The Rhizobium-Legume Nitrogen-Fixing Symbiosis

Gary Stacey , in Biological science of the Nitrogen Bike, 2007

10.2 An overview of nodule germination

Nodulation is a host-specific process with each rhizobium having a defined host-institute range ( Table 10-ane). Rhizobia, normally institute in the soil, answer to the plant-root environment (rhizosphere) by increasing their population levels and attaching to the root surface. When in a uniform interaction (i.east., an advisable symbiont with an appropriate host), the bacteria attach to root hairs and recognize flavonoid (secondary plant metabolites) signals excreted past the plant. Each rhizobium has co-evolved to recognize the specific flavonoid mixture excreted by its compatible host. Recognition of this bespeak leads to de novo transcription of nodulation (nod) genes in the symbiont. These nod genes, in plough, encode enzymes that synthesize a unique signal molecule, the Nod bespeak, which is excreted from the bacterium and recognized by the found host. The Nod indicate is a modified lipo-chitin molecule and rhizobia appear to be the only prokaryotes with the power to make such a molecule. Indeed, possession of the nod genes defines, in big part, whether a rhizobium is competent for forming an Nii-fixing symbiosis.

Table 10-1. Selected rhizobial species and examples of host range.

Species Host range
Rhizobium leguminosarum bv. phaseoli Mutual bean
Rhizobium leguminosarum bv. trifolii Clover
Rhizobium leguminosarum bv. viceae a Pea, vetch
Rhizobium tropici Mutual edible bean
Rhizobium etli a Common bean
Mesorhizobium loti a Lotus japonicus
Azorhizobium caulinodans Sesbania
Sinorhizobium meliloti a Alfalfa, Medicago truncatula
Sinorhizobium fredii Soybean
Bradyrhizobium japonicum a Soybean, cowpea, mungbean
Bradyrhizobium elkanii Soybean, cowpea, mungbean
a
Species for which the genomic sequence is known or for which sequencing projects are underway.

The specific chemistry of the Nod signal is determined by the rhizobial symbiont and has co-evolved to exist recognized only past the compatible legume host. Therefore, it is this chemistry and the recognition systems involving diffusible signals that make up one's mind host specificity. Recognition of the lipo-chitin Nod signal by the host initiates a chain of events leading ultimately to the formation of the nodule structure. Every bit described in item below, Nod-indicate recognition leads to rapid events in the root hair of the compatible host and a modification of normal, polar root-hair growth, such that the pilus cell curls at its tip. This often results in a "cork-screw" like appearance and the rhizobia colonize the cavity created by the curl. After, the leaner gain entry into the root hair through a mechanism that is nevertheless not well understood. Entry appears to involve a form of endocytosis, such that the bacteria are never costless in the cytoplasm just are bars inside a membrane and cell-wall-enclosed tube (infection thread) that initiates at the whorl so progresses intracellularly downwards the root-hair cell. The infection thread then continues its growth into the cortex, where it ramifies into the newly forming nodule primordium. The cell-wall biosynthetic machinery that is normally used to produce the division between ii dividing daughter cells is adapted by the invading rhizobia to form the infection thread.

The lipo-chitin molecules produced by rhizobia are biologically active at <ane nM. Addition of either purified or chemically synthesized Nod signal can induce root-hair deformation merely not root-hair curling, which seems to crave the presence of the bacteria. The Nod betoken alone can trigger cell division in the root cortex and lead to the germination of a nodule primordium. This outcome involves activation of the cell wheel in quiescent cortical cells. Indeed, cells in the zone of the nodule, which will ultimately be infected, undergo rounds of endoreduplication leading to polyploidy. The mechanisms past which the Nod signal causes these changes are all the same unknown.

At some point, the rhizobia within the infection thread reach a cell that they will infect. Release of the leaner from the infection thread again resembles endocytosis by resulting in the formation of a membrane-spring compartment in which the bacteria exist as intracellular symbionts. This membrane-spring compartment has been termed the "symbiosome" to draw attention to both its quasi-organellar nature and its similarity to structures found in a multifariousness of intracellular bacterial symbionts both in plants and in animals. The symbiosome is the unit of biological N2 fixation because its membrane mediates interaction with the host jail cell (e.thou., for nutrient uptake and ammonia excretion), whereas the leaner induce all of the machinery for Due northtwo fixation. Rhizobia within the symbiosomes differentiate and induce a diverseness of new enzyme systems and often have on a larger, more than extended (sometimes branched) shape. For these reasons, the special term "bacteroid" is used to ascertain the intracellular symbiont.

