Plant Microbial Interactions in the Rhizosphere: Associations to Plant Growth Promoting Rhizosphere Microorganisms, Genetic Diversity, Competition and Interactions with Host Plants

Review Article

Ann Agric Crop Sci. 2023; 8(4): 1142.

Plant Microbial Interactions in the Rhizosphere: Associations to Plant Growth Promoting Rhizosphere Microorganisms, Genetic Diversity, Competition and Interactions with Host Plants

Zehara Mohammed Damtew*

Ethiopian Institute of Agricultural Research, Debre Zeit Research Center, Addis Ababa, Ethiopia

*Corresponding author: Zehara Mohammed Damtew Ethiopian Institute of Agricultural Research, Debre Zeit Research Center, PO Box 32, Hora road, Debre Zeit, Ethiopia. Email: zeharamicro2015@gmail.com

Received: July 15, 2022 Accepted: December 18, 2023 Published: December 23, 2023

Summary

Microbial interactions are crucial for successful establishment and maintenance of a microbial population. These interactions occur by the environmental recognition followed by transference of molecular and genetic information that include many mechanisms and classes of molecules. Microorganisms are rarely encountered as single species populations in the environment, since studies in different habitats have shown that an enormous richness and abundance variation are usually detected in a small sample. The rhizosphere is known to be a hot spot of microbial activities. Therefore, rhizosphere is an environment with a high microbial diversity. Rhizobacteria as PGPR can play an important role in promoting nutrient acquisition by plants, favoring factors inducing root biomass accumulation and/or hindering those that could have detrimental effects on root system development. This role of PGPR can be achieved via either an indirect (antagonism against pathogens) or direct (e.g., phytoharmones production) mode of action. Plant growth-promoting mechanisms differ between bacterial strains and to a great extent depend on the type of organic compounds released by these strains. For example, plant growth-promoting hormones and other secondary metabolites released by the bacteria can alter plant growth and development. Recently, it has been reported that associations between plant and associated bacteria have reached such levels that the host plant cannot develop properly without their associated bacteria.

Keywords: Interaction; Microorganisms; Phytoharmones; Rhizosphere; Species

Introduction

The positive plant-microbe relationship established in the rhizosphere could, moreover, sustain an additional service, which is phyto-rhizoremediation which constitutes a promising sustainable approach for the in-situ remediation of polluted soils and sediments. It relies on the stimulation by the plant of the degrading microbes in its rhizosphere, in a complex interaction involving roots, root exudates, rhizosphere soils, and microbial communities [56]. Microbial interactions are crucial for a successful establishment and maintenance of a microbial population. These interactions occur by the environmental recognition followed by transference of molecular and genetic information that include many mechanisms and classes of molecules. These mechanisms allow microorganisms to establish in a community, which depending on the multi-trophic interaction could result in high diversity. The result of this multiple interaction is frequently related to pathogenic or beneficial effect to a host. In humans, for example, the microbial community plays an important role in protection against diseases, caused by microbial pathogens or physiological disturbances. Soils microbial communities also play a major role in protecting plants from diseases and abiotic stresses or increasing nutrient uptake [19,56].

The interaction among microbe, plants and environmental factors are determinant for the colonization and development of microorganisms and plants in ecosystem. These interaction can be refelected in different aspect, such as variation in cellular morphology, changes in physicochemical traits, exchange and conversion of metabolite, molecular dialoog, gene transfer [62]. Suggesting that microbial interaction are inherent to the establishment of populations in the environment, which includes soil, sediment, animal, and plants, including also fungi and protozoa cells. The many years of coevolution of the different species lead to adaptation and specialization and resulted in a large variety of relationships that can facilitate cohabitation, such as mutualistic and endosymbiotic relationships, or competitive, antagonistic, pathogenic, and parasitic relationships [18,62].

Many secondary metabolites have been reported to be involved in the microbial interactions. These compounds are usually bioactive and can perform important functions in ecological interactions. A widely studied mechanism of microbial interaction is quorum sensing, which consists of a stimulation response system related to cellular concentration. The production of signaling molecules (auto-inducers) allows cells to communicate and respond to the environment in a coordinated way [2,48]. During interaction with the host cells, plant-associated molecular patterns (PAMP or MAMP microbial-associated molecular pattern) are conserved throughout different microbial taxon increasing the fitness during interaction with plant or animal cells and regulating the microbial interactions with different hosts [2,60]. There are many microbe host interactions, which can be related to beneficial or pathogenic interactions in plants and animals. In these interactions, the microbial cells may be found in biofilm or planktonic state, which result in different genetic and physiological states [9,30].

A fundamental knowledge on plants’ physiological properties and their associated microorganisms in the undisturbed natural environments is necessary to understand the impact of microorganisms on the plant development in general. The existence of positive plant-microbial interactions also in disturbed soils is unquestionable, but the mechanisms are often scarcely known. Microorganisms contribute essentially to the protection of plants against unfavorable soil conditions. In this chapter a selection of possible unfavorable soil properties in disturbed soils will be focused to analyze the possible impact of associated microorganisms on plants growth and vitality. Applicability of microbial inoculum for an improved remediation of such disturbed soils will be presented [1,27].

Literature of Review

Plant Microbe Interactions and the Rhizosphere Diversity

A narrow zone of soil affected by the presence of plant roots is defined as rhizosphere. The rhizosphere is known to be a hot spot of microbial activities. This is caused by an increased nutrient supply for microorganisms; since roots release a multitude of organic compounds (e.g., exudates and mucilage) derived from photosynthesis and other plant processes [10,26]. Therefore, rhizosphere is an environment with a high microbial diversity. An important consequence of the high diversity is an intense microbial activity with feedback effects on root development and plant growth in general. In general, the microbes serve as intermediary between the plant, which requires soluble mineral nutrients, and the soil, which contains the necessary nutrients but often in low concentrations and/or complex and inaccessible forms. Thus, rhizosphere microorganisms provide a critical link between plants and soil [26,36].

The highest portions of microorganisms, which inhabit the rhizosphere, are fungi and bacteria. When considering the rhizosphere effect on their abundance, the fungal abundance is 10–20 times higher and the bacterial abundance 2–20 times higher in the rhizosphere than in the bulk soil [2,42]. Competition for nutrient sources in the rhizosphere is very high. Therefore, different microorganisms have developed distinct strategies, giving rise to a range of antagonistic to synergistic interactions, both among themselves and with the plant [2,47]. A very high diversity of interactions can be assumed on the basis of the tremendous diversity of soil microorganisms and plants. The understanding of fundamentals of these interactions is critical for their use in plant growth promotion and remediation of disturbed soils.

The rhizosphere can be defined as the soil region where processes mediated by microorganisms are specifically influenced by the root system (Figure 1). This area includes the soil connected to the plant roots and often extends a few millimeters off the root surface, being an important environment for the plant and microorganism inter- actions [23,36,56], because plant roots release a wide range of compounds involved in attracting organisms which may be beneficial, neutral or detrimental to plants [4,36,56]. The plant growth-promoting bacteria (or PGPB) belong to a beneficial and heterogeneous group of microorganisms that can be found in the rhizosphere, on the root surface or associated to it, and are capable of enhancing the growth of plants and protecting them from disease and abiotic stresses [16,22,25,56]. The mechanisms by which PGPB stimulate plant growth involve the availability of nutrients originating from genetic processes, such as biological nitrogen fixation and phosphate solubilization, stress alleviation through the modulation of ACC deaminase expression, and production of phytohormones and siderophores, among several others.