularly at 48 h of imbibition, and several antioxidant enzymes were induced in most cases. NMR spectroscopy followed by principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that a large proportion of polar metabolites, mainly amino acids and organic acids, were decreased under stress conditions, while carbohydrates were increased at 48 h of imbibition, with significant increases in glucose and raffinose for treated embryos relatively to controls. We demonstrated that maize embryo axes were capable of shifting their metabolism to improve their antioxidant defense system, at the expense of their growth. Under these adverse conditions, proteolysis seems to play a key role by providing free amino acids needed for the de novo synthesis of defense-related proteins.

Exogenous spermidine elevating cadmium tolerance in Salix matsudana involves cadmium detoxification and antioxidant defense

In this study, exogenous spermidine role on Salix matsudana tolerance to cadmium was evaluated. Spermidine and cadmium presented antagonistic effects on the biomass, copper and zinc concentrations in S. matsudana. cadmium mainly distributed in the cell wall of subcellular fraction; 46.97%-60.43% of cadmium existed in a sodium chloride-extracted form. Cadmium contents in roots, leaves, and twigs ranged from 2002.67 to 3961.00, 111.59 to 229.72, and 102.56 to 221.27 mg/kg, respectively. Spermidine application elevated cadmium concentrations in the roots, cuttings, and cell wall and the ratio of deionized water-extracted cadmium, but decreased cadmium levels in the twigs and leaves and the fractions of cadmium extracted by ethanol and sodium chloride, respectively. Putrescine and malondialdehyde were important indicators of cadmium-induced oxidative damage. Exogenous spermidine alleviated the accumulation of superoxide anion, hydrogen peroxide, malondialdehyde via promoting the levels of spermidine, soluble protein, superoxide dismutase, reductive ascorbate, glutathione reductase, and glutathione peroxidase in S. matsudana leaves under the corresponding cadmium stress. The results indicated that S. matsudana was a candidate for cadmium rhizoremediation and extraction in leaves; the spermidine application enhanced the cadmium tolerance of S. matsudana through promoting cadmium accumulation in roots, cell wall, and less bioactive chemical forms and the antioxidative ability.

Orchestration of Cu-Zn SOD and class III peroxidase with upstream interplay between NADPH oxidase and PM H+-ATPase mediates root growth in Vigna radiata (L.) Wilczek

Post-germination plant growth depends on the regulation of reactive oxygen species (ROS) metabolism, spatiotemporal pH changes and Ca⁺² homeostasis, whose potential integration has been studied during Vigna radiata (L.) Wilczek root growth. The dissipation of proton (H⁺) gradients across plasma membrane (PM) by CCCP (protonophore) and the inhibition of PM H⁺-ATPase by sodium orthovanadate repressed SOD (superoxide dismutase; EC 1.15.1.1) activity as revealed by spectrophotometric and native PAGE assay results. Similar results derived from treatment with DPI (NADPH oxidase inhibitor) and Tiron (O₂- scavenger) denote a functional synchronization of SOD, PM H⁺-ATPase and NOX, as the latter two enzymes are substrate sources for SOD (H⁺ and O₂-, respectively) and are involved in a feed-forward loop. After SOD inactivation, a decline in apoplastic H₂O₂ content was observed in each treatment group, emerging as a possible cause of the diminution of class III peroxidase (Prx; EC 1.11.1.7), which utilizes H₂O₂ as a substrate. In agreement with the pivotal role of Ca⁺² in PM H⁺-ATPase and NOX activation, Ca⁺² homeostasis antagonists, i.e., LaCl₃ (Ca⁺² channel inhibitor), EGTA (Ca⁺² chelator) and LiCl (endosomal Ca⁺² release blocker), inhibited both SOD and Prx. Finally, a drastic reduction in apoplastic OH (hydroxyl radical) concentrations (induced by each treatment, leading to Prx inhibition) was observed via fluorometric analysis. A consequential inhibition of root growth observed under each treatment denotes the importance of the orchestrated functioning of PM H⁺-ATPase, NOX, Cu-Zn SOD and Prx during root growth. A working model demonstrating postulated enzymatic synchronization with an intervening role of Ca⁺² is proposed.

