Management of soil phosphorous using phosphate solubilizing microorganisms: A sustainable approach

Chapter 11 - Phosphate solubilization by microorganisms: Overview, mechanisms, applications and advances

Plant growth promoting rhizobacteria (PGPR) are a group of free-living bacteria that colonize the rhizosphere and benefit the root growth in plants. Bacteria of diverse genera such as Azospirillium, Bacillus, Burkholderia, Klebsiella, Pseudomonas, etc., were identified as PGPR. These PGPR exert a direct effect on plant growth by inducing the production of phytohormones, supplying biologically fixed nitrogen, and increasing the phosphorous uptake by the solubilization of inorganic phosphates. These bacteria affect plant growth by indirect mechanisms that involve suppression of bacterial, fungal, viral, and nematode pathogens. A lot of study showed that inoculation with PGPR resulted in significant yield increases in different crops, rooting of hardwood and semi-hardwood cuttings, increased germination and enhanced emergence of seeds under different conditions, promoted nutrient uptake of roots, total biomass of the plants, increased seed weight, induced early flowering, etc. In this review, the importance of PGPR is discussed for agricultural innovations with special references that utilises direct and indirect plant growth promotion.

Plant Growth Promoting Rhizobacteria (PGPR) or Rhizobacteria Plant Growth Boosters (RPPT) are non-pathogenic soil microbes found in plant roots that can increase plant growth and stimulate plant growth, PGPR's role in increasing growth and production, especially plant germination is the ability PGPR in synthesizing growth hormones and can provide protection against pathogens that attack plants so it is very good to be applied to a variety of plants. One of them is the banjar orange which is a typical fruit of South Kalimantan which has polyembryony seeds which are several embryos in one seed. Efforts in developing the productivity of banjar siam citrus cultivation are using PGPR. This study aims to determine the effect of the application of PGPR concentrations in spurring the germination of polyembryony seeds in banjar siam beans and to determine the best PGPR concentrations in stimulating the germination of polyembryony seeds in banjar siam seeds. This study uses a completely randomized design (CRD) consisting of a single factor with six replications and five treatments namely P 0 (concentration without PGPR), P 1 (PGPR concentration 15ml.1-1), P 2 (PGPR concentration 30ml.1-1), P 3 (PGPR concentration 45ml.1-1), and P 4 (PGPR concentration 60ml.1-1). This research was conducted at the Production Laboratory and Integrated Laboratory of Agroecotechnology Department, Faculty of Agriculture, Lambung Mangkurat University, Banjarbaru. The research lasted for two months, starting from April to May 2019. The results showed that the application of PGPR concentrations had an effect on spurring the germination of polyembryony seeds on Banjar siam beans at the age of 7 days and 14 days, but the treatment had no effect on the germination of polyembryony seeds at the age of 21 HST, percentage of germination age 7 HST, 14 HST, and 21 HST, the speed of germination either normally or polyembryony, germination length, number of germination, and root length. The best concentration of PGPR in stimulating polyembryony germination in banjar siam seeds is in the control treatment or P 0 (concentration without PGPR) at the age of 7 HST and 14 HST.

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.

Tomatoes are agriculturally and gastronomically significant and serve as model organisms in scientific research. This study examined plant-phosphorus interactions, evaluated two P sources for fertilization, and analyzed the effects of rhizobacteria on plant growth. Phosphorus (P) is an essential yet limited nutrient for plants. Microbial inoculants formulated with plant growth-promoting rhizobacteria (PGPR) enhance plant health and growth and improve P solubility. Tomato seedlings were inoculated with the following PGPR strains capable of P solubilization: Paenibacillus spp. BSP 1.1, Arthrobacter enclensis JN24, and Arthrobacter pokkalii JLB4. Morphological and physiological analyses were used to assess nitrogen (N) and P intake and developmental differences among treatments. The P source, bacterial strain, and their interactions influenced plant development differently. Plants treated with phosphate rock exhibited a higher greenness index and root volume, whereas those with soluble phosphate had increased leaf area when inoculated with BSP and JLB4. Strain JLB4 specifically increased leaf area when combined with soluble phosphate. P concentration was lower in phosphate rock-treated plants, although deficiency symptoms were absent. N concentrations and growth-related variables were diminished in the early stages but improved to the end of the experiment. In conclusion, P demand was constant, but its availability increased with the effect of P solubilization throughout the experiment. Therefore, the addition of phosphate rock as a source of P in soilless agriculture may be a viable alternative for tomato cultivation.

