OsEIL1 is involved in the response to heterogeneous high ammonium in rice: A split-root analysis

Ammonium (NH4+) concentrations in rice fields show heterogeneous spatial distribution under the combined influences of nitrogen fertilizer application and modern agronomic practices. However, the characteristics and mechanisms of rice roots in response to heterogeneous NH4+ supply are not well understood. Here, we found a systemic response of rice roots to heterogeneous and high (10 mM) NH4+ supply using a split-root experiment, and show root growth on the NH4+-free (NO3-) side was also inhibited by localized high-NH4+ supply. Moreover, OsEIL1 (encoding a core transcription factor in the ethylene signaling pathway) was found to be involved in the response of rice roots to heterogeneous NH4+. OsEIL1 mutation significantly increased the inhibitory effect of localized high-NH4+ on root growth of the NO3- side, as well as significantly increased NH4+ efflux there. Furthermore, our results indicate that the mitigating effect of OsEIL1 on NH4+ efflux is related to the regulated expression of OsVTC1-3 (encoding a GDP-mannose pyrophosphorylase). These findings provide insight into the mechanisms by which OsEIL1 responds to heterogeneous high NH4+ and contribute to our understanding of rice adaptation to heterogeneous NH4+ supply.

Nitrogen-loss and carbon-footprint reduction by plant-rhizosphere exudates

Low-carbon approaches to agriculture constitute a pivotal measure to address the challenge of global climate change. In agroecosystems, rhizosphere exudates are significantly involved in regulating the nitrogen (N) cycle and facilitating belowground chemical communication between plants and soil microbes to reduce direct and indirect emissions of greenhouse gases (GHGs) and control N runoff from cultivated sites into natural water bodies. Here, we discuss specific rhizosphere exudates from plants and microorganisms and the mechanisms by which they reduce N loss and subsequent N pollution in terrestrial and aquatic environments, including biological nitrification inhibitors (BNIs), biological denitrification inhibitors (BDIs), and biological denitrification promoters (BDPs). We also highlight promising application scenarios and challenges in relation to rhizosphere exudates in terrestrial and aquatic environments.

Plant iron status regulates ammonium-use efficiency through protein N-glycosylation

Improving nitrogen-use efficiency (NUE) is an important path toward enhancing crop yield and alleviating the environmental impacts of fertilizer use. Ammonium (NH4+) is the energetically preferred inorganic N source for plants. The interaction of NH4+ with other nutrients is a chief determinant of ammonium-use efficiency (AUE) and of the tipping point toward ammonium toxicity, but these interactions have remained ill-defined. Here, we report that iron (Fe) accumulation is a critical factor determining AUE and have identified a substance that can enhance AUE by manipulating Fe availability. Fe accumulation under NH4+ nutrition induces NH4+ efflux in the root system, reducing both growth and AUE in Arabidopsis (Arabidopsis thaliana). Low external availability of Fe and a low plant Fe status substantially enhance protein N-glycosylation through a Vitamin C1 (VTC1)-independent pathway, thereby reducing NH4+ efflux to increase AUE during the vegetative stage in Arabidopsis under elevated NH4+ supply. We confirm the validity of the iron-ammonium interaction in the important crop species lettuce (Lactuca sativa). We further show that dolomite can act as an effective substrate to subdue Fe accumulation under NH4+ nutrition by reducing the expression of Low Phosphate Root 2 (LPR2) and acidification of the rhizosphere. Our findings present a strategy to improve AUE and reveal the underlying molecular-physiological mechanism.

Smallholder vegetable farming produces more soil microplastics pollution than large-scale farming

Microplastics (MPs) accumulation in farmland has attracted global concern. Smallholder farming is the dominant type in China's agriculture. Compared with large-scale farming, smallholder farming is not constrained by restrictive environmental policies and public awareness about pollution. Consequently, the degree to which smallholder farming is associated with MP pollution in soils is largely unknown. Here, we collected soil samples from both smallholder and large-scale vegetable production systems to determine the distribution and characteristics of MPs. MP abundance in vegetable soils was 147.2-2040.4 MP kg^-1 (averaged with 500.8 MP kg^-1). Soil MP abundance under smallholder cultivation (730.9 MP kg^-1) was twice that found under large-scale cultivation (370.7 MP kg^-1). MP particle sizes in smallholder and large-scale farming were similar, and were mainly <1 mm. There were also differences in MP characteristics between the two types of vegetable soils: fragments (60%) and fibers (34%) were dominant under smallholder cultivation, while fragments (42%), fibers (42%), and films (11%) were dominant under large-scale cultivation. We observed a significant difference in the abundance of fragments and films under smallholder versus large-scale cultivation; the main components of MPs under smallholder cultivation were PP (34%), PE (28%), and PE-PP (10%), while these were PE (29%), PP (16%), PET (16%), and PE-PP (13%) under large-scale cultivation. By identifying the shape and composition of microplastics, it can be inferred that agricultural films were not the main MP pollution source in vegetable soil. We show that smallholder farming produces more microplastics pollution than large-scale farming in vegetable soil.

The nitrification inhibitor 1,9-decanediol from rice roots promotes root growth in Arabidopsis through involvement of ABA and PIN2-mediated auxin signaling

1,9-decanediol (1,9-D) is a biological nitrification inhibitor secreted in roots, which effectively inhibits soil nitrifier activity and reduces nitrogen loss from agricultural fields. However, the effects of 1,9-D on plant root growth and the involvement of signaling pathways in the plant response to 1,9-D have not been investigated. Here, we report that 1,9-D, in the 100-400 μM concentration range, promotes primary root length in Arabidopsis seedlings at 3d and 5d, by 10.1%-33.3% and 6.9%-32.6%, and, in a range of 50-200 μM, leads to an increase in the number of lateral roots. 150 μM 1,9-D was found optimum for the positive regulation of root growth. qRT-PCR analysis reveals that 1,9-D can significantly increase AtABA3 gene expression and that a mutation in ABA3 results in insensitivity of root growth to 1,9-D. Moreover, through pharmacological experiments, we show that exogenous addition of ABA (abscisic acid) with 1,9-D enhances primary root length by 23.5%-63.3%, and an exogenous supply of 1,9-D with the ABA inhibitor Flu reduces primary root length by 1.0%-14.3%. Primary root length of the pin2/eir1-1 is shown to be insensitive to both exogenous addition of 1,9-D and ABA, indicating that the auxin carrier PIN2/EIR1 is involved in promotion of root growth by 1,9-D. These results suggest a novel for 1,9-D in regulating plant root growth through ABA and auxin signaling.

Syringic acid from rice roots inhibits soil nitrification and N2O emission under red and paddy soils but not a calcareous soil

Syringic acid (SA) is a novel biological nitrification inhibitor (BNIs) discovered in rice root exudates with significant inhibition of Nitrosomonas strains. However, the inhibitory effect of SA on nitrification and nitrous oxide (N₂O) emissions in different soils and the environmental factors controlling the degree of inhibition have not been studied. Using 14-day microcosm incubation, we investigated the effects of different concentrations of SA on nitrification activity, abundance of ammonia-oxidizing microorganisms, and N₂O emissions in three typical agricultural soils. The nitrification inhibitory efficacy of SA was strongest in acidic red soil, followed by weakly acidic paddy soil, with no significant effect in an alkaline calcareous soil. Potential nitrification activity (PNA) were also greatly reduced by SA additions in paddy and red soil. Pearson correlation analysis showed that the inhibitory efficacy of SA might be negatively correlated with soil pH and positively correlated with clay percentage. SA treatments significantly reduced N₂O emissions by 69.1-79.3% from paddy soil and by 40.8%-46.4% from red soil, respectively, but no effect was recorded in the calcareous soil. SA addition possessed dual inhibition of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) abundance in paddy and red soil. Structural equation modelling revealed that soil ammonium (NH₄ ⁺) and dissolved organic carbon content (DOC) were the key variables explaining AOA and AOB abundance and subsequent N₂O emissions. Our results support the potential for the use of the BNI SA in mitigating N₂O emissions and enhancing N utilization in red and paddy soils.

