How Plant Root Exudates Shape the Nitrogen Cycle Coskun, Devrim; Britto, Dev T.; Shi, Weiming ...
Trends in plant science,
August 2017, 2017-08-00, 20170801, Volume:
22, Issue:
8
Journal Article
Peer reviewed
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.
Major advances in understanding the complexity of the N cycle have recently been made, with the discovery of previously unknown microbial players and N transformations.
The study of plant root exudates and their influence on the plant–soil microbiome in shaping nutrient cycles has greatly intensified in recent years.
Root exudates that specifically inhibit soil nitrification have been identified in important crop species, including rice, wheat, and sorghum, while others have been shown to stimulate root nodulation and N2 fixation, even in neighboring plants.
By influencing soil N cycle dynamics, root exudates have been shown to improve N use efficiency and can help to mitigate environmental pollution and climate change.
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 90μl−1. Plant N-use efficiency (NUE) was determined using a 15N-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.
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.
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.
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•Rhizospheric Pseudomonas inoculant strain RWX31 inoculation induces stigmasterol exudation in duckweed.•Stigmasterol at low dose can function as an N-removal stimulant for strain RWX31.•Stigmasterol enhances bacterial biofilm formation and nitrite reductase.•Stigmasterol alters the composition of the nitrite-reducing bacterial community responsible for N removal.•The stigmasterol/RWX31 pair significantly improves the rate of N removal in duckweed systems.
Maintenance of root growth is essential for plant adaptation to soil drying. Here, we tested the hypothesis that auxin transport is involved in mediating ABA's modulation by activating proton ...secretion in the root tip to maintain root growth under moderate water stress.
Rice and Arabidopsis plants were raised under a hydroponic system and subjected to moderate water stress (−0.47 MPa) with polyethylene glycol (PEG). ABA accumulation, auxin transport and plasma membrane H+-ATPase activity at the root tip were monitored in addition to the primary root elongation and root hair density.
We found that moderate water stress increases ABA accumulation and auxin transport in the root apex. Additionally, ABA modulation is involved in the regulation of auxin transport in the root tip. The transported auxin activates the plasma membrane H+-ATPase to release more protons along the root tip in its adaption to moderate water stress. The proton secretion in the root tip is essential in maintaining or promoting primary root elongation and root hair development under moderate water stress.
These results suggest that ABA accumulation modulates auxin transport in the root tip, which enhances proton secretion for maintaining root growth under moderate water stress.
A combination of nitrogen (N) fertilizer side-deep placement and mechanical transplanting of rice seedling (MSDF) has been recommended as an effective alternative technique to conventional ...broadcasting of fertilizer. However, its comprehensive interactions with N-fertilizer type, split ratio, environmental impact, and profitability are unclear. A three-year field experiment was conducted using MSDF and three fertilizer types (NPK briquette, F1; NPK briquette with nitrification inhibitor, F2; and controlled-released N fertilizer, F3) with 200 kg N hm−2 at two split ratios (a one-time basal application (N200) and basal plus supplementary application at the rice tillering stage (N140 + 60)). Conventional fertilization (conventional fertilizer using NPK briquettes by broadcasting with 270 kg N hm−2 at three split ratios (CF1N270)) and a no-N-added treatment were established as two controls. Directly reducing the N-application rate by 26% (CF1N200) decreased grain production by 13.1%. However, MSDF management (MF1N200, MF2N200, MF3N200, MF1N160 + 40, MF2N160 + 40, MF3N160 + 40) maintained high yield, increased NUE by 24.8–40.9%, and decreased NH3 volatilization and total N concentration in runoff by 39.0–65.6% and 29.1–59.3%, respectively. Moreover, there was no difference with fertilizer type and split ratio design among these MSDF treatments. When using a lower N-application rate (200 kg N hm−2), compared with CF1N200, MSDF treatments increased NUE by 43.6% and net economic benefit by 74.9%, and decreased NH3 volatilization and total runoff N concentration by 35.2% and 78.4%, respectively. MSDF at a reduced N-application rate minimizes NH3 volatilization and N runoff and increases profitability, independent of fertilizer type and split ratio.
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•Benefits of mechanical side-deep fertilization (MSDF) are demonstrated in a rice production system.•Changing the surface broadcast regimen to MSDF can reduce nitrogen rate by 26% without yield loss.•MSDF mitigates ammonia volatilization and nitrogen runoff and increases profitability.•The positive effects of MSDF are independent of nitrogen-fertilizer type and split ratio.•Improving nitrogen-use efficiency by MSDF saves more energy than common practice.
