Endophytic establishment of Azorhizobium caulinodans in wheat

The biopesticide market for global agricultural use

Among the biotic stresses, plant pathogens can reduce yield crop which affected potential loss to crop productivity. Plant growth-promoting rhizobacteria (PGPR) can help plants to be resistant against biotic stress via direct antagonism to pathogens or by induction of systemic resistance to pathogens. The presence of high levels of nutrients exuded from various roots of most plants can support bacterial growth and metabolism as well as maintain health of the plant in the growth process. PGPR promote plant growth due to their abilities in phytohormone production, nitrogen fixation, and phosphorus solubilization; produce several substances which are related to pathogen control, i.e., exhibiting competition with plant pathogens, synthesis of antibiotics, antifungal metabolites and defense enzymes, and secretion of iron-chelating siderophores; and trigger induced systemic resistance (ISR) via methyl jasmonate and methyl salicylate in plants. The ISR resembles pathogen-induced systemic acquired resistance (SAR) through the salicylic acid-dependent SAR pathway under conditions where the inducing bacteria and the challenging pathogen remain spatially separated. The use of PGPR combinations of different mechanisms of action, i.e., induced resistance and antagonistic PGPR, might be useful in formulating inoculants leading to a more efficient use for biological control strategies to improve crop productivity. Many PGPR have been isolated from the tissues of many plants, and various species of bacteria, i.e., Azotobacter, Azospirillum, Alcaligenes, Arthrobacter, Bacillus, Burkholderia, Enterobacter, Klebsiella, Pseudomonas, and Serratia, have been reported to control several diseases and enhance plant growth. PGPR belonging to the genera Pseudomonas and Bacillus are also well known for their antagonistic effects and their ability to trigger ISR. An increasingly successful study to reduce disease severity is the use of bacteria, namely, Bacillus subtilis, P. fluorescens, Serratia, and the fungus Trichoderma. Tea and rice plants are cultivated in Indonesia predominantly in Java and Sumatra islands. Major constraints of cultivation include low fertility of soils, poor input management, low germination, and high susceptibility to the diseases. The strategies employed by PGPR provide promising approaches to alter agricultural crops and plantation practices toward sustainable environmental development. Research has been conducted to know the effect of PGPR on tea plant growth that can work optimally as a biological fertilizer and plant-induced resistance to suppress blister blight (Exobasidium vexans Massee), a major disease in tea plantation that can decrease yield loss up to 50%. Individual PGPR strains for in vitro broad-spectrum pathogen suppression and production of several physiological/biochemical activities related to plant growth promotion have been screened. Numerous bacterial isolates have been found to function both as biofertilizers and biological control agents, namely, Chryseobacterium sp. AzII-1, Acinetobacter sp., Alcaligenes sp. E5, Bacillus E65, and Burkholderia E76. Study about synergism among bacteria has been carried out in the laboratory test using four combinations, i.e., (a) Chryseobacterium sp. AzII-1 + Acinetobacter sp., (b) Chryseobacterium sp. AzII-1 + Alcaligenes sp. E5, (c) Chryseobacterium sp. AzII-1 + Bacillus E65, and (d) Chryseobacterium sp. AzII-1 + Burkholderia E76. All bacterial combinations had a synergistic effect. It was shown that the bacterial population was not significantly different with the average of the total bacterial population (4.62 × 108 CFU/ml). The effect of bacterial combinations to blister blight and plant growth under a tea nursery trial revealed that combination of Chryseobacterium sp. AzII-1 75% + Alcaligenes sp. E5 25% could increase the growth of tea plant and suppress the intensity of blister blight up to 1.27%. The disease intensity of blister blight decreased in all treatments under field trial, while the Acinetobacter sp. treatment in tea shoots was 17.26% higher than the control. PGPR have also been isolated from cultivated rice. Serratia SKM, Burkholderia E76, and Bacillus E65 have the potential for controlling rice diseases and induce plant growth promotion. Under in vitro antagonistic assay, it was shown that these isolates could suppress effectively the growth of rice pathogens Xanthomonas oryzae pv. oryzae, the causal agent of bacterial blight (BB). Kaolin formulation of these three isolates was evaluated as a foliar application on rice. PGPR application under experimental plots resulted in enhancement of rice growth and yield, with the yield increment on cv. Sintanur being 12.8 percent higher compared with control (cv. Ciherang). Based on PGPR application technology which is demonstrated in farmers’ plots, the severity of BB disease was reduced to 76.8 percent compared with the untreated plot. The farmers were convinced with the beneficial effects of PGPR on both plant growth and yield and reduction of BB disease incidence. PGPR technologies have the potential to reduce agrochemical application. They can also be exploited as low in input and environmentally friendly for sustainable plant management. PGPR is highly diverse, and in this review, we focus on PGPR in plant growth promotion, as well as understanding the role of PGPR in crop protection.