OsEIL1 protects rice growth under NH4+ nutrition by regulating OsVTC1-3-dependent N-glycosylation and root NH4+ efflux

Rice is known for its superior adaptation to ammonium (NH₄⁺ ) as a nitrogen source. Compared to many other cereals, it displays lower NH₄⁺ efflux in roots and higher nitrogen-use efficiency on NH₄⁺ . A critical role for GDP-mannose pyrophosphorylase (VTC1) in controlling root NH₄⁺ fluxes was previously documented in Arabidopsis, but the molecular pathways involved in regulating VTC1-dependent NH₄⁺ efflux remain unclear. Here, we report that ETHYLENE-INSENSITIVE3-LIKE1 (OsEIL1) acts as a key transcription factor regulating OsVTC1-3-dependent NH₄⁺ efflux and protein N-glycosylation in rice grown under NH₄⁺ nutrition. We show that OsEIL1 in rice plays a contrasting role to Arabidopsis-homologous ETHYLENE-INSENSITIVE3 (AtEIN3) and maintains rice growth under NH₄⁺ by stabilizing protein N-glycosylation and reducing root NH₄⁺ efflux. OsEIL1 constrains NH₄⁺ efflux by activation of OsVTC1-3, but not OsVTC1-1 or OsVTC1-8. OsEIL1 binds directly to the promoter EIN3-binding site (EBS) of OsVTC1-3 in vitro and in vivo and acts to increase the transcription of OsVTC1-3. Our work demonstrates an important link between excessive root NH₄⁺ efflux and OsVTC1-3-mediated protein N-glycosylation in rice grown under NH₄⁺ nutrition and identifies OsEIL1 as a direct genetic regulator of OsVTC1-3 expression.

The Role of Plant Growth Regulators in Modulating Root Architecture and Tolerance to High-Nitrate Stress in Tomato

Plant growth regulators are known to exert strong influences on plant performance under abiotic stress, including exposure to high nitrate, as occurs commonly in intensive vegetable production. However, direct comparative evaluations of growth regulators under otherwise identical conditions in major crop species are scarce. In this study, tomato (Solanum lycopersicum L.) was used as a model crop, and the roles of four common exogenously applied plant growth regulators (MT, melatonin; SA, salicylic acid; HA, humic acid; SNP, sodium nitroprusside) in regulating crop growth were studied under high-nitrate stress. We provide a particular focus on root system architecture and root physiological responses. Our data show that all four growth regulators improve tomato tolerance under high nitrate, but that this occurs to differing extents and via differing mechanisms. Optimal concentrations of MT, SA, HA, and SNP were 50 μmol L^-1, 25 μmol L^-1, 25 mg L^-1, and 50 μmol L^-1, respectively. MT and SNP produced the strongest effects. MT enhanced root growth while SNP enhanced above-ground growth. Growth of coarse and thin lateral roots was significantly improved. Furthermore, an enhancement of root vitality and metabolism, improved integrity of root cell membranes, and an increase in antioxidant enzyme activities were found, but regulatory mechanisms were different for each growth regulator. Our results show that in particular the application of MT and SNP can improve growth of tomato in intensive vegetable production under high-nitrate stress and that root growth stimulation is of special importance in procuring these beneficial effects.

The good and the bad of preprint servers in plant physiology

Preprint servers allow rapid publication of research findings by eliminating the time gap between submission and publication associated with editorial and peer review of scientific works. Consequently, non-peer-reviewed articles are essentially accessible immediately to researchers and the public. There are many valid justifications for sharing work on preprint servers, such as the ability to collect feedback from the research community and improve work prior to journal submission and a reduced risk of work being "scooped" by competitors. Rapid access to the latest scientific developments can furthermore expedite progress in important research areas. Significant downsides of preprint servers, however, are that the public, including members of the media and policy makers, cannot judge the quality of such non-reviewed publications and that misinformation may be spread. Balancing the good and the bad of preprint servers as opposed to classic peer review, we provide guidance for authors of the Journal of Plant Physiology.

OsGF14b is involved in regulating coarse root and fine root biomass partitioning in response to elevated [CO2] in rice

Elevated [CO₂] can increase rice biomass and yield, but the degree of this increase varies substantially among cultivars. Little is known about the gene loci involved in the acclimation and adaptation to elevated [CO₂] in rice. Here, we report on a T-DNA insertion mutant in japonica rice exhibiting a significantly enhanced response to elevated [CO₂] compared with the wild type (WT). The root biomass response of the mutant was higher than that of the WT, and this manifested in the number of adventitious roots, the average diameter of roots, and total root length. Furthermore, coarse roots (>0.6 mm) and thin lateral roots (<0.2 mm) were more responsive to elevated [CO₂] in the mutant. When exposed to lower light intensity, however, the response of the mutant to elevated [CO₂] was not superior to that of the WT, indicating that the high response of the mutant under elevated [CO₂] was dependent on light intensity. The T-DNA insertion site was located in the promoter region of the OsGF14b gene, and insertion resulted in a significant decrease in OsGF14b expression. Our results indicate that knockout of OsGF14b may improve the response to elevated [CO₂] in rice by enhancing carbon allocation to coarse roots and to fine lateral roots.

Coordination of nitrogen uptake and assimilation favours the growth and competitiveness of moso bamboo over native tree species in high-NH4+ environments

Phenotypic plasticity and competitive strength are major mechanisms determining the success of invasive species and are influenced by abiotic factors. A rise in the ratio of ammonium (NH₄⁺) to nitrate (NO₃^-) in soils is frequently associated with the invasion of bamboo into broad-leaved evergreen forests. However, the influence of soil nitrogen (N) chemistry on plant growth and interspecific competition in the context of invasion remains insufficiently studied. In the present work, differences in plasticity and interspecific competition between native tree species in broad-leaved evergreen forests and invasive bamboo in response to different N forms were investigated using seedlings grown in a controlled environment. We show that moso bamboo responded positively and strongly to increased soil NH₄⁺/NO₃^- ratios, while the native tree species Sapium sebiferum, Camellia oleifera, and Machilus pauhoi responded negatively and exhibited limited plasticity. Native tree species growth was significantly inhibited in the presence of moso bamboo under high-NH₄⁺ conditions, whereas native tree species were less affected by interspecific competition when NO₃^- was supplied as the sole N source. By contrast, moso bamboo growth was significantly inhibited, followed by seedling death, in both monoculture and in mixed culture with prolonged NO₃^- treatment. All species tested exhibited significantly higher rates of ¹⁵NH₄⁺ than ¹⁵NO₃^- uptake, but the Michaelis constant (K_m) for ¹⁵NH₄⁺ uptake was lower in moso bamboo, indicating higher substrate affinity. Nitrate reductase (NR) and nitrite reductase (NiR) activities showed no inducible effects in moso bamboo compared to the induction response seen in the native tree species in response to NO₃^-. Activities of glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) significantly increased with NH₄⁺ provision in roots of moso bamboo, contrasted by a less plastic response in the native tree species. Enhanced ammonification and reduced nitrification in soils is typically observed during bamboo invasion and appears to create a positive soil-plant feedback loop that, due to highly flexible and opportunistic NH₄⁺-acquisition pathways, favours bamboo fitness and invasion into native forests when NH₄⁺ is the dominant N form.