The nodule is a highly specialized organ [i]. 2 morphological types of nodules are known and they are determined by the plant host (Figure 10-i). The outset type is chosen indeterminate and these nodules occur on clover and alfalfa, for example. They appear every bit modified lateral roots with a final apical meristem, but with lateral vascular tissue. Considering these nodules grow from their tip by means of new cell divisions, the total developmental series of the nodule tin be seen in a cross-section. In club from the tip, such nodules exhibit a preinfection zone, infection zone, fixation zone, and, finally, a zone of senescent tissue. In contrast, the 2nd blazon, called determinate nodules, which form on plants such every bit soybean and mutual bean, is initiated from cortical cell divisions simply largely grows through cell expansion and results in a globular nodule construction. Determinate nodules as well possess peripheral vascular tissue but, considering development is in a radial design, distinct zones are more difficult to distinguish.

Effigy 10-1. Graphical representation of the anatomical construction of determinate (left) and indeterminate (right) nodules. The peripheral vascular tissue divides the outer and inner cortex. Clear developmental zones are difficult to discern in determinate nodules. Indeterminate nodules have a persistent apical meristem (M) and bear witness clear zones marking stages in nodule development: 1, preinfection zone; two, infection zone; iii, fixation zone; 4, senescent zone. Baseline represents the root surface.

The interior of both determinate and indeterminate nodules contains very depression Oii levels. A physical barrier to O2 permeation exists within the outer cortex of the nodule and leghemoglobin, which is specifically produced in nodules, binds available Otwo within the nodule. The low Oii concentration is essential because nitrogenase is quickly denatured (inactivated) by O2. Paradoxically, rhizobia are obligate aerobes then require Oii for respiration. The bacteroids circumvent this seeming paradox by expressing a high-affinity respiration system that functions using the O2 transferred directly from leghemoglobin. Hence, the bacteroids are able to carry out aerobic metabolism, even though the free Oii levels are very depression.

The nodule is a true organ considering it exhibits cellular specialization. The infected cells, which contain the symbiosomes, carry out biological N2 fixation. Each infected cell is besides in contact with at to the lowest degree one uninfected cell. Northward fixed in the infected cell is transferred to the uninfected cell where it is incorporated. Temperate-climate legumes, which mainly form indeterminate nodules, largely incorporate fixed N into amides (e.g., asparagine), which are used to transport the stock-still N to the upper office of the establish. Tropical legumes, which mainly form determinate nodules, comprise the fixed N into ureides (east.g., allantoin) that as well deed as stock-still-North carriers. The transcription of enzymes for North assimilation, carbon utilization, nodule development, etc., is oft induced to high levels during nodulation. Originally, some of these genes were thought to exist nodule specific and, therefore, their products were given the special name, nodulins. Still, more research has shown that these genes were likely recruited from pre-existing systems and it is just their regulation that is nodulation responsive. Nodulins produced inside the first 48 h or so later on rhizobial inoculation are termed "early nodulins" and thought to largely function in infection and nodule development. The "belatedly nodulins" include gene products idea to function in nodule metabolism, N2 fixation, and nodule maintenance. However, although these distinctions were historically useful, recent data has blurred them and nodulation is more correctly viewed as a continuum.

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Agricultural and Related Biotechnologies

J.Thou. Bergthorson , ... J.K. Vessey , in Comprehensive Biotechnology (Tertiary Edition), 2011

iv.08.4.2 Microbial Point Compounds and Plant Growth Promotion

4.08.four.2.1 Lipo-Chitooligosaccharides (LCOs)