The application of biological nitrification inhibitors (BNIs) is considered an important new strategy to mitigate nitrogen losses from agricultural soils. 1,9-decanediol was recently identified as a ...new BNI in rice root exudates and was shown to inhibit nitrification in bioassays using Nitrosomonas. However, the effect of this compound on nitrification and ammonia oxidizers in soils remained unknown. In this study, three typical agriculture soils were collected to investigate the impact of 1,9-decanediol on nitrification and ammonia oxidizers in a 14-day microcosm incubation. High doses of 1,9-decanediol showed strong soil nitrification inhibition in all three agricultural soils, with the highest inhibition of 58.1% achieved in the acidic red soil, 37.0% in the alkaline fluvo-aquic soil, and 35.7% in the neutral paddy soil following 14 days of incubation. Moreover, the inhibition of 1,9-decanediol was superior to the widely used synthetic nitrification inhibitor, dicyandiamide (DCD) and two other BNIs, methyl 3-(4-hydroxyphenyl) propionate (MHPP) and α-linolenic acid (LN), in all three soils. The abundance of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) was significantly inhibited by 1,9-decanediol addition across the three soils. All AOB sequences fell within the Nitrosospira group, and the dominant AOA sequences belonged to the Nitrososphaera cluster in all three soils. Changes in the community composition of AOB were more pronounced than AOA after the application of 1,9-decanediol. The AOB community structure shifted from Nitrosospira cluster 2 and cluster 3a toward Nitrosospira clusters 8a and 8b. As for AOA, no significant impact on the proportion of the dominant Nitrososphaera cluster was observed in the fluvo-aquic soil and paddy soil while only the Nitrosopumilus cluster decreased in the red soil. 1,9-decanediol could also significantly reduce soil N2O emissions, especially in acidic red soil. Our results provide evidence that 1,9-decanediol is capable of suppressing nitrification in agricultural soils through impeding both AOA and AOB rather than affecting soil NH4+ availability. 1,9-decanediol holds promise as an effective biological nitrification inhibitor for soil ammonia-oxidizing bacteria and archaea.
•1,9-decanediol can inhibit soil nitrification.•The inhibition of 1,9-decanediol is superior to that of DCD and two other BNIs.•1,9-decanediol inhibits nitrification by impeding both AOA and AOB.•1,9-decanediol can reduce N2O emissions, especially in acidic red soils.
► We studied N leaching in vegetable production field for 4 consecutive years and its relationship to rainfall and irrigation. ► We identified the period when leaching is most likely to occur. ► We ...found N leaching loss could be reduced by 40% or more if fertilizer and water management are improved.
Overuse or misuse of nitrogen (N) fertilizers in intensive greenhouse vegetable production regions has been recognized as a non-point source pollution to environmental quality. The objectives of this study were to study the potential of N leaching in intensive greenhouse vegetable systems of southern China and to investigate strategies in minimizing the impact of N loss on water quality. A consecutive four-year field experiment was conducted with five N (manure+urea) application rates (234+0, 234+348, 234+522, 234+696, and 234+870kgNha−1a−1) in a tomato, cucumber, and celery annual rotation system. The results demonstrated that the amount of N leached was 181.6–276.9kgNha−1a−1 under traditional N rates of 1104kgNha−1a−1 used by local farmers; this leaching loss mainly occurred during the open-field (the polyethylene-cover was not in use) periods. The leached water flux and the total N concentration in the leachates determined by a lysimeter were 205.1–288.4mma−1 and 36.6–171.1mgL−1 under the traditional N rate, respectively; the flux produced during the open-field was 40.9–58.9% of that for the whole year. By decreasing traditional N rate of synthetic fertilizer by 40%, N leaching loss was reduced by 39.6% without any yield loss in intensive greenhouse vegetable production systems.
Iron (Fe) is essential for life, but in excess can cause oxidative cytotoxicity through the generation of Fe-catalyzed reactive oxygen species. It is yet unknown which genes and mechanisms can ...provide Fe-toxicity tolerance. Here, we identify S-nitrosoglutathione-reductase (GSNOR) variants underlying a major quantitative locus for root tolerance to Fe-toxicity in Arabidopsis using genome-wide association studies and allelic complementation. These variants act largely through transcript level regulation. We further show that the elevated nitric oxide is essential for Fe-dependent redox toxicity. GSNOR maintains root meristem activity and prevents cell death via inhibiting Fe-dependent nitrosative and oxidative cytotoxicity. GSNOR is also required for root tolerance to Fe-toxicity throughout higher plants such as legumes and monocots, which exposes an opportunity to address crop production under high-Fe conditions using natural GSNOR variants. Overall, this study shows that genetic or chemical modulation of the nitric oxide pathway can broadly modify Fe-toxicity tolerance.
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