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, Letnik:
22, Številka:
8
Journal Article
Recenzirano
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.
Although deemed a "non-essential" mineral nutrient, silicon (Si) is clearly beneficial to plant growth and development, particularly under stress conditions, including salinity and drought. Here, we ...review recent research on the physiological, biochemical, and molecular mechanisms underlying Si-induced alleviation of osmotic and ionic stresses associated with salinity and drought. We distinguish between changes observed in the apoplast (i.e., suberization, lignification, and silicification of the extracellular matrix; transpirational bypass flow of solutes and water), and those of the symplast (i.e., transmembrane transport of solutes and water; gene expression; oxidative stress; metabolism), and discuss these features in the context of Si biogeochemistry and bioavailability in agricultural soils, evaluating the prospect of using Si fertilization to increase crop yield and stress tolerance under salinity and drought conditions.
BackgroundPlants can utilize two major forms of inorganic N: NO3− (nitrate) and NH4+ (ammonium). In some cases, the preference of one form over another (denoted as β) can appear to be quite ...pronounced for a plant species, and can be an important determinant and predictor of its distribution and interactions with other species. In many other cases, however, assignment of preference is not so straightforward and must take into account a wide array of complex physiological and environmental features, which interact in ways that are still not well understood.ScopeThis Viewpoint presents a discussion of the key, and often co-occurring, factors that join to produce the complex phenotypic composite referred to by the deceptively simple term ‘N-source preference’.ConclusionsN-source preference is much more complex a biological phenomenon than is often assumed, and general models predicting how it will influence ecological processes will need to be much more sophisticated than those that have been so far developed.
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.
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.
Abstract
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.
Given the critical importance and interconnectedness of water and nitrogen in determining crop yield, there is great impetus to understand and optimize their uptake. We review this intersection and provide proposals for improving these critical crop traits.
CONTENTS: Summary 54 I. Introduction 55 II. The role of nonselective cation channels in primary sodium influx - a solid consensus. How solid is the evidence? 55 III. Low-affinity cation transporter 1 ...- a forgotten link? 61 IV. Are potassium transporters implicated in sodium influx? 62 V. HKT: a saga of twists and turns - where do we stand? 64 VI. SOS: an ambiguous tale 67 VII. Vacuolar storage via NHX: some lingering questions 68 VIII. Other pathways - the apoplast and possibilities of symport with chloride 69 IX. ‘Toxic' Na⁺ fluxes, Na⁺‘homeostasis', and the question of cytosolic Na⁺ 71 X. Concluding remarks 72 Acknowledgements 73 References 73 SUMMARY: Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na⁺ entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na⁺ partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na⁺ fluxes and cytosolic Na⁺ concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na⁺ transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.
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.
This review summarizes fundamental intersections between the pathways of inorganic nitrogen (NH4+ and NO3) and potassium (K+) acquisition in plants. Uptake, storage, translocation and metabolism are discussed at levels of organization ranging from molecular‐genetic processes to whole‐plant physiology. The regulation and optimization of plant growth, yield, metabolism and water‐use efficiency are discussed in this nutritional context.
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.
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