The process of nodulation in legumes begins with a circuitous network of signaling between host plants and rhizobia. The offset footstep in the rhizobial establishment in plant roots is production of isoflavanoids as plant-to-bacterial signals; the most common in the soybean– B. japonicum symbiosis are genestin and diadzein, 51,52 which trigger the nod genes in the leaner to produce lipo-chitooligosaccharide (LCO), or nod factors that human activity equally return signals to the plants to offset the process of root hair crimper, leading to nodule formation. LCOs are oligosaccharides of β-1,4-linked N-acetyl-d-glucosamine coded past a series of nod genes and are rhizobia specific. 51,52 The nodDABCIJ genes are conserved in all nodulating rhizobia, organized as a transcriptional unit and regulated by plant-to-rhizobia signals such a isoflavanoids. 51,52 Nodulation and subsequent nitrogen fixation are affected past ecology factors. It has been observed for soybean that nether suboptimal root zone temperatures (1–17   °C), pH stress, and the presence of nitrogen, isoflavanoid signal levels are reduced; while high temperature (39   °C) increased nonspecific isoflavanoid production and reduced nod gene activation, thereby affecting nodulation. Researchers have isolated and purified the major LCO molecule produced by B. japonicum 532C (Nod Bj Five (C18:1; MeFuc)). 59 This Nod gene contains a methyl-fucose group at the reducing end that is encoded by the host-specific nodZ gene, which is an essential component for soybean–rhizobia interactions.

LCOs also positively bear upon constitute growth and development in nonlegumes. The potential function of LCOs in institute growth regulation was first reported past Denarie and Cullimore. xx Nod genes A and B from Rhizobium meliloti, when introduced into tobacco, contradistinct the phenotype past producing bifurcated leaves and stems, suggesting a role for nod genes in plant morphogenesis. 51,52 The evolution of somatic embryos of Norway bandbox is enhanced when treated with purified Nod factor from Rhizobium sp. NGR234. Information technology is suggested that these Nod factors can substitute for auxin and cytokinin-similar activities in promoting embryo development, and that the chitin core of the nod gene is an essential component for such developmental regulation. 25,26 Some of the LCO-induced enod genes in nonlegumes seem to encode for defense-related responses, such as chitinase and PR proteins, peroxidase, and enzymes of phenylpropanoid pathway, such as l-phenylalanine ammonia-lyase (PAL). 51,52 Treatment with LCOs enhanced seed germination and seedling institution of soybean, common bean, maize, rice, B. napus, apple tree, and grape, accompanied by an increase in photosynthetic rates. Hydroponically grown maize showed an increase in root growth when LCO was applied through the food solution, 51,52 and foliar awarding to green-firm-grown maize resulted in increases in photosynthetic rate, leafage surface area, and dry out matter. Foliar application on tomato plant during early and belatedly flowering stages increased flowering and fruiting and also fruit yield. 13 An increase in mycorrhizal colonization (Gigaspora margarita) was observed in Pinus abies. 26,57 Recent research on soybean leaves treated with LCOs under suboptimal growth weather condition has revealed the upregulation of over 600 genes of which many are defense- and stress-related genes and transcription factors. The microarray results bear witness that the transcriptome of the leaves are highly responsive to LCO treatment at 48   h posttreatment (Wang, MSc Thesis; unpublished data). The results of the microarray analysis accept suggested the need to investigate more carefully the mechanisms past which microbe-to-plant signals help plants accommodate abiotic and biotic stress conditions. Initial experiments with Arabidopsis thaliana showed an early germination pattern and an increase in total phenolics, PAL, and tyrosine ammonia lyase (TAL) upon treatment with LCO (Subramanian, unpublished data).

A number of authors three,13,25,26,52,57,60 bear witness that LCOs can alter the class of growth and development in a range of plants. Enhanced germination and seedling growth, along with the mitogenic nature of LCOs, suggest accelerated meristem activity. LCOs take been isolated from B. japonicum and information technology has been confirmed that these compounds accelerate seed germination, seedling emergence, and root growth and development in soybean and also in nonleguminous plants. 51,52 Oláh et al. 57 confirmed LCO stimulation of root growth in Medicago truncatula. Chen et al. 13 showed that LCO spray on lycopersicon esculentum accelerates flowering (a typical response to stress) and increases yield. Foliar awarding of LCOs induces resistance of soybean plants to powdery mildew. 24 Given that LCOs induce defense responses in Medicago cell cultures and roots, 65 that LCOs testify structural similarity to chitin (they have a chitin courage), and that chitin induces defense responses in plants, 41 it is reasonable to hypothesize that LCOs induce aspects of constitute defense responses, similar to chitin.

