Plant-Microbe Interactions in Ecosystems Functioning and Sustainability

C H A P T E R

18 Plant-Microbe Interactions in Ecosystems Functioning and Sustainability Paramanantham Parasuraman, Subhaswaraj Pattnaik, and Siddhardha Busi Department of Microbiology, School of Life Sciences, Pondicherry University, Puducherry, India

O U T L I N E 18.1 Introduction

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18.2 Plant-Microbe Relationship in Ecosystem

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18.3 Impact of Plant-Microbe Interactions

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18.4 Plant-Microbe Interaction on Productivity and Disease Management

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18.5 Plant Growth Promotion and Improved Productivity

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18.6 Biological Control and Pest Management

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18.7 The Factors Affecting the Plant-Microbe Interactions and Ecosystem Sustainability

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18.8 Recent Advancement in Enhancement of Plant-Microbe Interactions and Sustainable Ecosystem

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18.9 Future Challenges in Attaining Ecosystem Sustainability

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18.10 Conclusions

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References

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Further Reading

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18.1 INTRODUCTION An ecosystem can be defined as a system formed by the interaction of a community of organisms with each other and their physical properties. It is being necessary to understand the functioning of the ecosystem for successive maintenance of the ecosystem. However, understanding the functioning of ecosystem finds difficult due to the greater complexity of multifactorial ecological interaction. It is highly required to specify the effect of different components of the ecosystem like both biotic and abiotic factors which significantly help to understand the functioning of ecosystem as a whole (Santoyo et al., 2017). Among several ecosystems, certain ecosystems are gaining significant attention for the study of functioning of the ecosystems, particularly soil ecosystem. Soil ecosystem is the highly complex ecosystem where both the biotic and abiotic components are involved in their regular interaction to maintain the ecosystem function. Soil ecosystem popularly defined as an interdependent life-supporting system that holds abiotic components like air, water, minerals, and organic matters function cumulatively and coordinates effectively with their biotic components such as macro and microorganisms. The biotic and abiotic components of the soil ecosystem regulate the sustainability of its own by preserving the soil fertility, soil health, and plant productivity (Singh and Seneviratne, 2017a, b). Biotic components of the soil play a significant role in the regulation of soil processes including nutrient recycling, decomposition of organic material, detoxification of toxic substance, and suppression of pathogenic macro and microorganisms. The successive sustainability of the soil ecosystem demands the abovementioned soil-processing events that could be executed by the biotic components of the soil ecosystem. Biotic components like plants, microorganisms, etc., which are placed in the soil food web, are interdependency for their carbon and energy sources with their

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organisms community in their living area in soil. Nevertheless, the abiotic components such as air, water, minerals, and organic matters also address soil food web directly or indirectly. For example, the requirement of water for the decomposition processes a major biotic event that subsequently regulates the soil food web. The soil ecosystem was considered as most lively as any other ecosystem due to the presence of diversified and abundant life in it. For an example, 1 g of soil could contain around 10 billion of microorganisms and thousands of different species. These numerous microorganism extent their diversity by creating relation among their neighboring microbial community or with other biotic components like plant by complex system of intra and interspecies communication (Moya-Laraño et al., 2014). In most cases, microbial communities play a significant role in every biogeochemical cycle in the soil ecosystem, also impelling global carbon and nutrient cycle that effectively affect the ecosystem functioning and productivity. The greater amount of microorganism in association with the higher organisms and collective function is called as microbiome. Till date, the microbiomes of several higher organisms such as plants, insects, and fish, the list extended even up to mice, apes, and human are investigated. Among mentioned higher organism microbiomes, plants microbiomes are the most effective association group which actively participates in balancing the ecosystem functioning (van der Heijden and Hartmann, 2016). For example, in recent years, chemical contaminants are the major concern for the human health and environmental stability. Indeed, the available contaminants in the soil are naturally attenuated or detoxified by soil microbiota, the process is also called as bioremediation. In this regard, most of the ecological processes are connected to plant-microbe interactions and are considered to be of crucial importance for understanding and managing a terrestrial ecosystem challenged by organic chemicals (Fester et al., 2014). The presence of diverse microbial consortium in relation with plant facilitates to increase or restore plant ecosystem productivity by carbon and water cycling, nutrient trapping, crop production, and carbon uptake and storage, improve plants response to a wide range of environmental stress factors, and mitigate effect of climate change by modulating ecosystem for a prolonged soil carbon storage (Ahkami et al., 2017). In recent years, natural biological processes have been mimicked by the scientific society for restoration of ecological balance in the environment. In this regard, manipulation and regulation of biotic interaction in functioning agroecosystem are experimented in order to enhance the productivity in agriculture and sustainable agroecosystem for a prolonged period of agricultural practices. Such kind of understanding is highly important for effective restoration of ecosystem where it is badly affected by the human activity (Gaba et al., 2014). This chapter aims to cover plant-microbe interactions in the ecosystem, impact of pant-microbe interaction, plantmicrobe interactions on productivity and disease management, the factors that affect the plant-microbe interaction and ecosystem sustainability, and recent advancement and future challenges in attaining ecosystem sustainability.

