N- and P-mediated seminal root elongation response in rice seedlings

Aims

In rice, seminal root elongation plays an important role in acquisition of nutrients such as N and P, but the extent to which different N forms and P concentrations affect root growth is poorly understood. This study aimed to examine N- and P-mediated seminal root elongation response and to identify putative QTLs associated with seminal root elongation.

Methods

Seminal root elongation was evaluated in 15 diverse wild and cultivated accessions of rice, along with 48 chromosome segment substitution lines (CSSLs) derived from a cross between the rice variety ‘Curinga’ and Oryza rufipogon (IRGC 105491). Root elongation in response to different forms of N (NH₄ ⁺, NO₃ ^? and NH₄NO₃) and concentrations of P was evaluated under hydroponic conditions, and associated putative QTL regions were identified.

Results

The CSSL parents had contrasting root responses to N and P. Root elongation in O. rufipogon was insensitive to N source and concentration, whereas Curinga was responsive. In contrast to N, seminal root elongation and P concentration was positively correlated. Three putative QTLs for seminal root elongation in response to N were detected on chromosome 1, and one QTL on chromosome 3 was associated with low P concentration.

Conclusions

Genetic variation in seminal root elongation and plasticity of nutrient response may be appropriate targets for marker-assisted selection to improve rice nutrient acquisition efficiency.     

Root growth inhibition by NH4+ in Arabidopsis is mediated by the root tip and is linked to NH4+ efflux and GMPase activity

Root growth in higher plants is sensitive to excess ammonium (NH₄⁺). Our study shows that contact of NH₄⁺ with the primary root tip is both necessary and sufficient to the development of arrested root growth under NH₄⁺ nutrition in Arabidopsis. We show that cell elongation and not cell division is the principal target in the NH₄⁺ inhibition of primary root growth. Mutant and expression analyses using DR5:GUS revealed that the growth inhibition is furthermore independent of auxin and ethylene signalling. NH₄⁺ fluxes along the primary root, measured using the Scanning Ion‐selective Electrode Technique, revealed a significant stimulation of NH₄⁺ efflux at the elongation zone following treatment with elevated NH₄⁺, coincident with the inhibition of root elongation. Stimulation of NH₄⁺ efflux and inhibition of cell expansion were significantly more pronounced in the NH₄⁺‐hypersensitive mutant vtc1‐1, deficient in the enzyme GDP‐mannose pyrophosphorylase (GMPase). We conclude that both restricted transmembrane NH₄⁺ fluxes and proper functioning of GMPase in roots are critical to minimizing the severity of the NH₄⁺ toxicity response in Arabidopsis.     

NH4 + induces antioxidant cellular machinery and provides resistance to salt stress in citrus plants

Key message

NH ₄ ⁺ acts as a mild oxidative stressor, which triggers antioxidant cellular machinery and provide resistance to salinity.

Abstract

NH₄ ⁺ nutrition in Carrizo citrange (Citrus sinensis L. Osbeck × Poncirus trifoliata L) plants acts as an inducer of resistance against salinity conditions. NH₄ ⁺ treatment triggers mild chronic stress that primes plant defence responses by stress imprinting and confers protection against subsequent salt stress. In this work, we studied the influence of NH₄ ⁺ nutrition on antioxidant enzymatic activities and metabolites involved in detoxification of reactive oxygen species (ROS) to clarify their involvement in NH₄ ⁺-mediated salt resistance. Our results showed that NH₄ ⁺ nutrition induces in citrus plants high levels of H₂O₂, strongly inhibits superoxide dismutase (SOD) and glutathione reductase (GR) activities, and leads to higher content of oxidised glutathione (GSSG) than in control plants in the absence of salt, thus providing evidence to confirm mild stress induced by NH₄ ⁺ nutrition. However, upon salinity, plants grown with NH₄ ⁺ (N-NH₄ ⁺ plants) showed a reduction of H₂O₂ levels in parallel to an increase of catalase (CAT), SOD, and GR activities compared with the control plants. Moreover, N-NH₄ ⁺ plants were able to keep high levels of reduced glutathione (GSH) upon salinity and were able to induce glutathione-S-transferase (GST) and phospholipid hydroperoxide glutathione peroxidise (PHGPx) mRNA accumulation. Based on this evidence, we confirm that sublethal concentrations of NH₄ ⁺ might act as a mild oxidative stressor, which triggers antioxidant cellular machinery that can provide resistance to subsequent salt stress.     

