Agricultural practices to improve nitrogen use efficiency through the use of arbuscular mycorrhizae: Basic and agronomic aspects

Navigating nitrogen sustainability with microbiome-associated phenotypes

Crop microbiomes promote plant health through various mechanisms, including nutrient provisioning. However, agriculture neglected the importance of these microbiome-associated phenotypes (MAPs) in conventional management approaches originating from the Green Revolution. Green Revolution innovations, such as nitrogen fertilizers and high-yielding germplasm, supported an increase in global crop yields. Yet these advances also led to many environmental issues, including disruptions in microbially mediated nitrogen transformations that have reduced reliance on microbiomes for sustainable nitrogen acquisition. Overcoming the challenges introduced by the Green Revolution requires a shift toward ecologically informed agronomic strategies that incorporate MAPs into breeding and management decisions. Agriculture in the Anthropocene needs to mindfully manage crop microbiomes to decouple agrochemical inputs from profitable yields, minimizing the environmental repercussions of modern agriculture.

Effect of AMF Inoculation on Reducing Excessive Fertilizer Use

Excessive use of chemical fertilizer is a global concern. Arbuscular mycorrhizal fungi (AMF) are considered a potential solution due to their symbiotic association with crops. This study assessed AMF's effects on maize yield, fertilizer efficiency, plant traits, and soil nutrients under different reduced-fertilizer regimes in medium-low fertility fields. We found that phosphorus supplementation after a 30% fertilizer reduction enhanced AMF's positive impact on grain yield, increasing it by 3.47% with pure chemical fertilizers and 6.65% with mixed fertilizers. The AMF inoculation did not significantly affect the nitrogen and phosphorus fertilizer use efficiency, but significantly increased root colonization and soil mycelium density. Mixed fertilizer treatments with phosphorus supplementation after fertilizer reduction showed greater mycorrhizal effects on plant traits and soil nutrient contents compared to chemical fertilizer treatments. This study highlights that AMF inoculation, closely linked to fertilization regimes, can effectively reduce fertilizer use while sustaining or enhancing maize yields.

A study of microbial diversity in a biofertilizer consortium

Biofertilizers supply living microorganisms to help plants grow and keep their health. This study examines the microbiome composition of a commercial biofertilizer known for its plant growth-promoting activity. Using ITS and 16S rRNA gene sequence analyses, we describe the microbial communities of a biofertilizer, with 163 fungal species and 485 bacterial genera found. The biofertilizer contains a variety of microorganisms previously reported to enhance nutrient uptake, phytohormone production, stress tolerance, and pathogen resistance in plants. Plant roots created a microenvironment that boosted bacterial diversity but filtered fungal communities. Notably, preserving the fungal-inoculated substrate proves critical for keeping fungal diversity in the root fraction. We described that bacteria were more diverse in the rhizosphere than in the substrate. In contrast, root-associated fungi were less diverse than the substrate ones. We propose using plant roots as bioreactors to sustain dynamic environments that promote the proliferation of microorganisms with biofertilizer potential. The study suggests that bacteria grow close to plant roots, while root-associated fungi may be a subset of the substrate fungi. These findings show that the composition of the biofertilizer may be influenced by the selection of microorganisms associated with plant roots, which could have implications for the effectiveness of the biofertilizer in promoting plant growth. In conclusion, our study sheds light on the intricate interplay between plant roots and the biofertilizer's microbial communities. Understanding this relationship can aid in optimizing biofertilizer production and application, contributing to sustainable agricultural practices and improved crop yields.

Role of mycorrhizas and root exudates in plant uptake of soil nutrients (calcium, iron, magnesium, and potassium): has the puzzle been completely solved?

Anthropogenic global change is driving an increase in the frequency and intensity of drought and flood events, along with associated imbalances and limitation of several soil nutrients. In the context of an increasing human population, these impacts represent a global-scale challenge for biodiversity conservation and sustainable crop production to ensure food security. Plants have evolved strategies to enhance uptake of soil nutrients under environmental stress conditions; for example, symbioses with fungi (mycorrhization) in the rhizosphere and the release of exudates from roots. Although crop cultivation is managed for the effects of limited availability of nitrogen (N) and phosphorus (P), there is increasing evidence for limitation of plant growth and fitness because of the low availability of other soil nutrients such as the metals potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe), which may become increasingly limiting for plant productivity under global change. The roles of mycorrhizas and plant exudates on N and P uptake have been studied intensively; however, our understanding of the effects on metal nutrients is less clear and still inconsistent. Here, we review the literature on the role of mycorrhizas and root exudates in plant uptake of key nutrients (N, P, K, Ca, Mg, and Fe) in the context of potential nutrient deficiencies in crop and non-crop terrestrial ecosystems, and identify knowledge gaps for future research to improve nutrient-uptake capacity in food crop plants.

