Nitric oxide generated by nitrate reductase increases nitrogen uptake capacity by inducing lateral root formation and inorganic nitrogen uptake under partial nitrate nutrition in rice

The hidden harmony: Exploring ROS-phytohormone nexus for shaping plant root architecture in response to environmental cues

Root system architecture, encompassing lateral roots and root hairs, plays a vital in overall plant growth and stress tolerance. Reactive oxygen species (ROS) and plant hormones intricately regulate root growth and development, serving as signaling molecules that govern processes such as cell proliferation and differentiation. Manipulating the interplay between ROS and hormones has the potential to enhance nutrient absorption, stress tolerance, and agricultural productivity. In this review, we delve into how studying these processes provides insights into how plants respond to environmental changes and optimize growth patterns to better control cellular processes and stress responses in crops. We discuss various factors and complex signaling networks that may exist among ROS and phytohormones during root development. Additionally, the review highlights possible role of reactive nitrogen species (RNS) in ROS-phytohormone interactions and in shaping root system architecture according to environmental cues.

Molecular hydrogen positively regulates nitrate uptake and seed size by targeting nitrate reductase

Although the sources of molecular hydrogen (H2) synthesis in plants remain to be fully elucidated, ample evidence shows that plant-based H2 can regulate development and stress responses. Here, we present genetic and molecular evidence indicating that nitrate reductase (NR) might be a target of H2 sensing that positively regulates nitrogen use efficiency (NUE) and seed size in Arabidopsis (Arabidopsis thaliana). The expression level of NR and changes of NUE under control and, in particular, low nitrogen supply were positively associated with H2 addition supplied exogenously or through genetic manipulation. The improvement in nitrate assimilation achieved by H2 was also mediated via NR dephosphorylation. H2 control of seed size was impaired by NR mutation. Further genetic evidence revealed that H2, NR, and nitric oxide can synergistically regulate nitrate assimilation in response to N starvation conditions. Collectively, our data indicate that NR might be a target for H2 sensing, ultimately positively regulating nitrate uptake and seed size. These results provide insights into H2 signaling and its functions in plant metabolism.

Early differentiation of Casparian strip mediated by nitric oxide is required for efficient K transport under low K conditions in Arabidopsis

The Casparian strip (CS) is a cell wall modification made of lignin that functions as an apoplastic barrier in the root endodermis to restrict nutrient and water transport between the soil and stele. CS formation is affected by nutritional conditions, and its physiological roles have been discussed. This study found that low K condition affects CS permeability, lignin deposition, and MYB36 mRNA accumulation. To understand the mechanism underlying these findings, we focused on nitric oxide (NO). NO is known to act as a signaling molecule and participates in cell wall synthesis, especially for lignin composition. However, the mechanism by which NO affects lignin deposition and corrects CS formation in the plant roots remains unclear. Through combining fluorescent observation with histological stains, we demonstrated that the root endodermal cell lignification response to low-potassium (K) conditions is mediated by NO through the MYB36-associated lignin-polymerizing pathway. Furthermore, we discovered the noteworthy ability of NO to maintain nutrient homeostasis for adaptation to low K conditions by affecting the correct apoplastic barrier formation of CS. Collectively, our results suggest that NO is required for the lignification and apoplastic barrier formation in the root endodermis during adaptation to low K conditions, which revealing the novel physiological roles of CS under low nutrient conditions and making a significant contribution to CS biology.

OsMADS5 interacts with OsSPL14/17 to inhibit rice root elongation by restricting cell proliferation of root meristem under ammonium supply

Nitrogen (N) is a vital major nutrient for rice (Oryza sativa). Rice responds to different applications of N by altering its root morphology, including root elongation. Although ammonium (NH 4 + ) is the primary source of N for rice,NH 4 + is toxic to rice roots and inhibits root elongation. However, the precise molecular mechanism thatNH 4 + -inhibited root elongation of rice is not well understood. Here, we identified a rice T-DNA insert mutant of OsMADS5 with a longer seminal root (SR) under sufficient N conditions. Reverse-transcription quantitative PCR analysis revealed that the expression level of OsMADS5 was increased underNH 4 + compared withNO 3 - supply. UnderNH 4 + conditions, knocking out OsMADS5 (cas9) produced a longer SR, phenocopying osmads5, while there was no significant difference in SR length between wild-type and cas9 underNO 3 - supply. Moreover, OsMADS5-overexpression plants displayed the opposite SR phenotype. Further study demonstrated that enhancement of OsMADS5 byNH 4 + supply inhibited rice SR elongation, likely by reducing root meristem activity of root tip, with the involvement of OsCYCB1;1. We also found that OsMADS5 interacted with OsSPL14 and OsSPL17 (OsSPL14/17) to repress their transcriptional activation by attenuating DNA binding ability. Moreover, loss of OsSPL14/17 function in osmads5 eliminated its stimulative effect on SR elongation underNH 4 + conditions, implying OsSPL14/17 may function downstream of OsMADS5 to mediate rice SR elongation underNH 4 + supply. Overall, our results indicate the existence of a novel modulatory pathway in which enhancement of OsMADS5 byNH 4 + supply represses the transcriptional activities of OsSPL14/17 to restrict SR elongation of rice.

Convergently selected NPF2.12 coordinates root growth and nitrogen use efficiency in wheat and barley

Understanding the genetic and molecular function of nitrate sensing and acquisition across crop species will accelerate breeding of cultivars with improved nitrogen use efficiency (NUE). Here, we performed a genome-wide scan using wheat and barley accessions characterized under low and high N inputs that uncovered the NPF2.12 gene, encoding a homolog of the Arabidopsis nitrate transceptor NRT1.6 and other low-affinity nitrate transporters that belong to the MAJOR FACILITATOR SUPERFAMILY. Next, it is shown that variations in the NPF2.12 promoter correlated with altered NPF2.12 transcript levels where decreased gene expression was measured under low nitrate availability. Multiple field trials revealed a significantly enhanced N content in leaves and grains and NUE in the presence of the elite allele TaNPF2.12^TT grown under low N conditions. Furthermore, the nitrate reductase encoding gene NIA1 was up-regulated in npf2.12 mutant upon low nitrate concentrations, thereby resulting in elevated levels of nitric oxide (NO) production. This increase in NO correlated with the higher root growth, nitrate uptake, and N translocation observed in the mutant when compared to wild-type. The presented data indicate that the elite haplotype alleles of NPF2.12 are convergently selected in wheat and barley that by inactivation indirectly contribute to root growth and NUE by activating NO signaling under low nitrate conditions.

Nitrate supply decreases fermentation and alleviates oxidative and ionic stress in nitrogen-fixing soybean exposed to saline waterlogging

Nitrate (NO3 - ) nutrition is known to mitigate the damages caused by individual stresses of waterlogging and salinity. Here, we investigated the role of NO3 - in soybean plants exposed to these stresses in combination. Nodulated soybean cultivated under greenhouse conditions and daily fertilised with a nutrient solution without nitrogen were subjected to the following treatments: Water, NO3 - , NaCl, and NaCl+NO3 - . Then, plants were exposed to waterlogging (6days) and drainage (2days). Compared to plants exposed to isolated stress, the saline waterlogging resulted in higher concentrations of H2 O2 , O2 ˙- , and lipid peroxidation at the whole-plant level, mainly during drainage. Furthermore, saline waterlogging increased fermentation and the concentrations of Na+ and K+ in roots and leaves both during waterlogging and drainage. NO3 - supplementation led to augments in NO3 - and NO levels, and stimulated nitrate reductase activity in both organs. In addition, NO3 - nutrition alleviated oxidative stress and fermentation besides increasing the K+ /Na+ ratio in plants exposed to saline waterlogging. In conclusion, NO3 - supplementation is a useful strategy to help soybean plants overcome saline waterlogging stress. These findings are of high relevance for agriculture as soybean is an important commodity and has been cultivated in areas prone to saline waterlogging.