18.2 PLANT-MICROBE RELATIONSHIP IN ECOSYSTEM In an ecosystem, plants harbor a variety of microorganisms and the dynamicity of the interactions between microorganisms and the respective plant hosts play a crucial role in maintaining ecosystem functioning. In the entire life cycle of any microorganisms, the microorganisms are generally associated with plant host in three different possible ways such as “pathogens,” “parasites,” or “mutualists” and it is important to understand these three toward achieving the sustainability of the ecosystem (Newton et al., 2010). In rhizospheric plant-microbe interactions, symbiotic and associative interactions with beneficial microbes constitute the positive interactions whereas association with parasitic plants, pathogenic bacteria, and fungi represents the negative interactions (Haldar and Sengupta, 2015). In the process of any plant-microbe interactions, microorganisms used to utilize plant-derived substrates for energy production play substantial roles in maintaining nutrient cycling and modifying plant environments (Fig. 18.1). In majority of plantmicrobe interactions, the association is either neutral or beneficial where microorganisms promote plant growth and/ or regulate plant disease management by improving the nutrient acquisition and production of specific growth regulators. As compared to these association types, the third type of association is relatively rare but proves to be detrimental in plant health and environmental stability (Schenk et al., 2012). From the ecological perspectives, the plant-microbe interactions are regulated by a variety of physiological process such as production and exchange of metabolites of ecological importance between the members associated with the interaction, physicochemical changes such as chemotaxis, genetic exchange, and signaling mechanisms (Braga et al., 2016). In recent years, the plant-microbe interactions are exploited for ecosystem resurgence by improving the bioremediation of toxic environmental pollutants in an efficient manner for achieving a sustainable environment (Abhilash et al., 2012; Fester et al., 2014). In this context, rhizospheric bacteria colonizing plant rhizosphere can establish neutral, beneficial, or deleterious association with respective plant host proved to be crucial in remediating organic and inorganic contaminants which negatively affects the plant growth and development as well as detrimental for environmental deterioration. It is imperative to understand the underlying mechanism of plant-rhizospheric microbe’s interaction which will provide larger scope

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FIG. 18.1 Schematic representation of plant-microbe interactions in different interfaces, microorganisms involved in the interactions and the application of plant-microbe interactions in ecosystem functioning and environmental sustainability.

and opportunities for improvement in the plant health and maintenance of ecosystem stability (Berendsen et al., 2012; Segura and Ramos, 2013). Plants growing in metal-contaminated soils harbor a wide variety of microbial communities capable of tolerating high concentration of organic and inorganic heavy metals. Such rhizospheric microbial communities provide potential benefits to the soil by minimizing the heavy metal concentration in the soil thereby providing systemic resistance to the respective plant. In this context, it is imperative to explore such microbial communities for improved phytoremediation efficacy by modulating soil pH, release of organic acids, and siderophores thereby minimizing the heavy metal availability (Rajkumar et al., 2012). From the past few years, plant growth-promoting rhizobacteria (PGPR) have gained considerable attention for their close association with plant rhizosphere and their ability to confer benefits to plant hosts by improving crop productivity, stress tolerance, and systemic resistance. In addition, PGPR also mitigate the environmental concerns by providing stability between plant and environment (Ahkami et al., 2017). The existence of endophytes, may it be endophytic bacteria and/or endophytic fungi within the niche of associated plant hosts forms a highly complex, dynamic, and fascinating cross-talk environment for widespread pharmaceutical applications as well as for achieving ecological sustainability (Kusari et al., 2014). Endophytic microbial communities have the inherent properties of synthesizing phytohormones, vitamins, and supply essential nutrients to the host plants thereby modulating the physiology of host plants. Apart from that, endophytic microbial communities also have a significant influence on the ecological functioning and environmental sustainability (Zhang et al., 2006). Bioprospecting the complex interplay between diverse array of microbial communities with host plant results in characteristic ecophysiological changes such as effect on host secondary metabolism, effect on microbial secondary metabolism, effect on plant nutrition, nutrition acquisition, improved productivity, and stress tolerance (Wani et al., 2015). Soil harbors a vast diversity of microorganisms of which bacteria are the most abundant and diverse group of microorganisms in soil. The soil microorganisms play a crucial role in driving proper functioning and sustainability of soil ecosystem thereby maintaining environmental stability. Any considerable changes in the soil type or unskilled agricultural practices and indiscriminate use of synthetic pesticides and insecticides have profound impact on soil sickness which is detrimental to soil microorganisms leading to serious deterioration of the environment (Huang et al., 2013). Heterotrophic microbial communities inhabiting the rhizosphere of plants greatly influence the carbon (C) and nitrogen (N) cycling for nutrient uptake by host plant and thereby constitute a mechanistic link between the plant diversity and proper ecosystem functioning (Zak et al., 2003). Mangroves are described as an important intertidal ecosystem