Aboveground herbivory does not affect mycorrhiza-dependent nitrogen acquisition from soil but inhibits mycorrhizal network-mediated nitrogen interplant transfer in maize

Arbuscular mycorrhizal fungi (AMF) are considered biofertilizers for sustainable agriculture due to their ability to facilitate plant uptake of important mineral elements, such as nitrogen (N). However, plant mycorrhiza-dependent N uptake and interplant transfer may be highly context-dependent, and whether it is affected by aboveground herbivory remains largely unknown. Here, we used ¹⁵N labeling and tracking to examine the effect of aboveground insect herbivory by Spodoptera frugiperda on mycorrhiza-dependent N uptake in maize (Zea mays L.). To minimize consumption differences and ¹⁵N loss due to insect chewing, insect herbivory was simulated by mechanical wounding and oral secretion of S. frugiperda larvae. Inoculation with Rhizophagus irregularis (Rir) significantly improved maize growth, and N/P uptake. The ¹⁵N labeling experiment showed that maize plants absorbed N from soils via the extraradical mycelium of mycorrhizal fungi and from neighboring plants transferred by common mycorrhizal networks (CMNs). Simulated aboveground leaf herbivory did not affect mycorrhiza-mediated N acquisition from soil. However, CMN-mediated N transfer from neighboring plants was blocked by leaf simulated herbivory. Our findings suggest that aboveground herbivory inhibits CMN-mediated N transfer between plants but does not affect N acquisition from soil solutions via extraradical mycorrhizal mycelium.

Rhizophagus irregularis and Azotobacter chroococcum Uphold Eggplant Production and Quality under Low Fertilization

Microorganisms are essential parts of soil and play an important role in mediating many processes and influencing plant health. Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB), the most common of such microorganisms, can benefit plants by enhancing the nutrient-absorbing ability of roots through bio-inoculation, also called biofertilization. Different methods have been tested and proven to be effective in the enhancement of soil nutrient availability. However, the effects of increased application of biological methods with minimal chemical fertilizers are still inconsistent. In this 2-year of fixed-point greenhouse test, we aimed to evaluate the impact of AMF (Rhizophagus irregularis) and/or NFB (Azotobacter) on growth, quality, and yield of eggplants under different N levels. Data showed that biofertilizer application with reduced chemical fertilizer had the highest impact on eggplant performance and yield. Indeed, low chemical fertilizers combined with adequate amounts of biofertilizers produced a higher plant height, length and width of leaves, dry matter, number of fruits per plant with better morphology, total yield per plant, and total soluble solids (TSS), suggesting that the use of Azotobacter and R. irregularis as biofertilizers could substantially reduce the use of chemical fertilizers without impairing the quality and yield of eggplant.

Temporal complementarity between roots and mycorrhizal fungi drives wheat nitrogen use efficiency

Improving nitrogen (N) use efficiency (NUE) to reduce the application of N fertilisers in a way that benefits the environment and reduces farmers' costs is an ongoing objective for sustainable wheat production. However, whether and how arbuscular mycorrhizal fungi (AMF) affect NUE in wheat is still not well explored. Three independent but complementary experiments were conducted to decipher the contribution of roots and AMF to the N uptake and utilisation efficiency in wheat. We show a temporal complementarity pattern between roots and AMF in shaping NUE of wheat. Pre-anthesis N uptake efficiency mainly depends on root functional traits, but the efficiency to utilise the N taken up during pre-anthesis for producing grains (E_N,g ) is strongly affected by AMF, which might increase the uptake of phosphorus and thereby improve photosynthetic carbon assimilation. Root association with AMF reduced the N remobilisation efficiency in varieties with high E_N,g ; whilst the overall grain N concentration increased, due to a large improvement in post-anthesis N uptake supported by AMF and/or other microbes. The findings provide evidence for the importance of managing AMF in agroecosystems, and an opportunity to tackle the contradiction between maximising grain yield and protein concentration in wheat breeding.