Magnesium deficiency stress in rice can be alleviated by partial nitrate nutrition supply

The problem of nitrogen (N) excess and magnesium (Mg) deficiency in farmland is becoming more common, severe, and widespread in southern China. Magnesium is known to be an essential nutrient for higher plants; however, the physiological responses of field crops to Mg deficiency, particularly to its interaction with N forms, remain largely unknown. In this study, a hydroponic experiment was conducted using three Mg levels (0.01, 1.00, and 5.00 mM) and three nitrate/ammonium ratios (NO₃^-/NH₄⁺ of 0/100, 25/75, and 50/50) under greenhouse conditions. The results show that Mg deficiency (0.01 mM) could result in yellow leaves, dwarf plants, and fewer tillers during rice growth. Furthermore, Mg deficiency induced a major reduction in root morphology and activity, photosynthetic properties, and nutrient accumulation, while it resulted in a clear increase in malondialdehyde, superoxide dismutase, peroxidase, and catalase activities in rice. However, under Mg-deficiency stress, the supply of partial NO₃^- led to a significant drop in these antioxidant enzyme activities. Moreover, partial NO₃^- supply significantly improved the net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO₂ concentrations under Mg-deficiency conditions. In particular, the supply of partial NO₃^- dramatically promoted the growth of the root system, boosted the occurrence of lateral roots, and enhanced root vitality under Mg-deficiency stress. Additionally, the supply of partial NO₃^- led to significant increases in dry weight and N and Mg contents under Mg deficiency. The results of this study suggest that the symptoms of Mg-deficiency stress in rice can be alleviated by partial NO₃^- supply.

New Paradigms in Brassinosteroids, Strigolactones, Sphingolipids, and Nitric Oxide Interaction in the Control of Lateral and Adventitious Root Formation

The root system is formed by the primary root (PR), which forms lateral roots (LRs) and, in some cases, adventitious roots (ARs), which in turn may produce their own LRs. The formation of ARs is also essential for vegetative propagation in planta and in vitro and for breeding programs. Root formation and branching is coordinated by a complex developmental network, which maximizes the plant's ability to cope with abiotic stress. Rooting is also a response caused in a cutting by wounding and disconnection from the donor plant. Brassinosteroids (BRs) are steroid molecules perceived at the cell surface. They act as plant-growth-regulators (PGRs) and modulate plant development to provide stress tolerance. BRs and auxins control the formation of LRs and ARs. The auxin/BR interaction involves other PGRs and compounds, such as nitric oxide (NO), strigolactones (SLs), and sphingolipids (SPLs). The roles of these interactions in root formation and plasticity are still to be discovered. SLs are carotenoid derived PGRs. SLs enhance/reduce LR/AR formation depending on species and culture conditions. These PGRs possibly crosstalk with BRs. SPLs form domains with sterols within cellular membranes. Both SLs and SPLs participate in plant development and stress responses. SPLs are determinant for auxin cell-trafficking, which is essential for the formation of LRs/ARs in planta and in in vitro systems. Although little is known about the transport, trafficking, and signaling of SPLs, they seem to interact with BRs and SLs in regulating root-system growth. Here, we review the literature on BRs as modulators of LR and AR formation, as well as their crosstalk with SLs and SPLs through NO signaling. Knowledge on the control of rooting by these non-classical PGRs can help in improving crop productivity and enhancing AR-response from cuttings.

Nitrate–Nitrite–Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants

Oxygen (O₂) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O₂ deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO₃⁻), nitrite (NO₂⁻), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N₂O) when supplied with NO₃⁻ and NO₂⁻, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N₂O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants’ survival during O₂-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N₂O budget.

Integrated physiological, proteome and gene expression analyses provide new insights into nitrogen remobilization in citrus trees

Nitrogen (N) remobilization is an important physiological process that supports the growth and development of trees. However, in evergreen broad-leaved tree species, such as citrus, the mechanisms of N remobilization are not completely understood. Therefore, we quantified the potential of N remobilization from senescing leaves of spring shoots to mature leaves of autumn shoots of citrus trees under different soil N availabilities and further explored the underlying N metabolism characteristics by physiological, proteome and gene expression analyses. Citrus exposed to low N had an approximately 38% N remobilization efficiency (NRE), whereas citrus exposed to high N had an NRE efficiency of only 4.8%. Integrated physiological, proteomic and gene expression analyses showed that photosynthesis, N and carbohydrate metabolism interact with N remobilization. The improvement of N metabolism and photosynthesis, the accumulation of proline and arginine, and delayed degradation of storage protein in senescing leaves are the result of sufficient N supply and low N remobilization. Proteome further showed that energy generation proteins and glutamate synthase were hub proteins affecting N remobilization. In addition, N requirement of mature leaves is likely met by soil supply at high N nutrition, thereby resulting in low N remobilization. These results provide insight into N remobilization mechanisms of citrus that are of significance for N fertilizer management in orchards.

PHB3 regulates lateral root primordia formation via NO-mediated degradation of AUXIN/INDOLE-3-ACETIC ACID proteins

We have previously shown that Arabidopsis thaliana Prohibitin 3 (PHB3) controls auxin-stimulated lateral root (LR) formation; however, the underlying molecular mechanism is unknown. Here, we demonstrate that PHB3 regulates lateral root (LR) development mainly through influencing lateral root primordia (LRP) initiation, via affecting nitric oxide (NO) accumulation. The reduced LRP in phb3 mutant was largely rescued by treatment with a NO donor. The decreased NO accumulation in phb3 caused a lower expression of GATA TRANSCRIPTION FACTOR 23 (GATA23) and LATERAL ORGAN BOUNDARIES DOMAIN 16 (LBD16) through inhibiting the degradation of INDOLE-3-ACETIC ACID INDUCIBLE 14/28 (IAA14/28). Overexpression of either GATA23 or LBD16 in phb3 mutant background recovered the reduced density of LRP. These results indicate that PHB3 regulates LRP initiation via NO-mediated auxin signalling, by modulating the degradation of IAA14/28.

Nitrate Uptake and Use Efficiency: Pros and Cons of Chloride Interference in the Vegetable Crops

Over the past five decades, nitrogen (N) fertilization has been an essential tool for boosting crop productivity in agricultural systems. To avoid N pollution while preserving the crop yields and profit margins for farmers, the scientific community is searching for eco-sustainable strategies aimed at increasing plants' nitrogen use efficiency (NUE). The present article provides a refined definition of the NUE based on the two important physiological factors (N-uptake and N-utilization efficiency). The diverse molecular and physiological mechanisms underlying the processes of N assimilation, translocation, transport, accumulation, and reallocation are revisited and critically discussed. The review concludes by examining the N uptake and NUE in tandem with chloride stress and eustress, the latter being a new approach toward enhancing productivity and functional quality of the horticultural crops, particularly facilitated by soilless cultivation.

Nitrate-dependent regulation of miR444-OsMADS27 signalling cascade controls root development in rice

Nitrate is an important nutrient and a key signalling molecule for plant development. A number of transcription factors involved in the response to nitrate and their regulatory mechanisms have been identified. However, little is known about the transcription factors involved in nitrate sensing and their regulatory mechanisms among crop plants. In this study, we identified functions of a nitrate-responsive miR444:MADS-box transcription factor OsMADS27 module and its downstream targets mediating rice root growth and stress responses. Transgenic rice plants expressing miR444 target mimic improved rice root growth. Although miR444 has the potential to target multiple genes, we identified OsMADS27 as the major miR444 target that regulates the expression of nitrate transporters, as well as several key genes including expansins, and those associated with auxin signalling, to promote root growth. In agreement with this, overexpression of miRNA-resistant OsMADS27 improved root development and tolerance to abiotic stresses, while its silencing suppressed root growth. OsMADS27 mediated robust stress tolerance in plants through its ability to bind to the promoters of specific stress regulators, as observed in ChIP-seq analysis. Our results provide evidence of a nitrate-dependent miR444-OsMADS27 signalling cascade involved in the regulation of rice root growth, as well as its surprising role in stress responses.

Dehydration-Responsive Element Binding Protein 1C, 1E, and 1G Promote Stress Tolerance to Chilling, Heat, Drought, and Salt in Rice

The dehydration-responsive element binding protein 1 (DREB1)/C-repeat-binding factor (CBF) genes are key regulators of cold acclimation and freezing tolerance in the chilling tolerant Arabidopsis thaliana. Here, we investigated the function of three members of the 10 rice DREB1 genes, OsDREB1C, E, and G, in the chilling sensitive rice plants. Their loss of function (LOF) mutants were each more chilling susceptible compared to the wild type, and the LOF mutants of all three genes, dreb1ceg, were more chilling susceptible than any of the single mutants. Strikingly, these mutants were capable of cold acclimation, indicating that these rice DREB1 genes are important for basal chilling tolerance but not cold acclimation. Transcriptome and physiology analyses suggest that the OsDREB1C/E/G genes are involved in reactive oxygen species (ROS) scavenging and cell death regulation under chilling. Furthermore, these three rice DREB1 genes are found to promote tolerance to other abiotic stresses: the OsDREB1C/E/G genes are positive regulators of heat tolerance, OsDREB1C and OsDREB1G are positive regulators of salt tolerance, and OsDREB1G is a positive regulator of drought tolerance. These findings expand our knowledge of the roles of DREB1 proteins in plants, enhance our mechanistic understanding of abiotic stress tolerance and will facilitate the generation of stress-tolerant crop plants.

