Phytoglobin overexpression promotes barley growth in the presence of enhanced level of atmospheric nitric oxide

Photocatalysis of nanoparticles mediates the response of plants towards nitric oxide in air

Nitric oxide (NO), a signaling molecule involved in plant growth and metabolism, can be synthesized endogenously or assimilated from the atmosphere. Although NO can be oxidized to nitrate or NO2 by phototactically induced hot charge carriers over nanoparticles (NPs) under insolation, the specific role of NP-mediated NO transformation on foliage surfaces in stimulating plant enzymes remains unclear. In this research, Trident (T) and Willow (W) types of water spinach (Ipomoea aquatica Forsk) were employed to probe the physiological alteration by NO transformation and oxidants over photocatalytic NPs. The biomass of T was significantly enhanced by the 240-ppb NO stress or NPs, while W was immune to either condition. According to in-situ diffuse-reflectance-infrared-Fourier-transform spectroscopy, foliar ZnO or TiO2 (2 mg per plant) stimulated the oxidation of NO to NO3- and the production of •OH. NPs enhanced the activity of antioxidant enzymes and NR (nitrate reductase [NAD(P)H]) in T with low SOD (superoxide dismutase, 32 ± 7 Umin-1g-1), NR activity (reaching 188 ± 4 nmol h-1g-1) was positively correlated with a 46.39 % increase in biomass. Conversely, W, endowed with ample SOD (502 ± 30 Umin-1g-1) to offset the stress caused by NO or NPs, displayed negligible growth or NR alterations. Our findings indicate that a high [SOD] could counterbalance the oxidizing NO stress, while the photocatalytic NO conversion into nitrate could boost the NR production in plant with low [SOD] and under NO stress. This research contributes to understanding the impact of NPs application and plant responses to pollutant gas stress.

Inhibition of flavohemeproteins enhances the emission and level of nitric oxide in barley root tips

In this study, using a pharmaceutical approach, we analyzed the NO accumulation and emission from the root tips of barley seedlings and the possible mechanisms of NO catabolism. Application of flavohemeprotein inhibitors, such as azide, cyanide, diphenyleneiodonium and dicumarol, an inhibitor of the plasma membrane electron transport chain, increased the NO level in root tissue and stimulated the NO emission from root tip cells. It can be concluded that barley root tips generate and, at the same time, consume a considerable amount of NO, probably by the plasma membrane flavohemeproteins. This high NO-consuming activity of barley root tips efficiently degraded even the externally applied high concentrations of NO without marked root growth inhibition. These results suggest that the root tip cells NO consumption activity plays an important role in the regulation of NO level in barley root tips.

Nitric oxide-mediated regulation of macronutrients in plants

In plant physiology, nitric oxide (NO) is a widely used signaling molecule. It is a free radical and an important component of the N-cycle. NO is produced endogenously inside plant cells, where it participates in multiple functions and provides protection against several abiotic and biotic stresses. NO and its interplay with macronutrients had remarkable effects on plant growth and development, the signaling pathway, and defense mechanisms. Its chemical properties, synthetic pathways, physiological effects, antioxidant action, signal transduction, and regulation of transporter genes and proteins have been studied. NO emerges as a key regulator under macronutrient deficiency. In plants, NO also affects reactive oxygen species (ROS), reactive nitrogen species (RNS), and post-translational modifications (PTMs). The function of NO and its significant control in the functions and adjustments of macronutrients under macronutrient deficit were summed up in this review. NO regulate functions of macronutrients and associated signaling events involved with macronutrient transporters in different plants.

Interorgan, intraorgan and interplant communication mediated by nitric oxide and related species

Plant survival to a potential plethora of diverse environmental insults is underpinned by coordinated communication amongst organs to help shape effective responses to these environmental challenges at the whole plant level. This interorgan communication is supported by a complex signal network that regulates growth, development and environmental responses. Nitric oxide (NO) has emerged as a key signalling molecule in plants. However, its potential role in interorgan communication has only recently started to come into view. Direct and indirect evidence has emerged supporting that NO and related species (S-nitrosoglutathione, nitro-linolenic acid) are mobile interorgan signals transmitting responses to stresses such as hypoxia and heat. Beyond their role as mobile signals, NO and related species are involved in mediating xylem development, thus contributing to efficient root-shoot communication. Moreover, NO and related species are regulators in intraorgan systemic defence responses aiming an effective, coordinated defence against pathogens. Beyond its in planta signalling role, NO and related species may act as ex planta signals coordinating external leaf-to-leaf, root-to-leaf but also plant-to-plant communication. Here, we discuss these exciting developments and emphasise how their manipulation may provide novel strategies for crop improvement.