Nutrition vs association: plant defenses are altered by arbuscular mycorrhizal fungi association not by nutritional provisioning alone

BACKGROUND

While it is known that arbuscular mycorrhizal fungi (AMF) can improve nutrient acquisition and herbivore resistance in crops, the mechanisms by which AMF influence plant defense remain unknown. Plants respond to herbivory with a cascade of gene expression and phytochemical biosynthesis. Given that the production of defensive phytochemicals requires nutrients, a commonly invoked hypothesis is that the improvement to plant defense when grown with AMF is simply due to an increased availability of nutrients. An alternative hypothesis is that the AMF effect on herbivory is due to changes in plant defense gene expression that are not simply due to nutrient availability. In this study, we tested whether changes in plant defenses are regulated by nutritional provisioning alone or the response of plant to AMF associations. Maize plants grown with or without AMF and with one of three fertilizer treatments (standard, 2 × nitrogen, or 2 × phosphorous) were infested with fall armyworm (Spodoptera frugiperda; FAW) for 72 h. We measured general plant characteristics (e.g. height, number of leaves), relative gene expression (rtPCR) of three defensive genes (lox3, mpi, and pr5), total plant N and P nutrient content, and change in FAW mass per plant.

RESULTS

We found that AMF drove the defense response of maize by increasing the expression of mpi and pr5. Furthermore, while AMF increased the total phosphorous content of maize it had no impact on maize nitrogen. Fertilization alone did not alter upregulation of any of the 3 induced defense genes tested, suggesting the mechanism through which AMF upregulate defenses is not solely via increased N or P plant nutrition.

CONCLUSION

This work supports that maize defense may be optimized by AMF associations alone, reducing the need for artificial inputs when managing FAW.

A response surface methodology approach to improve nitrogen use efficiency in maize by an optimal mycorrhiza-to-Bacillus co-inoculation rate

Co-inoculation of arbuscular mycorrhizal fungi (AMF) and bacteria can synergically and potentially increase nitrogen use efficiency (NUE) in plants, thus, reducing nitrogen (N) fertilizers use and their environmental impact. However, limited research is available on AMF-bacteria interaction, and the definition of synergisms or antagonistic effects is unexplored. In this study, we adopted a response surface methodology (RSM) to assess the optimal combination of AMF (Rhizoglomus irregulare and Funneliformis mosseae) and Bacillus megaterium (a PGPR-plant growth promoting rhizobacteria) formulations to maximize agronomical and chemical parameters linked to N utilization in maize (Zea mays L.). The fitted mathematical models, and also 3D response surface and contour plots, allowed us to determine the optimal AMF and bacterial doses, which are approximately accorded to 2.1 kg ha^-1 of both formulations. These levels provided the maximum values of SPAD, aspartate, and glutamate. On the contrary, agronomic parameters were not affected, except for the nitrogen harvest index (NHI), which was slightly affected (p-value of < 0.10) and indicated a higher N accumulation in grain following inoculation with 4.1 and 0.1 kg ha^-1 of AMF and B. megaterium, respectively. Nonetheless, the identification of the saddle points for asparagine and the tendency to differently allocate N when AMF or PGPR were used alone, pointed out the complexity of microorganism interaction and suggests the need for further investigations aimed at unraveling the mechanisms underlying this symbiosis.

The Critical Role of Arbuscular Mycorrhizal Fungi to Improve Drought Tolerance and Nitrogen Use Efficiency in Crops

Drought stress (DS) is a serious abiotic stress and a major concern across the globe as its intensity is continuously climbing. Therefore, it is direly needed to develop new management strategies to mitigate the adverse effects of DS to ensure better crop productivity and food security. The use of arbuscular mycorrhizal fungi (AMF) has emerged as an important approach in recent years to improve crop productivity under DS conditions. AMF establishes a relationship with 80% of land plants and it induces pronounced impacts on plant growth and provides protection to plants from abiotic stress. Drought stress significantly reduces plant growth and development by inducing oxidative stress, disturbing membrane integrity, plant water relations, nutrient uptake, photosynthetic activity, photosynthetic apparatus, and anti-oxidant activities. However, AMF can significantly improve the plant tolerance against DS. AMF maintains membrane integrity, improves plant water contents, nutrient and water uptake, and water use efficiency (WUE) therefore, improve the plant growth under DS. Moreover, AMF also protects the photosynthetic apparatus from drought-induced oxidative stress and improves photosynthetic efficiency, osmolytes, phenols and hormone accumulation, and reduces the accumulation of reactive oxygen species (ROS) by increasing anti-oxidant activities and gene expression which provide the tolerance to plants against DS. Therefore, it is imperative to understand the role of AMF in plants grown under DS. This review presented the different functions of AMF in different responses of plants under DS. We have provided a detailed picture of the different mechanisms mediated by AMF to induce drought tolerance in plants. Moreover, we also identified the potential research gaps that must be fulfilled for a promising future for AMF. Lastly, nitrogen (N) is an important nutrient needed for plant growth and development, however, the efficiency of applied N fertilizers is quite low. Therefore, we also present the information on how AMF improves N uptake and nitrogen use efficiency (NUE) in plants.