COE2 Is Required for the Root Foraging Response to Nitrogen Limitation

There are numerous exchanges of signals and materials between leaves and roots, including nitrogen, which is one of the essential nutrients for plant growth and development. In this study we identified and characterized the Chlorophyll A/B-Binding Protein (CAB) (named coe2 for CAB overexpression 2) mutant, which is defective in the development of chloroplasts and roots under normal growth conditions. The phenotype of coe2 is caused by a mutation in the Nitric Oxide Associated (NOA1) gene that is implicated in a wide range of chloroplast functions including the regulation of metabolism and signaling of nitric oxide (NO). A transcriptome analysis reveals that expression of genes involved in metabolism and lateral root development are strongly altered in coe2 seedlings compared with WT. COE2 is expressed in hypocotyls, roots, root hairs, and root caps. Both the accumulation of NO and the growth of lateral roots are enhanced in WT but not in coe2 under nitrogen limitation. These new findings suggest that COE2-dependent signaling not only coordinates gene expression but also promotes chloroplast development and function by modulating root development and absorption of nitrogen compounds.

Higher nitrogen content and auxin export from rice tiller enhance low-ammonium-dependent tiller outgrowth

In the early growth stage, nutrient uptake by rice roots is weak. However, rice tillering at this stage would require high N input. Thus, it is vital to clarify the mechanism involved in tillering capacity with low N inputs. In this report, two widely-planted japonica cultivars (cvs Yangyujing 2 and Nanjing 45) were selected mainly because, unlike cv. Nanjing 45, cv. Yangyujing 2 shows low-N-induced tiller outgrowth. Responses of tillers in two rice cultivars to mixture of N forms versus sole NH₄⁺ supply were similar, suggesting that NH₄⁺ plays a pivotal role in N-modulated rice tillering. Under low NH₄⁺ supply, higher expression of OsAMT1.2, OsAMT1.3, OsGS1;2, and OsGS2 was recorded in the roots of cv. Yangyujing 2 in comparison with cv. Nanjing 45, ultimately resulting in higher N content and dry weight in cv. Yangyujing 2. Stronger ³H-IAA export from tiller stems was observed in cv. Yangyujing 2, mainly due to higher expression level of auxin efflux transporters. Moreover, tillers in auxin efflux transporter mutant ospin9 did not respond to NH₄⁺ supply relative to wild-type plants. These findings can be used in the molecular breeding of rice varieties to simultaneously improve rice population productivity and reduce N fertilizer input.

Increased glutamine synthetase by overexpression of TaGS1 improves grain yield and nitrogen use efficiency in rice

Improving nitrogen use efficiency (NUE) has been a focal point for crop growth and yield throughout the world. Glutamine synthetase (GS), which plays a fundamental role in N metabolism, has been exploited to improve crop NUE. However, increased GS activity in rice by overexpressing its own GS genes hasn't shown superior plant productivity. Here, transgenic rice plants with increased GS activity by overexpressing TaGS1 were analyzed under field and culture conditions at two N rates. Transgenic expression of TaGS1 significantly increases GS activity in leaves, junctions and roots of rice plants relative to wide-type plants. When rice plants grown under consecutive field trials with N rates of 60 and 240 kg/ha, three transgenic lines have higher grain yield than wild-type plants, with increment of 15%-22% in T2 generation and with that of 28%-36% in T3 generation, respectively. And increased panicle numbers (effective tiller numbers) mainly contribute to the advantage of grain yield in transgenic plants. Analysis of N use-related traits shows that transgenic plants with enhanced GS activity promote root capacity to obtain N, N accumulation during growth stages and N remobilization to grains, ultimately conferring 31%-40% improvement of NUE relative to wild-type rice plants.

Nitric oxide and hydrogen sulfide: an indispensable combination for plant functioning

Nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters, which are involved in almost all plant physiological and stress-related processes. With its antioxidant regulatory properties, NO on its own ameliorates plant stress, while H₂S, a foul-smelling gas, has differential effects. Recent studies have shown that these signaling molecules are involved in intertwined pathway networks. This is due to the contrasting effects of NO and H₂S depending on cell type, subcellular compartment, and redox status, as well as the flux and dosage of NO and H₂S in different plant species and cellular contexts. Here, we provide a comprehensive review of the complex networks of these molecules, with particular emphasis on root development, stomatal movement, and plant cell death.

Characteristics and functions of glyceraldehyde 3-phosphate dehydrogenase S-nitrosylation during controlled aging of elm and Arabidopsis seeds

Seed aging is the gradual decline in seed vigor, during which programmed cell death (PCD) occurs. The functions of nitric oxide (NO) are exerted through protein S-nitrosylation, a reversible post-translational modification. During seed aging, more than 80 proteins are S-nitrosylated, but the particular role of individual proteins is unknown. Here, we showed that the S-nitrosylation level of glyceraldehyde 3-phosphate dehydrogenase (UpGAPDH) in elm (Ulmus pumila L.) seeds increased after controlled deterioration treatment. UpGAPDH was S-nitrosylated at Cys154 during S-nitrosoglutathione (GSNO) treatment, and its oligomerization was triggered both in vitro and in elm seeds. Interestingly, UpGAPDH interacted with the mitochondrial voltage-dependent anion channel in an S-nitrosylation-dependent way. Some UpGAPDH-green fluorescent protein in Arabidopsis protoplasts co-localized with mitochondria during the GSNO treatment, while the S-nitrosylation-defective UpGAPDH C154S-GFP protein did not. Seeds of oxUpGAPDH lines showed cell death and lost seed vigor rapidly during controlled deterioration treatment-triggered seed aging, while those overexpressing S-nitrosylation-defective UpGAPDH-Cys154 did not. Our results suggest that S-nitrosylation of UpGAPDH may accelerate cell death and seed deterioration during controlled deterioration treatment. These results provide new insights into the effects of UpGAPDH S-nitrosylation on protein interactions and seed aging.

Nitrate Regulates Maize Root Transcriptome through Nitric Oxide Dependent and Independent Mechanisms

Maize root responds to nitrate by modulating its development through the coordinated action of many interacting players. Nitric oxide is produced in primary root early after the nitrate provision, thus inducing root elongation. In this study, RNA sequencing was applied to discover the main molecular signatures distinguishing the response of maize root to nitrate according to their dependency on, or independency of, nitric oxide, thus discriminating the signaling pathways regulated by nitrate through nitric oxide from those regulated by nitrate itself of by further downstream factors. A set of subsequent detailed functional annotation tools (Gene Ontology enrichment, MapMan, KEGG reconstruction pathway, transcription factors detection) were used to gain further information and the lateral root density was measured both in the presence of nitrate and in the presence of nitrate plus cPTIO, a specific NO scavenger, and compared to that observed for N-depleted roots. Our results led us to identify six clusters of transcripts according to their responsiveness to nitric oxide and to their regulation by nitrate provision. In general, shared and specific features for the six clusters were identified, allowing us to determine the overall root response to nitrate according to its dependency on nitric oxide.