Understanding soil–plant–microbe inter- and interactions is essential for ensuring proper soil health, quality, and soil-mediated ecosystem services (e.g., nutrient cycling) required for human–plant–animal life. Intensive and unsustainable farming practices can decrease soil microbial biodiversity, fertility, and quality leading to soil degradation, impaired nutrient cycling, and the incapability of soil to support plant growth. Under such a context, soil biological fertility can appear as a regenerative component that has the potential to harmonize and improve soil’s physical, chemical, and biological parameters. The review study expresses the micro biome in the rhizosphere, microbial nutrient cycling, and biological soil crusts as the major components of soil biological fertility, and explores the answers to the questions: (i) How does the rhizosphere promote plant growth, development, and nutrient cycling through soil microorganisms, (ii) How can soil microorganisms regulate macronutrient cycling and facilitate bio crust formation. Soil biological fertility is crucial for increasing crop resilience and productivity as well as sustainability in agriculture. Additionally, the reintroduction of plant growth promoting rhizo bacteria, a quantitative estimation of the root exudate’s composition, identifying the spatiotemporal dynamics of potassium solubilizing bacteria, and establishing biological soil crusts in agricultural lands remain the major tasks for improving soil biological fertility and the transition towards regenerative agriculture. Despite the promising potential of conservation tillage and biochar, the study meets problems such as diversity in soil types, climatic circumstances, and the long-term stability of biochar effects, which limit the generalization of results. However, by combining the most recent research on how these approaches boost crop productivity, encourage the formation of biocrusts, and enhance rhizospheric nutrient cycling, this review makes a substantial contribution and provides insightful information for long-term soil restoration plans.

Inoculation of Pseudomonas mutant strains can improve growth of soybean and corn plants in soils under salt stress

Currently arid or semi-arid land areas are increasing worldwide due to global warming and the soil is becoming saline because of the use of intensive irrigation in the crop fields. Consequently, the proportion of plants living under water shortage conditions is increasing. This phenomenon is limiting seriously crop production in such areas. In many cases, the fields are being abandoned and become uncultivable again in a period of time due to erosion. Although plants have their own mechanisms to cope with drought stress, they become more tolerant to drought when associated with different soil microorganisms. Among these soil microorganisms, the most abundant and effective are rhizobia, plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF). Rhizobia fix atmospheric nitrogen and transport this fixed nitrogen to the legume host plant. PGPR promote plant growth by means of several mechanisms. They are present in almost all ecosystems across the world. AMF were vital to plants when starting to colonize dry land surface, and hence for improving plant mineral nutrition, especially uptake of phosphorous, among other factors. Here we detail the most recent advances about how these microorganisms enhance plant drought tolerance at physiological and molecular levels, including decreased oxidative stress, improved water status or regulation of aquaporins. It has been found that legume plants inoculated with rhizobia grow faster under drought conditions than non-inoculated ones. However, how rhizobial symbiosis affects root water transport has not been addressed yet. At the same time, it seems that there is a correlation between drought tolerance in rhizobial bacteria and rhizobia-induced plant drought tolerance, at least in terms of reducing plant oxidative stress. Under drought conditions, PGPR regulate the levels of stress-related hormones, i.e. abscisic acid and ethylene. The regulation of these hormones could be the cause of an enhancement of plant drought tolerance mediated by PGPR. However, a more detailed molecular approach is still needed to fully understand this process. Arbuscular mycorrhizal symbiosis improves almost every physiological parameter of the host plant under drought stress, i.e. water status, leaf transpiration, photosynthesis or root water uptake. However, the molecular basis for this improvement is far to be understood. At the same time, AMF in combination with rhizobia or PGPR usually have an accumulative beneficial effect on plant drought tolerance, although this depends on the specific pair of strains inoculated. Therefore, although there are many studies in order to understand at the physiological level how beneficial soil microorganisms induce plant drought tolerance, there is still a lack of knowledge about the molecular basis behind this improvement.

