Interaction between Plants and Growth-Promoting Rhizobacteria (PGPR) for Sustainable Development

The relationship between plants and microorganisms is of paramount importance in maintaining the delicate balance of life on Earth, as evidenced by their interconnectedness in the intricate tapestry of nature [...]


Introduction
The relationship between plants and microorganisms is of paramount importance in maintaining the delicate balance of life on Earth, as evidenced by their interconnectedness in the intricate tapestry of nature [1,2].The relationship between plants and growth-promoting rhizobacteria (PGPR) is a noteworthy symbiotic partnership that holds significant implications for sustainable development [3,4].In light of the pressing issues of food security, environmental degradation, and climate change, the potential of PGPR has emerged as a favorable solution for addressing these challenges [5].PGPR, which are capable of displaying both antagonistic and synergistic interactions that ultimately lead to the enhancement of plant growth, offer a more comprehensive understanding of plant activity in conjunction with microbes [6][7][8].PGPR significantly influence soil properties and play a critical role in transforming uncultivated, low-quality soil into cultivable soil [9].To boost agricultural productivity, numerous regions around the globe are actively harnessing the potential of PGPR to improve soil quality and promote plant growth [10,11].Generally, this strategy encompasses direct or indirect techniques.The direct method entails providing compounds that stimulate plant growth directly to plants [12].This is accomplished through the application of methods like bio-fertilization, rhizo-remediation, and plant stress management [13].The acquisition of water and nutrients from the soil is often the primary environmental factor that restricts the development of land-based plant species [14].PGPR enhance plant growth by increasing the accessibility or absorption of nutrients from a finite pool of nutrients present in the soil [15].PGPR can alleviate plant stress, both biotic and abiotic, which can negatively impact plant growth and development.By promoting plant health and resilience, PGPR can help to improve crop yields and quality [13].Abiotic stress is a type of stress that is inflicted upon plants by their environment and can take the form of either physical or chemical stress [16].Biotic stress poses a significant biological threat to humans.Nevertheless, cutting-edge biotechnology methods and the utilization of technologies like nano-and microencapsulation may help address these challenges [17].This Special Issue has showcased a diverse array of studies investigating the possible function of PGPR, which exhibit both antagonistic and synergistic interactions that lead to improved plant growth.These studies offer a comprehensive understanding of the complex interplay between plants and microorganisms.The Special Issuecontains three articles and three reviews, which are briefly described in the following paragraphs.All published articles in this Special Issue highlight the importance of PGPR in agriculture, their mechanisms of action in disease control and plant growth promotion, and the current state of research in the field.