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which play a vital ecological role in the tropical and subtropical environments. The rhizosphere of mangrove plants harbors a wide variety of microbial communities capable of surviving in harsh environmental conditions such as salinity, aridity, and water temperature. However, the study on these microbial communities remains underexplored and provides new avenues for metagenomic studies of those microbial communities colonizing the rhizosphere of mangrove plants and their role in maintaining the ecosystem stability (Alzubaidy et al., 2016). The plant root-soil interfaces constitute a dynamic microbial habitat where the microorganisms, respective plant roots and soil constituents intricately associated with each other resulting in improved plant growth and health, enhanced soil quality, improved soil fertility, enhanced nutrient uptake, improved agroecosystem sustainability, and proper ecosystem functioning (Barea et al., 2002; Johansson et al., 2004). From the past few years, severe anthropogenic activities such as indiscriminate and improper agricultural practices, overgrazing, and deforestation result in serious decline in soil productivity and fertility leading to soil degradation and deterioration of agroecosystem. In this context, microbial communities colonizing the rhizosphere and phyllosphere of plants can be explored for achieving sustainable restoration of degraded soils and maintenance of agroecosystem which are proved to be safe and eco-friendly. These microbial communities have the inherent properties of increasing the nutrient bioavailability to host plants by fixing atmospheric nitrogen and mobilization of essential nutrients such as phosphorus, potassium, and iron thereby improving soil aggregation and stability (Rashid et al., 2016). It is imperative that plant growth and development have benefited from the diversity and complexity of the microbial society. The interspecies or interkingdom cross talk occurring in the rhizospheric/phyllospheric interface between all the interacting organisms is essential for the function, health, stability, and sustainability of ecosystems, including the agroecosystem (Da-le-Pena and Loyola-Vargas, 2014). In plant-microbe interactions, not only the species richness and diversity of microbial communities affect the cumulative output to the agroecosystems but also plant diversity has a significant role to play by modulating soil microbial community thereby significantly affecting the ecosystem dynamics and stability (Schlatter et al., 2015). The abiotic factors such as pH, temperature, soil type, soil geography, and climatic conditions greatly influence the microbial communities which ultimately affect the growth and development of plants. In other words, the mutualistic interplay between plants, soil microbial communities, and edaphic factors in soil has a profound role in maintaining ecosystem functioning and managing environmental stress conditions (Santoyo et al., 2017).

18.3 IMPACT OF PLANT-MICROBE INTERACTIONS As discussed interaction between plant and microorganisms facilitates successive functioning of soil ecosystem, being involved in soil nutrient cycling, suppression of soil-borne pathogenic microorganisms, and also mediate the decomposition of organic matter that is effectively associated with the aboveground performance of the plant. The interaction between plant and microorganism most often has any one of the following relationship like mutualistic, commensalism, and parasitic. These interactions influenced by plant metabolism by releasing carbon dioxide and phytohormones effectively enhance the interaction by providing energy sources for microorganisms and also act as chemical attractants and repellents. In certain cases, they also participate as signaling molecules for the communication to initiate biological and physiological interaction between microorganisms and plant that indirectly alter the chemical and physical properties of the soil and the community of soil microbes, inhibiting growth of competing plant species and providing beneficial symbioses including nitrogen fixation, preventing the pathogenic microorganisms (Ahkami et al., 2017; Li et al., 2018). Due to climate change, some of the abiotic factors including drought, high temperature, and salinity greatly affect the natural vegetation and crop production. Drought stress mostly inhibits photosynthesis and root development whereas salinity stress mediates to ion toxicity because of excessive amounts of Na+ and Cl that potentially inhibit the plant growth and root elongation and also affect several plant physiological pathways. Moreover, climate change negatively impacts plant development in several ways like altering hormone balance and more prominent susceptibility to diseases (Imam et al., 2016; Rezzonico et al., 2017). Under abiotic stress conditions, plants are able to acclimate themselves by phenotypic plasticity. Similarly, they associate with the naturally occurring microorganisms which can facilitate other means of stress tolerance capacity and can promote resistance to different abiotic stress factors (Farrar et al., 2014; Schlaeppi and Bulgarelli, 2015). The microbiomes of the plants, like the microbiota of human, cumulative of diversifying microorganism that can reside in the inside and outside of the plant tissues. This association creates certain physiological and molecular alteration that even the genome of the plant microbiomes is also referred to as second genome of the plants (Berg, 2014;