Rapid Ammonia Gas Transport Accounts for Futile Transmembrane Cycling under NH3/NH4+ Toxicity in Plant Roots

Futile transmembrane NH₃/NH₄⁺ cycling in plant root cells, characterized by extremely rapid fluxes and high efflux to influx ratios, has been successfully linked to NH₃/NH₄⁺ toxicity. Surprisingly, the fundamental question of which species of the conjugate pair (NH₃ or NH₄⁺) participates in such fluxes is unresolved. Using flux analyses with the short-lived radioisotope ¹³N and electrophysiological, respiratory, and histochemical measurements, we show that futile cycling in roots of barley (Hordeum vulgare) seedlings is predominately of the gaseous NH₃ species, rather than the NH₄⁺ ion. Influx of ¹³NH₃/¹³NH₄⁺, which exceeded 200 µmol g^–1 h^–1, was not commensurate with membrane depolarization or increases in root respiration, suggesting electroneutral NH₃ transport. Influx followed Michaelis-Menten kinetics for NH₃ (but not NH₄⁺), as a function of external concentration (K_m = 152 µm, V_max = 205 µmol g^–1 h^–1). Efflux of ¹³NH₃/¹³NH₄⁺ responded with a nearly identical K_m. Pharmacological characterization of influx and efflux suggests mediation by aquaporins. Our study fundamentally revises the futile-cycling model by demonstrating that NH₃ is the major permeating species across both plasmalemma and tonoplast of root cells under toxicity conditions.Ammonia/ammonium (NH₃/NH₄⁺) toxicity in higher plants has resulted in crop reduction and forest decline (Pearson and Stewart, 1993; Vitousek et al., 1997; Britto and Kronzucker, 2002), biodiversity loss (Stevens et al., 2004; Bobbink et al., 2010), and species extirpation (de Graaf et al., 1998; McClean et al., 2011). These major ecological and economic problems have been aggravated by an accelerated global nitrogen (N) cycle caused primarily by the industrialized production and use of N fertilizers (Galloway et al., 2008; Gruber and Galloway, 2008). With increasing global population and demands on agricultural production, there is no sign of this trend easing: anthropogenic N fixation has reached 210 teragrams year^–1, an approximately 12% increase from 2005 and an approximately 1,300% rise from 150 years ago (Galloway et al., 2008; Fowler et al., 2013).Although considerable knowledge of the causes and mechanisms of NH₃/NH₄⁺ toxicity has accrued in recent years, our understanding of the key processes remains rudimentary (Gerendas et al., 1997; Britto and Kronzucker, 2002). A major hypothesis is that of futile transmembrane NH₄⁺ cycling, which proposes a pathological inability of root cells to restrict the primary entry of NH₄⁺ at high external concentrations ([NH₄+]_ext); many downstream toxicological events are contingent upon this entry (Britto et al., 2001b). In this model, a rapid, thermodynamically passive influx of NH₄⁺ is coupled to an active efflux of NH₄⁺ that is nearly as rapid, constraining normal cellular function and energetics and resulting in plant growth decline and mortality. This phenomenon is thought to occur in NH₄⁺-sensitive species such as barley (Hordeum vulgare) and, to a lesser extent, in tolerant species such as rice (Oryza sativa), which can be susceptible at higher thresholds (Balkos et al., 2010; Chen et al., 2013).Most soils are typically acidic, especially when [NH₄⁺] is high (i.e. in the millimolar range; Van Breemen et al., 1982; Bobbink et al., 1998; Britto and Kronzucker, 2002), and given the pKa of 9.25 for the conjugate pair NH₃/NH₄⁺, [NH₃] is generally low (Izaurralde et al., 1990; Weise et al., 2013). Consequently, the fluxes of NH₃ have largely been considered negligible (Britto et al., 2001a; Britto and Kronzucker, 2002; Loqué and von Wirén, 2004), in contrast to NH₄⁺ fluxes, which are well characterized physiologically (Lee and Ayling, 1993; Wang et al., 1993a, 1993b; Kronzucker et al., 1996) and at the molecular level (Rawat et al., 1999; von Wirén et al., 2000; Ludewig et al., 2007), at least at lower concentrations. However, the transport of NH₃ across membranes has received new attention in the light of evidence that some members of the aquaporin (AQP) family of transporters, a diverse and ubiquitous class of major intrinsic proteins (Maurel et al., 2008; Hove and Bhave, 2011), can mediate NH₃ fluxes in single-cell systems (Jahn et al., 2004; Holm et al., 2005; Loqué et al., 2005; Saparov et al., 2007). However, a convincing demonstration that AQPs transport NH₃ in planta is currently lacking. Given the unusually high capacity of AQP-mediated fluxes relative to those of ion channels and other transporters (Kozono et al., 2002), it is possible that sizable NH₃ fluxes can be conducted through AQPs, even at very low external NH₃ concentration ([NH₃]_ext).Here, we have critically reexamined the hypothesis that futile cycling is composed of cationic NH₄⁺ fluxes across the plasmalemma, of which an active efflux mechanism accounts for energetic demands directly contributing to toxicity (Britto et al., 2001b). We present evidence for the following alternative scenario: 1) futile cycling consists mainly of the passive electroneutral flux of the conjugate base NH₃; 2) such fluxes rapidly span both major membrane systems in root cells (i.e. plasmalemma and tonoplast); 3) AQPs mediate such fluxes; and 4) a thermodynamic equilibrium of NH₃ is established throughout the cell, resulting in hyperaccumulation of NH₄⁺ in the acidic vacuole. This evidence comes primarily from positron emission tracing with the short-lived radioisotope ¹³N, used to characterize the component fluxes of futile cycling at the cellular level in the model species barley. We have coupled this with ⁴²K⁺ radiotracing, to provide comparison with a well-understood cationic flux, as well as electrophysiological, respiratory, pharmacological, and histochemical analyses.     