An Overview of Published Articles
The study conducted by Al Amin et al. (2021) aimed to investigate a significant disease affecting Oryza sativa L. crops in Bangladesh, known as "Bakanae".This disease causes considerable yield loss annually.The researchers collected thirty isolates of Fusarium spp.from infected O. sativa L. plants across various agroecological zones in Bangladesh and analyzed the variations in morphological and cultural characteristics and pathogenicity.The study found that the isolates varied in terms of cultural characteristics such as shape, phialide, colony features, size of macro-andmicroconidia and chlamydospore formation.Based on these characteristics, the researchers identified threevariants of Fusarium species, including F. proliferatum, F. moniliforme, and F. fujikuroi, on PDA media.Furthermore, Al Amin et al.'s research assessed the efficacy of various bioagents, including Pseudomonas fluorescens, Bacillus subtilis, Trichoderma spp., and Achromobacter spp., against the virulent isolate of F. moniliforme in order to devise a strategy for managing bakanae disease in rice.The study's findings indicate that the use of Achromobacter spp.and B. subtilis as biological control agents holds great promise for achieving environmentally friendly and effective control of bakanae disease in rice crops.
The article authored by Khoshru et al. (Contribution 2) delves into the crucial role that manganese (Mn) plays in plant growth, as it functions as a co-factor for enzymes involved in antioxidant synthesis, photosynthesis, and defense against pathogens.Mn plays a vital role in plant growth and development, as it is also involved in root growth, nutrient uptake, and soil microbial communities.However, the availability of Mn in the soil can be limited due to factors such as organic matter content, soil pH, mineralogy, and redox potential.The excessive application of chemical fertilizers containing Mn can lead to negative consequences for the environment and human health, such as water and soil pollution.According to a review report by Khoshru et al., recent research emphasizes the significance of microbial interactions in enhancing Mn uptake in plants, contributing to an environmentally friendlier approach to addressing Mn deficiencies.Beneficial microbes take onvarious roles and employ various strategies, including organic acid production, pH reduction, and the promotion of root growth, to enhance Mn bioavailability.They also produce anti-pathogenic compounds, siderophores, and form symbiotic relationships with plants, thereby facilitating the transport, uptake of Mn, and stimulating plant growth while reducing negative environmental impacts.This review examines the influences impacting the mobility of Mn in the rhizosphere, soil, and plants and highlights the problems caused by Mn scarcity in the soil and the use of chemical fertilizers, including their consequences.The authors investigate the potential of different beneficial soil microorganisms in addressing these challenges using eco-friendly methods.The study concludes that microbial interactions could be a promising approach toimproving the uptake of Mn in plants, resulting in enhanced crop productivity and maintaining environmental sustainability.
The research from Boya et al. (Contribution 3) exploresthe establishment of permanent preservation plots (PPPs) in natural forests and their significance in assessing the impact of climate change on forests.Two major forest areas in Bangalore were designated as PPPs in 2016 to facilitate long-term studies of climate change and monitor ecological changes, including vegetation.The primary objective of Boya's research was to investigate the factors contributing to the diversity of microorganisms, such as arbuscular mycorrhizal fungi (AMF), in terms of biological nutrients and physical properties.The native tropical forest of Bangalore, located near Bannerghatta National Park (BNP), has not been extensively studied in terms of its microflora, particularly AMF.Therefore, the study aimed to quantify the AMF in the three one-hectare plots established in BNP and the Doresanipalya Reserve Forest (DRF) using the Centre for Tropical Forest Sciences (CTFS) protocol.The two plots established in BNP were located in the Thalewood House area (mixed, moist, deciduous type) and the Bugurikallu area (dry, deciduous type), while the plot established in DRF was located in dry, deciduous vegetation.Each plot was divided into twenty-five subplots (20 m × 20 m), and composite soil samples were collected during dry and wet seasons and analyzed for spore of AMF and available P content.The results of the study revealed the occurrence of AMF in all threeplots, with Doresanipalya being the highest, followed by the ThalewoodHouse plot and the Bugurikallu plot.The available P and AMF spore numbers presented correlations in all threeplots.Among the spores of AMF, the Glomus species were found to be the most dominant in all three plots.The study concluded that dry, deciduous forests accommodated more AMF spores than mixed, moist forests.