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TABLE 18.1

Some of the Examples of Plant-Microbe Interaction Their Beneficial Aspects

Action

Plants

Microorganism

References

Indole acetic acid (IAA) aminocyclopropane1-carboxylate (ACC) deaminase

Triticum, Cicer arietinum Pisum sativum L, Brassica napus

Bacillus cereus, Pseudomonas sp. Pseudomonas sp., Pseudomonas putida

Malik and Sindhu (2011) and Mohite (2013)) Arshad et al. (2008) and Cheng et al. (2007)

Siderophore N2 fixation Phosphate solubilization

Solanum lycopersicum, Festuca rubra, Brassica napus L Glycine max L, Saccharum officinarum Pisum sativum L., Triticum

Pseudomonas aeruginosa, P. fluorescens, Pseudomonas sp., Bacillus sp. Bradyrhizobium japonicum, Pseudomonas sp. Pseudomonas sp., Aspergillus niger

Grobelak and Hiller (2017) and Solanki et al. (2014)) Brunner et al. (2015) and Li et al. (2017)) Oteino et al. (2015) and Xiao et al. (2013)

Biological control Immune system response

Ochrobactrum lupine, Novosphingobium pentaromativorans, Paenibacillus polymyxa, Solanum melongena L. Lycopersicon esculentum Mill.

Piper nigrum, Staphylococcus sp., Streptomyces sp., Agrobacterium sp., Enterobacter sp. Pseudomonas putida, P. fluorescens, Trichoderma harzianum

Achari and Ramesh (2014) and Hahm et al. (2012)) Hol et al. (2013) and Martínez-Medina et al. (2013)

Vandenkoornhuyse et al., 2015). This relationship mediated the plant health and productivity, particularly, certain bacteria widely observed as plant growth-promoting bacteria that effectively involves in the production of plant growth regulators, organic acids, and volatiles organic matters as well as production of lytic enzymes like chitinase and glucanase. They also participate in the mechanisms including nutrient solubilization, biological nitrogen fixation, and induction of systemic resistance (Paramanandham et al., 2017). Some of the example of plant-microbe interactions and their beneficial aspects are shown in Table 18.1. The nature of the soil also participates in the selection of the microbial community that associates with the plants. Usually, soil shows its versatility by the following factors including pH, structure, texture, organic matter, microaggregate stability, and availability of nutrient. The mentioned physiochemical properties of soil can partially benefit certain group of microbial community by creating niche environments that selectively facilitate the growth of certain microorganism and influence the availability of plant root exudates affecting microbial recruitment by the plant. For example, in certain cases, the soil pH and nutrient availability have been determined to affect the abundance of plant pathogenic microorganism as well as beneficial microbes (Lareen et al., 2016). PGPR are the group of microorganisms that reside on the rhizophere of the plant they potentially stimulate plant growth and protection of minor and major pathogen and significantly enhance the crop productivity. These microbial group colonized around the root of the plants and undergoes directly or indirectly mechanisms to promote the plant development (Yuttavanichakul et al., 2012; Park et al., 2015). They effectively participate in the enrichment of the soil fertility and maintenance of chemical composition of the soil. Moreover, PGPR strains are known to help the plants for their growth, yield, and nutrient uptake by an array of mechanisms. In certain cases, PGPR strains directly regulate the physiology of the plant and mimicking the production or synthesis of the plant hormones, meanwhile others strains are involved in enhancing the availability of minerals, nitrogen, and chemical composition in the soil for the betterment of the plant development (Kesaulya et al., 2015; Wang et al., 2015). These PGPR strains execute different biochemical reaction including solubilization of phosphate which can be used to synthesize plant growth-promoting hormones like indole acetic acid. Nevertheless, they actively participate in the antagonistic activity against plant pathogen by producing antibiotics and siderophore and nutrition or site competition. In addition, there are certain PGPR strains that are able to produce hydrogen cyanide which is potentially toxic to certain pathogenic fungi. The production of siderophores by PGPR strains significantly facilitates sequestering the available soluble iron that effectively creates impact in plant growth and function. Additionally, siderophores play a potential role in competing among the beneficial microorganisms and plant pathogen as well as behave as potential plant promoters (Kumar et al., 2012; Selvakumar et al., 2013). In recent years, natural and artificial plant-microbe interactions are employed to enhance the nutrient bioavailability and crop production. In certain cases, the key compounds that occur in the natural plant-microbe interactions are used directly as targets for genetic engineering to further strengthen the interaction (Ahkami et al., 2017).