Expression dynamics of genes encoding nitrate and ammonium assimilation enzymes in rice genotypes exposed to reproductive stage salinity stress

Understanding the reproductive stage salinity stress tolerance is a key target for breeding stress tolerant rice genotypes. Nitrate and ammonium are equally important nitrogen forms utilized by rice. We evaluated nitrate and ammonium assimilation during reproductive stage in control and salinity (10dSm^-1 using NaCl) stressed rice plants. Osmotic stress tolerant rice genotype Shabhagidhan (SD) and high yielding yet osmotic and salinity stress sensitive genotype Pusa sugandh-5 (PS5) were evaluated. Salinity stress was given to plants during panicle emergence and flag leaves was collected after 1d, 3d 5d, 7d, 9d,12d and 15d after anthesis. Reproductive stage salinity stress resulted in decrease of membrane stability, relative water content and osmotic potential of rice plants. Reproductive stage salinity stress decreased the expression of nitrate reductase (OsNIA), nitrite reductase (OsNiR), Glutamine synthetase (OsGLN1.1, OsGLN1.2, OsGLN2) and glutamate synthase/GOGAT (OsFd-GOGAT, OsNADH-GOGAT) in flag leaves. In response to stress, SD showed better stress tolerance than PS5 in terms of higher yield stability. Variety SD showed higher leaf nitrate and ammonium content and maintained comparatively higher nitrate and ammonia metabolism enzyme activity than PS5. Salinity stress upregulated the activity of glutamate dehydrogenase enzyme and indirectly contributed to the higher proline content and maintenance of favourable osmotic potential in SD. Expression of GS2 which has role in photo respiratory ammonia assimilation was upregulated by salinity stress in PS5 in comparison to SD. Rice genotype showing better induction of nitrogen assimilatory genes will be more tolerant to reproductive stage salinity stress.

DIFFERENT NITRATE AND AMMONIUM LEVELS MEDIA ON CHANGES OF NITROGEN ASSIMILATION ENZYMES IN RICE

Nitrogen (N) is an important nutrient for the growth and development of rice plants, required in large quantity and often limiting factor of rice yields. The research was to understand the different sources and levels of nitrogen in rice plant on the activity of N assimilation enzymes, including nitrate reductase (NR), glutamine synthase (GS) content, glutamate synthase (Gogat) content, content, ammonium (NH4+) and nitrate (NO3-) content on the leaves. Paddy (Ciherang variety) was grown in sand media containing Hoagland solution with different sources (ammonium and nitrate) and levels (0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 mM) of nitrogen. Nitrogen assimilation was observed from leaves at one month of age. The NR activity increased on both Nitrogen sources, it was a higher activity in media contained nitrate. Also, the activity of GS showed higher in media contains nitrate, but its activity was decreased after application 1.6 mM of nitrate and 3.2 mM of ammonium. Western blot analysis of GS1 and GS2 showed that the band pattern of protein was similar to these enzyme activities. Nitrate content in leaves gradually increased in both sources of nitrogen and higher than 3.2 mM ammonium application caused an increase in ammonium content in leaves, but the nitrate content decreased. This research resulted that the available source of N for rice was in nitrate form, easily by the rice plants during the growth stage.

Silicon induces adventitious root formation in rice under arsenate stress with involvement of nitric oxide and indole-3-acetic acid

Arsenic (As) negatively affects plant development. This study evaluates how the application of silicon (Si) can favor the formation of adventitious roots in rice under arsenate stress (AsV) as a mechanism to mitigate its negative effects. The simultaneous application of AsV and Si up-regulated the expression of genes involved in nitric oxide (NO) metabolism, cell cycle progression, auxin (IAA, indole-3-acetic acid) biosynthesis and transport, and Si uptake which accompanied adventitious root formation. Furthermore, Si triggered the expression and activity of enzymes involved in ascorbate recycling. Treatment with L-NAME (NG-nitro L-arginine methyl ester), an inhibitor of NO generation, significantly suppressed adventitious root formation, even in the presence of Si; however, supplying NO in the growth media rescued its effects. Our data suggest that both NO and IAA are essential for Si-mediated adventitious root formation under AsV stress. Interestingly, TIBA (2,3,5-triiodobenzoic acid), a polar auxin transport inhibitor, suppressed adventitious root formation even in the presence of Si and SNP (sodium nitroprusside, an NO donor), suggesting that Si is involved in a mechanism whereby a cellular signal is triggered and that first requires NO formation, followed by IAA biosynthesis.

SPL14/17 act downstream of strigolactone signalling to modulate rice root elongation in response to nitrate supply

Nitrogen (N) is an essential major nutrient for food crops. Although ammonium (NH₄⁺ ) is the primary N source of rice (Oryza sativa), nitrate (NO₃^- ) can also be absorbed and utilized. Rice responds to NO₃^- application by altering its root morphology, such as root elongation. Strigolactones (SLs) are important modulators of root length. However, the roles of SLs and their downstream genes in NO₃^- -induced root elongation remain unclear. Here, the levels of total N and SL (4-deoxyorobanchol) and the responses of seminal root (SR) lengths to NH₄⁺ and NO₃^- were investigated in rice plants. NO₃^- promoted SR elongation, possibly due to short-term signal perception and long-term nutrient function. Compared with NH₄⁺ conditions, higher SL signalling/levels and less D53 protein were recorded in roots of NO₃^- -treated rice plants. In contrast to wild-type plants, SR lengths of d mutants were less responsive to NO₃^- conditions, and application of rac-GR24 (SL analogue) restored SR length in d10 (SL biosynthesis mutant) but not in d3, d14, and d53 (SL-responsive mutants), suggesting that higher SL signalling/levels participate in NO₃^- -induced root elongation. D53 interacted with SPL17 and inhibited SPL17-mediated transactivation from the PIN1b promoter. Mutation of SPL14/17 and PIN1b caused insensitivity of the root elongation response to NO₃^- and rac-GR24 applications. Therefore, we conclude that perception of SLs by D14 leads to degradation of D53 via the proteasome system, which releases the suppression of SPL14/17-modulated transcription of PIN1b, resulting in root elongation under NO₃^- supply.

Molecular functions of nitric oxide and its potential applications in horticultural crops

Nitric oxide (NO) regulates plant growth, enhances nutrient uptake, and activates disease and stress tolerance mechanisms in most plants, making NO a potential tool for use in improving the yield and quality of horticultural crop species. Although the use of NO in horticulture is still in its infancy, research on NO in model plant species has provided an abundance of valuable information on horticultural crop species. Emerging evidence implies that the bioactivity of NO can occur through many potential mechanisms but occurs mainly through S-nitrosation, the covalent and reversible attachment of NO to cysteine thiol. In this context, NO signaling specifically affects crop development, immunity, and environmental interactions. Moreover, NO can act as a fumigant against a wide range of postharvest diseases and pests. However, for effective use of NO in horticulture, both understanding and exploring the biological significance and potential mechanisms of NO in horticultural crop species are critical. This review provides a picture of our current understanding of how NO is synthesized and transduced in plants, and particular attention is given to the significance of NO in breaking seed dormancy, balancing root growth and development, enhancing nutrient acquisition, mediating stress responses, and guaranteeing food safety for horticultural production.

Elevated Nitrogen Priming Induced Oxinitro-Responses and Water Deficit Tolerance in Rice

Under water deficit conditions, the essential macronutrient nitrogen becomes limited as a result of reduced dissolved nitrogen and root nitrogen uptake. An elevated nitrogen level might be able to mitigate these effects, integrated with the idea of using nitric oxide as abiotic stress tolerant inducers. In this study, we evaluated the potential of using elevated nitrogen priming prior to water shortage to mitigate plant stress through nitric oxide accumulation. We grew rice plants in 300 mg L^-1 nitrogen for 10 weeks, then we primed plants with four different nitrogen concentrations: 100, 300 (control), 500 and 1000 mg L^-1 nitrogen prior to inducing water deficit conditions. Plants primed with 500 mg L^-1 nitrogen possessed a higher photosynthetic rate, relative water content, electrolyte leakage and lipid peroxidation under water deficit conditions, compared to control plants. The induction of water deficit tolerance was supported with the activation of antioxidant defense system, induced by the accumulation of nitric oxide in leaves and roots of rice plants. We originally demonstrated the accumulation of nitric oxide in leaves of rice plants. The elevated nitrogen priming can be used to enhance water deficit tolerance in irrigated paddy fields, instead of nitric oxide donors.