EMD Crop Biosciences is at present marketing products based on LCO effects on ingather growth and development (www.emdcropbioscience.com/homepage.cfm). A related technology, based on activation of rhizobacterial signaling through use of a plant-to-microbe signal (jasmonate), is too being marketed by Becker–Underwood (http://world wide web.beckerunderwood.com/en/newsreleases/vaulthprelease). The exploitation of signals involved in interactions between PGPR and host plants constitutes a new depression-input method to increment crop growth.

four.08.4.2.2 Bacteriocins

The PGPR Bacillus thuringiensis NEB17 was originally isolated from soybean nodules 52 and enhances nodulation when practical equally a coinoculant with B. japonicum 532C. It has been shown that this bacterium produces a novel antimicrobial peptide (bacteriocin), now named thuricin-17, which has a molecular weight of 3.1   kDa. 33,35 When sprayed on leaves or applied to roots, thuricin-17 stimulates growth of corn and soybean. This growth stimulation is similar in nature to that caused by LCOs. Thuricin-17 and bacthuricin-F4 are quite like and a new class of bacteriocins, course IId, has been proposed to adapt them. 33 These compounds, similar the LCOs, represent a novel low-input method for enhancing crop growth, especially the growth of biofuel feedstock crops.

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Nitrogen-Use Efficiency Nether Irresolute Climatic Weather

Amitav Bhattacharya , in Changing Climate and Resource Employ Efficiency in Plants, 2019

Chickpea

NaCl stress results in the reduction of nodulation and inhibition of nitrogen-fixing activeness in legumes. Garg and Chandel (2011) conducted an experiment to study the interaction between mycorrhizal mucus, Glomus mosseae, and salinity stress in relation to nitrogen fixation, plant growth, and nutrient accumulation in chickpea. The performances of ii genotypes of chickpea (Pusa-329, Pusa-240) were compared under different levels of salinity with and without mycorrhizal inoculations. They concluded that nitrogen and phosphorus levels in the leaves and roots were reduced with increasing salinity in both chickpea genotypes; the refuse was more distinct in Pusa-240 than Pusa-329. Nitrogenase activeness was reduced with increasing salt concentrations that ultimately resulted in a reduction of nitrogen fixation, plant growth, and yield. The mineralization of organic nitrogen and the immobilization of mineral nitrogen are important for the nitrogen supply of the institute. The mineralization depends on the C:N ratio of the organic affair, the temperature, the h2o stress (Rodrigo et al., 1997), the water table through its effect on soil aeration (Van Hoorn, 1958), and salinity, affecting the nitrification (Mengel and Kirkby, 1982). Immobilization is the inversed process by which mineral nitrogen is transformed in organic nitrogen. Moreover, mineral nitrogen can be lost through denitrification and under aerobic conditions. The residuum between mineralization and immobilization is estimated at a supply of about 0.5   kg/ha per 24-hour interval. Co-ordinate to Van Hoorn (1958) a lowering of the groundwater table from 0.4   thousand to one.5   m depth in clay soil correspond with an increase of most 100   kg   North/ha. No data are available about the salinity consequence on the remainder betwixt mineralization and immobilization, simply salinity could bear upon information technology directly through nitrification or indirectly through a change in the C:Due north ratio or through water stress. The nitrogen uptake of the found decreased with increasing salinity. The nitrogen contribution of the soil decreased stronger than the institute uptake, pointing to a salinity effect on the mineral nitrogen production by biological activity in the soil through nitrogen fixation and transformation of organic nitrogen. A salinity effect on nitrogen fixation could explain, at least partly, the salt sensitivity of grain legumes.