Alteration of nitrogen metabolism in rice variety 'Nipponbare' induced by alkali stress

Aims

Alkali stress (AS) is an important agricultural contaminant and has complex effects on plant metabolism, specifically root physiology. The aim of this study was to test the role of nitrogen metabolism regulation in alkali tolerance of rice variety 'Nipponbare'.

Methods

In this study, the rice seedlings were subjected to salinity stress (SS) or AS. Growth, the contents of inorganic ions, NH ₄ ⁺ -nitrogen (free amino acids), and NO ₃ ^? -nitrogen in the stressed seedlings were then measured. The expression of some critical genes involved in nitrogen metabolism were also assayed to test their roles in the regulation of nitrogen metabolism during adaptation of rice variety 'Nipponbare' to AS.

Results

AS showed a stronger inhibiting effect on rice variety 'Nipponbare' growth than SS. AS may have more complex effects on nitrogen metabolism than SS.

Conclusions

Effects of AS on the nitrogen metabolism of rice variety 'Nipponbare' mainly comprised two mechanisms. Firstly, in roots, AS caused the reduction of NO ₃ ^? content, which caused two harmful consequences, the large downregulation of OsNR1 expression and the subsequent reduction of NH ₄ ⁺ production in roots. On the other hand, under AS (pH, 9.11), almost all the NH ₄ ⁺ was changed to NH₃, which caused a severe deficiency of NH ₄ ⁺ surrounding the roots. Both events might cause a severe deficiency of NH ₄ ⁺ in roots. Under AS, the increased expression of several OsAMT family members in roots might be an adaptative response to the reduction of NH ₄ ⁺ content in roots or the NH ₄ ⁺ deficiency in rhizosphere. Also, the down-regulation of OsNADH-GOGAT and OsGS1;2 in roots might be due to NH ₄ ⁺ deficiency in roots. Secondly, in shoots, AS caused a larger acuumulatiuon of Na⁺, which possibly affected photorespiration and led to a continuous decrease of NH ₄ ⁺ production in shoots, and inhibited the expression of OsFd-GOGAT and OsGS2 in chloroplasts.     