Interplay between nitric oxide and inorganic nitrogen sources in root development and abiotic stress responses

Nitrogen (N) is an essential nutrient for plants, and the sources from which it is obtained can differently affect their entire development as well as stress responses. Distinct inorganic N sources (nitrate and ammonium) can lead to fluctuations in the nitric oxide (NO) levels and thus interfere with nitric oxide (NO)-mediated responses. These could lead to changes in reactive oxygen species (ROS) homeostasis, hormone synthesis and signaling, and post-translational modifications of key proteins. As the consensus suggests that NO is primarily synthesized in the reductive pathways involving nitrate and nitrite reduction, it is expected that plants grown in a nitrate-enriched environment will produce more NO than those exposed to ammonium. Although the interplay between NO and different N sources in plants has been investigated, there are still many unanswered questions that require further elucidation. By building on previous knowledge regarding NO and N nutrition, this review expands the field by examining in more detail how NO responses are influenced by different N sources, focusing mainly on root development and abiotic stress responses.

Integrated physiological, biochemical, and transcriptomics analyses reveal the underlying mechanisms of high nitrogen use efficiency of black sesame

Cultivating high nitrogen use efficient varieties is a sustainable solution to mitigating adverse effects on the environment caused by excessive nitrogen fertilizer application. However, in sesame, although immoderate nitrogen fertilizers are used to promote yield, the molecular basis of high nitrogen use efficiency (NUE) is largely unknown. Hence, this study aimed to identify high NUE black sesame variety and dissect the underlying physiological and molecular mechanisms. To achieve this, seventeen seedling traits of 30 black sesame varieties were evaluated under low nitrogen (LN) and high nitrogen (HN) conditions. Dry matter accumulation, root parameters, shoot nitrogen accumulation, and chlorophyll content are important factors for evaluating the NUE of sesame genotypes. The variety 17-156 was identified as the most efficient for N utilization. Comparative physiological and transcriptomics analyses revealed that 17-156 possesses a sophisticated nitrogen metabolizing machinery to uptake and assimilate higher quantities of inorganic nitrogen into amino acids and proteins, and simultaneously improving carbon metabolism and growth. Specifically, the total nitrogen and soluble protein contents significantly increased with the increase in nitrogen concentrations. Many important genes, including nitrate transporters (NPFs), amino acid metabolism-related (GS, GOGAT, GDH, etc.), phytohormone-related, and transcription factors, were significantly up-regulated in 17-156 under HN condition. In addition, 38 potential candidate genes were identified for future studies toward improving sesame's NUE. These findings offer valuable resources for deciphering the regulatory network of nitrogen metabolism and developing sesame cultivars with improved NUE.

Nitric Oxide and Globin Glb1 Regulate Fusarium oxysporum Infection of Arabidopsis thaliana

Plants continuously interact with fungi, some of which, such as Fusarium oxysporum, are lethal, leading to reduced crop yields. Recently, nitric oxide (NO) has been found to play a regulatory role in plant responses to F. oxysporum, although the underlying mechanisms involved are poorly understood. In this study, we show that Arabidopsis mutants with altered levels of phytoglobin 1 (Glb1) have a higher survival rate than wild type (WT) after infection with F. oxysporum, although all the genotypes analyzed exhibited a similar fungal burden. None of the defense responses that were analyzed in Glb1 lines, such as phenols, iron metabolism, peroxidase activity, or reactive oxygen species (ROS) production, appear to explain their higher survival rates. However, the early induction of the PR genes may be one of the reasons for the observed survival rate of Glb1 lines infected with F. oxysporum. Furthermore, while PR1 expression was induced in Glb1 lines very early on the response to F. oxysporum, this induction was not observed in WT plants.