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Office of rhizosphere microbiome during phytoremediation of heavy metals

Fifty. Breton-Deval , ... E. Tovar-Sanchez , in Microbial Biodegradation and Bioremediation (2nd Edition), 2022

xiii.2.5 Indirect mechanisms

It has been reported that HMs reduce the efficiency of symbiotic nodulation with Rhizobium and inhibit plant growth. Some rhizobacteria can improve institute growth indirectly by promoting clan with other beneficial rhizobacteria (i.eastward., nitrogen-fixing bacteria), by protecting plants from pathogen set on (Jian, Bai, Zhang, Song, & Li, 2019), and also by enhancing "symbiotic N fixation through promoting root development in general" (Ahemad & Kibret, 2014). Laboratory studies under axenic atmospheric condition help to understand the effect of individual strains and to decipher specific mechanisms; still, complex relationships occur in rhizosphere between plants roots, bacteria protozoa, fungi, nematodes, and annelids. These relationships brainstorm to develop from seed germination and to empathize these complex interactions global studies are required (Chen et al., 2018).

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Biological North Inputs

Peter J. Bottomley , David D. Myrold , in Soil Microbiology, Ecology and Biochemistry (Fourth Edition), 2015

B Rhizobial Nodulation Genes

The rhizobial genes needed to found a symbiosis have been identified, and some are listed in Tabular array fifteen.4 . Genes referred to every bit common nodulation genes ( nod A, B, C, and D) are found in all rhizobia and are essential for initiation of nodulation. Many other genes have been identified that define the legume host range of specific rhizobia and are referred to equally either nod, nol, or noe genes. These genes are involved in the synthesis and ship of an unusual grade of compounds referred to as "nod factors" that induce nodule formation. Nod factors are equanimous of an oligosaccharide backbone of βane   -4 linked N-acetyl glucosamine residues with a fatty acid acyl group attached to the N atom of the nonreducing acetyl glucosamine rest (Supplemental Fig. 15.one; run across online supplemental material at http://booksite.elsevier.com/9780124159556). These compounds are referred to equally lipo-chitin oligosaccharides or LCOs. LCOs usually contain iii to vi Northward-acetylglucosamine residues. The common nod genes A, B, and C encode enzymes that play key roles in synthesis of the LCO backbone structure and include chitin oligosaccharide synthase (nod C), chitin oligosaccharide deacetylase (nod B), and acyl transferase (nod A). A large number of structural variants exist among the LCOs (Table 15.5). Host specificity is based on the structural variations amid the nod factors. A number of specific substituents ("decorations") are plant on positions R1 through R9, and different rhizobial genes (referred to as host range genes) are involved in the attachment of these moieties. For example, the placement of a And so4 grouping at position R5 of the LCO from Ensifer meliloti is essential for nodulation of alfalfa (Medicago sativa), whereas O-methyl fucosylation of R5 is essential for nodulation of soybean (Glycine max) by Bradyrhizobium japonicum. Nod factors are produced by rhizobia in response to inducers secreted by germinating seedlings and past institute roots. The most potent inducers are flavones and isoflavones, which are phenolic compounds collectively referred to as flavanoids. The flavanoids demark to the nod D cistron production, which acts as a transcriptional activator of the other nod genes. Different types of flavanoid molecules are involved in nod gene consecration in unlike legume species. For instance, luteolin in Medicago sativa (alfalfa) and genistein in Glycine max (soybeans).

Table 15.four. Examples of Nodulation (nod) Genes and Their Proposed Functions

Gene Proposed Function
Regulatory genes
nodD1 Transcriptional activator
nodFive Two-component regulator
nodWestward Ii-component regulator
nolA Transcriptional regulator
nolR Repressor, DNA binding poly peptide
syrM Transcriptional regulator
Nod factor core synthesis
nodA Acetyltransferase
nodB Deacetylase
nodC Chitin synthase
nodYard d-glucosamine synthase
Nod factor core modifications
nodE Ketoacyl synthase
nodF Acyl carrier protein
nodH Sulfotransferase
nodL Acetyl transferase
nodS Methyl transferase
nodL ATP-sulfurylase
nodP ATP-sulfurylase
nodZ Fucosyl transferase
nolL O-acetyl transferase
nolO Carbamoyl transferase
noeC Arabinosylation
noeI 2-O-methylation
Nod factor send
nodI Integral membrane protein
nodJ Outer membrane transport
nodO Pore-forming protein

This listing of genes is not inclusive; some genes coding for the aforementioned function in different organisms are named differentially; see online supplemental material at http://booksite.elsevier.com/9780124159556.