ROS-Activated Ion Channels in Plants: Biophysical Characteristics, Physiological Functions and Molecular Nature

Ion channels activated by reactive oxygen species (ROS) have been found in the plasma membrane of charophyte Nitella flixilis, dicotyledon Arabidopsis thaliana, Pyrus pyrifolia and Pisum sativum, and the monocotyledon Lilium longiflorum. Their activities have been reported in charophyte giant internodes, root trichoblasts and atrichoblasts, pollen tubes, and guard cells. Hydrogen peroxide and hydroxyl radicals are major activating species for these channels. Plant ROS-activated ion channels include inwardly-rectifying, outwardly-rectifying, and voltage-independent groups. The inwardly-rectifying ROS-activated ion channels mediate Ca2+-influx for growth and development in roots and pollen tubes. The outwardly-rectifying group facilitates K⁺ efflux for the regulation of osmotic pressure in guard cells, induction of programmed cell death, and autophagy in roots. The voltage-independent group mediates both Ca2+ influx and K⁺ efflux. Most studies suggest that ROS-activated channels are non-selective cation channels. Single-channel studies revealed activation of 14.5-pS Ca2+ influx and 16-pS K⁺ efflux unitary conductances in response to ROS. The molecular nature of ROS-activated Ca2+ influx channels remains poorly understood, although annexins and cyclic nucleotide-gated channels have been proposed for this role. The ROS-activated K⁺ channels have recently been identified as products of Stellar K⁺ Outward Rectifier (SKOR) and Guard cell Outwardly Rectifying K⁺ channel (GORK) genes.

Early plant growth and biochemical responses induced by Azospirillum brasilense Sp245 lipopolysaccharides in wheat (Triticum aestivum L.) seedlings are attenuated by procyanidin B2

This study analyzes the effects of procyanidin B2 on early wheat plant growth and plant biochemical responses promoted by lipopolysaccharides (LPS) derived from the rhizobacteria Azospirillum brasilense Sp245. Measurements of leaf, root length, fresh weight, and dry weight showed in vitro plant growth stimulation 4 days after treatment with A. brasilense as well as LPS. Superoxide anion (O₂^·-) and hydrogen peroxide (H₂O₂) levels increased in seedling roots treated with LPS (100 μg mL^-1). The chlorophyll content in leaf decreased while the starch content increased 24 h after treatment in seedling roots. The LPS treatment induced a high increase in total peroxidase (POX) (EC 1.11.1.7) activity and ionically bound cell wall POX content in roots, when compared to respective controls. Early plant growth and biochemical responses observed in wheat seedlings treated with LPS were inhibited by the addition of procyanidin B2 (5 μg mL^-1), a B type proanthocyanidin (PAC), plant-derived polyphenolic compound with binding properties of LPS. All results suggest first that the ionically bound cell wall POX enzymes could be a molecular target of A. brasilense LPS, and second that the recognition or association of LPS by plant cells is required to activate plant responses. This last event could play a critical role during plant growth regulation by A. brasilense LPS.

OsPhyB-Mediating Novel Regulatory Pathway for Drought Tolerance in Rice Root Identified by a Global RNA-Seq Transcriptome Analysis of Rice Genes in Response to Water Deficiencies

Water deficiencies are one of the most serious challenges to crop productivity. To improve our understanding of soil moisture stress, we performed RNA-Seq analysis using roots from 4-week-old rice seedlings grown in soil that had been subjected to drought conditions for 2-3 d. In all, 1,098 genes were up-regulated in response to soil moisture stress for 3 d, which causes severe damage in root development after recovery, unlikely that of 2 d. Comparison with previous transcriptome data produced in drought condition indicated that more than 68% of our candidate genes were not previously identified, emphasizing the novelty of our transcriptome analysis for drought response in soil condition. We then validated the expression patterns of two candidate genes using a promoter-GUS reporter system in planta and monitored the stress response with novel molecular markers. An integrating omics tool, MapMan analysis, indicated that RING box E3 ligases in the ubiquitin-proteasome pathways are significantly stimulated by induced drought. We also analyzed the functions of 66 candidate genes that have been functionally investigated previously, suggesting the primary roles of our candidate genes in resistance or tolerance relating traits including drought tolerance (29 genes) through literature searches besides diverse regulatory roles of our candidate genes for morphological traits (15 genes) or physiological traits (22 genes). Of these, we used a T-DNA insertional mutant of rice phytochrome B (OsPhyB) that negatively regulates a plant's degree of tolerance to water deficiencies through the control of total leaf area and stomatal density based on previous finding. Unlike previous result, we found that OsPhyB represses the activity of ascorbate peroxidase and catalase mediating reactive oxygen species (ROS) processing machinery required for drought tolerance of roots in soil condition, suggesting the potential significance of remaining uncharacterized candidate genes for manipulating drought tolerance in rice.