Crops' response to the emergent air pollutants

Consequences of air pollutants on physiology, biology, yield and quality in the crops are evident. Crop and soil management can play significant roles in attenuating the impacts of air pollutants. With rapid urbanization and industrialization, air pollution has emerged as a serious threat to quality crop production. Assessing the effect of the elevated level of pollutants on the performance of the crops is crucial. Compared to the soil and water pollutants, the air pollutants spread more rapidly to the extensive area. This paper has reviewed and highlighted the major findings of the previous research works on the morphological, physiological and biochemical changes in some important crops and fruits exposed to the increasing levels of air pollutants. The crop, soil and environmental factors governing the effect of air pollutants have been discussed. The majority of the observations suggest that the air pollutants alter the physiology and biochemical in the plants, i.e., while some pollutants are beneficial to the growth and yields and modify physiological and morphological processes, most of them appeared to be detrimental to the crop yields and their quality. A better understanding of the mechanisms of the uptake of air pollutants and crop responses is quite important for devising the measures ‒ at both policy and program levels ‒ to minimize their possible negative impacts on crops. Further research directions in this field have also been presented.

Belowground plant-microbe communications via volatile compounds

Volatile compounds play important roles in rhizosphere biological communications and interactions. The emission of plant and microbial volatiles is a dynamic phenomenon that is affected by several endogenous and exogenous signals. Diffusion of volatiles can be limited by their adsorption, degradation, and dissolution under specific environmental conditions. Therefore, rhizosphere volatiles need to be investigated on a micro and spatiotemporal scale. Plant and microbial volatiles can expand and specialize the rhizobacterial niche not only by improving the root system architecture such that it serves as a nutrient-rich shelter, but also by inhibiting or promoting the growth, chemotaxis, survival, and robustness of neighboring organisms. Root volatiles play an important role in engineering the belowground microbiome by shaping the microbial community structure and recruiting beneficial microbes. Microbial volatiles are appropriate candidates for improving plant growth and health during environmental challenges and climate change. However, some technical and experimental challenges limit the non-destructive monitoring of volatile emissions in the rhizosphere in real-time. In this review, we attempt to clarify the volatile-mediated intra- and inter-kingdom communications in the rhizosphere, and propose improvements in experimental design for future research.

Cyanobacterial NOS expression improves nitrogen use efficiency, nitrogen-deficiency tolerance and yield in Arabidopsis

Developing strategies to improve nitrogen (N) use efficiency (NUE) in plants is a challenge to reduce environmental problems linked to over-fertilization. The nitric oxide synthase (NOS) enzyme from the cyanobacteria Synechococcus PCC 7335 (SyNOS) has been recently identified and characterized. SyNOS catalyzes the conversion of arginine to citrulline and nitric oxide (NO), and then approximately 75 % of the produced NO is rapidly oxidized to nitrate by an unusual globin domain in the N-terminus of the enzyme. In this study, we assessed whether SyNOS expression in plants affects N metabolism, NUE and yield. Our results showed that SyNOS-expressing transgenic Arabidopsis plants have greater primary shoot length and shoot branching when grown under N-deficient conditions and higher seed production both under N-sufficient and N-deficient conditions. Moreover, transgenic plants showed significantly increased NUE in both N conditions. Although the uptake of N was not modified in the SyNOS lines, they showed an increase in the assimilation/remobilization of N under conditions of low N availability. In addition, SyNOS lines have greater N-deficiency tolerance compared to control plants. Our results support that SyNOS expression generates a positive effect on N metabolism and seed production in Arabidopsis, and it might be envisaged as a strategy to improve productivity in crops under adverse N environments.