Stigmasterol root exudation arising from Pseudomonas inoculation of the duckweed rhizosphere enhances nitrogen removal from polluted waters

Rhizospheric microorganisms such as denitrifying bacteria are able to affect 'rhizobioaugmention' in aquatic plants and can help boost wastewater purification by benefiting plant growth, but little is known about their effects on the production of plant root exudates, and how such exudates may affect microorganismal nitrogen removal. Here, we assess the effects of the rhizospheric Pseudomonas inoculant strain RWX31 on the root exudate profile of the duckweed Spirodela polyrrhiza, using gas chromatography/mass spectrometry. Compared to untreated plants, inoculation with RWX31 specifically induced the exudation of two sterols, stigmasterol and β-sitosterol. An authentic standard assay revealed that stigmasterol significantly promoted nitrogen removal and biofilm formation by the denitrifying bacterial strain RWX31, whereas β-sitosterol had no effect. Assays for denitrifying enzyme activity were conducted to show that stigmasterol stimulated nitrogen removal by targeting nitrite reductase in bacteria. Enhanced N removal from water by stigmasterol, and a synergistic stimulatory effect with RWX31, was observed in open duckweed cultivation systems. We suggest that this is linked to a modulation of community composition of nirS- and nirK-type denitrifying bacteria in the rhizosphere, with a higher abundance of Bosea, Rhizobium, and Brucella, and a lower abundance of Rubrivivax. Our findings provide important new insights into the interaction of duckweed with the rhizospheric bacterial strain RWX31 and their involvement in the aquatic N cycle and offer a new path toward more effective bio-formulations for the purification of N-polluted waters.

WRKY46 promotes ammonium tolerance in Arabidopsis by repressing NUDX9 and indole-3-acetic acid-conjugating genes and by inhibiting ammonium efflux in the root elongation zone

Ammonium (NH₄⁺ ) is toxic to root growth in most plants, even at moderate concentrations. Transcriptional regulation is one of the most important mechanisms in the response of plants to NH₄⁺ toxicity, but the nature of the involvement of transcription factors (TFs) in this regulation remains unclear. Here, RNA-seq analysis was performed on Arabidopsis roots to screen for ammonium-responsive TFs. WRKY46, the member of the WRKY transcription factor family most responsive to NH₄⁺ , was selected. We defined the role of WRKY46 using mutation and overexpression assays, and characterized the regulation of NUDX9 and indole-3-acetic acid (IAA)-conjugating genes by WRKY46 via yeast one-hybrid and electrophoretic mobility shift assays and chromatin immunoprecipitation-quantitative real-time polymerase chain reaction (ChIP-qPCR). Knockout of WRKY46 increased, while overexpression of WRKY46 decreased, NH₄⁺ -suppression of the primary root. WRKY46 is shown to directly bind to the promoters of the NUDX9 and IAA-conjugating genes (GH3.1, GH3.6, UGT75D1, UGT84B2) and to inhibit their transcription, thus positively regulating free IAA content and stabilizing protein N-glycosylation, leading to an inhibition of NH₄⁺ efflux in the root elongation zone (EZ). We identify TF involvement in the regulation of NH₄⁺ efflux in the EZ, and show that WRKY46 inhibits NH₄⁺ efflux by negative regulation of NUDX9 and IAA-conjugating genes.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

High ammonium inhibits root growth in Arabidopsis thaliana by promoting auxin conjugation rather than inhibiting auxin biosynthesis

Ammonium (NH₄⁺) inhibits primary root (PR) growth in most plant species when present even at moderate concentrations. Previous studies have shown that transport of indole-3-acetic acid (IAA) is critical to maintaining root elongation under high-NH₄⁺ stress. However, the precise regulation of IAA homeostasis under high-NH₄⁺ stress (HAS) remains unclear. In this study, qRT-PCR, RNA-seq, free IAA and IAA conjugate and PR elongation measurements were conducted in genetic mutants to investigate the role of IAA biosynthesis and conjugation under HAS. Our data clearly show that HAS decreases free IAA in roots by increasing IAA inactivation but does not decrease IAA biosynthesis, and that the IAA-conjugating genes GH3.1, GH3.2, GH3.3, GH3.4, and GH3.6 function as the key genes in regulating high-NH₄⁺ sensitivity in the roots. Furthermore, the analysis of promoter::GUS staining in situ and genetic mutants reveals that HAS promotes IAA conjugation in the elongation zone (EZ), which may be responsible for the PR inhibition observed under HAS. This study provides potential new insight into the role of auxin in the improvement of tolerance to NH₄⁺.

Induction of S-nitrosoglutathione reductase protects root growth from ammonium toxicity by regulating potassium homeostasis in Arabidopsis and rice

Ammonium (NH4+) is toxic to root growth in most plants already at moderate levels of supply, but mechanisms of root growth tolerance to NH4+ remain poorly understood. Here, we report that high levels of NH4+ induce nitric oxide (NO) accumulation, while inhibiting potassium (K+) acquisition via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4), leading to the arrest of primary root growth. High levels of NH4+ also stimulated the accumulation of GSNOR (S-nitrosoglutathione reductase) in roots. GSNOR overexpression improved root tolerance to NH4+. Loss of GSNOR further induced NO accumulation, increased SNO1/SOS4 activity, and reduced K+ levels in root tissue, enhancing root growth sensitivity to NH4+. Moreover, the GSNOR-like gene, OsGSNOR, is also required for NH4+ tolerance in rice. Immunoblotting showed that the NH4+-induced GSNOR protein accumulation was abolished in the VTC1- (vitamin C1) defective mutant vtc1-1, which is hypersensititive to NH4+ toxicity. GSNOR overexpression enhanced vtc1-1 root tolerance to NH4+. Our findings suggest that induction of GSNOR increases NH4+ tolerance in Arabidopsis roots by counteracting NO-mediated suppression of tissue K+, which depends on VTC1 function.

Nitrogen depletion enhances endodermal suberization without restricting transporter-mediated root NO3- influx

Roots vary their permeability to aid radial transport of solutes towards xylem vessels in response to nutritional cues. Nitrogen (N) depletion was previously shown to induce early suberization of endodermal cell walls and reduce hydraulic conductivity of barley roots suggesting reduced apoplastic transport of ions (Armand et al., 2019). Suberization may also limit transcellular ion movement by blocking access to transporters (Barberon et al., 2016). The aim of this study was to confirm that N depletion induced suberization in the roots of barley and demonstrate that this was a specific effect in response to NO₃^- depletion. Furthermore, in roots with early and enhanced suberization, we assessed their ability for transporter-mediated NO₃^- influx. N depletion induced lateral root elongation and early and enhanced endodermal suberization of the seminal root of each genotype. Both root to shoot NO₃^- translocation and net N uptake was half that of plants supplied with steady-state NO₃^-. Genes with predicted functions in suberin synthesis (HvHORST) and NO₃^- transport (HvNRT2.2) were induced under N-deplete conditions. N-deplete roots had a higher capacity for high-affinity NO₃^- influx in early suberized roots than under optimal NO₃^-. In conclusion, NO₃^- depletion induced early and enhanced suberization in the roots of barley, however, suberization did not restrict transcellular NO₃^- transport.

Continuous monitoring of plant sodium transport dynamics using clinical PET

BACKGROUND

The absorption, translocation, accumulation and excretion of substances are fundamental processes in all organisms including plants, and have been successfully studied using radiotracers labelled with ¹¹C, ¹³N, ¹⁴C and ²²Na since 1939. Sodium is one of the most damaging ions to the growth and productivity of crops. Due to the significance of understanding sodium transport in plants, a significant number of studies have been carried out to examine sodium influx, compartmentation, and efflux using ²²Na- or ²⁴Na-labeled salts. Notably, however, most of these studies employed destructive methods, which has limited our understanding of sodium flux and distribution characteristics in real time, in live plants. Positron emission tomography (PET) has been used successfully in medical research and diagnosis for decades. Due to its ability to visualise and assess physiological and metabolic function, PET imaging has also begun to be employed in plant research. Here, we report the use of a clinical PET scanner with a ²²Na tracer to examine ²²Na-influx dynamics in barley plants (Hordeum vulgare L. spp. Vulgare-cultivar Bass) under variable nutrient levels, alterations in the day/night light cycle, and the presence of sodium channel inhibitors.