Integrated role of ROS and Ca+2 in blue light-induced chloroplast avoidance movement in leaves of Hydrilla verticillata (L.f.) Royle

Directional chloroplast photorelocation is a major physio-biochemical mechanism that allows these organelles to realign themselves intracellularly in response to the intensity of the incident light as an adaptive response. Signaling processes involved in blue light (BL)-dependent chloroplast movements were investigated in Hydrilla verticillata (L.f.) Royle leaves. Treatments with antagonists of actin filaments [2,3,5-triiodobenzoic acid (TIBA)] and microtubules (oryzalin) revealed that actin filaments, but not microtubules, play a pivotal role in chloroplast movement. Involvement of reactive oxygen species (ROS) in controlling chloroplast avoidance movement has been demonstrated, as exogenous H₂O₂ not only accelerated chloroplast avoidance but also could induce chloroplast avoidance even in weak blue light (WBL). Further support came from experiments with different ROS scavengers, i.e., dimethylthiourea (DMTU), KI, and CuCl₂, which inhibited chloroplast avoidance, and from ROS localization using specific stains. Such avoidance was also partially inhibited by ZnCl₂, an inhibitor of NADPH oxidase (NOX) as well as 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a photosynthetic electron transport chain (ETC) inhibitor at PS II. However, methyl viologen (MV), a PS I ETC inhibitor, rather accelerated avoidance response. Exogenous calcium (Ca⁺²) induced avoidance even in WBL while inhibited chloroplast accumulation partially. On the other hand, chloroplast movements (both accumulation and avoidance) were blocked by Ca⁺² antagonists, La³⁺ (inhibitor of plasma membrane Ca⁺² channel) and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, Ca⁺² chelator) while LiCl that affects Ca⁺² release from endosomal compartments did not show any effect. A model on integrated role of ROS and Ca⁺² (influx from apolastic space) in actin-mediated chloroplast avoidance has been proposed.

Loss of Gravitropism in Farnesene-Treated Arabidopsis Is Due to Microtubule Malformations Related to Hormonal and ROS Unbalance

Mode of action of farnesene, a volatile sesquiterpene commonly found in the essential oils of several plants, was deeply studied on the model species Arabidopsis thaliana. The effects of farnesene on the Arabidopsis root morphology were evaluated by different microscopic techniques. As well, microtubules immunolabeling, phytohormone measurements and ROS staining helped us to elucidate the single or multi-modes of action of this sesquiterpene on plant metabolism. Farnesene-treated roots showed a strong growth inhibition and marked modifications on morphology, important tissue alterations, cellular damages and anisotropic growth. Left-handed growth of farnesene-treated roots, reverted by taxol (a known microtubule stabilizer), was related to microtubule condensation and disorganization. As well, the inhibition of primary root growth, lateral root number, lateral root length, and both root hairs length and density could be explained by the strong increment in ethylene production and auxin content detected in farnesene-treated seedlings. Microtubule alteration and hormonal unbalance appear as important components in the mode of action of farnesene and confirm the strong phytotoxic potential of this sesquiterpene.