Ammonium nutrition increases water absorption in rice seedlings (Oryza sativa L.) under water stress

Water stress is a primary limitation on plant growth. In previous studies, it has been found that ammonium enhances the tolerance of rice plants to water stress, but how water is related to nitrogen form and water stress remains unknown. To study the effects of nitrogen form (NH ₄ ⁺ , NO ₃ ^? , and a mixture of NH ₄ ⁺ and NO ₃ ^? ) on the growth and water absorption of rice (Oryza sativa L.) seedlings, a hydroponic experiment with water stress, simulated by the addition of polyethylene glycol (PEG, 10% w/v, MW 6000), was conducted in a greenhouse. The results showed that, compared with non-water stress, under water stress, the fresh weight of rice seedlings increased by 14% with NH ₄ ⁺ nutrition, whereas it had decreased by about 20% with either NO ₃ ^? or mixed nitrogen nutrition. No significant difference was found in the transpiration rate of excised shoots or in xylem exudation of excised roots in NH ₄ ⁺ supply between the two water situations, whereas xylem flow decreased by 57% and 24% under water stress in NO ₃ ^? and mixed nutrition, and root hydraulic conductivity decreased by 29% and 54% in plants in NH ₄ ⁺ and NO ₃ ^? nutrition conditions, respectively. Although water absorption ability decreased in both NH ₄ ⁺ and NO ₃ ^? nutrition, aquaporin activity was higher in NH ₄ ⁺ than in NO ₃ ^? nutrition, regardless of water stress. We conclude that NH ₄ ⁺ nutrition can improve water handling in rice seedlings and subsequently enhance their resistance to drought.     

Biofortification of rice grain with zinc through zinc fertilization in different countries

Aims and background

The ability to suppress soil nitrification through the release of nitrification inhibitors from plant roots is termed ‘biological nitrification inhibition’ (BNI). Earlier, we reported that sorghum roots release higher BNI-activity when grown with NH ₄ ⁺ , but not with NO ₃ ^- as N source. Also for BNI release, rhizosphere pH of <5.0 is needed; beyond this, a negative effect on BNI release was observed with nearly 80% loss of BNI activity at pH >7.0. This study is aimed at understanding the inter-functional relationships associated with NH ₄ ⁺ uptake, rhizosphere-pH and plasma membrane H⁺-ATPase (PM H⁺-ATPase) activity in regulating the release of BNIs (biological nitrification inhibitors) from sorghum roots.

Methods

Sorghum was grown hydroponically and root exudates were collected from intact plants using a pH-stat system to separate the secondary acidification effects by NH ₄ ⁺ uptake on BNIs release. A recombinant luminescent Nitrosomonas europaea bioassay was used to determine BNI-activity. Root plasma membrane was isolated using a two-phase partitioning system. Hydrolytic H⁺-ATPase activity was determined. Split-root system setup was deployed to understand the localized responses to NH ₄ ⁺ , H⁺-ATPase-stimulator (fusicoccin) or H⁺-ATPase-inhibitor (vanadates) on BNI release by sorghum.

Results

Presence of NH ₄ ⁺ in the rhizosphere stimulated the expression of H⁺-ATPase activity and enhanced the release of BNIs from sorghum roots. Fusicoccin, which stimulates H⁺-ATPase activity, also stimulated BNIs release in the absence of NH ₄ ⁺ ; vanadate, which suppresses H⁺-ATPase activity, also suppressed the release of BNIs. NH ₄ ⁺ levels (in rhizosphere) positively influenced BNIs release and root H⁺-ATPase activity in the concentration range of 0-1.0 mM, indicating a close relationship between BNI release and root H⁺-ATPase activity with a possible involvement of carrier-mediated transport for the release of BNIs in sorghum.