RESULTS

3D dynamic PET images of whole plants show readily visible ²²Na translocation from roots to shoots in each examined plant, with rates influenced by both nutrient status and channel inhibition. PET images show that plants cultivated in low-nutrient media transport more ²²Na than plants cultivated in high-nutrient media, and that ²²Na uptake is suppressed in the presence of a cation-channel inhibitor. A distinct diurnal pattern of ²²Na influx was discernible in curves displaying rates of change of relative radioactivity. Plants were found to absorb more ²²Na during the light period, and anticipate the change in the light/dark cycle by adjusting the sodium influx rate downward in the dark period, an effect not previously described experimentally.

CONCLUSIONS

We demonstrate the utility of clinical PET/CT scanners for real-time monitoring of the temporal dynamics of sodium transport in plants. The effects of nutrient deprivation and of ion channel inhibition on sodium influx into barley plants are shown in two proof-of-concept experiments, along with the first-ever 3D-imaging of the light and dark sodium uptake cycles in plants. This method carries significant potential for plant biology research and, in particular, in the context of genetic and treatment effects on sodium acquisition and toxicity in plants.

Comparative analysis reveals gravity is involved in the MIZ1-regulated root hydrotropism

Hydrotropism is the directed growth of roots toward the water found in the soil. However, mechanisms governing interactions between hydrotropism and gravitropism remain largely unclear. In this study, we found that an air system and an agar-sorbitol system induced only oblique water-potential gradients; an agar-glycerol system induced only vertical water-potential gradients; and a sand system established both oblique and vertical water-potential gradients. We employed obliquely oriented and vertically oriented experimental systems to study hydrotropism in Arabidopsis and tomato plants. Comparative analyses using different hydrotropic systems showed that gravity hindered the ability of roots to search for obliquely oriented water, whilst facilitating roots' search for vertically oriented water. We found that the gravitropism-deficient mutant aux1 showed enhanced hydrotropism in the oblique orientation but impaired root elongation towards water in the vertical orientation. The miz1 mutant exhibited deficient hydrotropism in the oblique orientation but normal root elongation towards water in the vertical orientation. Importantly, in contrast to miz1, the miz1/aux1 double mutant exhibited hydrotropic bending in the oblique orientation and attenuated root elongation towards water in the vertical orientation. Our results suggest that gravitropism is required for MIZ1-regulated root hydrotropism in both the oblique orientation and the vertical orientation, providing further insight into the role of gravity in root hydrotropism.

Selenium Biofortification and Interaction With Other Elements in Plants: A Review

Selenium (Se) is an essential element for humans and animals and its deficiency in the diet is a global problem. Crop plants are the main source of Se for consumers. Therefore, there is much interest in understanding the factors that govern the accumulation and distribution of Se in the tissues of crop plants and the mechanisms of interaction of Se absorption and accumulation with other elements, especially with a view toward optimizing Se biofortification. An ideal crop for human consumption is rich in essential nutrient elements such as Se, while showing reduced accumulation of toxic elements in its edible parts. This review focuses on (a) summarizing the nutritional functions of Se and the current understanding of Se uptake by plant roots, translocation of Se from roots to shoots, and accumulation of Se in grains; and (b) discussing the influence of nitrogen (N), phosphorus (P), and sulfur (S) on the biofortification of Se. In addition, we discuss interactions of Se with major toxicant metals (Hg, As, and Cd) frequently present in soil. We highlight key challenges in the quest to improve Se biofortification, with a focus on both agronomic practice and human health.

Superior growth, N uptake and NH4+ tolerance in the giant bamboo Phyllostachys edulis over the broad-leaved tree Castanopsis fargesii at elevated NH4+ may underlie community succession and favor the expansion of bamboo

The unbridled expansion of bamboo has imposed serious threats on ecosystem processes and functions. Considerable evidence indicates that bamboo invasions can alter plant-available soil nitrogen (N) pools and rates of N cycling, but the consequences of altered N availability for plant growth and community structure have thus far been poorly characterized. The primary soil-accessible N forms for most plants are ammonium (NH4+) and nitrate (NO3-), but plants differ in their ability to use the different N forms, and these differences can be related to their ecological characteristics and drive community structure. In this context, we evaluated the growth response, N uptake and interspecific competition in two subtropical species, Phyllostachys edulis (Carrière) J. Houzeau (Synonym Phyllostachys heterocycla Carrière) and Castanopsis fargesii Franch., dominant species of bamboo and secondary evergreen broad-leaved forests, respectively, under changing N availability in seedlings supplied with different N concentrations and NH4+/NO3- proportions, in vermiculite culture, in a controlled environment. The results show that (i) both species display an NH4+ preference at elevated N concentrations. The growth of P. edulis seedlings was strongly enhanced at increased ratios of NH4+ to NO3- especially at higher N concentrations, but to a much lesser extent in C. fargesii. (ii) NH4+ preference at the level of N uptake in both species was confirmed by the Non-invasive Micro-test Technology and by examining 15N signatures. Phyllostachys edulis had higher NH4+ net fluxes and N concentration under NH4+ treatments than C. fargesii. (iii) NH4+ at higher concentrations caused toxicity in both species as it inhibited root growth and even caused seedling death, but P. edulis had a higher NH4+-tolerance threshold (24 mM) than C. fargesii (16 mM). (iv) When mixed-species cultures were examined in an NH4+-rich medium, the growth of C. fargesii, but not P. edulis, was significantly inhibited compared with growth in monoculture. Therefore, P. edulis exhibited stronger plasticity and adaptation to changing N availability, whereas C. fargesii had low responsiveness and capacity to acclimate to soil N changes. Phyllostachys edulis displayed a significant competitive growth advantage compared with C. fargesii on NH4+-dominated substrates.

Root-Apex Proton Fluxes at the Centre of Soil-Stress Acclimation

Proton (H⁺) fluxes in plant roots play critical roles in maintaining root growth and facilitating plant responses to multiple soil stresses, including fluctuations in nutrient supply, salt infiltration, and water stress. Soil mining for nutrients and water, rates of nutrient uptake, and the modulation of cell expansion all depend on the regulation of root H⁺ fluxes, particularly at the root apex, mediated primarily by the activity of plasma membrane (PM) H⁺-ATPases. Here, we summarize recent findings on the regulatory mechanisms of H⁺ fluxes at the root apex under three abiotic stress conditions - phosphate deficiency, salinity stress, and water deficiency - and present an integrated physiomolecular view of the functions of H⁺ fluxes in maintaining root growth in the acclimation to soil stress.

Higher nitrogen use efficiency (NUE) in hybrid "super rice" links to improved morphological and physiological traits in seedling roots

Great progress has been achieved in developing hybrid "super rice" varieties in China. Understanding morphological root traits in super rice and the mechanisms of nitrogen acquisition by the root system are of fundamental importance to developing proper fertilisation and nutrient management practices in their production. The present study was designed to study morphological and physiological traits in hybrid super rice roots that are associated with nitrogen use efficiency (NUE). Two hybrid super rice varieties (Yongyou12, YY; Jiayou 6, JY) and one common variety (Xiushui 134, XS) with differing NUE were cultivated hydroponically, and morphological and physiological traits of seedling roots in response to varying nitrogen conditions were investigated. Our results show that the hybrid cultivars YY and JY exhibit larger root systems, arising from a maximisation of root tips and from longer roots without changes in root diameter. The cross-sectional proportion of aerenchyma was significantly higher in super rice roots. The larger root system of super hybrid rice contributed to higher N accumulation and resulted in higher N uptake efficiency. ¹⁵N (¹⁵NH₄⁺) labeling results show that YY and JY had an enhanced capacity for ammonium (NH₄⁺) uptake. Moreover, YY and JY were more tolerant to high NH₄⁺ and showed reduced futile NH₄⁺ efflux. NH₄⁺ efflux in the root elongation zone, measured by Non-invasive Micro-test Technology, was significantly lower than in XS. Taken together, our results suggest that a longer root, a larger number of tips, a better developed aerenchyma, a higher capacity for N uptake, and reduced NH₄⁺ efflux from roots are associated with higher NUE and growth performance in hybrid super rice.