Microorganisms are the key components of the soil biodiversity. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth-promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are associated with the rhizosphere, which is an important soil ecological environment and plant health for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria, viz. Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium, and Mesorhizobium, and free-living nitrogen-fixing bacteria or associative nitrogen fixers, viz. Azospirillum, Enterobacter, Klebsiella, and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Due to increase inputs of pesticides and fertilizers, the role of these microorganisms is marginalized in conventional agricultural, leading to loss of biodiversity as well as its function. However, increased awareness in many countries, including India, is progressively leading to a development from conventional intensive agriculture to sustainable agriculture. Sustainable agriculture refers to farming systems where the use of mineral fertilizers and pesticides is restricted. These agro-ecosystems are consequently more dependent upon biological control of pests and organic fertilizers to maintain crop health and productivity. In this chapter, PGPR role has been discussed in the process of plant growth promotion, their mechanisms, and their importance in crop production on sustainable basis.

erhaps the most widely used agents of biological plant protection are endophytic fungal symbionts (endophytes) of forage and turfgrasses. These are fungi of the family Clavicipitaceae, which grow between host cells in vegetative tissues, ovules, and seeds of systemically infected grass plants. The existence of these endophytes was not fully appreciated until recent years, although the protection they provide against insect damage (Fig. 1) and drought contributes to the superior agronomic qualities of favorite pasture grasses in North America, Australia, and New Zealand. Unfortunately for livestock farmers, these endophytes also provide a degree of protection from grazing mammals. In 1977, Bacon et al. (2) reported that the grass Festuca arundinacea var. genuina Schreb. (hexaploid tall fescue) had a fungal endophyte related to Epichloe typhina (Pers.:Fr.) Tul., and that this endophyte— now known as Neotyphodium coenophialum (Morgan-Jones & Gams) Glenn, Bacon, & Hanlin—was responsible for toxicosis suffered by livestock grazing the grass. Epichloe species were known for many decades (44), but reports relating that nonpathogenic endophytes could be detrimental to livestock provided new impetus for intensive studies, making the grass–endophyte associations among the best characterized symbioses in biology. Less than two decades of research have yielded a rich body of knowledge about these symbioses: their secondary product chemistry, ecology, evolution, genetics, and molecular biology; their ecological roles as protectants from insect and vertebrate herbivores, pathogenic fungi and nematodes, and drought; and their effects on host growth and competitiveness. The endophytes produce numerous alkaloids, some of which are unrelated to any known from plants or other fungi. Their genetic and evolutionary complexity is extraordinary. They were the first fungi genetically documented as interspecific hybrids (47,53). Meanwhile, molecular genetic techniques were applied to the endophytes of tall fescue and other grasses, bringing us closer to the prospect of reducing or eliminating their toxicosis to livestock while continuing to employ their bioprotective qualities.