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18.4 PLANT-MICROBE INTERACTIONS ON PRODUCTIVITY AND DISEASE MANAGEMENT In the first half of the twenty first century, one of the major global challenges is to provide the growing population with environmentally sound and sustainable crop production. However, use of conventional agricultural strategies which focus on the indiscriminate use of chemical pesticides and fertilizers leads to severe health consequences and environmental hazards. In this context, exploitation of beneficial plant-microbe interactions sounds promising, effective and environmentally friendly strategy to overcome the limitations associated with conventional agricultural methods (Berg, 2009). Although certain plant-microbe interactions lead to the emergence of pathogenic determinants; majority of plant-microbe interactions account for either neutral or beneficial roles to the host plants by maintaining nutrient acquisition, increased agricultural productivity, maintaining systemic resistance, and promoting environmental stress tolerance (Andreote et al., 2014).

18.5 PLANT GROWTH PROMOTION AND IMPROVED PRODUCTIVITY In recent times, majority of Methylobacterium sp. are exploited for their keen association with plant phyllosphere of a variety of plant species and their efficacy to promote plant growth and development, enhanced productivity of economically important crops, better nutrient acquisition by respective host plants, and increased seed vigor and germination percentage (Tani et al., 2015). The plant growth promotion and development by close association between microbial communities with their respective plant host are generally associated with direct production of plant growth-promoting hormones and mediate the synthesis of certain metabolic intermediates that serve as precursors for the biosynthesis of phytohormones (Dourado et al., 2015). Potassium solubilizing microorganisms (KSMs) constitute an integral component of soil microbial community and play an important role in the potassium (K) cycle in soil rendering the unavailable K form to plants for the growth and development. The plant growth-promoting properties of KSMs can be attributed to their ability to induce the synthesis of plant growth hormones such as auxins, gibberellins, cytokines, and ethylene as well as the ability to solubilize the nutrient and make it available for plant uptake (Singh and Seneviratne, 2017a, b). In addition, their ability to induce production of siderophores, hydrogen cyanides, and specific enzymes provides systemic resistance to plants against phytopathogens (Meena et al., 2014). Methylobacterium, inhabiting the phyllosphere and rhizosphere of any plant species is generally involved in maintaining the plant health and increase the growth by supplementing essential nutrients to the hosts by virtue of its close association with the host plant. Different species of Methylobacterium are involved in various functions such as maintaining the balance in carbon cycling between plant and environment, increased efficacy in nitrogen fixation, enhanced phosphate acquisition efficacy, maintaining abiotic stress tolerance mechanisms, and their synergistic behavior with other microbiotas in plant growth and promotion (Kumar et al., 2016). The pink pigmented facultative methylotrophs (PPFMs) isolated from the surface of young leaves of sugarcane (Saccharum officinarum L.) have significantly influenced the growth, germination, and yield of sugarcane by modulating the growth-promoting traits (Madhaiyan et al., 2005). The genus Methylobacterium is among the most commonly observed leaf epiphytes and represents an abundant and stable members of the phyllospheric microbial community of a wide range of crop plants such as sugarcane (S. officinarum L.), pigeonpea (Cajanus cajan L.), mustard (Brassica campestris L.), potato (Solanum tuberosum L.), and radish (Raphanus sativus L.) (Meena et al., 2012). The phyllospheric Methylobacterium spp. has a rich heritage of producing plant growth-promoting traits like seedling length, seed vigor index, production of growth-promoting phytohormones, and is associated with plant growth promotion and development of economically important crops irrespective of its isolation source. The association of PGPR, especially Methylobacterim sp. with the plant hosts greatly benefits the plant growth by production of phytohormones like auxins, cytokinins, and by increased activity of enzymes such as urease, 1-aminocyclopropane-1-carboxylate deaminase (ACCD) which promotes growth and enhances production of siderophores thereby enhancing the uptake of essential nutrients. The benefits associated with plant-microbe interaction are also dependent on the variety of inoculation methods such as soil inoculation, foliar inoculation, and combination of both soil and foliar inoculation (Lee et al., 2011).