Endogenous ABA alleviates rice ammonium toxicity by reducing ROS and free ammonium via regulation of the SAPK9-bZIP20 pathway

Ammonium (NH4+) is one of the principal nitrogen (N) sources in soils, but is typically toxic already at intermediate concentrations. The phytohormone abscisic acid (ABA) plays a pivotal role in responses to environmental stresses. However, the role of ABA under high-NH4+ stress in rice (Oryza sativa L.) is only marginally understood. Here, we report that elevated NH4+ can significantly accelerate tissue ABA accumulation. Mutants with high (Osaba8ox) and low levels of ABA (Osphs3-1) exhibit elevated tolerance or sensitivity to high-NH4+ stress, respectively. Furthermore, ABA can decrease NH4+-induced oxidative damage and tissue NH4+ accumulation by enhancing antioxidant and glutamine synthetase (GS)/glutamate synthetasae (GOGAT) enzyme activities. Using RNA sequencing and quantitative real-time PCR approaches, we ascertain that two genes, OsSAPK9 and OsbZIP20, are induced both by high NH4+ and by ABA. Our data indicate that OsSAPK9 interacts with OsbZIP20, and can phosphorylate OsbZIP20 and activate its function. When OsSAPK9 or OsbZIP20 are knocked out in rice, ABA-mediated antioxidant and GS/GOGAT activity enhancement under high-NH4+ stress disappear, and the two mutants are more sensitive to high-NH4+ stress compared with their wild types. Taken together, our results suggest that ABA plays a positive role in regulating the OsSAPK9-OsbZIP20 pathway in rice to increase tolerance to high-NH4+ stress.

The intersection of nitrogen nutrition and water use in plants: new paths toward improved crop productivity

Water and nitrogen availability limit crop productivity globally more than most other environmental factors. Plant availability of macronutrients such as nitrate is, to a large extent, regulated by the amount of water available in the soil, and, during drought episodes, crops can become simultaneously water and nitrogen limited. In this review, we explore the intricate relationship between water and nitrogen transport in plants, from transpiration-driven mass flow in the soil to uptake by roots via membrane transporters and channels and transport to aerial organs. We discuss the roles of root architecture and of suberized hydrophobic root barriers governing apoplastic water and nitrogen movement into the vascular system. We also highlight the need to identify the signalling cascades regulating water and nitrogen transport, as well as the need for targeted physiological analyses of plant traits influencing water and nitrogen uptake. We further advocate for incorporation of new phenotyping technologies, breeding strategies, and agronomic practices to improve crop yield in water- and nitrogen-limited production systems.

Transcriptome analysis of rice (Oryza sativa L.) in response to ammonium resupply reveals the involvement of phytohormone signaling and the transcription factor OsJAZ9 in reprogramming of nitrogen uptake and metabolism

NH₄⁺ is not only the primary nitrogen for rice, a well-known NH₄⁺ specialist, but is also the chief limiting factor for its production. Limiting NH₄⁺ triggers a series of physiological and biochemical responses that help rice optimise its nitrogen acquisition. However, the dynamic nature and spatial distribution of the adjustments at the whole plant level during this response are still unknown. Here, nitrogen-starved rice seedlings were treated with 0.1 mM (NH₄)₂SO₄ for 4 or 12 h, and then the shoots and roots were harvested for RNA-Seq analysis. We identified 138 and 815 differentially expressed genes (DEGs) in shoots, and 597 and 1074 in roots following 4 and 12 h treatment, respectively. Up-regulated DEGs mainly participated in phenylpropanoid, sugar, and amino acid metabolism, which was confirmed by chemical content analysis. The transcription factor OsJAZ9 was the most pronouncedly induced component under low NH₄⁺ in roots, and a significant increase in root growth, NH₄⁺ absorption, amino acid, and sugar metabolism in response to resupplied NH₄⁺ following nitrogen starvation was identified in JAZ9ox (OsJAZ9-overexpressed) and coi1 (OsCOI1-RNAi). Our data provide comprehensive insight into the whole-plant transcriptomic response in terms of metabolic processes and signaling transduction to a low-NH₄⁺ signal, and identify the transcription factor OsJAZ9 and its involvement in the regulation of carbon/nitrogen metabolism as central to the response to low NH₄⁺.

TaANR1-TaBG1 and TaWabi5-TaNRT2s/NARs Link ABA Metabolism and Nitrate Acquisition in Wheat Roots

Nitrate is the preferred form of nitrogen for most plants, acting both as a nutrient and a signaling molecule. However, the components and regulatory factors governing nitrate uptake in bread wheat (Triticum aestivum), one of the world's most important crop species, have remained unclear, largely due to the complexity of its hexaploid genome. Here, based on recently released whole-genome information for bread wheat, the high-affinity nitrate transporter2 (NRT2) and the nitrate-assimilation-related (NAR) gene family are characterized. We show that abscisic acid (ABA)- Glc ester deconjugation is stimulated in bread wheat roots by nitrate resupply following nitrate withdrawal, leading to enhanced root-tissue ABA accumulation, and that this enhancement, in turn, affects the expression of root-type NRT2/NAR genes. TaANR1 is shown to regulate nitrate-mediated ABA accumulation by directly activating TaBG1, while TaWabi5 is involved in ABA-mediated NO3 - induction of NRT2/NAR genes. Building on previous evidence establishing ABA involvement in the developmental response to high-nitrate stress, our study suggests that ABA also contributes to the optimization of nitrate uptake by regulating the expression of NRT2/NAR genes under limited nitrate supply, offering a new target for improvement of nitrate absorption in crops.

Energy costs of salt tolerance in crop plants

Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H⁺ -ATPase also is a critical component. One proposed leak, that of Na⁺ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na⁺ and Cl^- concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assessment of the energy costs of NaCl tolerance to guide breeding and engineering of molecular components.

N and P runoff losses in China's vegetable production systems: Loss characteristics, impact, and management practices

Vegetable production systems involve the use of high rates of fertilizer application, leading to significant losses of nitrogen (N) and phosphorus (P) to the environment, resulting in water body eutrophication through surface runoff and leaching. We here quantify, at the national level, total N and P runoff losses and the key factors controlling runoff in open-field vegetable systems in China, by summarizing data from 151 publications pertaining to 13 Chinese provinces using literature dating back to 1990. Rank analysis was employed to clarify the impact of N and P runoff losses in vegetable systems, and different strategies for controlling N and P entering into water bodies are being compared. Vegetable production systems have higher fertilizer inputs (264.3 kg N ha^-1, 101.0 kg P ha^-1) compared with upland crop and rice cultivation. As a result, annual total nitrogen (TN) and total phosphorus (TP) losses via runoff from vegetable systems reached 16.5 kg ha^-1 and 3.45 kg ha^-1, respectively, and the N and P loss ratio for fruit vegetable systems reached 13.1% and 3.95% of the total fertilizer input, respectively. In the summer-autumn growing season, soil nutrient losses were the highest, accounting for 44% to 89% of the whole year. Redundancy analysis revealed that the most critical factor determining runoff losses was runoff volume. N and P runoff losses were also largely dependent on total soil N (TSN) and Olsen-P, respectively. Therefore, quantitative data for the national N and P runoff losses in vegetable production systems provide a scientific basis for an effective optimization of fertilizer applications.