Signaling Responses to N Starvation: Focusing on Wheat and Filling the Putative Gaps With Findings Obtained in Other Plants. A Review

Wheat is one of the most important food crops worldwide. In recent decades, fertilizers, especially nitrogen (N), have been increasingly utilized to maximize wheat productivity. However, a large proportion of N is not used by plants and is in fact lost into the environment and causes serious environmental pollution. Therefore, achieving a low N optimum via efficient physiological and biochemical processes in wheat grown under low-N conditions is highly important for agricultural sustainability. Although N stress-related N capture in wheat has become a heavily researched subject, how this plant adapts and responds to N starvation has not been fully elucidated. This review summarizes the current knowledge on the signaling mechanisms activated in wheat plants in response to N starvation. Furthermore, we filled the putative gaps on this subject with findings obtained in other plants, primarily rice, maize, and Arabidopsis. Phytohormones have been determined to play essential roles in sensing environmental N starvation and transducing this signal into an adjustment of N transporters and phenotypic adaptation. The critical roles played by protein kinases and critical kinases and phosphatases, such as MAPK and PP2C, as well as the multifaceted functions of transcription factors, such as NF-Y, MYB, DOF, and WRKY, in regulating the expression levels of their target genes (proteins) for low-N tolerance are also discussed. Optimization of root system architecture (RSA) via root branching and thinning, improvement of N acquisition and assimilation, and fine-tuned autophagy are pivotal strategies by which plants respond to N starvation. In light of these findings, we attempted to construct regulatory networks for RSA modification and N uptake, transport, assimilation, and remobilization.

OsPIN9, an auxin efflux carrier, is required for the regulation of rice tiller bud outgrowth by ammonium

The degree of rice tillering is an important agronomic trait that can be markedly affected by nitrogen supply. However, less is known about how nitrogen-regulated rice tillering is related to polar auxin transport. Compared with nitrate, ammonium induced tiller development and was paralleled with increased ³ H-indole-acetic acid (IAA) transport and greater auxin into the junctions. OsPIN9, an auxin efflux carrier, was selected as the candidate gene involved in ammonium-regulated tillering based on GeneChip data. Compared with wild-type plants, ospin9 mutants had fewer tillers, and OsPIN9 overexpression increased the tiller number. Additionally, OsPIN9 was mainly expressed in vascular tissue of the junction and tiller buds, and encoded a membrane-localised protein. Heterologous expression in Xenopus oocytes and yeast demonstrated that OsPIN9 is a functional auxin efflux transporter. More importantly, its RNA and protein levels were induced by ammonium but not by nitrate, and tiller numbers in mutants did not respond to nitrogen forms. Further advantages, including increased tiller number and grain yield, were observed in overexpression lines grown in the paddy field at a low-nitrogen rate compared with at a high-nitrogen rate. Our data revealed that ammonium supply and an auxin efflux transporter co-ordinately control tiller bud elongation in rice.

Effects of nitric oxide on nitrogen metabolism and the salt resistance of rice (Oryza sativa L.) seedlings with different salt tolerances

Salt stress inhibits rice productivity seriously. Nitric oxide (NO) is an endogenous signaling molecule in plants that can improve the resistance of rice to abiotic stresses. Previous studies also showed that nitrogen metabolism is essential for rice stress-tolerance. However, the physiological and molecular mechanisms by how NO affects the nitrogen metabolisms of rice seedlings remain unclear. A hydroponic experiment with two rice varieties, Jinyuan85 (salt tolerant) and Liaojing763 (salt sensitive), was carried out to explore whether NO could alleviate the negative effects of salt stress on nitrogen metabolism and increase salt resistance of rice seedlings. The results showed that (1) the application of NO alleviated the inhibitory effects of salt stress on plant height and biomass accumulation, and increased the nitrogen content of rice leaf. (2) the accumulation of the sucrose and proline was markedly increased in salt stress after application of NO, and peroxidase activities was increased by 107% and 67.7% for Jinyuan85 and Liaojing763, respectively. (3) NO significantly increased the activities of glutamate dehydrogenase, sucrose synthase and sucrose phosphate synthase in both rice varieties under salt stress. (4) Additionally, NO regulated the expression levels of AMT, NIA and SUT genes, but these regulation effects are different with rice varieties and treatments. The results suggested that NO mainly increased the glutamate dehydrogenase and peroxidase activities and sucrose accumulation to enhance the nitrogen metabolism and antioxidative capacity, and alleviated the negative effects of salt stress on rice performance.

Nitric oxide regulates water status and associated enzymatic pathways to inhibit nutrients imbalance in maize (Zea mays L.) under drought stress

Nitric oxide (NO) is a key signaling molecule that instigates significant changes in plant metabolic processes and promotes tolerance against various environmental stresses including drought. In this study, we focused on NO-mediated physiological mechanisms and enzymatic activities that influence the nutrient concentrations and yield in maize under drought stress. The drought-tolerant (NK-8711) and sensitive (P-1574) maize hybrids were sown in lysimeter tanks and two levels of water stress (well-watered at100% field capacity and drought stress at 60% field capacity) were applied at three-leaves stage of maize. Foliar treatment of sodium nitroprusside (SNP), the donor of NO was applied at the cob development stage. The results showed that the foliar spray of NO regulated water relations by increasing proline content and improved drought tolerance in water stressed maize plants. In addition, it stimulated the activity of antioxidative enzymes which reduced the production of free radicals and lipid peroxidation. The activities of nitrate assimilation enzymes were considerably increased by NO spray which, in turn, increased nutrient accumulation and yield in maize under water deficit conditions. These results acknowledge the importance of NO as a stress-signaling molecule that positively regulates defense mechanisms in maize to withstand water-limited conditions.

Transcriptomic Study for Identification of Major Nitrogen Stress Responsive Genes in Australian Bread Wheat Cultivars

High nitrogen use efficiency (NUE) in bread wheat is pivotal to sustain high productivity. Knowledge about the physiological and transcriptomic changes that regulate NUE, in particular how plants cope with nitrogen (N) stress during flowering and the grain filling period, is crucial in achieving high NUE. Nitrogen response is differentially manifested in different tissues and shows significant genetic variability. A comparative transcriptome study was carried out using RNA-seq analysis to investigate the effect of nitrogen levels on gene expression at 0 days post anthesis (0 DPA) and 10 DPA in second leaf and grain tissues of three Australian wheat (Triticum aestivum) varieties that were known to have varying NUEs. A total of 12,344 differentially expressed genes (DEGs) were identified under nitrogen stress where down-regulated DEGs were predominantly associated with carbohydrate metabolic process, photosynthesis, light-harvesting, and defense response, whereas the up-regulated DEGs were associated with nucleotide metabolism, proteolysis, and transmembrane transport under nitrogen stress. Protein–protein interaction and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways analysis further revealed that highly interacted down-regulated DEGs were involved in light-harvesting and photosynthesis, and up-regulated DEGs were mostly involved in steroid biosynthesis under N stress. The common down-regulated genes across the cultivars included photosystem II 10 kDa polypeptide family proteins, plant protein 1589 of uncharacterized protein function, etc., whereas common up-regulated genes included glutamate carboxypeptidase 2, placenta-specific8 (PLAC8) family protein, and a sulfate transporter. On the other hand, high NUE cultivar Mace responded to nitrogen stress by down-regulation of a stress-related gene annotated as beta-1,3-endoglucanase and pathogenesis-related protein (PR-4, PR-1) and up-regulation of MYB/SANT domain-containing RADIALIS (RAD)-like transcription factors. The medium NUE cultivar Spitfire and low NUE cultivar Volcani demonstrated strong down-regulation of Photosystem II 10 kDa polypeptide family protein and predominant up-regulation of 11S globulin seed storage protein 2 and protein transport protein Sec61 subunit gamma. In grain tissue, most of the DEGs were related to nitrogen metabolism and proteolysis. The DEGs with high abundance in high NUE cultivar can be good candidates to develop nitrogen stress-tolerant variety with improved NUE.