From Soil Microbiology, Ecology, and Biochemistry, third ed., Paul, Eastward.A., Biological Northward inputs. (2007). pp. 365–387.

Table fifteen.v. Examples of Modifications of the Rhizobial Cadre nod Factors and the Genes and Their Products Involved

Bacterium Cadre Substituents Genes
E. meliloti R4:Ac, R5:S, C16:2 R4:nod50, R5:nodH, fat acid:nodAFEG
1000. loti R1:Me, R3:Cb, R5:AcFuc R1:nodS, R3:nolO,R5: nodZ &amp; nolL
B. japonicum R5:MeFuc R5:nodZ &amp; noeI
A. caulinodans R1:Me, R4:Cb, R5:Fuc,R8:Ara R1:nodS, R4:nodU, R5:nodZ, R8: noeC

These examples practise not represent nol factors produced past all species of a named genus, nor all strains of the same species.

From Soil Microbiology, Environmental, and Biochemistry, third ed., Paul, Due east.A., Biological Northward inputs. (2007). pp. 365–387.

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Seaweeds and Cancer Prevention

Jinju Jiang , Shaojuan Shi , in Bioactive Seaweeds for Food Applications, 2018

14.vi Effect of Seaweed Iodine on Cancer Prevention

Iodine-rich seaweeds have long been used as a breast cancer treatment in traditional East Asian medicine to soften tumors and reduce nodulation ( Lin et al., 2011; Aceves et al., 2005). Modern research shows that iodine intake tin offer protection against chest cancer (Cann et al., 2000). Majem et al. (1988) reported that low iodine intake was associated with an increased risk of chest cancer mortality in a correlation study conducted in northeastern Espana. Furthermore, clinical trials of iodine supplementation have shown significant reductions in the symptoms of fibrocystic breast disease, considered a precursor to ductal carcinoma, in up to lxx% of the patients (Cann et al., 2000; Ghent et al., 1993). In dimethylbenzanthracene-induced mammary carcinoma in rats, iodine supplementation suppressed the affliction's development (Cann et al., 2000). Researchers observed that Lugol'southward iodine or iodine-rich U. pinnatifida administration to rats treated with the carcinogen 7,12-dimethylbenzanthracene suppressed mammary tumor evolution. Those researchers additionally demonstrated that seaweed induced a greater caste of apoptosis in human breast cancer cells than fluorouracil, a chemotherapeutic agent used to care for breast cancer. This finding led the authors to speculate that iodine-rich seaweed may be applicable for prevention of chest cancer (Funahashi et al., 2001).

In the developed world, Japan has the everyman age-adapted breast cancer mortality rates (Walsh, 1998; Rebecca et al., 2012). The incidence of breast cancer in Japanese immigrants to the United States, and in their successive generations, has gradually reached the rates of white United states of america women, which suggests a dietary link (Ziegler et al., 1993). Loftier iodine intake may exist a central protective gene against breast cancer development in Japanese women. Seaweed consumption is a major source of iodine in the Japanese diet, with the iodine content of the most commonly consumed seaweeds, Porphyra (nori), Undaria (wakame), and Laminaria (kombu), ranging from lxxx to 2500   μg/g (Cann et al., 2000). On average, the Japanese consume ≥12   mg of iodine per day, a much greater amount than the quantities consumed in the Westward, e.g., 166   μg/twenty-four hour period in the United Kingdom and 240   μg/day in the United States.

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Growth and carbon economic system of VA mycorrhizal plants