Conclusion

Our results suggest that NH ₄ ⁺ uptake, PM H⁺-ATPase activity, and rhizosphere acidification are functionally inter-connected with BNI release in sorghum. Such knowledge is critical to gain insights into why BNI function is more effective in light-textured, mildly acidic soils compared to other soil types.     

Increased photosynthetic capacity in response to nitrate is correlated with enhanced cytokinin levels in rice cultivar with high responsiveness to nitrogen nutrients

Background and aims

Ammonium (NH₄ ⁺) is the preferred nitrogen nutrient over nitrate (NO₃ ^–) in Oryza sativa L. (rice), but photosynthetic capacity is enhanced by partial NO₃ ^– nutrition (PNN). The role of cytokinin in the effects of PNN on photosynthetic capacity is unknown.

Methods

We investigated effects of PNN on six cytokinin fractions in roots, xylem sap, and leaves and on the expression of eight cytokinin synthesis genes in the roots of Nanguang and Elio rice cultivars. The effect of exogenous cytokinin (6-BA) on leaf growth and photosynthetic activity was examined.

Results

Cell expansion and CO₂ assimilation in the first fully expanded leaf were enhanced by PNN in Nanguang but not in Elio. The concentrations of cytokinins in roots, xylem sap, and leaves of Nanguang increased approximately 25–34 % with PNN compared with sole NH₄ ⁺, but no difference was observed in Elio. Exogenous 6-BA counteracted the effects of sole NH₄ ⁺ on leaf growth and photosynthetic activity in both cultivars. OsIPT3 was the key NO₃ ^–-responsive cytokinin synthesis gene in cv. Nanguang.

Conclusions

High NO₃ ^– responsiveness is associated with increased cytokinin synthesis and transport from the root to the leaf and is strongly related to a higher photosynthetic capacity in cv. Nanguang.     

Importance of the nitrogen source in the grass species Brachiaria brizantha responses to sulfur limitation

Background and aims

Plant responses to S supply are highly dependent on N nutrition. We investigated the effect of S status on metabolic, nutritional, and production variables in Brachiaria brizantha treated with different N forms. Additionally, ¹⁵N and ³⁴S root influx were determined in plants under short- and long-term S deprivation.

Methods

Plants were submitted to soil fertilization treatments consisted of combinations of N forms [without N, ammonium (NH₄ ⁺), nitrate (NO₃ ^?) or NH₄ ⁺+NO₃ ^?] at S rates (0, 15, 30, or 45 mg dm^?3). N and S influx capacity was determined in hydroponically-grown plants.

Results

Shoot production due to S supply increased 53, 145 and 196 % with NH₄ ⁺, NH₄ ⁺+NO₃ ^? and NO₃ ^? treatments, respectively. No or low S impaired protein synthesis and led to high accumulation of N-NO₃ ^? and asparagine in NO₃ ^?-fed plants, both alone and with NH₄ ⁺. Proline accumulation was observed in NH₄ ⁺-fed plants. Short- and long-term S deprivation did not promote considerable changes in ¹⁵N influx. ³⁴S absorption decreased depending on the N form provided: NH₄ ⁺+NO₃ ^? > only NH₄ ⁺ > only NO₃ ^? > low N.

Conclusions

Including both NH₄ ⁺ and NO₃ ^? forms in fertilizer increases N and S intake potential and thereby enhances plant growth, nutritional value and production.     

Hydrogen sulfide alleviates aluminum toxicity in barley seedlings

Aims

Aluminum (Al) toxicity is one of the major factors that limit plant growth. Low concentration of hydrogen sulfide (H₂S) has been proven to function in physiological responses to various stresses. The objective of this study is to investigate the possible role of H₂S in Al toxicity in barley (Hordeum vulgare L) seedlings.