18.6 BIOLOGICAL CONTROL AND PEST MANAGEMENT In addition to plant growth promotion and enhanced productivity, plant-microbe interactions also have the ability to promote plant protection from a variety of phytopathogens and maintain systemic resistance of the host plant by

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altering the level of antioxidant enzymes such as polyphenol oxidase (PPO), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), and superoxide dismutase (SOD) (El-Gawad et al., 2015). The influence of colonization of aerobic phyllospheric methylobacteria on pea (Pisum sativum L.) plant is generally associated with systemic resistance from abiotic stress conditions by significantly modulating the activity of antioxidant enzymes such as SOD, CAT and PODs, and lipid peroxidation (Agafonova et al., 2016). The phyllosphere microbiotas especially that normally live inside the plant phyllosphere without harming the host plant seem to be a highly promising approach for biological disease control and management. The disease control capacities and induction of systemic resistance shown by phyllospheric microbial communities depend on the plant cultivar, pathogen, and on the density of Methylobacterium sp. inoculums (Ardanov et al., 2012). The phyllospheric microbes mediated defense strategies adopted by host plants include activation of antioxidant status of the plant by activation of defense-related enzymes, modulation of quorum sensing phenomenon in the microbial inhabitant, and activation of specific pathway leading to production of specific secondary metabolites of antagonistic stature (Mishra et al., 2015).

18.7 THE FACTORS AFFECTING THE PLANT-MICROBE INTERACTIONS AND ECOSYSTEM SUSTAINABILITY In the soil ecosystem, plant-microbe interactions are influenced by a variety of biotic and abiotic factors which shows versatility in time and space with respect to the location of the ecosystem (Fig. 18.2). The wide quantitative and qualitative variations were observed in plant microbiomes with different biotic and abiotic environments. Biotic factors including plant pests, which are feeding on specific plant tissues and selectively interrupt water and nutrient uptake as well as plant stability by destroying parts of the root system. Additionally certain weeds participate in the competition of the utilization of water and nutrients from the ecosystem with plants and alter the plant physiology as well as productivity. Generally, the plant community composition can also play a significant role in altering the microbial community in both direct and indirect altering mechanism (Reese et al., 2018). Moreover, specific group of microorganism dominant over the beneficial microorganism in the soil ecosystem effectively disturbs positive impact of plant-microbe interactions. Furthermore, plant react to the dynamic attack of different biotic factor by producing specific blends of nonvolatile and volatile secondary metabolites that greatly enhances the plant’s defense mechanisms. Unfortunately, the plant’s defense mechanisms specifically target not only the pathogenic microorganism but also showed adverse effect on the beneficial microorganism (Erb and Lu, 2013). The mentioned biotic factors are potentially altering the interaction between plant and microorganism. In addition to these biotic factors, several abiotic factors are also significant contributes to their adverse effect to affect the plant microbe interaction. In plant-microbe interactions, in addition to climate, temperature, salinity, light, and humidity; abiotic factors like soil texture, pH, and moisture content also play crucial role. Excess of water content mediates hypoxia that causes an anaerobic respiration rate, accumulation of ethanol, lactic acid, and alanine in the soil ecosystem. The availability or unavailability of these minerals or toxic substance in the soil ecosystem potentially affects the microbial community and the plant development (Burpee, 1990). Due to these environmental factors, plants tend to synthesize certain acids like citric, malic, and oxalic acid to demineralize aluminum in the environment; availability of phenolic compounds

FIG. 18.2

Biotic and abiotic factors that affect the plant-microbe interactions.