Emerge E Smith , David J Read , in Mycorrhizal Symbiosis (Second Edition), 2002

Plant Growth

The clan between the development of VA mycorrhizas and increased growth of the host was made by Asai (1944) in his studies of mycorrhizal colonization and nodulation in a big number of legumes. He concluded that colonization was important both in institute growth and in the evolution of nodules. Subse-quently, many investigators have carried out a big number of experiments which in general demonstrate that colonization is followed by considerable stimulation of growth. This early on piece of work has been extensively reviewed, with particular emphasis on the importance of mycorrhizas in P nutrition (e.k. Gerdemann, 1968, 1975; Mosse, 1973; Tinker, 1975a,b; Smith, 1980; Gianinazzi-Pearson and Gianinazzi, 1983; Harley and Smith, 1983; Hayman, 1983; Smith and Gianinazzi-Pearson, 1988; Koide, 1991a). Pioneering work on the potential significance of mycorrhizas in establish nutrition was carried out by Mosse (1957) on apples, Baylis (1959, 1967, 1972a) on Griselinia and other New Zealand plants, and Gerdemann (1968) on Liquidambar and maize. Subsequently, Daft and Nicolson (1966, 1969a, b, 1972), and Hayman and Mosse (1971, 1972; see also Mosse and Hayman, 1971) independently investigated the footing for this growth response in a number of constitute species, in particular with respect to soil conditions and inoculum density. They demonstrated that development of mycorrhizal roots and their event on plant growth is greater in soils of low or imbalanced food condition, in particular if P is in short supply, and they made valuable advances in interpretation of the mechanisms of these effects, which are discussed in Chapter 5.

Increased growth has been demonstrated for a very wide variety of plant species including many crop plants and trees (Plate 2); it is manifest as increased growth of roots equally well as shoots, reduced root:shoot ratio and increased tissue P concentrations (encounter Table 4.1 and Fig. iv.1). In a few plant species increased bloom production and yield take likewise been demonstrated. Nodulation and Northward fixation in mycorrhizal legumes and dually colonized actinorrhizal plants are as well increased and have been shown to result in higher tissue North concentrations in some experiments (see Barea and Azcón-Aguilar, 1983). Indeed, so many pot experiments have been carried out with such a wide variety of institute species that many of them cannot be cited. Most of the effects on growth can be attributed, directly or indirectly, to improved mineral nutrition and in many cases similar changes have been shown to take place in response to awarding of fertilizer in the absence of mycorrhizal colonization. Even so, it must exist stressed here that in discussing C use past the fungi it is essential to distinguish direct mycorrhizal furnishings from those which would inevitably follow any changes in size, form or nutrient content.

Plate ii. Effects of mycorrhizal inoculation of a range of crop plants in fumigate soil. Right-hand cake, inoculated with VA mycorrhizal fungi. Left-manus block, not inoculated. Crops (forepart to back): Al-lium, Catalpa, Pisum, Yicia, Zea. Non-host plants trimmed.

 Photograph courtesy of V. Gianinazzi-Pearson.

Tabular array 4.i. Effects of mycorrhizal colonisation and P diet on growth and P concentrations of roots and shoots of Trifolium subterraneum (31 to 35 days old)

Added P (mmol kg−1) Colonization (%) Dry weight of tissue (mg plant−1) Root:shoot ratio P concentration μg mg−1 dry out wt Response (%)
Root Shoot Total Root Shoot
0 0 36 (half dozen) 39 (4) 75 (ix) 0.92 0.53 0.79
0 74 51 (i) 58 (3) 109 (2) 0.88 1.34 1.98 45
0.2 0 57 (4) 63 (6) 120 (x) 0.90 0.75 1.02
0.2 72 57 (three) 90 (three) 147 (5) 0.63 2.12 2.83 22
0.4 0 70 (eight) 97 (half dozen) 172 (fourteen) 0.72 1.twenty ane.29
0.4 63 60 (2) 104 (3) 164 (five) 0.58 three.00 3.11 0
0.67 0 87 (11) 132 (8) 218 (xx) 0.66 1.77 i.57
0.67 53 67 (ane) 120 (2) 187 (2) 0.56 two.33 2.83 –14

Ways of iii replicate determinations are given, with standard errors of means in parentheses.

Percentage response (calculated from fresh weight data)=(+M)− (−M)/(−Yard) × 100

Data from Oliver et al. (1983, and unpublished).

Copyright © 1983

Figure 4.1. (a) Dry weight changes and (b) P contents of Allium cepa, colonized by 4 VA mycorrhizal fungi, compared with controls (C). YV, Glomus mosseae; LAM, G. macrocarpus var. geosporus; MIC, Thou. microcarpus; BR, Scutellospora (Gigaspora) calospora.