Improving Air Quality by Nitric Oxide Consumption of Climate-Resilient Trees Suitable for Urban Greening

Nitrogen oxides (NO_x), mainly a mixture of nitric oxide (NO) and nitrogen dioxide (NO₂), are formed by the reaction of nitrogen and oxygen compounds in the air as a result of combustion processes and traffic. Both deposit into leaves via stomata, which on the one hand benefits air quality and on the other hand provides an additional source of nitrogen for plants. In this study, we first determined the NO and NO₂ specific deposition velocities based on projected leaf area (sV _d) using a branch enclosure system. We studied four tree species that are regarded as suitable to be planted under predicted future urban climate conditions: Carpinus betulus, Fraxinus ornus, Fraxinus pennsylvanica and Ostrya carpinifolia. The NO and NO₂ sV_d were found similar in all tree species. Second, in order to confirm NO metabolization, we fumigated plants with ¹⁵NO and quantified the incorporation of ¹⁵N in leaf materials of these trees and four additional urban tree species (Celtis australis, Alnus spaethii, Alnus glutinosa, and Tilia henryana) under controlled environmental conditions. Based on these ¹⁵N-labeling experiments, A. glutinosa showed the most effective incorporation of ¹⁵NO. Third, we tried to elucidate the mechanism of metabolization. Therefore, we generated transgenic poplars overexpressing Arabidopsis thaliana phytoglobin 1 or 2. Phytoglobins are known to metabolize NO to nitrate in the presence of oxygen. The ¹⁵N uptake in phytoglobin-overexpressing poplars was significantly increased compared to wild-type trees, demonstrating that the NO uptake is enzymatically controlled besides stomatal dependence. In order to upscale the results and to investigate if a trade-off exists between air pollution removal and survival probability under future climate conditions, we have additionally carried out a modeling exercise of NO and NO₂ deposition for the area of central Berlin. If the actually dominant deciduous tree species (Acer platanoides, Tilia cordata, Fagus sylvatica, Quercus robur) would be replaced by the species suggested for future conditions, the total annual NO and NO₂ deposition in the modeled urban area would hardly change, indicating that the service of air pollution removal would not be degraded. These results may help selecting urban tree species in future greening programs.

Plant hemoglobins: a journey from unicellular green algae to vascular plants

Globins (Glbs) are widely distributed in archaea, bacteria and eukaryotes. They can be classified into proteins with 2/2 or 3/3 α-helical folding around the heme cavity. Both types of Glbs occur in green algae, bryophytes and vascular plants. The Glbs of angiosperms have been more intensively studied, and several protein structures have been solved. They can be hexacoordinate or pentacoordinate, depending on whether a histidine is coordinating or not at the sixth position of the iron atom. The 3/3 Glbs of class 1 and the 2/2 Glbs (also called class 3 in plants) are present in all angiosperms, whereas the 3/3 Glbs of class 2 have been only found in early angiosperms and eudicots. The three Glb classes are expected to play different roles. Class 1 Glbs are involved in hypoxia responses and modulate NO concentration, which may explain their roles in plant morphogenesis, hormone signaling, cell fate determination, nutrient deficiency, nitrogen metabolism and plant-microorganism symbioses. Symbiotic Glbs derive from class 1 or class 2 Glbs and transport O₂ in nodules. The physiological roles of class 2 and class 3 Glbs are poorly defined but could involve O₂ and NO transport and/or metabolism, respectively. More research is warranted on these intriguing proteins to determine their non-redundant functions.

Nitrogen Depletion Blocks Growth Stimulation Driven by the Expression of Nitric Oxide Synthase in Tobacco

Nitric oxide (NO) is a messenger molecule widespread studied in plant physiology. Latter evidence supports the lack of a NO-producing system involving a NO synthase (NOS) activity in higher plants. However, a NOS gene from the unicellular marine alga Ostreococcus tauri (OtNOS) was characterized in recent years. OtNOS is a genuine NOS, with similar spectroscopic fingerprints to mammalian NOSs and high NO producing capacity. We are interested in investigating whether OtNOS activity alters nitrogen metabolism and nitrogen availability, thus improving growth promotion conditions in tobacco. Tobacco plants were transformed with OtNOS under the constitutive CaMV 35S promoter. Transgenic tobacco plants expressing OtNOS accumulated higher NO levels compared to siblings transformed with the empty vector, and displayed accelerated growth in different media containing sufficient nitrogen availability. Under conditions of nitrogen scarcity, the growth promoting effect of the OtNOS expression is diluted in terms of total leaf area, protein content and seed production. It is proposed that OtNOS might possess a plant growth promoting effect through facilitating N remobilization and nitrate assimilation with potential to improve crop plants performance.