Methods

Barley seedlings pre-treated with sodium hydrosulfide (NaHS), a H₂S donor, and subsequently exposed to Al treatment were studied for their effects on root elongation, Al accumulation in seedlings, Al-induced citrate secretion and oxidative stress, and plasma membrane (PM) H⁺-ATPase expression.

Results

Our results showed that H₂S had significant rescue effects on Al-induced inhibition of root elongation which was correlated well with the decrease of Al accumulation in seedlings. Meanwhile, Al-induced citrate secretion was also significantly enhanced by NaHS pretreatment. Al-induced oxidative stress as indicated by lipid peroxidation and reactive oxygen species burst was alleviated by H₂S through the activation of the antioxidant system. Moreover, Al-induced reduction in PM H⁺-ATPase expression was reversed by exogenous NaHS.

Conclusions

Altogether, our results suggest H₂S plays an ameliorative role in protecting plants against Al toxicity by inducing the activities of antioxidant enzymes, increasing citrate secretion and citrate transporter gene expression, and enhancing the expression of PM H⁺-ATPase.     

Influence of different mineral nitrogen sources (NO 3 − -N vs. NH 4 + -N) on arbuscular mycorrhiza development and N transfer in a Glomus intraradices–cowpea symbiosis

Labeled nitrogen (¹⁵?N) was applied to a soil-based substrate in order to study the uptake of N by Glomus intraradices extraradical mycelium (ERM) from different mineral N (NO ₃ ^? vs. NH ₄ ⁺ ) sources and the subsequent transfer to cowpea plants. Fungal compartments (FCs) were placed within the plant growth substrate to simulate soil patches containing root-inaccessible, but mycorrhiza-accessible, N. The fungus was able to take up both N-forms, NO ₃ ^? and NH ₄ ⁺ . However, the amount of N transferred from the FC to the plant was higher when NO ₃ ^? was applied to the FC. In contrast, analysis of ERM harvested from the FC showed a higher ¹⁵?N enrichment when the FC was supplied with ¹⁵NH ₄ ⁺ compared with ¹⁵NO ₃ ^? . The ¹⁵?N shoot/root ratio of plants supplied with ¹⁵NO ₃ ^? was much higher than that of plants supplied with ¹⁵NH ₄ ⁺ , indicative of a faster transfer of ¹⁵NO ₃ ^? from the root to the shoot and a higher accumulation of ¹⁵NH ₄ ⁺ in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH ₄ ⁺ preferentially over NO ₃ ^? but that export of N from the hyphae to the root and shoot may be greater following NO ₃ ^? uptake. The need for NH ₄ ⁺ to be assimilated into organically bound N prior to transport into the plant is discussed.     

Effects of NH4+ concentration on growth,morphology and NH4+ uptake kinetics of Salvinia natans

Many plants develop toxicity symptoms and have reduced growth rates when supplied with ammonium (NH₄⁺) as the only source of inorganic nitrogen. In the present study, the growth, morphology, NH₄⁺ uptake kinetics and mineral concentrations in the tissues of the free-floating aquatic plant Salvinia natans (water fern) supplied exclusively with NH₄⁺–N at concentrations of 0.25–15 mM were investigated. S. natans grew well, with relative growth rates of c. 0.25 g g^?1 d^?1 at external NH₄⁺ concentrations up to 5 mM, but at higher levels growth was suppressed and the plants had small leaves and short roots with stunted growth. The high-affinity transport system (HATS) that mediate NH₄⁺ uptake at dilute NH₄⁺ levels was downregulated at high NH₄⁺ concentrations with lower velocities of maximum uptake (V_max) and higher half-saturation constants (K_1/2). High NH₄⁺ levels also barely affected the concentrations of mineral cations and anions in the plant tissue. It is concluded that S. natans can be characterized as NH₄⁺-tolerant in line with a number of other species of wetland plants as growth was unaffected at NH₄⁺ concentrations as high as 5 mM and as symptoms of toxicity at higher concentrations were relatively mild. Depolarization of the plasma membrane to the equilibrium potential for NH₄⁺ at high external concentrations may be a mechanism used by the plant to avoid excessive futile transmembrane cycling. S. natans is tolerant to the high NH₄⁺ levels that prevail in domestic and agricultural wastewaters, and the inherent high growth rate and the ease of biomass harvesting make S. natans a primary candidate for use in constructed wetland systems for the treatment of various types of nitrogen-rich wastewaters.     