A field experiment was conductedduring the summer season of 2022 to study the effect of two forms of phosphorus (super phosphate and rock phosphate), humic acids, and plant growth-promotingrhizobacteria(PGPR) on peanut yield (Arachishypogaea L.) cv. Giza 6. Data showed that humic acids, a mixture of superphosphate and rock phosphate, and PGPR especially those produce exopolysaccharides are crucial for the growth and yield of peanut plants. A superior increase in the effect of humic acids, followed by phosphorus applications on plant height, number of pods per plant, pod weight per plant, seed weight per plant, 100 pod weight, 100 seed weight, pods and seed yields, and shelling percentage was recorded. The co-inoculation with PGPR especiallythatproduces exopolysaccharides, can affect peanut yield and the abovementioned yield attributes and shelling percentage.Moreover, the application of humic substance resulted in significant increases in protein and oil content compared to untreated samples. Finally, the combination ofhumic acids,PGPR, and mixture of super phosphate and rock phosphate resulted in significant increases in all quality parameters of peanuts as compared to untreated ones.

Among the diverse soil microflora, plant growth promoting rhizobacteria (PGPR) mark an important role in enhancing plant growth through a range of beneficial effects. This is often achieved by forming biofilms in the rhizosphere, which has advantages over planktonic mode of bacterial existence. However, the biofilm formation of PGPR has been overlooked in past research. This chapter focuses on new insights and concepts with reference to improved PGPR effects caused by the biofilm formation by PGPR and its impact on overall plant growth promotion, compared with the planktonic lifestyle of PGPR. Beneficial PGPR play a key role in agricultural approaches through quorum sensing in their biofilm mode. The in vitro production of biofilmed PGPR can be used to give increased crop yields through a range of plant growth mechanisms. They can be used as biofertilizers through improved N2 fixation and micro- and macronutrient uptake. Further, higher levels of plant growth with PGPR have been observed due to their production of plant growth regulators and their abilities to act as biocontrol agents, which are carried out by the production of antibiotics and other antimicrobial compounds. The microbial inoculant industry would also benefit greatly by developing biofilmed PGPR with N2 fixing microbes. Biofilmed PGPR can be manipulated to achieve results in novel agricultural endeavors and hence is as an area which needs a deeper probing into its potential.

Chapter17 - Plant growth promoting rhizobacteria from the perspectives of tea plantations and diseases

Reducing the use of chemicals in vegetable cultivation is an environmentally friendly cultivation technology that is expected to reduce negative impacts on the environment. One of the efforts to reduce chemicals in cultivation is to use bacteria that live in plant roots as plant growth promoters or biopesticides. This study aims to determine the number of endophytic bacteria from bamboo and leguminosae roots that have the potential as plant growth promoting agents. The making of plant growth-promoting rhizobacteria (PGPR) starter which was carried out in this study used the roots of bamboo, lemongrass and Mimosa Pudica Linn. Furthermore, it is applied to vegetable cultivation (Chinese cabbage, tomato, carrot and green mustard/caisim) to determine the physical characteristics of vegetables from the result of PGPR application compared to results from conventional farmers. This study used a factorial randomized block design with 2 factors. The first factor is the plant roots used, while the second factor is the length of the plant roots soaking in water, namely 72, 96, 120 hours. The data from the PGPR starter analysis results are made into a table, then a descriptive discussion is carried out. For vegetable data from the cultivation results using PGPR compared to vegetable from conventional farmer results. Based on laboratory analysis, obtained the total microbes in samples from bamboo roots with a soaking time of 72 hours were higher than the other samples, namely 8.49 x 106 cfu ml-1; pH in samples of PGPR from the roots of Mimosa Pudica Linn, bamboo, lemongrass, commercial PGPR (from Central Java farmers) ranged from 3.0 to 6.7; while the total dissolved solids content ranged from 0.2 to 2.6%. Based on the graph of plant growth in the PGPR treatment and with no treatment, there is a slight difference, not much increase or decrease. In tomatoes with PGPR treatment, plant growth was higher than without the use of PGPR, while for Chinese cabbage and caisim the growth was almost the same between PGPR treatment and without PGPR. For carrots, it was seen that with PGPR treatment, plant growth was lower than without PGPR. The results of the analysis of texture, brightness level and total dissolved solids in Chinese cabbage, caisim, tomatoes and carrots with PGPR application, the characteristics are almost the same as the results of conventional farmers. By looking at these results, it is hoped that in the future cultivation innovation by utilizing bacteria around the roots, is one of the environmentally friendly cultivation applications and begins to reduce the use of chemicals in cultivation.