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significantly enhances the deficiency of phosphorous in the soil. Additionally, releasing of signaling molecules like flavanone and flavones increases nitrogen limitation in the soil (Haldar and Sengupta, 2015). This condition significantly unfavors a potential relation between plant and microorganism. Nevertheless, at the time of adverse environmental condition, plants develop the stress-tolerance capacity by utilizing the beneficial microorganisms and developing mutualistic interaction between plant and microorganism. This relationship promotes the plants to enhance the root nutrient uptake, biomass productivity, and develop the potential plant acclimation to abiotic stress factors. In addressing the microbe-mediated stress tolerance of plants, PGPR strains confer more than one type of abiotic stress tolerance and potentially create a relation in greater diversified plants. In recent years, PGPR strains gain significant attraction toward bio-fertilizers due to their ability to confer benefits to crop production and stress tolerance, similar to that of time-consuming plant breeding technology (Ahkami et al., 2017). Among different abiotic factors, drought causes serious ecological stress to the plant that gains significant attention of the ecologist and agro-based researchers, because it introduces major constraints on plant growth and productivity. The stress condition alters the plant physiology like reduces cell size, membrane integrity, produce reactive oxygen species, and promotes leaf senescence, which subsequently reduce the plant productivity. In the less water condition, plant undergoes self-physiological and molecular modification including enhanced ethylene production, change in the chlorophyll content, disturbing the photosynthesis apparatus, and inhibit photosynthesis process. Furthermore, this condition leads to accumulation of free radicals which can effectively create the modification in membrane function, protein conformation, and lipid peroxidation and accelerate the cell death. To overcome drought, plants associated microorganisms to undergo several mechanisms that significantly reduce the negative impact of the drought on plants and soil. Usually, the beneficial microorganisms proliferate their colonies around the root of the plant and extent their support to promote plant growth and development by different direct and indirect mechanisms including production of phytohormones like indole acetic acid, cytokinins and abscisic acid, and bacterial exopolysaccharides, which induce systemic tolerance. The association or interaction between drought-tolerant microorganisms with plants can balance proper growth and survival under drought conditions (Aung et al., 2018; Kumar and Verma, 2018; Naylor and Coleman-Derr, 2018). Salinity is another abiotic factor that provides a highly unfavorable ecosystem for the plant that subsequently limits the productivity of the crop plant; this occurs because most of the plants are sensitive to salinity, that is, high salt concentrations in the soil. Soil salinity has become the major adverse effect to balance the ecosystem in the recent years. The salinity imposes toxic effect by perturbing potassium (K+)-dependent processes, inducing deleterious protein confirmation, and leads to osmotic stress that causes growth inhibition and subsequently mediates to cell death (Zhang et al., 2008). To overcome or to manage the salinity stress in a simple and low-cost biological process is by exploiting the microorganisms. Certain bacterial group, in particularly, PGPR induce physical and chemical change that potentially increases tolerance to abiotic stress factors. They actively participate in the promotion of plant growth indirectly by reducing plant pathogen or directly by enhancing the nutrient uptake by producing phytohormones. It is widely accepted and demonstrated that inoculation of PGPR stains on the plant root will potentially overcome from salinity stress and allows the plant to grow in a high salt-containing soil (Shrivastava and Kumar, 2015; Yaish et al., 2016). Temperature is another significant abiotic environmental factor that affects both plants and microorganisms. Once again, the plant growth-promoting microorganisms are investigated for their efficacy in facilitating the plant to grow in a high-temperature environment once it gains association with plants and potentially expands productivity of the crop. Additionally, other abiotic factors such as chemical contaminants, heavy metals, and elevation of CO2 level all effectively interfere with the plant growth and the development. On the other hand, plants form a spectrum of association like mutualistic, commensalistic, and parasitic with microorganisms and overcomes from the mentioned abiotic stress and improves their productivity (Curiel Yuste et al., 2010; Rajkumar et al., 2013; Ahkami et al., 2017).

18.8 RECENT ADVANCEMENT IN ENHANCEMENT OF PLANT-MICROBE INTERACTIONS AND SUSTAINABLE ECOSYSTEM Apart from the conventional culture-dependent approach to study the plant-microbe interactions and their specific role toward environmental stability, the current trends focus on advanced high-throughput approaches such as metabolomics, proteomics, and metagenomics/metatranscriptomics. The study of metabolomics will give an insight into the identification and characterization of novel secondary metabolites from microorganisms associated with plant hosts for the improvement of plant growth and development. Meanwhile, proteomics study specifically focus on the biochemical and physiological parameters influencing the plant-microbe interactions (Singh, 2013a, b, 2014). The metagenomics/metatranscriptomics approach will give novel insights into specific genes or gene products from