From Sanders et al. (1977), with permission. Copyright © 1977

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Biological dinitrogen fixation: symbiotic

Janice E. Thies , in Principles and Applications of Soil Microbiology (Third Edition), 2021

Methods of inoculation

When inoculation is necessary, there are a number of approaches that can be used. The goals of inoculation are to:

•

supply the number of rhizobia needed for good nodulation and constructive Due north 2 fixation

•

ensure that the highly constructive rhizobial strains used in inoculants form well-nigh of the nodules produced

•

favor inoculant strain persistence in soil and its domination in nodules over subsequent years

Prior to the 20th century, inoculation was often accomplished by taking soil from a productive site and applying it to a new crop or revegetation area. This often resulted in the transfer of undesirable soil organisms as well as those that were beneficial. Inoculant rhizobia are at present commercially produced and packaged, with different formulations bachelor to meet unlike seeding requirements. Four inoculation procedures are common.

Seed inoculation

The inoculant is mixed with milk or another slightly-adhesive material, and the seed is uniformly covered with this break. The seed is dried in the shade and sown the same 24-hour interval. This procedure requires that the inoculant strain be packaged in a relatively fine carrier textile (usually finely ground and limed peat) or liquid that volition adhere to the seed.

Seed pelleting

A stronger agglutinative, such as gum arabic or methyl cellulose, is used with the inoculated seed so rolled in ground limestone, stone phosphate, or biochar. Pelleting combats unfavorable soil weather condition such equally low pH or high temperature and is used for aerial sowing. The aim is to provide a microenvironment around the seed favorable to rhizobial survival. Preinoculation of legume seed, advocated for and sold past commercial vendors every bit a time savings, is not recommended because rhizobial numbers on the seed tin decline dramatically during storage.

Soil inoculation with a granular peat or liquid

The inoculant is banded into the seed furrow and so that it can make contact with the emerging root. The inoculant is packaged in a coarse peat or a liquid that is dribbled into the row alongside or beneath the seed. Granular soil inoculation is less time consuming, allows college rates of inoculation when soil atmospheric condition are unfavorable, and permits separation of seed and inoculants, when the seed has been treated with fungicides.

Inoculation in the planter seed box

The inoculant is mixed straight with seeds in the planter box. This simple method is sometimes used as insurance when soils already contain rhizobia. However, inoculant and seed tend to carve up providing uneven coverage, and the practice is not recommended.

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Root system architectural and growth responses of crop plants to mineral nutrition under wet stress and its implications in drought tolerance

Kirti Bardhan , ... Duwini Padukkage , in Climate change and Ingather Stress, 2022

7.4.8 Cobalt (Co)

While working on the consequence of Co on pea plants, Gad (2006) demonstrated that addition of Co improvised root growth. Tenywa, 1997 augmented urea, ammonium nitrate, and peanut compost efficiency along with root nodulation in pea plants with cobalt supplementation. Its application can reduce the requirement of organic and inorganic fertilizers up to 67% and 25%, respectively, of the recommended doses. In improver to this, uptake of N, P, K, Iron, Mn, and Zn was also increased. Conclusively, supplementing pea plants with Co improved nutrient uptake, water uptake, and pod yield under low moisture growing conditions.

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Found Abiotic Stress: Salt

A. Läuchli S.R. Grattan , in Encyclopedia of Agronomics and Nutrient Systems, 2014

Constitute pathogens

Salinity tin impact the soil microbe populations in the rhizospere and their interaction with roots. For instance, Rhizobium spp., which are integral to legume production, seem more table salt tolerant than their host plants, but testify indicates that nodulation and North 2 fixation by some crops are dumb by salinity (Läuchli, 1984). Other investigators have suggested that mycorrhizal symbioses improve the ability of some crops to tolerate common salt by improving phosphorus diet (meet review by Läuchli and Grattan, 2007).

Table salt-stressed plants may be predisposed to infection by soil pathogens. Salinity has been reported to increase the incidence of phytophthora root rot in chrysanthemum (MacDonald, 1982) and tomato (Snapp et al., 1991). The combined effects significantly reduced fruit size and yield of tomatoes (Snapp et al., 1991), but wetter soil nether salt-stunted plants, due to less evapotranspiration than nonsaline command plants, may contribute to increased susceptibility to fungal diseases. Research on salinity–pathogen interactions is rather limited despite its potential economical impact in table salt-affected areas many of which are also decumbent to waterlogging. Therefore further inquiry is warranted in this area.

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