Global gene expression profiling under nitrogen stress identifies key genes involved in nitrogen stress adaptation in maize (Zea mays L.)

Maize is a heavy consumer of fertilizer nitrogen (N) which not only results in the high cost of cultivation but may also lead to environmental pollution. Therefore, there is a need to develop N-use efficient genotypes, a prerequisite for which is a greater understanding of N-deficiency stress adaptation. In this study, comparative transcriptome analysis was performed using leaf and root tissues from contrasting inbred lines, viz., DMI 56 (tolerant to N stress) and DMI 81 (susceptible to N stress) to delineate the differentially expressed genes (DEGs) under low-N stress. The contrasting lines were grown hydroponically in modified Hoagland solution having either sufficient- or deficient-N, followed by high-throughput RNA-sequencing. A total of 8 sequencing libraries were prepared and 88–97% of the sequenced raw reads were mapped to the reference B73 maize genome. Genes with a p value ≤ 0.05 and fold change of ≥ 2.0 or ≤ − 2 were considered as DEGs in various combinations performed between susceptible and tolerant genotypes. DEGs were further classified into different functional categories and pathways according to their putative functions. Gene Ontology based annotation of these DEGs identified three different functional categories: biological processes, molecular function, and cellular component. The KEGG and Mapman based analysis revealed that most of the DEGs fall into various metabolic pathways, biosynthesis of secondary metabolites, signal transduction, amino acid metabolism, N-assimilation and metabolism, and starch metabolism. Some of the key genes involved in N uptake (high-affinity nitrate transporter 2.2 and 2.5), N assimilation and metabolism (glutamine synthetase, asparagine synthetase), redox homeostasis (SOD, POX), and transcription factors (MYB36, AP2-EREBP) were found to be highly expressed in the tolerant genotype compared to susceptible one. The candidate genes identified in the present study might be playing a pivotal role in low-N stress adaptation in maize and hence could be useful in augmenting further research on N metabolism and development of N-deficiency tolerant maize cultivars.

Nitrogen Use Efficiency Definitions of Today and Tomorrow

Crop production has a large impact on the nitrogen (N) cycle, with consequences to climate, environment, and public health. Designing better N management will require indicators that accurately reflect the complexities of N cycling and provide biological meaning. Nitrogen use efficiency (NUE) is an established metric used to benchmark N management. There are numerous approaches to calculate NUE, but it is difficult to find an authoritative resource that collates the various NUE indices and systematically identifies their assets and shortcomings. Furthermore, there is reason to question the usefulness of many traditional NUE formulations, and to consider factors to improve the conceptualization of NUE for future use. As a resource for agricultural researchers and students, here we present a comprehensive list of NUE indices and discuss their functions, strengths, and limitations. We also suggest several factors-which are currently ignored in traditional NUE indices-that will improve the conceptualization of NUE, such as: accounting for a wider range of soil N forms, considering how plants mediate their response to the soil N status, including the below-ground/root N pools, capturing the synchrony between available N and plant N demand, blending agronomic performance with ecosystem functioning, and affirming the biological meaning of NUE.