Characterization and comparison of nitrate fluxes in Tamarix ramosissima and cotton roots under simulated drought conditions

Tamarix ramosissima Ledeb., a major host plant for the parasitic angiosperm Cistanche tubulosa, and known for its unique drought tolerance, has significant ecological and economic benefits. However, the mechanisms of nitrogen acquisition by the T. ramosissima root system under drought have remained uncharacterized. Here, uptake of nitrate (NO3-) in various regions of the root system was measured in T. ramosissima using Non-invasive Micro-test Technology at the cellular level, and using a 15NO3--enrichment technique at the whole-root level. These results were compared with responses in the model system cotton (Gossypium hirsutum L.). Tamarix ramosissima had lower net NO3- influx and a significantly lower Km (the apparent Michalis-Menten constant; 8.5 μM) for NO3- uptake than cotton under normal conditions. Upon simulated drought conditions, using polyethylene glycol (PEG), NO3- flux in cotton switched from net influx to net efflux, with a substantive peak in the white zone (WZ) of the root. There were no significant NO3- influx signals observed in the WZ of T. ramosissima under control conditions, whereas PEG treatment significantly enhanced NO3- influx in the WZ of T. ramosissima. The effect of PEG application on NO3- fluxes was highly localized, and the increase in net NO3- influx in response to PEG stimulation was also found in C. tubulosa-inoculated T. ramosissima. Consistently, root nitrogen (N) content and root biomass were higher in T. ramosissima than in cotton under PEG treatment. Our study provides insights into NO3- uptake and the influence of C. tubulosa inoculation in T. ramosissima roots during acclimation to PEG-induced drought stress and provides guidelines for silvicultural practice and for breeding of T. ramosissima under coupled conditions of soil drought and N deficiency.

Plasma-membrane electrical responses to salt and osmotic gradients contradict radiotracer kinetics, and reveal Na+-transport dynamics in rice (Oryza sativa L.)

A systematic analysis of NaCl-dependent, plasma-membrane depolarization (∆∆Ψ) in rice roots calls into question the current leading model of rapid membrane cycling of Na⁺ under salt stress. To investigate the character and mechanisms of Na⁺ influx into roots, Na⁺-dependent changes in plasma-membrane electrical potentials (∆∆Ψ) were measured in root cells of intact rice (Oryza sativa L., cv. Pokkali) seedlings. As external sodium concentrations ([Na⁺]_ext) were increased in a step gradient from 0 to 100 mM, membrane potentials depolarized in a saturable manner, fitting a Michaelis-Menten model and contradicting the linear (non-saturating) models developed from radiotracer studies. Clear differences in saturation patterns were found between plants grown under low- and high-nutrient (LN and HN) conditions, with LN plants showing greater depolarization and higher affinity for Na⁺ (i.e., higher V_max and lower K_m) than HN plants. In addition, counterion effects on ∆∆Ψ were pronounced in LN plants (with ∆∆Ψ decreasing in the order: Cl^- > SO₄^2- > HPO ₄ ^2- ), but not seen in HN plants. When effects of osmotic strength, Cl^- influx, K⁺ efflux, and H⁺-ATPase activity on ∆∆Ψ were accounted for, resultant K_m and V_max values suggested that a single, dominant Na⁺-transport mechanism was operating under each nutritional condition, with K_m values of 1.2 and 16 mM for LN and HN plants, respectively. Comparing saturating patterns of depolarization to linear patterns of ²⁴Na⁺ radiotracer influx leads to the conclusion that electrophysiological and tracer methods do not report the same phenomena and that the current model of rapid transmembrane sodium cycling may require revision.

The Arabidopsis AMOT1/EIN3 gene plays an important role in the amelioration of ammonium toxicity

Ammonium (NH4+) toxicity inhibits shoot growth in Arabidopsis, but the underlying mechanisms remain poorly characterized. Here, we show that a novel Arabidopsis mutant, ammonium tolerance 1 (amot1), exhibits enhanced shoot growth tolerance to NH4+. Molecular cloning revealed that amot1 is a new allele of EIN3, a key regulator of ethylene responses. The amot1 mutant and the allelic ein3-1 mutants show greater NH4+ tolerance than the wild type. Moreover, transgenic plants overexpressing EIN3 (EIN3ox) are more sensitive to NH4+ toxicity The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) increases shoot sensitivity to NH4+, whereas the ethylene perception inhibitor Ag+ decreases sensitivity. NH4+ induces ACC and ethylene accumulation. Furthermore, ethylene-insensitive mutants such as etr1-3 and ein3eil1 display enhanced NH4+ tolerance. In contrast, the ethylene overproduction mutant eto1-1 exhibits decreased ammonium tolerance. AMOT1/EIN3 positively regulates shoot ROS accumulation, leading to oxidative stress under NH4+ stress, a trait that may be related to increased expression of peroxidase-encoding genes. These findings demonstrate the role of AMOT1/EIN3 in NH4+ tolerance and confirm the strong link between NH4+ toxicity symptoms and the accumulation of hydrogen peroxide.

Overexpression of rice aquaporin OsPIP1;2 improves yield by enhancing mesophyll CO2 conductance and phloem sucrose transport

Aquaporins are involved in CO2 transport from the leaf intercellular air space to the chloroplast, which contributes to CO2 assimilation. However, the mechanism of CO2 transport by rice (Oryza sativa L.) aquaporins is unknown. Here, we investigated the function of the aquaporin OsPIP1;2 in CO2 diffusion-associated photosynthesis and phloem sucrose transport. Moreover, the grain yield of rice lines overexpressing OsPIP1;2 was determined. OsPIP1;2 was localized to the plasma membrane and the relative expression of OsPIP1;2 was approximately 5-fold higher in leaves in the presence of an elevated CO2 concentration. Overexpression of OsPIP1;2 increased mesophyll conductance by approximately 150% compared with wild-type (WT) rice. The OsPIP1;2-overexpressing lines had higher biomass than the WT, possibly due to increased phloem sucrose transport. In addition, the grain yield of OsPIP1;2-overexpressing lines was approximately 25% higher than that of the WT in three-season field experiments, due to the increased numbers of effective tillers and spikelets per panicle. Our results suggest that OsPIP1;2 modulates rice growth and grain yield by facilitating leaf CO2 diffusion, which increases both the net CO2 assimilation rate and sucrose transport.

The controversies of silicon's role in plant biology

Contents Summary 67 I. Introduction 68 II. Silicon transport in plants: to absorb or not to absorb 69 III. The role of silicon in plants: not just a matter of semantics 71 IV. Silicon and biotic stress: beyond mechanical barriers and defense priming 76 V. Silicon and abiotic stress: a proliferation of proposed mechanisms 78 VI. The apoplastic obstruction hypothesis: a working model 79 VII. Perspectives and conclusions 80 Acknowledgements 81 References 81 SUMMARY: Silicon (Si) is not classified as an essential plant nutrient, and yet numerous reports have shown its beneficial effects in a variety of species and environmental circumstances. This has created much confusion in the scientific community with respect to its biological roles. Here, we link molecular and phenotypic data to better classify Si transport, and critically summarize the current state of understanding of the roles of Si in higher plants. We argue that much of the empirical evidence, in particular that derived from recent functional genomics, is at odds with many of the mechanistic assertions surrounding Si's role. In essence, these data do not support reports that Si affects a wide range of molecular-genetic, biochemical and physiological processes. A major reinterpretation of Si's role is therefore needed, which is critical to guide future studies and inform agricultural practice. We propose a working model, which we term the 'apoplastic obstruction hypothesis', which attempts to unify the various observations on Si's beneficial influences on plant growth and yield. This model argues for a fundamental role of Si as an extracellular prophylactic agent against biotic and abiotic stresses (as opposed to an active cellular agent), with important cascading effects on plant form and function.

From aquaporin to ecosystem: Plants in the water cycle

Vascular plants are major intermediaries in the global water cycle, and are highly adapted to both facilitate and resist water fluxes, such as during root uptake, translocation in the xylem, and transpiration by leaves. Here, we summarize the contributions to a Special Issue on water in the Journal of Plant Physiology, which cluster around the theme of control and facilitation of water movement in plants. We conclude with an editorial view of the need for plant physiologists to consider larger cultural issues surrounding water use, especially in terms of the increasing agricultural demand for water to produce animal feed, with its associated trophic nutritive losses and environmental damage.