The paper by Hasan et al. (Contribution 4) emphasizes the significance of the rhizosphere, which is the region surrounding the plant's root system and is inhabited by beneficial microorganisms known as PGPR.The review covers various plant growth processes, such as hormone secretion, phosphate solubilization, and nitrogen fixation, and emphasizes the potential benefits of PGPR.These benefits include increased plant tolerance to biotic andabiotic stress, reduced reliance on chemical fertilizers and pesticides, and enhanced soil fertility, absorption and nutrient availability.PGPR has multiple ecological and functions properties in the soil's rhizosphere and soil, amajor being to increase the phytohormones synthesis and other metabolites that directly impact on plant growth.Additionally, PGPR can help to combat plant pathogens and bolster the plant's natural defenses.Moreover, PGPR also aid in soil remediation through a process called bioremediation.Themultiple functions of PGPR include ammonia (NH 3 ) production, indole acetic acid (IAA) production, catalase production, and hydrogen cyanide (HCN) production, among others.Also, PGPR help with nutrient uptake and control the production of a hormone that increases root size and strength.The study concludes that using PGPR for sustainable agriculture can provide ecological and economic benefits such as improving crop yield, reducing environmental pollution, and ensuring food security.
The article authored by Singh et al. (Contribution 5) highlights the indiscriminate application of various agrochemicals, particularly fertilizers, by farmers across trapezoidal landscapes in order to boost productivity and cater to the growing demand for food.A significant portion of the population in developing countries is vulnerable to zinc (Zn) deficiency due to their major dependence on cereals as their primary source of calories.Zn, an essential major micronutrient for crops/plants, plays an important role throughout the plant lifecycle.Despite its crucial role in enhancing yield, it is often overlooked.One of the most commonly occurring micronutrient deficiencies, soil Zn deficiency, has a detrimental effect on crop yields.The inaccessibility of Zn in fixed forms to plants is the primary reason for Zn deficiency in both plants and soils, which characterizes the majority of agricultural soils.As a result, it is essential to employ sustainable and environmentally friendly methods to meet the demand for food.One feasible solution is the implementation of Zn-solubilizing bacteria (ZSB) for sustainable agriculture.This method involves inoculating plants and application with ZSB, which is a more efficient strategy for increasing the translocation of Zn in various edible plant components.ZSB possess crop growth promoting characteristics or attributes, making them effective bio-elicitors for promoting sustainable growth of crops through various mechanisms that are crucial for plant health and productivity.This article provides a comprehensive analysis of the effectiveness of ZSB, the practical characteristics of ZSB-mediated Zn localization, the mechanisms and roles underlying Zn solubilization, and the application of ZSB to enhance crop productivity and yield.
The work by Khoshru et al. (Contribution 6) highlighted the importance of phosphorus (P) for crop production, emphasizing that it is one of the most crucial elements required for plant growth.The optimal soil pH for P absorption by plants is approximately 6.5; however, in acidic and alkaline soils, the P becomes insoluble and unavailable for plant absorption due to its interaction with calcium, iron, and aluminum elements.Although chemical fertilizers are primarily used to supply P to plants, their high cost and long-term environmental damage make them an unsustainable option.Consequently, researchers have explored alternative methods, such as using rock phosphate (RP), which is abundant and inexpensive.However, solubilizing P from RP has been a challenge.To address this, physical and chemical treatments have been employed, but recent attention has focused on the potential of rhizobacteria to solubilize RP and provide P to plants.Microorganisms employ various mechanisms, including the production of organic and mineral acids, proton secretion, and siderophore production, to solubilize RP, thereby enhancing the quality and quantity of agricultural products.The study by Khoshru and colleagues examined the potential of rhizosphere microbes, particularly rhizobacteria, as an environmentally friendly strategy for RP solubilization, in addition to physical and chemical solutions.

Conclusions
The symbiotic relationship between plants and PGPR has immense potential for sustainable agriculture and environmental interaction.By promoting plant growth and enhancing nutrient uptake efficiency, PGPR offer a natural and eco-friendly alternative to chemical fertilizers and pesticides, thereby reducing the environmental footprint of agricultural practices.Furthermore, their ability to confer resistance to biotic and abiotic stresses can bolster crop resilience in the face of climate variability and emerging pathogens, contributing to food security and agricultural sustainability.This Special Issue aims to investigate the multifaceted relationship between plants and PGPR, with a focus on elucidating the underlying mechanisms and potential applications for sustainable development.By examining recent advancements in research and innovative approaches in agricultural practices, this endeavor seeks to illuminate the transformative role of PGPR in fostering resilient and sustainable agroecosystems.Through interdisciplinary collaboration and concerted efforts, harnessing the symbiotic potential of plants and PGPR holds promise for addressing the intertwined challenges of food security, environmental sustainability, and climate resilience in the 21st century.

Conflicts of Interest:
The authors declare no conflicts of interest.