Diversity and species composition of arbuscular mycorrhizal fungi across maize fields in the southern part of Belgium

Arbuscular mycorrhizal fungi (AMF) are key actors among soil microbial inhabitants, forming beneficial associations with most horticultural plants and crops (e.g., maize). For maize, the world most cultivated cereal, data on AMF species diversity in fields is sparse and even totally nonexistent in the southern part of Belgium where maize represents 8% of the cultivated area. In the present study, 14 maize fields in South Belgium under conventional, conversion, or organic management were analyzed for AMF diversity and species composition using 454 pyrosequencing. A large part (54%) of the 49 AMF species observed were unknown or have not been described in the literature. AMF diversity highly varied among fields, with the number of species ranging between 1 and 37 according to the field. A statistically significant effect of management was measured on AMF diversity, with the highest Hill index values (diversity and richness) under the organic management system compared with conventional management or conversion. Our results suggest a positive effects of organic management on AMF diversity in maize. They also highlight the rather high diversity or richness of AMF and the large portion of sequences not yet ascribed to species, thereby emphasizing a need to intensify AMF identification in cropping systems.

Biostimulant Effects of Glutacetine® and Its Derived Formulations Mixed With N Fertilizer on Post-heading N Uptake and Remobilization, Seed Yield, and Grain Quality in Winter Wheat

Biostimulants could play an important role in agriculture particularly for increasing N fertilizer use efficiency that is essential for maintaining both yield and grain quality in bread wheat, which is a major global crop. In the present study, we examined the effects of mixing urea-ammonium-nitrate fertilizer (UAN) or urea with five new biostimulants containing Glutacetine® or its derivative formulations (VNT1, 2, 3, and 4) on the physiological responses, agronomic traits, and grain quality of winter wheat. A first experiment under greenhouse conditions showed that VNT1, VNT3, and VNT4 significantly increased the seed yield and grain numbers per ear. VNT4 also enhanced total plant nitrogen (N) and total grain N, which induced a higher N Harvest Index (NHI). The higher post-heading N uptake (for VNT1 and VNT4) and the acceleration of senescence speed with all formulations enabled better nutrient remobilization efficiency, especially in terms of N mobilization from roots and straw toward the grain with VNT4. The grain ionome was changed by the formulations with the bioavailability of iron improved with the addition of VNT4, and the phytate concentrations in flour were reduced by VNT1 and VNT4. A second experiment in three contrasting field trials confirmed that VNT4 increased seed yield and N use efficiency. Our investigation reveals the important role of these new formulations in achieving significant increases in seed yield and grain quality.

Productivity and quality of horticultural crops through co-inoculation of arbuscular mycorrhizal fungi and plant growth promoting bacteria

Associations between plants and microorganisms exist in nature, and they can either be beneficial or detrimental to host plants. Promoting beneficial plant-microbe interaction for increased crop yield and quality is one pathway to eco-friendly and sustainable crop production. Arbuscular mycorrhizal fungi (AMF) and plant growth promoting bacteria (PGPB) are microorganisms that are beneficial to horticultural crops. Arbuscular mycorrhizal fungi establish symbioses with plant roots which help to improve nutrient uptake by the host plant and alter its physiology to withstand external abiotic factors and pathogens. Plant growth promoting bacteria promote plant growth either directly by aiding resource acquisition and controlling the levels of plant hormones or indirectly by reducing the inhibitory effects of phytopathogens. Co-inoculation of both organisms combines the benefits of each for increased crop productivity. Even though the co-inoculation of PGPB and AMF have been shown to enhance the yield and quality of crops, its benefits have fully not been exploited for horticultural crops. In this review, the response of horticultural crops to co-inoculation with PGPB and AMF with particular interest to the impact on the yield and crop quality was discussed. We explained some of the mechanisms responsible for the synergy between AMF and PGPB in plant growth promotion. Finally, suggestions on areas that need to be researched further to exploit and improve the effects of these organisms were highlighted.

The endosphere microbial communities, a great promise in agriculture

Agricultural food production and sustainability need intensification to address the current global food supply to meet human demand. The continuous human population increase and other anthropogenic activities threaten food security. Agrochemical inputs have long been used in conventional agricultural systems to boost crop productivity, but they are disadvantageous to a safe environment. Towards developing environmentally friendly agriculture, efforts are being directed in exploring biological resources from soil and plant microbes. The survival of the rhizosphere and endosphere microbiota is influenced by biotic and abiotic factors. Plant microbiota live interdependently with the host plants. Endophytes are regarded as colonizer microbes inhabiting and establishing microbial communities within the plant tissue. Their activities are varied and include fixing atmospheric nitrogen, solubilizing phosphate, synthesis of siderophores, secretion of metabolite-like compounds containing active biocontrol agents in the control of phytopathogens, and induced systemic resistance that stimulates plant response to withstand stress. Exploring beneficial endophyte resources in the formulation of bio-inoculants, such as biofertilizers, as an alternative to agrochemicals (fertilizers and pesticides) in developing environmentally friendly agriculture and for incorporation into crop breeding and disease control program is promising. Therefore, in this review, endosphere microbial ecology, associating environmental factors, and their roles that contribute to their effectiveness in promoting plant growth for maximum agricultural crop productivity were highlighted.