Excess iron stress reduces root tip zone growth through nitric oxide-mediated repression of potassium homeostasis in Arabidopsis

The root tip zone is regarded as the principal action site for iron (Fe) toxicity and is more sensitive than other root zones, but the mechanism underpinning this remains largely unknown. We explored the mechanism underpinning the higher sensitivity at the Arabidopsis root tip and elucidated the role of nitric oxide (NO) using NO-related mutants and pharmacological methods. Higher Fe sensitivity of the root tip is associated with reduced potassium (K⁺ ) retention. NO in root tips is increased significantly above levels elsewhere in the root and is involved in the arrest of primary root tip zone growth under excess Fe, at least in part related to NO-induced K⁺ loss via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4) and reduced root tip zone cell viability. Moreover, ethylene can antagonize excess Fe-inhibited root growth and K⁺ efflux, in part by the control of root tip NO levels. We conclude that excess Fe attenuates root growth by effecting an increase in root tip zone NO, and that this attenuation is related to NO-mediated alterations in K⁺ homeostasis, partly via SNO1/SOS4.

Tomato plants ectopically expressing Arabidopsis GRF9 show enhanced resistance to phosphate deficiency and improved fruit production in the field

Agronomic performance of transgenic tomato overexpressing functional genes has rarely been investigated in the field. In an attempt to improve low-phosphate (P) stress tolerance of tomato (Solanum lycopersicum) plants and promote tomato fruit production in the field, an expression vector containing cDNA to an Arabidopsis 14-3-3 protein, General Regulatory Factor 9 (GRF9), driven by a cauliflower mosaic virus 35S promoter, was transferred into tomato plants. Transgenic expression of GRF9 was ascertained by quantitative real-time PCR analysis. The degree of low-P tolerance in transgenic plants was found to be significantly greater than that in wild-type plants, and reflected in improved root development and enhanced P content under hydroponic conditions. For transgenic tomato, roots had higher P uptake, as evidenced by tissue P content and relative expression of the genes LePT1 and LePT2 in both normal and low-P hydroponic solutions. GRF9 overexpressors had greatly enhanced proton extrusion from roots and heightened activity of the plasma-membrane H⁺-ATPase (PM H⁺-ATPase) in roots under low-P hydroponic conditions. Thus, in addition to enhanced root development, higher expression of genes coding for phosphate transporters and improved capacity for acidification in the rhizosphere emerged as key mechanisms underpinning improved P acquisition in transgenic tomato plants in soil. Subsequent field trials measuring tomato fruit production at two P levels, indicated that GRF9 can indeed improve total tomato production and may play a role in early fruit maturity. Our results suggest that the heterologous Arabidopsis GRF9 gene can confer resistance to P deficiency in transgenic tomato plants and promote fruit production.

Dynamic analysis of the impact of free-air CO2 enrichment (FACE) on biomass and N uptake in two contrasting genotypes of rice

Elevated CO2 concentrations ([CO2]) in the atmosphere often increase photosynthetic rates and crop yields. However, the degree of the CO2 enhancement varies substantially among cultivars and with growth stage. Here, we examined the responses of two rice cultivars, Wuyunjing23 (WYJ) and IIyou084 (IIY), to two [CO2] (~400 vs ~600) and two nitrogen (N) provision conditions at five growth stages. In general, both seed yield and aboveground biomass were more responsive to elevated [CO2] in IIY than WYJ. However, the responses significantly changed at different N levels and growth stages. At the low N input, yield response to elevated [CO2] was negligible in both cultivars while, at the normal input, yield in IIY was 18.8% higher under elevated [CO2] than ambient [CO2]. Also, responses to elevated [CO2] significantly differed among various growth stages. Elevated [CO2] tended to increase aboveground plant biomass in both cultivars at the panicle initiation (PI) and the heading stages, but this effect was significant only in IIY by the mid-ripening and the grain maturity stages. In contrast, CO2 enhancement of root biomass only occurred in IIY. Elevated [CO2] increased both total N uptake and seed N in IIY but only increased seed N in WYJ, indicating that it enhanced N translocation to seeds in both cultivars but promoted plant N acquisition only in IIY. Root C accumulation and N uptake also exhibited stronger responses in IIY than in WYJ, particularly at the heading stage, which may play a pivotal role in seed filling and seed yield. Our results showed that the more effective use of CO2 in IIY compared with WYJ results in a strong response in root growth, nitrogen uptake, and in yield. These findings suggest that selection of [CO2]-responsive rice cultivars may help optimise the rice yield under future [CO2] scenarios.

Comparative Analysis of Arabidopsis Ecotypes Reveals a Role for Brassinosteroids in Root Hydrotropism

Plant roots respond to soil moisture gradients and remodel their growth orientation toward water through hydrotropism, a process vital for acclimation to a changing soil environment. Mechanisms underlying the root hydrotropic response, however, remain poorly understood. Here, we examined hydrotropism in 31 Arabidopsis (Arabidopsis thaliana) ecotypes collected from different parts of the world and grown along moisture gradients in a specially designed soil-simulation system. Comparative transcriptome profiling and physiological analyses were carried out on three selected ecotypes, Wassilewskija (Ws), Columbia (Col-0) (strongly hydrotropic), Col-0 (moderately hydrotropic), and C24 (weakly hydrotropic), and in mutant lines with altered root hydrotropic responses. We show that H+ efflux, Ca2+ influx, redox homeostasis, epigenetic regulation, and phytohormone signaling may contribute to root hydrotropism. Among phytohormones, the role of brassinosteroids (BRs) was examined further. In the presence of an inhibitor of BR biosynthesis, the strong hydrotropic response observed in Ws was reduced. The root H+ efflux and primary root elongation also were inhibited when compared with C24, an ecotype that showed a weak hydrotropic response. The BR-insensitive mutant bri1-5 displayed higher rates of root growth inhibition and root curvature on moisture gradients in vertical or oblique orientation when compared with wild-type Ws. We also demonstrate that BRI1 (a BR receptor) interacts with AHA2 (a plasma membrane H+-ATPase) and that their expression patterns are highly coordinated. This synergistic action may contribute to the strong hydrotropism observed in Ws. Our results suggest that BR-associated H+ efflux is critical in the hydrotropic response of Arabidopsis roots.

Membrane fluxes, bypass flows, and sodium stress in rice: the influence of silicon

Provision of silicon (Si) to roots of rice (Oryza sativa L.) can alleviate salt stress by blocking apoplastic, transpirational bypass flow of Na+ from root to shoot. However, little is known about how Si affects Na+ fluxes across cell membranes. Here, we measured radiotracer fluxes of 24Na+, plasma membrane depolarization, tissue ion accumulation, and transpirational bypass flow, to examine the influence of Si on Na+ transport patterns in hydroponically grown, salt-sensitive (cv. IR29) and salt-tolerant (cv. Pokkali) rice. Si increased growth and lowered [Na+] in shoots of both cultivars, with minor effects in roots; neither root nor shoot [K+] were affected. In IR29, Si lowered shoot [Na+] via a large reduction in bypass flow, while in Pokkali, where bypass flow was small and not affected by Si, this was achieved mainly via a growth dilution of shoot Na+. Si had no effect on unidirectional 24Na+ fluxes (influx and efflux), or on Na+-stimulated plasma-membrane depolarization, in either IR29 or Pokkali. We conclude that, while Si can reduce Na+ translocation via bypass flow in some (but not all) rice cultivars, it does not affect unidirectional Na+ transport or Na+ cycling in roots, either across root cell membranes or within the bulk root apoplast.

The nitrogen-potassium intersection: membranes, metabolism, and mechanism

Nitrogen (N) and potassium (K) are the two most abundantly acquired mineral elements by plants, and their acquisition pathways interact in complex ways. Here, we review pivotal interactions with respect to root acquisition, storage, translocation and metabolism, between the K⁺ ion and the two major N sources, ammonium (NH₄⁺ ) and nitrate (NO₃^- ). The intersections between N and K physiology are explored at a number of organizational levels, from molecular-genetic processes, to compartmentation, to whole plant physiology, and discussed in the context of both N-K cooperation and antagonism. Nutritional regulation and optimization of plant growth, yield, metabolism and water-use efficiency are also discussed.