Growth-Stimulatory Effect of Quorum Sensing Signal Molecule N-Acyl-Homoserine Lactone-Producing Multi-Trait Aeromonas spp. on Wheat Genotypes Under Salt Stress

Salinity is one of the major threats to agricultural productivity worldwide. Soil and plant management practices, along with inoculation with plant-beneficial bacteria, play a key role in the plant’s tolerance toward salinity stress. The present study demonstrates the potential of acyl homoserine lactone (AHL)-producing plant growth promoting rhizobacteria (PGPR) strains of Aeromonas sp., namely, SAL-17 (accession no. HG763857) and SAL-21 (accession no. HG763858), for growth promotion of two wheat genotypes inherently different for salt tolerance potential. AHLs are the bacterial signal molecules that regulate the expression of various genes in bacteria and plants. Both Aeromonas spp., along with innate plant-growth-promoting (PGP) and salt tolerance traits, showed AHL production which was identified on tandem mass spectrometry as C6-HSL, 3-OH-C5-HSL, 3-OH-C6-HSL, 3-oxo-C7-HSL C10-HSL, 3-oxo-C10-HSL, 3-OH-C10-HSL, 3-oxo-C12-HSL and C6-HSL, and 3-oxo-C10-HSL. The exogenous application of purified AHLs (mix) significantly improved various root parameters at 200 mM NaCl in both salt-sensitive (SSG) and salt-tolerant (STG) genotypes, where the highest increase (≈80%) was observed where a mixture of both strains of AHLs was used. Confocal microscopic observations and root overlay assay revealed a strong root colonization potential of the two strains under salt stress. The inoculation response of both STG and SSG genotypes was evaluated with two AHL-producing strains (SAL-17 and SAL-21) and compared to non-AHL-producing Aeromonas sp. SAL-12 (accession no. HG763856) in saline (EC = 7.63 ms/cm²) and non-saline soil. The data reveal that plants inoculated with the bacterial consortium (SAL-21 + SAL-17) showed a maximum increase in leaf proline content, nitrate reductase activity, chlorophyll a/b, stomatal conductance, transpiration rate, root length, shoot length, and grain weight over non-inoculated plants grown in saline soil. Both STG and SSG showed relative effectiveness toward inoculation (percent increase for STG: 165–16%; SSG: 283–14%) and showed a positive correlation of grain yield with proline and nitrate reductase activity. Furthermore, principal component analysis (PCA) and categorical PCA analysis clearly showed an inoculation response in both genotypes, revealing the effectiveness of AHL-producing Aeromonas spp. than the non-AHL-producing strain. The present study documents that the consortium of salt-tolerant AHL-producing Aeromonas spp. is equally effective for sustaining the growth of STG as well as SSG wheat genotypes in saline soil, but biosafety should be fully ensured before field release.

Protective role of nitric oxide on nitrogen-thiol metabolism and amino acids profiling during arsenic exposure in Oryza sativa L

Nitric oxide (NO) being a signaling molecule inside the plant cells, play significant role in signaling cascades and protection against environmental stresses. However, the protective role of NO in alleviating As toxicity in rice plants is currently not available. In the present study, the level of NO, nitrogen (N), inorganic N (nitrate, ammonium), thiols {TT (Total thiols), NPT (Nonprotein thiol)} and AAs contents along with N assimilating enzymes (NR, GDH, GOGAT) were analyzed after exposure of As^III/NO treatment alone, and in combination. NO supplementation enhanced the content of N, inorganic N & thiol contents, NR, GOGAT activities, when compared with As^III exposure alone. In As^III exposed rice seedlings, content of AAs (except His, Arg, Met) reduced over the control, while supplementation of SNP improved AAs contents, compared to As^III treatment alone. In conclusion, rice seedlings supplemented with NO tolerate the As^III toxicity by reducing the N related parameters, thiol contents, altering the AA profile and enhanced the nutritional quality by increasing EAAs (essential amino acids) and NEAAs (non-essential amino acids).

Silicon tackles butachlor toxicity in rice seedlings by regulating anatomical characteristics, ascorbate-glutathione cycle, proline metabolism and levels of nutrients

Reckless use of herbicides like butachlor (Buta) in the fields represents a serious threat to crop plants, and hence to their productivity. Silicon (Si) is well known for its implication in the alleviation of the effects of abiotic stresses; however, its role in mitigating Buta toxicity is not yet known. Therefore, this study was carried out to explore the role of Si (10 µM) in regulating Buta (4 µM) toxicity in rice seedlings. Buta reduced growth and photosynthesis, altered nitric oxide (NO) level and leaf and root anatomy, inhibited enzyme activities of the ascorbate-glutathione cycle (while transcripts of associated enzymes, increased except OsMDHAR), as well as its metabolites (ascorbate and glutathione) and uptake of nutrients (Mg, P, K, S, Ca, Fe, etc. except Na), while addition of Si reversed Buta-induced alterations. Buta stimulated the expression of Si channel and efflux transporter genes- Lsi1 and Lsi2 while the addition of Si further greatly induced their expression under Buta toxicity. Buta increased free proline accumulation by inducing the activity of Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and decreasing proline dehydrogenase (PDH) activity, while Si reversed these effects caused by Buta. Our results suggest that Si-governed mitigation of Buta toxicity is linked with favorable modifications in energy flux parameters of photosynthesis and leaf and root anatomy, up-regulation of Si channel and transporter genes, ascorbate-glutathione cycle and nutrient uptake, and lowering in oxidative stress. We additionally demonstrate that NO might have a crucial role in these responses.

Enhanced Nitric Oxide Synthesis Through Nitrate Supply Improves Drought Tolerance of Sugarcane Plants

Nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. Nitrate reductase (NR) not only mediates the reduction of NO3 - to NO2 - but also reduces NO2 - to NO, a relevant pathway for NO production in higher plants. Herein, we hypothesized that sugarcane plants supplied with more NO3 - as a source of N would produce more NO under water deficit. Such NO would reduce oxidative damage and favor photosynthetic metabolism and growth under water limiting conditions. Sugarcane plants were grown in nutrient solution and received the same amount of nitrogen, with varying nitrate:ammonium ratios (100:0 and 70:30). Plants were then grown under well-watered or water deficit conditions. Under water deficit, plants exhibited higher root [NO3 -] and [NO2 -] when supplied with 100% NO3 -. Accordingly, the same plants also showed higher root NR activity and root NO production. We also found higher photosynthetic rates and stomatal conductance in plants supplied with more NO3 -, which was associated with increased root growth. ROS accumulation was reduced due to increases in the activity of catalase in leaves and superoxide dismutase and ascorbate peroxidase in roots of plants supplied with 100% NO3 - and facing water deficit. Such positive responses to water deficit were offset when a NO scavenger was supplied to the plants, thus confirming that increases in leaf gas exchange and plant growth were induced by NO. Concluding, NO3 - supply is an interesting strategy for alleviating the negative effects of water deficit on sugarcane plants, increasing drought tolerance through enhanced NO production. Our data also provide insights on how plant nutrition could improve crop tolerance against abiotic stresses, such as drought.

Nitrate Signaling, Functions, and Regulation of Root System Architecture: Insights from Arabidopsis thaliana

Root system architecture (RSA) is required for the acquisition of water and mineral nutrients from the soil. One of the essential nutrients, nitrate (NO₃⁻), is sensed and transported by nitrate transporters NRT1.1 and NRT2.1 in the plants. Nitrate transporter 1.1 (NRT1.1) is a dual-affinity nitrate transporter phosphorylated at the T101 residue by calcineurin B-like interacting protein kinase (CIPKs); it also regulates the expression of other key nitrate assimilatory genes. The differential phosphorylation (phosphorylation and dephosphorylation) strategies and underlying Ca²⁺ signaling mechanism of NRT1.1 stimulate lateral root growth by activating the auxin transport activity and Ca²⁺-ANR1 signaling at the plasma membrane and the endosomes, respectively. NO₃⁻ additionally functions as a signal molecule that forms a signaling system, which consists of a vast array of transcription factors that control root system architecture that either stimulate or inhibit lateral and primary root development in response to localized and high nitrate (NO₃⁻), respectively. This review elucidates the so-far identified nitrate transporters, nitrate sensing, signal transduction, and the key roles of nitrate transporters and its downstream transcriptional regulatory network in the primary and lateral root development in Arabidopsis thaliana under stress conditions.

Hydrogen sulfide and rhizobia synergistically regulate nitrogen (N) assimilation and remobilization during N deficiency-induced senescence in soybean

Hydrogen sulfide (H2 S) is emerging as an important signalling molecule that regulates plant growth and abiotic stress responses. However, the roles of H2 S in symbiotic nitrogen (N) assimilation and remobilization have not been characterized. Therefore, we examined how H2 S influences the soybean (Glycine max)/rhizobia interaction in terms of symbiotic N fixation and mobilization during N deficiency-induced senescence. H2 S enhanced biomass accumulation and delayed leaf senescence through effects on nodule numbers, leaf chlorophyll contents, leaf N resorption efficiency, and the N contents in different tissues. Moreover, grain numbers and yield were regulated by H2 S and rhizobia, together with N accumulation in the organs, and N use efficiency. The synergistic effects of H2 S and rhizobia were also demonstrated by effects on the enzyme activities, protein abundances, and gene expressions associated with N metabolism, and senescence-associated genes (SAGs) expression in soybeans grown under conditions of N deficiency. Taken together, these results show that H2 S and rhizobia accelerate N assimilation and remobilization by regulation of the expression of SAGs during N deficiency-induced senescence. Thus, H2 S enhances the vegetative and reproductive growth of soybean, presumably through interactions with rhizobia under conditions of N deficiency.