Beneficial soil-borne bacteria and fungi: a promising way to improve plant nitrogen acquisition

Nitrogen (N) is an essential element for plant productivity, thus, it is abundantly applied to the soil in the form of organic or chemical fertilizers that have negative impacts on the environment. Exploiting the potential of beneficial microbes and identifying crop genotypes that can capitalize on symbiotic associations may be possible ways to significantly reduce the use of N fertilizers. The best-known example of symbiotic association that can reduce the use of N fertilizers is the N2-fixing rhizobial bacteria and legumes. Bacterial taxa other than rhizobial species can develop associative symbiotic interactions with plants and also fix N. These include bacteria of the genera Azospirillum, Azotobacter, and Bacillus, some of which are commercialized as bio-inoculants. Arbuscular mycorrhizal fungi are other microorganisms that can develop symbiotic associations with most terrestrial plants, favoring access to nutrients in a larger soil volume through their extraradical mycelium. Using combinations of different beneficial microbial species is a promising strategy to boost plant N acquisition and foster a synergistic beneficial effect between symbiotic microorganisms. Complex biological mechanisms including molecular, metabolic, and physiological processes dictate the establishment and efficiency of such multipartite symbiotic associations. In this review, we present an overview of the current knowledge and future prospects regarding plant N nutrition improvement through the use of beneficial bacteria and fungi associated with plants, individually or in combination.

Genetic Variation in Response to N, P, or K Deprivation in Baby Leaf Lettuce

Lettuce harvested at the baby leaf stage is a popular component of mixed salads in ready-to-use packages. Little is known, however, about response of baby leaf lettuce to nitrogen (N), phosphorus (P), and potassium (K) fertilization. Eight lettuce accessions were subjected to five fertilization treatments to investigate genetic differences in reaction to N, P, and K fertilization. The control treatment provided optimal levels of macronutrients for plant growth, while other treatments deprived plants of either N, P, or K. Potassium deprivation had no obvious effect on plant weight or composition, apart from substantially decreased potassium content. Nitrogen and phosphorus deprivations, however, extensively decreased fresh weight and affected plant composition. Phosphorus and nitrogen deprivation considerably increased anthocyanin content in red-colored accessions, but anthocyanin was decreased in dark green-colored accessions, indicating differences in regulation of anthocyanin biosynthesis. Correlations between fresh weight, chlorophyll, anthocyanin, nitrogen, phosphorus, and potassium content were substantially affected by selection of datasets used for analyses; some relationships were revealed when analyzed separately by individual treatments, while others were more likely to be detected when analyzed by individual accessions. Absolute (ΔABS) and relative (2ΔREL) parameters described in this study were suitable for detecting over- and underperforming accessions. The ΔABS identified the absolute Lb-fold (logarithm to the base of 2, binary logarithm) change in performance of an accession in a treatment as compared to its performance in control conditions. The 2ΔREL parameter showed relative Lb-fold change for an accession as compared to the overall mean of ΔABS values of all accessions tested in control and treatment conditions.

Isolation and Culture of Arbuscular Mycorrhizal Fungi from Field Samples

The isolation of arbuscular mycorrhizal fungi from different land use is the starting point for selecting and producing inoculants. There are different techniques to isolate and produce large-scale arbuscular mycorrhizal fungi-based inoculum, being soil, inert substrate, and in vitro culture techniques among the most used by different biofertilizer producers. This chapter describes an active operating method to isolate and produce large-scale fungal inoculant in substrate-based manufacturing. In addition, critical parameters are presented for the optimal production of arbuscular mycorrhizal fungal inoculum. All the steps of the process are enlisted: from choosing the source of inoculum, its production, scaling, sustaining quality control, to shelf life.