The effects of root surface charge and nitrogen forms on the adsorption of aluminum ions by the roots of rice with different aluminum tolerances

Aim

Our objectives were to compare effects of root charge properties on Al adsorption by the roots of rice that differed in Al-tolerance, and to examine effects of different nitrogen forms on charge properties of rice roots and Al adsorption.

Methods

Streaming potential and chemical methods were used to measure root zeta potential and investigate Al chemical forms adsorbed on the roots of rice obtained from solution culture experiments.

Results

Rice roots of the Al-sensitive variety Yangdao-6 carried greater negative charge than the Al-tolerant variety Wuyunjing-7, which meant the roots of Yangdao-6 adsorbed more exchangeable and complexed Al. When both rice varieties were grown in NH₄ ⁺-containing nutrient solutions, there were less functional groups and lower negative surface charge on their roots, which reduced Al adsorption compared to the rice grown in NO₃ ^? containing nutrient solutions. The decline in nutrient solution pH due to NH₄ ⁺ uptake by rice roots was responsible for the reduced numbers of functional groups and the lower negative surface charge on the roots compared to the rice grown in NO₃ ^? containing solutions.

Conclusions

Integrated root surface charge, as expressed by zeta potential, played an important role in Al adsorption by the roots of rice with different Al-tolerance.

     

Cell type distinct accumulations of mRNA and protein for NADH-dependent glutamate synthase in rice roots in response to the supply of NH4+

Transient and NH₄⁺-inducible accumulation of the mRNA for NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14) in the roots of rice seedlings was analyzed in situ to identify the cell types responsible for the induction. The mRNA was detected specifically in sclerenchyma cells (the third cell-layer from the root surface), and the maximal accumulation was seen at 3–6 h following the supply of NH₄⁺ ions. Expression of the NADH-GOGAT gene in sclerenchyma cells was also confirmed using transgenic rice plants expressing GUS reporter gene under the control of rice NADH-GOGAT promoter. On the other hand, clear signals for the NADH-GOGAT protein were detected in epidermial cells and exodermal cells (the first and second cell layers from the root surface) at 12 h, following the supply of NH₄⁺ ions. The distinct localization of mRNA and protein for NADH-GOGAT suggests that either the mRNA or the translated protein in the sclerenchyma cells is migrated to the root surface. In contrast to NADH-GOGAT protein, Fd-GOGAT (EC 1.4.7.1) protein was detected in sclerenchyma cells, cortex cells, and stele in the rice roots. The distinct localization of the two GOGAT species indicates that they have different roles in the nitrogen metabolism in rice roots.     

Diel plant water use and competitive soil cation exchange interact to enhance NH<Subscript>4</Subscript><Superscript>+</Superscript> and K<Superscript>+</Superscript> availability in the rhizosphere

Aims

Hydro-biogeochemical processes in the rhizosphere regulate nutrient and water availability, and thus ecosystem productivity. We hypothesized that two such processes often neglected in rhizosphere models — diel plant water use and competitive cation exchange — could interact to enhance availability of K⁺ and NH₄ ⁺, both high-demand nutrients.

Methods

A rhizosphere model with competitive cation exchange was used to investigate how diel plant water use (i.e., daytime transpiration coupled with no nighttime water use, with nighttime root water release, and with nighttime transpiration) affects competitive ion interactions and availability of K⁺ and NH₄ ⁺.