In order to achieve maximum crop yields, excessive amounts of expensive fertilizers are applied in intensive farming practices. However, the biological nitrogen fixation via symbiotic and nonsymbiotic bacteria can play a significant role in increasing soil fertility and crop productivity, thereby reducing the need for chemical fertilizers. It is well known that a considerable number of bacterial species, mostly those associated with the plant rhizosphere, are able to exert a beneficial effect on plant growth. The use of those bacteria, often called plant growth-promoting rhizobacteria (PGPR), as biofertilizers in agriculture has been the focus of research for several years. The beneficial impact of PGPR is due to direct plant growth promotion by the production of growth regulators, enhanced access to soil nutrients, disease control, and associative nitrogen fixation. Legumes play a crucial role in agricultural production due to their capability to fix nitrogen in association with rhizobia. Inoculation with nodule bacteria called rhizobia has been found to increase plant growth and seed yields in many legume species such as chickpea, common bean, lentil, pea, soybean, and groundnut. However, both rhizobia and legumes suffer heavily and adversely from various abiotic factors. The impact of different stress factors on both PGPR and legume production is critically reviewed and discussed.

Azospirillum is a well-studied genus of plant growth-promoting rhizobacteria (PGPR) and one of the most extensively researched diazotrophs. This genus can colonize rhizosphere soil and enhance plant growth and productivity by supplying essential nutrients to the host. Azospirillum–plant interactions involve multiple mechanisms, including nitrogen fixation, the production of phytohormones (auxins, cytokinins, indole acetic acid (IAA), and gibberellins), plant growth regulators, siderophore production, phosphate solubilization, and the synthesis of various bioactive molecules, such as flavonoids, hydrogen cyanide (HCN), and catalase. Thus, Azospirillum is involved in plant growth and development. The genus Azospirillum also enhances membrane activity by modifying the composition of membrane phospholipids and fatty acids, thereby ensuring membrane fluidity under water deficiency. It promotes the development of adventitious root systems, increases mineral and water uptake, mitigates environmental stressors (both biotic and abiotic), and exhibits antipathogenic activity. Biological nitrogen fixation (BNF) is the primary mechanism of Azospirillum, which is governed by structural nif genes present in all diazotrophic species. Globally, Azospirillum spp. are widely used as inoculants for commercial crop production. It is considered a non-pathogenic bacterium that can be utilized as a biofertilizer for a variety of crops, particularly cereals and grasses such as rice and wheat, which are economically significant for agriculture. Furthermore, Azospirillum spp. influence gene expression pathways in plants, enhancing their resistance to biotic and abiotic stressors. Advances in genomics and transcriptomics have provided new insights into plant-microbe interactions. This review explored the molecular mechanisms underlying the role of Azospirillum spp. in plant growth. Additionally, BNF phytohormone synthesis, root architecture modification for nutrient uptake and stress tolerance, and immobilization for enhanced crop production are also important. A deeper understanding of the molecular basis of Azospirillum in biofertilizer and biostimulant development, as well as genetically engineered and immobilized strains for improved phosphate solubilization and nitrogen fixation, will contribute to sustainable agricultural practices and help to meet global food security demands.

Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

A significant part of the nitrogen in living organisms is derived from atmospheric dinitrogen which gets incorporated into organic compounds by biological or chemical nitrogen fixation. Biological nitrogen fixation is an energy-consuming process performed by the enzyme nitrogenase, which is irreversibly denatured by oxygen. Nitrogenase is formed only by prokaryotes, which in some cases fix nitrogen in symbiosis with higher plants. Two groups of bacteria can enter symbioses with higher plants which lead to the formation of special organs, the root nodules, where the bacteria fix nitrogen while being hosted inside plant cells. The product of nitrogen fixation, ammonium, is exported to the plant, while the plant in turn provides its symbiont with energy sources. Azorhizobium, Bradyrhizobium, and Rhizobium enter symbioses with leguminous plants (with one exception, Parasponia from the Ulmaceae family; Trinick, 1979), and actinomycetes of the genus Frankia induce nodules on the roots of a diverse group of dicotyledonous plants, mostly woody shrubs, which are collectively referred to as actinorhizal plants (Benson and Silvester, 1993). While rhizobia are unicellular, Frankia usually grows in hyphal form, but can also form sporangia and, under certain conditions, nitrogen-fixing vesicles (Benson and Silvester, 1993). Vesicles are formed at the ends of hyphae or on short side branches either in the free-living state under nitrogen starvation and aerobic conditions, or in symbiosis. Within the vesicles, nitrogenase is protected from oxygen (Parsons et al., 1987) by the vesicle envelope, which consists of multiple layers of hopanoids, bacterial steroid lipids (Berry et al., 1993). In contrast to Frankia, rhizobia can use N2 as nitrogen source only in symbiosis (with one exception, Azorhizobium caulinodans ORS571; reviewed by de Bruijn, 1989).