The physiological mechanism underlying root elongation in response to nitrogen deficiency in crop plants

In response to low nitrogen stress, multiple hormones together with nitric oxide signaling pathways work synergistically and antagonistically in crop root elongation. Changing root morphology allows plants to adapt to soil nutrient availability. Nitrogen is the most important essential nutrient for plant growth. An important adaptive strategy for crops responding to nitrogen deficiency is root elongation, thereby accessing increased soil space and nitrogen resources. Multiple signaling pathways are involved in this regulatory network, working together to fine-tune root elongation in response to soil nitrogen availability. Based on existing research, we propose a model to explain how different signaling pathways interact to regulate root elongation in response to low nitrogen stress. In response to a low shoot nitrogen status signal, auxin transport from the shoot to the root increases. High auxin levels in the root tip stimulate the production of nitric oxide, which promotes the synthesis of strigolactones to accelerate cell division. In this process, cytokinin, ethylene, and abscisic acid play an antagonistic role, while brassinosteroids and auxin play a synergistic role in regulating root elongation. Further study is required to identify the QTLs, genes, and favorable alleles which control the root elongation response to low nitrogen stress in crops.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.

Exogenous Nitric Oxide and Phosphorus Stress Affect the Mycorrhization, Plant Growth, and Associated Microbes of Carya illinoinensis Seedlings Colonized by Tuber indicum

In the artificial cultivation of truffles, ectomycorrhizal colonization level, host plant quality, and the associated microbes in the rhizosphere soil are vitally important. To explore the effects of nitric oxide (NO) and phosphorus (P) stress on the early symbiosis of truffles and host plants, different concentrations of exogenous NO donor sodium nitroprusside (SNP) and P were applied to Carya illinoinensis seedlings inoculated with the Chinese black truffle (Tuber indicum). The growth of T. indicum-mycorrhized seedlings and their mycorrhizal colonization rate were investigated. Additionally, the denitrifying bacterial community harboring NO reductase (norB) genes and the fungal community in the rhizosphere of the host were analyzed by high-throughput sequencing. The results showed that the colonization rate of T. indicum was significantly influenced by SNP treatments and P stress, with the highest level being obtained when the SNP was 100 μmol/L under low P stress (5 μmol/L). Treatment with 100 μmol/L SNP alone also increased the colonization rate of T. indicum and had positive effects on the plant height, stem circumference, biomass, root-shoot ratio and root POD activity of the seedlings at different times after inoculation. Under low P stress, the 100 μmol/L SNP increased the richness of the norB-type denitrifying bacterial community. Interestingly, the diversity and richness of norB-type denitrifying bacteria were significantly positively correlated with the colonization rate of T. indicum. SNP treatments under low P stress altered the abundance of some dominant taxa such as Alphaproteobacteria, Gammaproteobacteria, Pseudomonas, Ensifer, and Sulfitobacter. Evaluation of the fungal community in the rhizosphere revealed that 100 μmol/L SNP treatment alone had no noticeable effect on their richness and diversity, but it did shape the abundance of some fungi. Buellia, Podospora, Phaeoisaria, Ascotaiwania, and Lophiostoma were more abundant following exogenous NO application, while the abundance of Acremonium, Monographella, and Penicillium were decreased. Network analysis indicated that T. indicum was positively and negatively correlated with some fungal genera when treated with 100 μmol/L SNP. Overall, these results revealed how exogenous NO and P stress influence the symbiosis of truffles and host plants, and indicate that application of SNP treatments has the potential for ectomycorrhizal synthesis and truffle cultivation.

Overexpression of OsPIN2 Regulates Root Growth and Formation in Response to Phosphate Deficiency in Rice

The response of root architecture to phosphate (P) deficiency is critical in plant growth and development. Auxin is a key regulator of plant root growth in response to P deficiency, but the underlying mechanisms are unclear. In this study, phenotypic and genetic analyses were undertaken to explore the role of OsPIN2, an auxin efflux transporter, in regulating the growth and development of rice roots under normal nutrition condition (control) and low-phosphate condition (LP). Higher expression of OsPIN2 was observed in rice plants under LP compared to the control. Meanwhile, the auxin levels of roots were increased under LP relative to control condition in wild-type (WT) plants. Compared to WT plants, two overexpression (OE) lines had higher auxin levels in the roots under control and LP. LP led to increased seminal roots (SRs) length and the root hairs (RHs) density, but decreased lateral roots (LRs) density in WT plants. However, overexpression of OsPIN2 caused a loss of sensitivity in the root response to P deficiency. The OE lines had a shorter SR length, lower LR density, and greater RH density than WT plants under control. However, the LR and RH densities in the OE lines were similar to those in WT plants under LP. Compared to WT plants, overexpression of OsPIN2 had a shorter root length through decreased root cell elongation under control and LP. Surprisingly, overexpression of OsPIN2 might increase auxin distribution in epidermis of root, resulting in greater RH formation but less LR development in OE plants than in WT plants in the control condition but levels similar of these under LP. These results suggest that higher OsPIN2 expression regulates rice root growth and development maybe by changing auxin distribution in roots under LP condition.

Hydrogen peroxide alleviates P starvation in rice by facilitating P remobilization from the root cell wall

Phosphorus (P) deficiency limits rice production. Increasing the remobilization of P stored in the root cell wall is an efficient way to alleviate P starvation in rice. In the current study, we found that the addition of 50 μM H₂O₂ significantly increased soluble P content in rice. H₂O₂ stimulated pectin biosynthesis and increased pectin methylesterase (PME) activity, thus stimulating the release of P from the cell wall in roots. H₂O₂ also regulates internal P homeostasis by increasing the expression of P transporter genes OsPT2, OsPT6, and OsPT8 at different treatment times. In addition, the H₂O₂ treatment increased the expression of nitrate reductase (NR) genes OsNIA1 and OsNIA2 and the activity of NR, then increased the accumulation of nitric oxide (NO) in the rice root. The application of the NO donor sodium nitroprusside (SNP) and the H₂O₂ scavenger 4-hydroxy-TEMPO significantly increased soluble P content by increasing pectin levels and PME activity to enhance the remobilization of P from the cell wall. However, the addition of NO scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) with and without H₂O₂ had the opposite effect, suggesting that NO functions downstream of H₂O₂ to increase the remobilization of cell wall P in rice.

Senescence is delayed by selenium in oilseed rape plants

Leaf senescence is a genetically programmed process that can also be induced by nitrogen (N) deficiency. Although selenium (Se) delays leaf senescence, the underlying mechanisms are still unknown. To explore the mechanisms of Se-mediated delay of leaf senescence, we studied the biochemical and molecular events that occur during developmental and N deficiency-induced senescence. Oilseed rape (Brassica napus L.) plants were grown under adequate N (AN, 16 mM) or low N (LN, 4 mM) conditions during the rosette growth stage and treated with Se (15 μg plant^-1 as Na₂SeO₄) either through roots or leaves for four weeks. Shoot dry matter production was not influenced, while the photosynthetic parameters were improved by Se application in both young and old leaves under both AN and LN conditions. The Se treatment rarely influenced the concentrations of reactive oxygen species (ROS), while it increased the nitric oxide (NO) levels in young and old leaves under both AN and LN conditions. The positive correlation between the NO level and leaf photosynthetic parameters in old leaves of LN plants suggested a role for NO boosting, mediated by Se, in the protection of aging leaves from LN-induced accelerated senescence. This implication was further supported by the clear down-regulation of SAG12-1 and up-regulation of Cab, particularly by root application of Se in old leaves of LN plants. Our results provide the first evidence that Se influences the expression of senescence-associated genes and delays senescence through NO signalling but is independent of the ROS defence system.

Nitric oxide and plant mineral nutrition: current knowledge

Plants under conditions of essential mineral deficiency trigger signaling mechanisms that involve common components. Among these components, nitric oxide (NO) has been identified as a key participant in responses to changes in nutrient availability. Usually, nutrient imbalances affect the levels of NO in specific plant tissues, via modification of its rate of synthesis or degradation. Changes in the level of NO affect plant morphology and/or trigger responses associated with nutrient homeostasis, mediated by its interaction with reactive oxygen species, phytohormones, and through post-translational modification of proteins. NO-related events constitute an exciting field of research to understand how plants adapt and respond to conditions of nutrient shortage. This review summarizes the current knowledge on NO as a component of the multiple processes related to plant performance under conditions of deficiency in mineral nutrients, focusing on macronutrients such as nitrogen, phosphate, potassium, and magnesium, as well as micronutrients such as iron and zinc.