Arbuscular mycorrhizal fungi improve the growth and disease resistance of the invasive plant Wedelia trilobata

AIMS

Arbuscular mycorrhizal fungi (AMF) are symbiotic partners of many invasive plants, however, it is still unclear how AMF contribute to traits that are important for the successful invasion of their host and how environmental factors, such as nutrient conditions, influence this. This study was to explore the effects of Glomus versiforme (GV) and Glomus mosseae (GM) on the growth and disease resistance of the invasive plant Wedelia trilobata under different nutrient conditions.

METHODS AND RESULTS

We found that GV and GM had higher root colonization rates resulting in faster W. trilobata growth under both low-N and low-P nutrient conditions compared to the normal condition. Also, the colonization of W. trilobata by GV significantly reduced the infection area of the pathogenic fungus Rhizoctonia solani under low-N conditions.

CONCLUSIONS

These results demonstrated that AMF can promote the growth and pathogenic defence of W. trilobata in a nutrient-poor environment, which might contribute to their successful invasion into certain type of habitats.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, we report for the first time that AMF can promote growth and disease resistance of W. trilobata under nutrient-poor environment, which contribute to a better understanding of plant invasion.

Comparative study of the mycorrhizal root transcriptomes of wild and cultivated rice in response to the pathogen Magnaporthe oryzae

BACKGROUND

Rice, which serves as a staple food for more than half of the world's population, is very susceptible to the pathogenic fungus, Magnaporthe oryzae. However, common wild rice (Oryza rufipogon), which is the ancestor of Asian cultivated rice (O. sativa), has significant potential as a genetic source of resistance to M. oryzae. Recent studies have shown that the domestication of rice has altered its relationship to symbiotic arbuscular mycorrhizae. A comparative response of wild and domestic rice inhabited by mycorrhizae to infection by M. oryzae has not been documented.

RESULTS

In the current study, roots of wild and cultivated rice colonized with the arbuscular mycorrhizal (AM) fungus (AMF) Rhizoglomus intraradices were used to compare the transcriptomic responses of the two species to infection by M. oryzae. Phenotypic analysis indicated that the colonization of wild and cultivated rice with R. intraradices improved the resistance of both genotypes to M. oryzae. Wild AM rice, however, was more resistant to M. oryzae than the cultivated AM rice, as well as nonmycorrhizal roots of wild rice. Transcriptome analysis indicated that the mechanisms regulating the responses of wild and cultivated AM rice to M. oryzae invasion were significantly different. The expression of a greater number of genes was changed in wild AM rice than in cultivated AM rice in response to the pathogen. Both wild and cultivated AM rice exhibited a shared response to M. oryzae which included genes related to the auxin and salicylic acid pathways; all of these play important roles in pathogenesis-related protein synthesis. In wild AM rice, secondary metabolic and biotic stress-related analyses indicated that the jasmonic acid synthesis-related α-linolenic acid pathway, the phenolic and terpenoid pathways, as well as the phenolic and terpenoid syntheses-related mevalonate (MVA) pathway were more affected by the pathogen. Genes related to these pathways were more significantly enriched in wild AM rice than in cultivated AM rice in response to M. oryzae. On the other hand, genes associated with the 'brassinosteroid biosynthesis' were more enriched in cultivated AM rice.

CONCLUSIONS

The AMF R. intraradices-colonized rice plants exhibited greater resistance to M. oryzae than non-AMF-colonized plants. The findings of the current study demonstrate the potential effects of crop domestication on the benefits received by the host via root colonization with AMF(s), and provide new information on the underlying molecular mechanisms. In addition, results of this study can also help develop guidelines for the applications of AMF(s) when planting rice.

Understanding the Mobilization of a Nitrification Inhibitor from Novel Slow Release Pellets, Fabricated through Extrusion Processing with PHBV Biopolymer

Dicyandiamide (DCD) has been studied as a stabilizer for nitrogen fertilizers for over 50 years. Its efficacy is limited at elevated temperatures, but this could be addressed by encapsulation to delay exposure. Here, poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) was investigated as a biodegradable matrix for the encapsulation of DCD. Cylindrical ∼3 mm × 3 mm pellets were fabricated through extrusion processing with 23 wt % DCD. Release kinetics were monitored in water, sand, and both active and γ-irradiated agricultural clay loam soils. Raman maps showed a wide particle size distribution of DCD crystals and indicated that Hitachi's classic moving front theory did not hold for this formulation. The inhibitor release kinetics were mediated by four distinct mechanisms: (i) initial rapid dissolution of surface DCD, (ii) channeling of water through voids and pores in the PHBV matrix, (iii) gradual diffusion of water and DCD through layers of PHBV, and (iv) biodegradation of the PHBV matrix. After ∼6 months, 45-100% release occurred, depending on the release media. PHBV is shown to be an effective, biodegradable matrix for the long-term slow release of nitrification inhibitors.