Nitrogen-fixing bacteria are able to enter into roots from the rhizosphere, particularly at the base of emerging lateral roots, between epidermal cells and through root hairs. In the rhizosphere growing root hairs play an important role in symbiotic recognition in legume crops. Nodulated legumes in endosymbiosis with rhizobia are amongst the most prominent nitrogen-fixing systems in agriculture. The inoculation of non-legumes, especially cereals, with various non-rhizobial diazotrophic bacteria has been undertaken with the expectation that they would establish themselves intercellularly within the root system, fixing nitrogen endophytically and providing combined nitrogen for enhanced crop production. However, in most instances bacteria colonize only the surface of the roots and remain vulnerable to competition from other rhizosphere micro-organisms, even when the nitrogen-fixing bacteria are endophytic, benefits to the plant may result from better uptake of soil nutrients rather than from endophytic nitrogen fixation. Azorhizobium caulinodans is known to enter the root system of cereals, other non-legume crops and Arabidopsis, by intercellular invasion between epidermal cells and to internally colonize the plant intercellularly, including the xylem. This raises the possibility that xylem colonization might provide a non-nodular niche for endosymbiotic nitrogen fixation in rice, wheat, maize, sorghum and other non-legume crops. A particularly interesting, naturally occurring, non-nodular xylem colonising endophytic diazotrophic interaction with evidence for endophytic nitrogen fixation is that of Gluconacetobacter diazotrophicus in sugarcane. Could this beneficial endophytic colonization of sugarcane by G. diazotrophicus be extended to other members of the Gramineae, including the major cereals, and to other major non-legume crops of the World?

10 - Soybean production and suboptimal root zone temperatures

under rainfed conditions, on the high plateaus, where drought represents a major limitation to crop production. Yield gap between irrigated and rainfed trials indicated that gain yield reduction owing to water deficit ranged from 24 to 80%, depending on rainfall amount and dis­tribution patterns. In addition, salinity stress, which in­duces both ionic, osmotic and oxidative damages, impairs plant growth and causes severe reductions in cropyield. To achieve the salt tolerance, damages must be firstly prohibited or all eviated; secondly, homeostatic conditions should be recovered in the new stressful con­dition; and finally, the growth must restart, although at al ower rate. The use of efficient micro-organisms like plant growth-pro-moting rhizobacteria (PGPR) is helpful in boosting and improving sustainable agriculture and environmen­tal stability. Currently, soil salinity is one of the major concerns in agriculture that limits water absorption and induces ac­cumulation of toxic ions in the different plants’ organs. The application of halotolerant and ther­mophilic plant-growth- promoting (PGPB) actinobacteria can be a valid tool to reduce the harmful effects of saline stress and to improve crop productivity. PGPR can promote growth via different mechanisms including phytohormones production (e.g. auxin, cytoki­nin, ethylene and gibberellins), nitrogen fixation, nutri­ent mobilization and siderophore production. PGPR can induce salinity stress tolerance through the modulation of physiological and biochemical process. They can also induce systemic resistance. The aim of the present work was to select actinomycete strains with plant-growth-promoting capacities, and to de­termine their effects on the growth of Triticum durum. Actinomycete isolates were firstly screened through sever­al in vitro plant-growth-promoting (PGP) traits (i.e., phos­phate solubilization ability, production of indole-3-acetic acid, hydrocyanic acid, and ammonia, nitrogen fixation, growth at different temperatures and NaCl concentra­tion, antifungal activities and several enzymatic activities). Strains with interesting traits were investigated for their biostimulant effects on Triticum durum.