Temporal genetic patterns of root growth in Brassica napus L. revealed by a low-cost, high-efficiency hydroponic system

Application of a low-cost and high-efficiency hydroponic system in a rapeseed population verified two types of genetic factors ("persistent" and "stage-specific") that control root development. The root system is a vital plant component for nutrient and water acquisition and is targeted to enhance plant productivity. Genetic dissection of the root system generally focuses on a single stage, but roots grow continuously during plant development. To reveal the temporal genetic patterns of root development, we measured nine root-related traits in a rapeseed recombinant inbred line population at six continuous stages during vegetative growth, using a modified hydroponic system with low-cost and high-efficiency features that could synchronize plant growth under controlled conditions. Phenotypic correlation and growth dynamic analysis suggested the existence of two types of genetic factors ("persistent" and "stage-specific") that control root development. Dynamic (unconditional and conditional) quantitative trait loci (QTL) mapping detected 28 stage-specific and 23 persistent QTLs related to root growth. Among them, 13 early stage-specific, 19 persistent and 8 later stage-specific QTLs were detected at 7 DAS (days after sowing), 16 DAS and 5 EL (expanding leaf stage), respectively, providing efficient and adaptable stages for QTL identification. The effective prediction of biomass accumulation using root morphological traits (up to 96.6% or 92.64% at a specific stage or the final stage, respectively) verified that root growth allocation with maximum root uptake area facilitated biomass accumulation. Furthermore, marker-assistant selection, which combined the "persistent" and "stage-specific" QTLs, proved their effectiveness for root improvement with an excellent uptake area. Our results highlight the potential of high-throughput and precise phenotyping to assess the dynamic genetics of root growth and provide new insights into ideotype root system-based biomass breeding.

Molybdenum-induced effects on photosynthetic efficacy of winter wheat (Triticum aestivum L.) under different nitrogen sources are associated with nitrogen assimilation

Different nitrogen (N) sources have been reported to significantly affect the photosynthesis (P_n) and its attributes. However, molybdenum (Mo) induced effects on photosynthetic efficacy of winter wheat under different N sources have not been investigated. A hydroponic study was carried out comprising of two winter wheat cultivars '97003' and '97014' as Mo-efficient and Mo-inefficient, respectively to underpin the effects of Mo supply (0 and 1 μM) on photosynthetic efficacy of winter wheat under different N sources (NO₃^̶, NH₄NO₃ or NH₄⁺). The results revealed that Mo-induced increases in dry weight, gas exchange parameters, chlorophyll contents, NR activities, NO₃^̶ assimilation, total N contents and transcripts of TaNR and TaNRT1.1 genes under different N sources followed the trend of NH₄NO₃ > NO₃^̶ > NH₄⁺, suggesting that Mo has more complementary effects to nitrate nutrition than sole ammonium. Interestingly, under Mo-deprivation environments, cultivar '97003' recorded more pronounced alterations in Mo-dependent parameters than '97014' cultivar. Moreover, Mo application significantly improved the chlorophyll contents and chloroplast configuration in all N sources showing that Mo has a key role in chlorophyll biosynthesis and chloroplast integrity. The results also highlighted that Mo-induced enhancements in total N contents and photosynthetic characteristics followed the same order as NH₄NO₃ > NO₃^- > NH₄⁺, suggesting that Mo might affect P_n through N metabolism. In crux, our study findings imply that Mo supply increased P_n not only through chlorophyll synthesis and chloroplast configuration but also by N uptake and assimilation which may represent a strategy of Mo fertilizer to strengthen the photosynthetic machinery.

Nitric oxide alters nitrogen metabolism and PIN gene expressions by playing protective role in arsenic challenged Brassica juncea L

Plants have ability to adapt themselves through altering their growth process. In the present study, we examined exogenous application of nitric oxide (NO) on nitrogen metabolism and auxin (PIN) gene expression, and its possible role in alleviation of arsenic (As) toxicity in Brassica juncea seedlings. Seven days old hydroponically grown B. juncea seedlings were exposed to As^III (150 μM), Sodium nitroprusside (NO donor, 100 μM), As^III + SNP and control (without metal)for 48 h. Experimental results revealed that As^III stress: enhanced the level of nitrite, NiR activity, NO₃^- and NH₄⁺content as well as NADH-GOGAT activity; but GDH level decreased; enhanced content of amino acids; upregulated gene expression level of N metabolism and downregulated polar auxin transporter genes (PIN); inhibited plant growth and morphological parameters; increased MDA, H₂O_2, cysteine, proline content, enzymatic antioxidants (SOD, CAT, APX; GSH, TT, NPT); and decreased nutrient content. As^III + SNP combination reduced the accumulation of As; improved growth; chlorophyll, protein and mineral nutrient content by scavenging ROS generation; maintained amino acids content; downregulated expression of N metabolism genes and upregulated expression of auxin transporter (PIN) genes . Additional biochemical data depicts reduction in the level of nitrogen related enzymatic activities, and other stress related parameters. Overall, this study provides an integrated view that exogenous SNP (NO donor) supplementation alleviated the inhibitory role of As^III in B. juncea seedlings by altering nutrients, amino acids and auxin redistribution via expression of nitrogen and PIN gene profiling.

Exploring the Potential of Overexpressed OsCIPK2 Rice as a Nitrogen Utilization Efficient Crop and Analysis of Its Associated Rhizo-Compartmental Microbial Communities

Nitrogen (N) is one of the indispensable factors in rice growth and development. China holds a premier position in the production of rice and at the same time also faces higher N fertilizer costs along with serious damage to the environment. A better solution is much needed to address these issues, without disrupting the production of rice as an important cereal, while minimizing all the deleterious effects on the environment. Two isogenic lines Kitaake (WT) and its genetically modified line CIPK2 (RC), overexpressing the gene for Calcineurin B-like interacting protein kinase 2 (OsCIPK2) with better nitrogen use efficiency (NUE), were compared for their growth and development under low versus normal levels of N. NUE is a complex trait mainly related to a plant’s efficiency in extraction, assimilation, and recycling of N from soil. The microbial population was analyzed using high-throughput Illumina Miseq 16S rRNA sequencing and found that RC with CIPK2, specifically expressed in rice root, not only performed better without nitrogen fertilizer (LN) but also increased the diversity of bacterial communities in rice rhizosphere compartments (rhizosphere, rhizoplane, and endosphere). The relative abundance of beneficial bacteria phyla increased, which are known to promote the circulation and transformation of N in rhizosphere soil. To further explore the potential of RC regarding better performance under LN, the ion fluxes in root apical were detected by non-invasive micro-test technique (NMT). We found that RC can absorb more Ca²⁺ and NO₃⁻ under LN as compared to WT. Finally, compared to WT, RC plants exhibited better growth of root and shoot, and increased yield and N uptake under LN, whereas there was no significant difference in the growth of two rice lines under normal nitrogen (NN) treatment. We are able to get preliminary results, dealing with the OsCIPK2 overexpressed rice line, by studying the rice molecular, physiological, and chemical parameters related to NUE. The results laid the foundation for further research on N absorption and utilization in rice from the soil and the interaction with microbial communities.

The Systems Biology of Lateral Root Formation: Connecting the Dots

The root system is a major determinant of a plant's access to water and nutrients. The architecture of the root system to a large extent depends on the repeated formation of new lateral roots. In this review, we discuss lateral root development from a systems biology perspective. We focus on studies combining experiments with computational modeling that have advanced our understanding of how the auxin-centered regulatory modules involved in different stages of lateral root development exert their specific functions. Moreover, we discuss how these regulatory networks may enable robust transitions from one developmental stage to the next, a subject that thus far has received limited attention. In addition, we analyze how environmental factors impinge on these modules, and the different manners in which these environmental signals are being integrated to enable coordinated developmental decision making. Finally, we provide some suggestions for extending current models of lateral root development to incorporate multiple processes and stages. Only through more comprehensive models we can fully elucidate the cooperative effects of multiple processes on later root formation, and how one stage drives the transition to the next.