Inorganic Nitrogen Application Affects Both Taxonomical and Predicted Functional Structure of Wheat Rhizosphere Bacterial Communities

The effects of fertilizer regime on bulk soil microbial communities have been well studied, but this is not the case for the rhizosphere microbiome. The aim of this work was to assess the impact of fertilization regime on wheat rhizosphere microbiome assembly and 16S rRNA gene-predicted functions with soil from the long term Broadbalk experiment at Rothamsted Research. Soil from four N fertilization regimes (organic N, zero N, medium inorganic N and high inorganic N) was sown with seeds of Triticum aestivum cv. Cadenza. 16S rRNA gene amplicon sequencing was performed with the Illumina platform on bulk soil and rhizosphere samples of 4-week-old and flowering plants (10 weeks). Phylogenetic and 16S rRNA gene-predicted functional analyses were performed. Fertilization regime affected the structure and composition of wheat rhizosphere bacterial communities. Acidobacteria and Planctomycetes were significantly depleted in treatments receiving inorganic N, whereas the addition of high levels of inorganic N enriched members of the phylum Bacteroidetes, especially after 10 weeks. Bacterial richness and diversity decreased with inorganic nitrogen inputs and was highest after organic treatment (FYM). In general, high levels of inorganic nitrogen fertilizers negatively affect bacterial richness and diversity, leading to a less stable bacterial community structure over time, whereas, more stable bacterial communities are provided by organic amendments. 16S rRNA gene-predicted functional structure was more affected by growth stage than by fertilizer treatment, although, some functions related to energy metabolism and metabolism of terpenoids and polyketides were enriched in samples not receiving any inorganic N, whereas inorganic N addition enriched predicted functions related to metabolism of other amino acids and carbohydrates. Understanding the impact of different fertilizers on the structure and dynamics of the rhizosphere microbiome is an important step toward developing strategies for production of crops in a sustainable way.

Interactive effects of compost and pre-planting soil moisture on plant biomass, nutrition and formation of mycorrhizas: a context dependent response

We aimed to investigate the combined impacts of compost addition and pre-planting soil moisture conditions, on plant-available nutrients, and subsequent impacts on the biomass, nutrition and formation of AM by two important crop species. A glasshouse study was undertaken in which wheat and tomato plants were grown in compost amended or un-amended soil that was subjected to different moisture regimes prior to planting. The availability of P was strongly influenced by compost addition, but not pre-planting moisture conditions. In contrast, mineral N pools were affected by compost addition and pre-planting soil moisture conditions in complex ways. These changes in nutrient availability affected plant biomass, nutrient uptake and formation of AM. In general, plant performance was better where pre-planting soil moisture conditions were wet or dry, and worse where they involved a wet/dry cycle, and mycorrhizal colonisation was lower where compost was added to the soil. That pre-planting moisture conditions affect the biomass of subsequent crops is an important finding, the potential implications of which are considered here.

Nitrogen Nutrition of Fruit Trees to Reconcile Productivity and Environmental Concerns

Although perennial fruit crops represent 1% of global agricultural land, they are of a great economic importance in world trade and in the economy of many regions. The perennial woody nature of fruit trees, their physiological stages of growth, the root distribution pattern, and the presence of herbaceous vegetation in alleys make orchard systems efficient in the use and recycling of nitrogen (N). The present paper intends to review the existing literature on N nutrition of young and mature deciduous and evergreen fruit trees with special emphasis to temperate and Mediterranean climates. There are two major sources of N contributing to vegetative tree growth and reproduction: root N uptake and internal N cycling. Optimisation of the use of external and internal N sources is important for a sustainable fruit production, as N use efficiency by young and mature fruit trees is generally lower than 55% and losses of fertilizer N may occur with the consequent economic and environmental concern. Organic alternatives to mineral N fertilizer like the application of manure, compost, mulching, and cover crops are scarcely used in perennial fruit trees, in spite of the fact that society’s expectations call for more sustainable production techniques and the demand for organic fruits is increasing.