Phytomicrobiome Interactions and Sustainable Agriculture. Группа авторов
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occurs between different species when the organism of one species is unable to fulfill its nourishment requirements and depends upon the other. This establishes a harmonious exchange of growth and enhancement benefits without breaching the living integrity of another. However, not all the species contains a harmonious tendency and tries to breach the living system of another to attain a competitive edge in survival. This acts as a detriment to the survival of other species on the whole.
Nonetheless, most of the species reside within an ecosystem with an effort of communal harmony among one another to support each other to sustain and maintain a healthy ecosystem. Hence an ecosystem comprises a consortium of species at the multitudes of genetic diversity existing with each other through a beautiful exchange of mutual benefits while preserving their interests.
Plant–microbe associations best exemplify such mutualistic associations in the ecosystem. The plants act as a meta‐organism harboring a consortium of microorganisms living in symbiosis with a variety of plant organ systems. Microbes may colonize the various parts of the plants, be it plant surfaces, areas adjoining the areas around the plant surface, or the interstitial spaces present between the adjoining cells. A phyto‐organ system apart from being the sheltered habitat and a future source of nutrition for the surrounding microbial population may attract the tiny creatures for their metabolic benefits by releasing such chemicals that stabilize the colonization of the beneficial microbial colonies. The growth of beneficial microbes, in turn, provides an increment in plant health. Moreover, the microbial signaling system is so developed that upon sensing some stress or threat, the microbial consortium confers changes in metabolism, thus rendering great adaptability and protection by releasing metabolites and antibiotic agents against pathogenic invasion.
Plants are of utmost importance from the view of the ecosystem as well as from the view of commercial viability. Plants form the framework of the ecosystem and provide nourishment and shelter to almost all living beings. Understanding the methods of the best sustenance of the plant within its natural system and providing further enhancement under stressed conditions become the foundation of studying mandatory growth parameters of plants. Since microbes work in close association of plants to enhance and enrich them, thus study of plant microbial associations are of great reputation to humankind as well as to the environment. Therefore, this chapter provides a crisp overview of the plant microbial associations and their manipulation through the proteomics approach, which shall further lead us to touch the proteomic perception of this association deciphered through technical knowhow.
2.2 Phytomicrobiome
Plants are meta‐organisms which are the multicellular organisms encompassing a synergistic association with a suite of microorganisms such as fungi, bacteria, archaea, and viruses, as well as several algal symbionts (Bosch and McFall‐Ngai 2011). Practically every plant component is colonized by a consortium of microorganisms, which in combination is designated as “plant microbiome” or“phytomicrobiome.” Based on their habitation, the microbes are categories as endophytic (within the plant tissue), epiphytic (on the surface of the above‐ground part), and rhizospheric (within or on the root surface) (McFall‐Ngai 2002, 2008). The microbiome composition and their metabolic activity determines the interaction and sustenance of the host plant within the surrounding environment (Kostic et al. 2013). The rhizospheric zone of the plant hosts a high density of microbial population enthralled with high microbial activity as compared with the other zones of plant habitation even within the surrounding bulk soil (Santoyo et al. 2016). Nonetheless, the microbial diversity is low as compared to the bulk soil and the composition within the bulk soil and rhizosphere are pretty distinct indicating stringency in the selection of microbial population (Pisa et al. 2011).
Years of research have contributed to identifying the associations of microbes with subtly distinguished parts of the plants such as epiphytic and endophytic regions of both stems and leaves of the phyllospheric region; the rhizosphere and the reproductive components such as seeds, fruits, flowers, etc. In the epidermal region of the grape plant along with the xylem of ovaries and ovules, two species of bacillus and the Pseudomonas species have specifically been found to colonize endophytically, whereas only the Bacillus species have been found to colonize the seed cell wall of berries (Lugtenberg and and Kamilova 2009; Compant et al. 2010a, 2010b). The plant growth–promoting rhizobacteria such as Diazotrophicus, Acetobacter, etc., (Quecine et al. 2012) assist the plant to fix nitrogen from the atmosphere and have an affinity to the plant roots (Pisa et al. 2011) and stem regions of the sugarcane plant and have been found to reside in the apoplastic zone under a low‐nitrogen and high‐sucrose situation.
The major hub of the interspecies associations that happen is at the rhizosphere which surrounds the roots area of the plants. The population of colonization depends upon the biochemistry of the plant and the microbes plus the available resources within the soil of that locality. One such species that is present in the majority of rhizosphere is the Streptomyces sp. They deserve a special mention because they can colonize not only the parts of the plants associated with the soil but also the parts above the ground. These microbes offer a variety of benefits to the host plant where they release antibiotic substances, thereby rendering the plant protected from the malicious conditions present in the surroundings. They are particularly useful as biocontrol agents where they protect various crop plants against pathogenic as well as environmental stresses. They serve as biofertilizers as well owing to their plant growth–promoting abilities. Their sporulating ability aids their survival in the adverse environment rendering them competent over the other microbes. This species releases various lytic enzymes that can metabolize insoluble organic substances to generate nutrients which are imperative for the proper growth and functioning of plants.
The aspect of microbial plant growth promotion can also be utilized for phytoremediation when dealing with a plant sown in a soil laden with high metal contamination. Bioethanol producers such as Helianthus tuberosus, a high biomass yielding crop acquires enhancement in the accumulation and sustenance of high concentrations of zinc and cadmium. In another case, the microbial population has been found to be colonizing the internal region of the roots as an endophyte; thereby resulting in increased assimilation of cadmium. The key feature of the presence of metal‐solubilizing bacteria is that it makes the host plant tolerant and decreases the effect of metal‐induced stress (Montalbán et al. 2017).
Interestingly, such a phenomenon has also been observed for the enhanced salt and drought resistance owing to the effects of plant growth–promoting bacteria as studied in Lolium perenne, also known as ryegrass. Unfortunately for this perennially relevant grass, as turf and forage, it is highly sensitive to drought‐induced stress and high salinity. However, Bacillus amyloliquefaciencs combined with hydrogels to prevent the soil from eroding can help the grass sustain the drought‐induced stress by the plant (Su et al. 2017).
In the cotton plant, arbuscular mycorrhizal fungus is a soil‐borne mutualistic symbiotic association with the terrestrial planets. One strain, Rhizophagusirregularis, acts antagonistically to the growth of Verticillium dahlia colonization and induces resistance by releasing the unknown volatiles thereby saving a most important crop plant (Zhang et al. 2018).
A beautiful symphony gets established to carry out mundane functions. The microbiome communities specific to the phyllosphere affect the development of plants along with its function within the ecosystem, whereas the host plant modulates the composition and effector functions of the phytomicrobiome. Environmentally governed factors have been found to amend several metabolite biosynthesis within the plant. Since the roots play a major role in establishing the health of the plant, the rhizomicrobial species participate in modulating plant growth, composition, and development. Interestingly enough, a study was found where just by treating the leaves with a specific consortium of phyllomicrobiome led to the prevention of feeding on the leaves by insect larvae (Badri et al. 2013).
However, it is believed that microbial community composition and their distribution in the phyllospheric region are somewhat arbitrary. Nonetheless, there is high‐level specificity in accommodation of the microbial communities within the endospheric and rhizospheric regions (Lebeis 2015).
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