Brassicaceae are scattered all over the world, where they exclusively grow on serpentine rocks in Western Australia, New Zealand, South Africa, Japan, Philippines, Brazil, Portugal, Italy, Turkey, Cuba, eastern Canada, and western north America. Although serpentine rocks cover only less than 1% of the earth’s surface their worldwide distribution has recently attracted many researchers in exploring their distinctive potential for phytoremediation plant communities, mainly members of Brassicaceae plant family inhabiting on serpentine rocks of these countries. On the other hand, the majority of Brassicaceae plant family are slow-growing plants producing little biomass and their use for phytoextraction purposes may not be practical, especially when bioavailable metal concentration is high in the contaminated conditions. Therefore, recently emerging practices in the field of phytoremediation have pointed out various focuses such as the utility of high-biomass crops such as maize, peas, oats and Indian mustard and associated soil practices including application of synthetic chelators such as ethylenediaminetetraacetic acid and nitrilotriacetate and elemental sulphur to enhance metal uptake by these plants. These approaches may meet the conditions required for the phytoremediation. However, one of the most critical components of phytoextraction process is the bioavailability of heavy metals meaning the portion of the metals that is available for absorption into living organisms such as plants. It has been known that various plant growth-promoting rhizobacteria (PGPR) associated with plant roots may provide some beneficial effects on plant growth and nutrition through a series of well known mechanisms, namely, nitrogen fixation, production of phytohormones and siderophores, and transformation of nutrients once they are either applied to seeds or incorporated into the soil. Similarly, heavy metal mobility and availability can substantially be driven by PGPR populations through their release of chelating agents, acidification, and phosphate solubilization in rhizosphere. Miscellaneous PGPR were also shown to tolerate heavy metals in different ways including the mechanisms of exclusion, active removal, biosorption, precipitation, and extra- or intracellular bioaccumulation. Since these processes may affect the solubility and the bioavailability of heavy metals to the plant and hence modifying their toxic effects, interactions between hyperaccumulator plants such as Brassicaceae spp., and metal tolerant or resistant PGPR are considered to have an increasing biotechnological potential in the remediation of anthropogenically polluted soils. Present chapter/review considers the role of PGPR on soil-heavy metal-plant interactions and more specifically bioaccumulation of toxic metals by Brassicaceae plant family.

Chapter 11 - Role of plant growth–promoting rhizobacteria in sustainable agriculture

Under green revolution practices, the imbalanced use of chemical fertilizers and pesticides causes a negative impact on soil health due to the loss of soil microbial flora and fauna. To overcome this negative impact of the green revolution and to increase sustainable agricultural production without damaging further agricultural lands, the only alternative and effective means is to reduce the use of chemicals in agriculture specifically for plant nutrition and plant protection. Under sustainable agricultural practices, plant growth-promoting rhizobacteria (PGPR) can be effective tools to increase productivity while ensuring sustainability in agriculture. PGPR colonize the rhizosphere zone and help in promoting plant growth and development by regulating nutrient acquisition, modulation of plant hormones, and ameliorating various negative effects of various pathogens. PGPR also help sustain the plant growth productivity and significantly increase soil fertility and health under different biotic and abiotic stresses. As per the literature, many studies prove to increase agriculture productivity due to the use of PGPR as eco-friendly microbial inoculants for promoting plant growth attaributes through various direct and indirect mechanisms. The mechanisms of PGPR include biological nitrogen fixation, phytohormones production, Phosphate, potassium, and zinc solubilization, siderophores production, and secretion of other secondary metabolites (phenolic compounds (phenylpropanoids and flavonoids)) that enhance crop productivity and control phytopathogens. Therefore, this chapter focuses on a detailed description of PGPR keeping in view their functional mechanisms as eco-friendly approaches to increase productivity and enhance soil fertility. PGPR can be used as an eco-friendly, socially acceptable, and cost-effective technology for challenges in the future.KeywordsPGPRProductivitySoil fertility and healthRhizosphereSustainable agriculture