How Plant Root Exudates Shape the Nitrogen Cycle

Although the global nitrogen (N) cycle is largely driven by soil microbes, plant root exudates can profoundly modify soil microbial communities and influence their N transformations. A detailed understanding is now beginning to emerge regarding the control that root exudates exert over two major soil N processes - nitrification and N2 fixation. We discuss recent breakthroughs in this area, including the identification of root exudates as nitrification inhibitors and as signaling compounds facilitating N-acquisition symbioses. We indicate gaps in current knowledge, including questions of how root exudates affect newly discovered microbial players and N-cycle components. A better understanding of these processes is urgent given the widespread inefficiencies in agricultural N use and their links to N pollution and climate change.

Nitrogen transformations in modern agriculture and the role of biological nitrification inhibition

The nitrogen (N)-use efficiency of agricultural plants is notoriously poor. Globally, about 50% of the N fertilizer applied to cropping systems is not absorbed by plants, but lost to the environment as ammonia (NH₃), nitrate (NO₃^-), and nitrous oxide (N₂O, a greenhouse gas with 300 times the heat-trapping capacity of carbon dioxide), raising agricultural production costs and contributing to pollution and climate change. These losses are driven by volatilization of NH₃ and by a matrix of nitrification and denitrification reactions catalysed by soil microorganisms (chiefly bacteria and archaea). Here, we discuss mitigation of the harmful and wasteful process of agricultural N loss via biological nitrification inhibitors (BNIs) exuded by plant roots. We examine key recent discoveries in the emerging field of BNI research, focusing on BNI compounds and their specificity and transport, and discuss prospects for their role in improving agriculture while reducing its environmental impact.

Drought stress obliterates the preference for ammonium as an N source in the C4 plant Spartina alterniflora

The C₄ grass Spartina alterniflora is known for its unique salt tolerance and strong preference for ammonium (NH₄⁺) as a nitrogen (N) source. We here examined whether Spartina's unique preference for NH₄⁺ results in improved performance under drought stress. Manipulative greenhouse experiments were carried out to measure the effects of variable water availability and inorganic N sources on plant performance (growth, photosynthesis, antioxidant, and N metabolism). Drought strongly reduced leaf number and area, plant fresh and dry weight, and photosynthetic activity on all N sources, but the reduction was most pronounced on NH₄⁺. Indeed, the growth advantage seen on NH₄⁺ in the absence of drought, producing nearly double the biomass compared to growth on NO₃^-, was entirely obliterated under both intermediate and severe drought conditions (50 and 25% field capacity, respectively). Both fresh and dry weight became indistinguishable among N sources under drought. Major markers of the antioxidant capacity of the plant, the activities of the enzymes superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, showed higher constitutive levels on NH₄⁺. Catalase and glutathione reductase were specifically upregulated in NH₄⁺-fed plants with increasing drought stress. This upregulation, however, failed to protect the plants from drought stress. Nitrogen metabolism was characterized by lower constitutive levels of glutamine synthetase in NH₄⁺-fed plants, and a rise in glutamate dehydrogenase (GDH) activity under drought, accompanied by elevated proline levels in leaves. Our results support postulates on the important role of GDH induction, and its involvement in the synthesis of compatible solutes, under abiotic stress. We show that, despite this metabolic shift, S. alterniflora's sensitivity to drought does not benefit from growth on NH₄⁺ and that the imposition of drought stress equalizes all N-source-related growth differences observed under non-drought conditions.

Spatio-temporal dynamics in global rice gene expression (Oryza sativa L.) in response to high ammonium stress

Ammonium (NH₄⁺) is the predominant nitrogen (N) source in many natural and agricultural ecosystems, including flooded rice fields. While rice is known as an NH₄⁺-tolerant species, it nevertheless suffers NH₄⁺ toxicity at elevated soil concentrations. NH₄⁺ excess rapidly leads to the disturbance of various physiological processes that ultimately inhibit shoot and root growth. However, the global transcriptomic response to NH₄⁺ stress in rice has not been examined. In this study, we mapped the spatio-temporal specificity of gene expression profiles in rice under excess NH₄⁺ and the changes in gene expression in root and shoot at various time points by RNA-Seq (Quantification) using Illumina HiSeqTM 2000. By comparative analysis, 307 and 675 genes were found to be up-regulated after 4h and 12h of NH₄⁺ exposure in the root, respectively. In the shoot, 167 genes were up-regulated at 4h, compared with 320 at 12h. According to KEGG analysis, up-regulated DEGs mainly participate in phenylpropanoid (such as flavonoid) and amino acid (such as proline, cysteine, and methionine) metabolism, which is believed to improve NH₄⁺ stress tolerance through adjustment of energy metabolism in the shoot, while defense and signaling pathways, guiding whole-plant acclimation, play the leading role in the root. We furthermore critically assessed the roles of key phytohormones, and found abscisic acid (ABA) and ethylene (ET) to be the major regulatory molecules responding to excess NH₄⁺ and activating the MAPK (mitogen-activated protein kinase) signal-transduction pathway. Moreover, we found up-regulated hormone-associated genes are involved in regulating flavonoid biosynthesis and are regulated by tissue flavonoid accumulation.

Biological nitrification inhibition by rice root exudates and its relationship with nitrogen-use efficiency

Microbial nitrification in soils is a major contributor to nitrogen (N) loss in agricultural systems. Some plants can secrete organic substances that act as biological nitrification inhibitors (BNIs), and a small number of BNIs have been identified and characterized. However, virtually no research has focused on the important food crop, rice (Oryza sativa). Here, 19 rice varieties were explored for BNI potential on the key nitrifying bacterium Nitrosomonas europaea. Exudates from both indica and japonica genotypes were found to possess strong BNI potential. Older seedlings had higher BNI abilities than younger ones; Zhongjiu25 (ZJ25) and Wuyunjing7 (WYJ7) were the most effective genotypes among indica and japonica varieties, respectively. A new nitrification inhibitor, 1,9-decanediol, was identified, shown to block the ammonia monooxygenase (AMO) pathway of ammonia oxidation and to possess an 80% effective dose (ED₈₀ ) of 90 ng μl^-1 . Plant N-use efficiency (NUE) was determined using a ¹⁵ N-labeling method. Correlation analyses indicated that both BNI abilities and 1,9-decanediol amounts of root exudates were positively correlated with plant ammonium-use efficiency and ammonium preference. These findings provide important new insights into the plant-bacterial interactions involved in the soil N cycle, and improve our understanding of the BNI capacity of rice in the context of NUE.

The Chloroplast Protease AMOS1/EGY1 Affects Phosphate Homeostasis under Phosphate Stress

Plastid intramembrane proteases in Arabidopsis (Arabidopsis thaliana) are involved in jasmonic acid biosynthesis, chloroplast development, and flower morphology. Here, we show that Ammonium-Overly-Sensitive1 (AMOS1), a member of the family of plastid intramembrane proteases, plays an important role in the maintenance of phosphate (P) homeostasis under P stress. Loss of function of AMOS1 revealed a striking resistance to P starvation. amos1 plants displayed retarded root growth and reduced P accumulation in the root compared to wild type (Col-0) under P-replete control conditions, but remained largely unaffected by P starvation, displaying comparable P accumulation and root and shoot growth under P-deficient conditions. Further analysis revealed that, under P-deficient conditions, the cell wall, especially the pectin fraction of amos1, released more P than that of wild type, accompanied by a reduction of the abscisic acid (ABA) level and an increase in ethylene production. By using an ABA-insensitive mutant, abi4, and applying ABA and ACC exogenously, we found that ABA inhibits cell wall P remobilization while ethylene facilitates P remobilization from the cell wall by increasing the pectin concentration, suggesting ABA can counteract the effect of ethylene. Furthermore, the elevated ABA level and the lower ethylene production also correlated well with the mimicked P deficiency in amos1 Thus, our study uncovers the role of AMOS1 in the maintenance of P homeostasis through ABA-antagonized ethylene signaling.