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microbes as well as from their respective plant hosts which play a crucial role in plant-microbe interactions and an advanced step toward achieving the ecological sustainability (Pii et al., 2015). The plant-associated microbial communities and their close association with the host plant represent the untapped source for novel secondary metabolites with widespread pharmaceutical and industrial applications. The microbial communities harboring a specific host are highly specialized and display specific genetic features that distinguish them from other group of microbial communities thereby providing ample information on exploring the specific biosynthetic gene clusters such as nonribosomal peptide synthetases (NRPS) and polyketide synthases (PKS) for biosynthesis of specific products of particular interests toward ecosystem sustainability (Muller et al., 2016). As evidenced from the earlier studies, plant-associated microbial communities especially soil microorganisms play fundamental roles in agriculture mainly by improving plant nutrition and maintaining plant health, and improving soil quality and fertility (Singh, 2015a, b, c, d, 2016). Accordingly, several strategies for a more effective exploitation of beneficial microbial services, as a low-input biotechnology, to help to sustain environmentally friendly agrotechnological practices should be taken into consideration for proper management of agroecosystems and environmental stability (Barea, 2015). In the cross talk between plant and associated microbial communities, it is important to understand the microbe’s behavior in a certain habitat, to understand the genes expressed and repressed, their functions, the gene products interact and affect the ecosystem sustainability, and how these processes depend on biotic and abiotic conditions (Lugtenberg et al., 2002). The intensive exploration and understanding the underlying mechanisms involved in plant-microbe interactions may it be in the rhizosphere or phyllosphere will undoubtedly improve our ability to guide the knowledge into high-throughput technological interventions and next-generation platforms toward achieving sustainability in agriculture to meet the need of the global population, restoring ecosystem stability, and functioning and implementing ecological engineering for a promising atmosphere for environmental management (Haldar and Sengupta, 2015). Although a wide range of studies are available describing the effect of soil microbial communities in response to different agricultural practices and their effect on ecosystem stability. However, current understanding as to whether such changes in microbial diversity are beneficial for the proper functioning of agroecosystems and environmental management is still underexplored. Hence, it is imperative to focus on exploring such understandings into practical aspect for improved ecosystem services and maintaining sustainability in the agroecosystems (van der Heijden and Wagg, 2013). From earlier studies, it is observed that the diversity of microbial communities in association with plants is responsible for ecosystem stability as well as positive effects on plant growth, disease resistance or tolerance toward abiotic as well as biotic stresses. However, further meticulous and scientific research is needed to understand the complex network of intricate interactions within microbial communities or with the host plants greatly influencing plant fitness and maintaining systemic resistance to varied level of stress conditions. Therefore, in spite of integrated studies, in depth research is required on plant-microbe interaction which will provide untapped potential of such close association for human welfare as well as for environmental sustainability and ecosystem functioning. This in turn will provide new horizons to optimize conventional plant cultivation methods and provide food for an ever-increasing population without harnessing ecosystem functioning and stability (Schirawski and Perlin, 2017).

18.9 FUTURE CHALLENGES IN ATTAINING ECOSYSTEM SUSTAINABILITY Industrialization and the modernization results in the promotion of excessive use of chemicals like fertilizers, pesticides, and other contaminants that negatively impact the ecosystem and the plant development. This condition results in the loss of soil fertility and greatly altered stability of the ecosystem as well as balance of our planet. Fortunately, soil microorganisms are endowed with different mechanisms to help as efficient candidates for sustainable agriculture and restoration of ecosystem (Weih et al., 2014; Usman and Kundiri, 2016). Understating of plant-microbe interactions could provide an alternative way to retain the ecological balance of the locality. Most of the beneficial microorganisms are potentially contributed as bio-fertilizers, biocontrol agent, and soil fertility enhancer that effectively replace usage of chemical fertilizers. The study on plant growth-promoting microorganism can be expanded to unrevealed concepts related to their ecology, population dynamics, and functionality over a range of environments. As a consequence of global increase of human population will create the situation, where agricultural sector completely depends on beneficial microorganisms in order to enhance the agricultural production in an eco-friendly manner (Perring et al., 2015; Mishra et al., 2016).

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18.10 CONCLUSIONS The intricate interplay between plants and different microbial communities at different interfaces, may it be rhizosphere or phyllosphere, is an important aspect of the ecology and evolutionary processes for improved plant productivity, ecosystem functioning, and environmental management. The understanding of such close association between plants and microbial communities will also provide new avenues for agricultural sustainability, conservation of evolutionarily important microbiomes, and preservation of soil quality and fertility. The advanced, high-throughput analytical tools such as genomics, proteomics, transcriptomics and metatranscriptomics, and other integrative approaches will also provide ample opportunities for environmental sustainability, ecosystem functioning, and sustainability in crop production for food, feed, fiber, and fuel.

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Further Reading Singh, J.S., 2012. Coal fly ash in agriculture: beneficial or risky? Sci. Rep. 43–45.