Results

Competitive cation exchange enabled low-demand cations that accumulate against roots (Ca²⁺, Mg²⁺, Na⁺) to desorb NH₄ ⁺ and K⁺ from soil, generating non-monotonic dissolved concentration profiles (i.e. ‘hotspots’ 0.1–1 cm from the root). Cation accumulation and competitive desorption increased with net root water uptake. Daytime transpiration rate controlled diel variation in NH₄ ⁺ and K⁺ aqueous mass, nighttime water use controlled spatial locations of ‘hotspots’, and day-to-night differences in water use controlled diel differences in ‘hotspot’ concentrations.

Conclusions

Diel plant water use and competitive cation exchange enhanced NH₄ ⁺ and K⁺ availability and influenced rhizosphere concentration dynamics. Demonstrated responses have implications for understanding rhizosphere nutrient cycling and plant nutrient uptake.

     

Human SLC4A11 Is a Novel NH3/H+ Co-transporter

SLC4A11 has been proposed to be an electrogenic membrane transporter, permeable to Na⁺, H⁺ (OH⁻), bicarbonate, borate, and NH₄⁺. Recent studies indicate, however, that neither bicarbonate or borate is a substrate. Here, we examined potential NH₄⁺, Na⁺, and H⁺ contributions to electrogenic ion transport through SLC4A11 stably expressed in Na⁺/H⁺ exchanger-deficient PS120 fibroblasts. Inward currents observed during exposure to NH₄Cl were determined by the [NH₃]_o, not [NH₄⁺]_o, and current amplitudes varied with the [H⁺] gradient. These currents were relatively unaffected by removal of Na⁺, K⁺, or Cl⁻ from the bath but could be reduced by inclusion of NH₄Cl in the pipette solution. Bath pH changes alone did not generate significant currents through SLC4A11, except immediately following exposure to NH₄Cl. Reversal potential shifts in response to changing [NH₃]_o and pH_o suggested an NH₃/H⁺-coupled transport mode for SLC4A11. Proton flux through SLC4A11 in the absence of ammonia was relatively small, suggesting that ammonia transport is of more physiological relevance. Methylammonia produced currents similar to NH₃ but with reduced amplitude. Estimated stoichiometry of SLC4A11 transport was 1:2 (NH₃/H⁺). NH₃-dependent currents were insensitive to 10 μm ethyl-isopropyl amiloride or 100 μm 4,4′- diisothiocyanatostilbene-2,2′-disulfonic acid. We propose that SLC4A11 is an NH₃/2H⁺ co-transporter exhibiting unique characteristics.     

Nitrate nutrition enhances nickel accumulation and toxicity in Arabidopsis plants

Background and aims

Nickel (Ni) has become a major heavy metal contaminant. The form of nitrogen nutrition remarkably affects IRT1 expression in roots. IRT1 has an activity of transporting Ni²⁺ into root cells. Therefore, nitrogen-form may affect Ni accumulation and toxicity in plants. The assumption was investigated in this study.

Methods

The Arabidopsis plants were treated in Ni-contained growth solutions with either nitrate (NO₃ ^?) or ammonium (NH₄ ⁺) as the sole N source. After 7-day treatments, Ni concentration, IRT1 expression, Ni-induced toxic symptoms and oxidative stress in plants were analyzed.

Results

The NO₃ ^?-fed plants contained a higher Ni concentration, had a greater IRT1 expression in roots, and developed more severe toxic symptoms in the youngest fully expanded leaves, compared with the NH₄ ⁺-fed plants. The Ni-induced growth inhibition was also more significant in NO₃ ^?-fed plants. Interestingly, Ni exposure resulted in greater hydrogen peroxide (H₂O₂) and superoxide radical (O₂ ^{. ?}) accumulations, more severe lipid peroxidation and more cell death in NO₃ ^?-fed plants, whereas the opposite was true for NH₄ ⁺-fed plants. Furthermore, the Ni-enhanced peroxidase (POD) and superoxide dismutase (SOD) activities were greater in NO₃ ^?-fed plants

Conclusion

NO₃ ^? nutrition promotes Ni uptake, and enhances Ni-induced growth inhibition and oxidative stress in plants compared with NH₄ ⁺ nutrition.