Potential role of reactive oxygen species and antioxidant genes in the regulation of peach fruit development and ripening

The roles of reactive oxygen species (ROS) as both toxic by-products and as signaling molecules have been reported in fruit development and ripening. Superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) play important roles in balancing the induction and removal of ROS in plants, and are respectively encoded by families of closely homologous genes. In the present study, we investigated the roles of ROS and the above-mentioned antioxidant genes during the development and ripening of peach fruit. The experimental results indicated that O2(-) and H2O2 acted as potential signaling molecules in the middle stage of fruit development, and only H2O2 might function as a main toxic molecule to stimulate lipid peroxidation and oxidative stress in the late stage of fruit ripening. PpaCu/Zn-SODs were the most abundant members in the PpaSOD gene family and they expressed steadily in peach fruit development and ripening. Low temperature (4 °C) postponed and suppressed the climacteric peaks of respiration and ethylene, significantly enhanced the activities of CAT and GPX, and up-regulated the expression of PpaCAT1 and PpaGPX6 in the late stage of fruit ripening. PpaCAT1 and PpaGPX6 were two key genes in alleviating oxidative stress in the late stage of fruit ripening.

The LIKE SEX FOUR2 regulates root development by modulating reactive oxygen species homeostasis in Arabidopsis

Maintaining reactive oxygen species (ROS) homeostasis plays a central role in plants, and is also critical for plant root development. Threshold levels of ROS act as signals for elongation and differentiation of root cells. The protein phosphatase LIKE SEX FOUR2 (LSF2) has been reported to regulate starch metabolism in Arabidopsis, but little is known about the mechanism how LSF2 affect ROS homeostasis. Here, we identified that LSF2 function as a component modulating ROS homeostasis in response to oxidative stress and, thus regulate root development. Compared with wild type Arabidopsis, lsf2-1 mutant exhibited reduced rates of superoxide generation and higher levels of hydrogen peroxide upon oxidative stress treatments. The activities of several antioxidant enzymes, including superoxide dismutase, catalase, and ascorbate peroxidase, were also affected in lsf2-1 mutant under these oxidative stress conditions. Consequently, lsf2-1 mutant exhibited the reduced root growth but less inhibition of root hair formation compared to wild type Arabidopsis plants. Importantly, protein phosphatase LSF2 interacted with mitogen-activated protein kinase 8 (MPK8), a known component of ROS homeostasis pathways in the cytoplasm. These findings indicated the novel function of LSF2 that controls ROS homeostasis to regulate root development.

Oxidative and antioxidative responses in the wheat-Azospirillum brasilense interaction

Azospirillum is a plant growth-promoting rhizobacteria (PGPR) able to enhance the growth of wheat. The aim of this study was to test the effect of Azospirillum brasilense cell wall components on superoxide (O2·(-)) production in wheat roots and the effect of oxidative stress on A. brasilense viability. We found that inoculation with A. brasilense reduced O2·(-) levels by approx. 30 % in wheat roots. Inoculation of wheat with papain-treated A. brasilense, a Cys protease, notably increased O2·(-) production in all root tissues, as was observed by the nitro blue tetrazolium (NBT) reduction. However, a 24-h treatment with rhizobacteria lipopolysaccharides (50 and 100 μg/mL) alone did not affect the pattern of O2·(-) production. Analysis of the effect of plant cell wall components on A. brasilense oxidative enzyme activity showed no changes in catalase activity but a decrease in superoxide dismutase activity in response to polygalacturonic acid treatment. Furthermore, A. brasilense growth was only affected by high concentrations of H2O2 or paraquat, but not by sodium nitroprusside. Our results suggest that rhizobacterial cell wall components play an important role in controlling plant cell responses and developing tolerance of A. brasilense to oxidative stress produced by the plant.

Superoxide and its metabolism during germination and axis growth of Vigna radiata (L.) Wilczek seeds