Plant growth-promoting rhizobacteria (PGPR) are the bacterial strains that are associated with the rhizospheric part of plants and are known to positively influence plant growth. These microorganisms induce plant growth promotion directly through production of phytohormones, solubilization of phosphates, biological fixation of nitrogen, inhibition of ethylene biosynthesis, and indirectly by imparting resistance to combat pathogenic attacks by the production of secondary metabolites such as antibiotics, siderophores, and many enzymes, and also by inducing tolerance to abiotic stress conditions. PGPR are especially used as bioinoculants due to their role in increasing crop production. PGPR are influenced, inter alia, by plant genotype, environmental conditions (both biotic and abiotic), and various biotic interactions (plant-microbe interactions and microbe-microbe interactions) taking place in the rhizosphere. Besides, agricultural activities, including use of chemical fertilizers, pesticides, and soil tillage, also have substantial influence not only on the PGPR population but also on physical, chemical, and biological characteristics of soil. In view of their profound growth-promoting characteristics and ever-increasing demand for adoption of ecologically compatible and eco-friendly practices in agriculture, PGPR could be an important tool in sustainable agriculture and can be safely relied upon for increasing productivity. It is in this backdrop, an attempt has been made in the present chapter to emphasize the importance of rhizobacteriain the promotion of plant development and sustainable agriculture.

The global demand for increasing agricultural productivity and declining farming land resource has posed a severe threat to crop production and agroecosystems. The use of chemical and mineral fertilizers has boosted up the agricultural productivity but considerably diminished the soil fertility, soil health, and sustainability. Improvement in agricultural sustainability requires the combined holistic approach integrating optimal use of soil fertilization, soil physical properties, soil biological processes, and soil microbial diversity, combining integrated plant nutrient management. Since past few decades, plant growth-promoting bacteria (PGPB) and plant growth-promoting rhizobacteria (PGPR) have replaced the conventional use of chemical fertilizers and pesticides in horticulture, silviculture, agriculture, environmental remediation, and cleanup strategies, and utilization of such microbial candidates for improving soil health and nutrient availability for plants is a vital practice since antiquity. Apart from the phytostimulatory effects on plants, PGPBs are potent colonizers of plant root or rhizosphere that improve both crop and soil health through various direct and indirect approaches such as nitrogen fixation, phosphate solubilization, quorum sensing, siderophore production, antimicrobials, volatile organically, mineral solubilization, induced systemic resistance, nutrient acquisition, modification of soil texture, soil porosity, etc. Increase in biomass, yield, seedling emergence, root proliferation, and timely flowering are the direct benefits that make these microbes most preferred in the agricultural crop production, with a high market demand. Researchers are now moving way forward to decipher their molecular mechanisms of plant beneficiation through genomic comparisons, real-time protein expressions revealing the ecophysiology, and niche adaptation that might facilitate functioning of these beneficial microbes. In this chapter, we have highlighted the status and recent trends of some important plant-beneficial bacterial members, their growth-promoting abilities, and genomic perspectives for sustainable use in crop productivity.

ABSTRACT: The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.

Rice is the most important cereal crop. In the next three decades, the world will need to produce about 60% more rice than today's global production to feed the extra billion people. Nitrogen is the major nutrient limiting rice production. Development of fertilizer-responsive varieties in the Green Revolution, coupled with the realization by farmers of the importance of nitrogen, has led to high rates of N fertilizer use on rice. Increased future demand for rice will entail increased application of fertilizer N. Awareness is growing, however, that such an increase in agricultural production needs to be achieved without endangering the environment. To achieve food security through sustainable agriculture, the requirement for fixed nitrogen must increasingly met by biological nitrogen fixation (BNF) rather than by using nitrogen fixed industrially. It is thus imperative to improve existing BNF systems and develop N2-fixing non-leguminous crops such as rice. Here we review the potentials and constraints of conventional BNF systems in rice agriculture, as well as the prospects of achieving in planta nitrogen fixation in rice.