ASH Rhizo-PGPR

Introduction: Soil is composed of minerals, organic matter, water, and microorganisms and it covers the surface of the earth. Soil not only provides an attachment surface for plants but also the necessary materials for their growth. It also acts as host to many types of bacteria. The number of bacterial species living in the soil varies according to the environmental conditions such as temperature of the soil, amount of salt, chemicals, and moisture in the soil, and plants growing nearby in the soil. Bacteria are usually found abundant around the rhizosphere. The term “rhizosphere” was first coined by Lorenz Hiltner in 1904 to define the layer of soil around the plant root that is populated by microorganisms.

The human population has been increasing rapidly and industrialization has various negative effects on the environment, such as decrease in available land for agriculture, decreased soil quality, global warming, and air and water pollution. New strategic solutions should be addressed to improve agricultural yields and sustainability so that the requirements for the human population will be met with the lowest environmental impacts.

A great solution can be the use of plant growth promoting rhizobacteria (PGPR), soil bacteria which colonize at the rhizosphere of the plants stimulating the plant growth and plant defenses.

Many microbes have the capacity to promote plant growth and microbial products that enhance plant health and growth have been commercialized. The beneficial effects of bacteria derived from the plant rhizosphere on roots and overall plant growth have been studied and demonstrated.

These types of bacteria have been designated as plant-growth-promoting rhizobacteria (PGPR). The significant beneficial effect of these rhizobacteria on plant growth are achieved by both direct and indirect mechanisms. The direct methods include the production of compounds that stimulate plant growth and ameliorate stress. PGPR exhibits a significant interaction with plant roots and has both direct and indirect positive effects on plant growth and the reduction of both biotic and abiotic stresses.

Plant growth is enhanced by the induction of systemic resistance, antibiosis, and competitive omission and other mechanisms. Viruses, bacteria, nematodes, weeds, and arachnids, all represent sources of biotic stress on plants. These agents injure their plant hosts, reduce plant vigor and can induce plant mortality. In addition, they also cause pre- and post-harvest losses in plants. Biotic and abiotic stresses negatively affect plant growth, development, yield, and biomass production.

The application of PGPR in the rhizosphere could be used to alleviate plant stresses due to their unique characteristics, diversity and relationship to plants. PGPR could be deployed in agricultural production systems to alleviate biotic and abiotic stresses and to produce sustainable, environmentally-friendly management tools. Plant roots are surrounded by a thin film of soil called the rhizosphere which represents the primary location of nutrient uptake, and is also where important physiological, chemical, and biological activities are occurring.

Bacteria are the most abundant microbes present in the rhizosphere. Rhizobacteria are capable of forming long-lived, stress tolerant spores and secreting metabolites that stimulate plant growth and prevent pathogen infection.

The Microbiome – The microbiome is a term that describes the collective genome of microbial communities, the so-called microbiota, which is associated with humans, animals, and plants. During recent years, the impact of microbial communities on shaping the host immune system and fitness of their host has gained attention. The composition of microbiota residing in a host is affected by environmental conditions such as temperature, pH, and nutrient availability. The overuse of xenobiotics in agriculture, along with the emergence of antibiotic and pesticide-resistance strains in agriculture and human medicine, can affect the host capacity to interact properly with the microbiota.

The Plant Microbiome – Plants, including cannabis, host distinct beneficial microbial communities on and inside their tissues, designated the plant microbiota from the moment that they are planted into the soil as seed. The plant microbiome is composed of specific microbial communities associated with the roots and the soil surrounding the roots (i.e., the
rhizosphere), the air-plant interface (i.e., the phyllosphere), and the internal tissues of the plant, the so-called endosphere. Seeds harbour diverse groups of microbiota that are a source of bio-inoculum for juvenile plants promoting protection against biotic and abiotic stress at seed germination and later stages.

Each of these microhabitats provides suitable conditions for microbial life, which also has a respective function for the host. Plant microbiome is a contributing factor to plant health and productivity. An increasing body of evidence highlights the importance of plant microbiome as a systemic booster of the plant immune system by priming accelerated activation of the defence system. Many studies focused on the rhizosphere microbiome due to the soil-derived microbial diversity surrounding the root, and a potential source for selecting beneficial microbesthat positively affect plant health.

Several reviews addressed the role of the rhizosphere microbiome in conferring disease suppressiveness and improving drought resistance; others studied contributing chemical components to selective enrichment of microorganisms in the rhizosphere. Generally, above-ground plant microbiota mostly originated from the soil, seed, and air adapt an endophytic lifestyle inhabiting tissues of the plant internally and play vital roles in plant development and fitness. These microbial communities that internally inhabit plant tissues, are referred to as endophytes, and play a crucial role in plant development and growth.

Root colonization – Colonization of roots by rhizobacteria are beneficial to both the bacterium and the host
plant.

Approximately 30% of the fixed carbon produced by plants is secreted through root exudates. Colonization of the roots by bacteria provides a nutrient source and in exchange, plants are the recipient of bacterial compounds and activities that stimulate plant growth and provide stress protection to their hosts. Rhizobacteria form a thin bio-film on the roots for long-term colonization of the rhizosphere. Chemotaxis is required for rhizobacteria to locate and colonize young roots. The chemotaxis machinery encoded in the bacterial genome is specific to individual species and is not associated with genome size.

The bacterial genome possesses several chemoreceptor genes along with genes that regulate cell differentiation and their mutual relationship with living organisms. The primary function of a bacterial chemoreceptor is to help in the establishment of strong beneficial interrelationship between the plant and the bacterium. The chemoreceptors enable it to find a specific environment, namely plant roots.

Rhizobacteria are an important component of the plant rhizosphere. Colonization of plant roots by rhizobacteria requires 24 hours to form a biofilm that is induced by the presence of plant molecules, such as cell wall polysaccharides and malic acid. Biofilms consist of a multicellular bacterial community covered in a self-secreted matrix.

The timing of the formation of a rhizobacteria biofilm on host roots is also dependent on the promoter of the genes responsible for the production of the matrix when the bacteria initially contacts a root.