Involvement of reactive oxygen species in regulation of plant growth and development is recently being demonstrated with various results depending on the experimental system and plant species. Role of superoxide and its metabolism in germination and axis growth was investigated in case of Vigna radiata seeds, a non-endospermous leguminous species having epigeal germination, by studying the effect of different reactive oxygen species (ROS) inhibitors, distribution of O2(•)- and H2O2 and ROS enzyme profile in axes. Germination percentage and axis growth were determined under treatment with ROS inhibitors and scavengers. Localization of O2(•)- and H2O2 was done using nitroblue tetrazolium (NBT) and 3,3',5,5'-tetramethyl benzidine dihydrochloride hydrate (TMB), respectively. Apoplastic level of O2(•)- was monitored by spectrophotometric analysis of bathing medium of axes. Profiles of NADPH oxidase and superoxide dismutase (SOD) were studied by in-gel assay. Germination was retarded by treatments affecting ROS level except H2O2 scavengers, while axis growth was retarded by all. Superoxide synthesis inhibitor and scavenger prevented H2O2 accumulation in axes in later phase as revealed from TMB staining. Activity of Cu/Zn SOD1 was initially high and declined thereafter. Superoxide being produced in apoplast possibly by NADPH oxidase activity is further metabolized to (•)OH via H2O2. Germination process depends possibly on (•)OH production in the axes. Post-germinative axis growth requires O2(•)- while the differentiating zone of axis (radicle) requires H2O2 for cell wall stiffening.

Willow seedlings from photooxidized seeds accelerate cotyledon death and anticipate first leaf emergence: a histological and biochemical study following germination

In willow seeds, photooxidative damage is mainly restricted to the outer cotyledonary tissues, significantly reducing normal germination. Here we analyzed the damage generated in cotyledonary tissues and investigated whether the increase in reactive oxygen species (ROS) generation in seedlings from photooxidized seeds can affect the morphogenetic capacity of the shoot apical meristem. Seeds were photooxidized under different light intensities and the evolution of the damage during seedling growth was studied by light and transmission electron microscopies. The level of lipid peroxidation and changes in antioxidant capacity were measured following the time course of superoxide dismutase, catalase, ascorbate peroxidase and guaiacol peroxidase enzyme activities, and the effect of photooxidative stress on the genesis of new leaf primordia and lateral roots was examined. Early and active endocytosis and autophagy, changes in chloroplast morphology, as well as the accumulation and diffusion of ROS all play important roles in the early cell death observed in cotyledonary tissues. Following germination, seedlings from photooxidized seeds anticipated the emergence of first leaves, which complemented the altered functionality of the damaged cotyledons.

Post-embryonic root organogenesis in cereals: branching out from model plants

The root architecture of higher plants is amazingly diverse. In this review, we compare the lateral root developmental programme in cereals and Arabidopsis thaliana. In cereals, cells in the endodermis are recruited to form the new root cap and overlying cortical cells divide to facilitate the emergence of the lateral root primordium. The TIR1/ABF2 auxin receptors and the AUX/IAA, ARF, and LBD transcriptional regulatory proteins are conserved in cereals and Arabidopsis. Several elements of this regulatory network are common to lateral and crown roots in cereals. Also, the ground meristem from which crown roots differentiate shows similarities with the root pericycle. Studies in cereals promise to give complementary insights into the mechanisms regulating the development of post-embryonic roots in plants.

Oxidative post translational modifications of proteins related to cell cycle are involved in cadmium toxicity in wheat seedlings

Abiotic stress is greatly associated with plant growth inhibition and redox cell imbalance. In the present work, we have investigated in which way oxidative posttranslational modifications (PTM) of proteins related to cell cycle may be implicated in post-germinative root growth reduction caused by cadmium, by methyl viologen (MV) and by hydrogen peroxide (H₂O₂) in wheat seedlings. Although cadmium is considered a redox inactive metal, reactive oxygen species were detected in the apex root of metal-treated seedlings. Oxidative stress hastened cells displacement from the cell division zone to elongation/differentiation zone, resulting in a shortened meristem. The number of cells in the proliferation zone was lower after MV, H₂O₂ and 10 μM Cd²⁺ treatments compared to control. All treatments increased protein carbonylation. Although no modification in total Ub-conjugated proteins was detected, oxidative treatments reduced cyclin D and CDKA protein ubiquitination, concomitantly with a decrease in expression of cyclin D/CDKA/Rb/E2F-regulated genes. We postulate that ROS and oxidative PTM could be part of a general mechanism, specifically affecting G1/S transition and progression through S phase. This would rapidly block cell cycle progression and would allow the cellular defence system to be activated.