Use of Rhizosphere Microorganisms in Plant Production – A Review Study

Minimizing or neutralizing the effects of environmental stresses on crop plants, protecting against pests and diseases, and at the same time ensuring optimal plant growth and development are currently the most important tasks faced by growers and plant producers around the world. Nowadays, the goal is to limit the use of chemicals as much as possible to protect the environment and improve the quality of food. The interest in the use of beneficial rhizosphere microorganisms is becoming global, as it can represent an environmentally friendly alternative to chemicalization in the era of threats to crop cultivation in the modern world (climate change, drought, salinity, introduction of plant pests).


INTRODUCTION
The microbiome communities living in an environment affects the health of plants, people, and other living things. In plants, different microbiomes colonize in various niches, in phyllosphere, endosphere (in the tissues) and rhizosphere (Berendsen et al. 2012).
The rhizosphere is the root zone where the interactions occurring at the plant-microorganism-soil level are influenced by a number of chemical (pH, nutrient content, exudates), physical (temperature, water availability, soil structure), and biological (bacteria and fungi) factors (Mimmo et al. 2018).
Rhizosphere microbial communities and their interactions have been the subject of research for many years, aimed at determining their influence on plant development (Philippot et al. 2013, Berg et al. 2014. Many authors showed that microorganisms bring many benefits to cultivated plants, such as: nutrient uptake (Berendsen et al. 2012), protection against soil pathogens (Mendes et al. mechanism that protects a plant against infections (biotic stress) or helps it develop under abiotic stress is an indirect mechanism. In contrast, the direct mechanism affects the plant growth through the supply of nutrients or the production of plant growth regulators (Goswami et al. 2016).
The interaction with PGPF also proves to be extremely beneficial for the flora. Fungi of the genera such as Aspergillus, Fusarium, Penicillium, Piriformospora, Phoma and Trichoderma are the strains most used in research (Hossain et al. 2017, Javaid et al. 2019). Comparison of the results of various experiments shows that the interactions at the plant-PGPF level can have a positive effect on the aerial and underground plant organs. According to Akhtar and Javaid (2018), PGPF provide plants with protection against diseases by limiting the penetration by pathogens. Yadav et al. (2017) showed in their study that application of fungi to the soil increased nutrient availability to plants, thus increasing plant growth and crop yields.
Mycorrhizal symbiosis is the most common and widespread synergy between microorganisms and plants. As reported by Bonfante and Genre (2010), endophytic fungi (endomycorrhiza, arbuscular mycorrhiza -AM, Arbuscular Mycorrhizal Fungi -AMF) are a group of fungi of the Glomeromycota genera that form symbiotic relationships with over 90% of higher plant families. According to many authors, inoculation with AMF provides plants with tolerance to various environmental stresses such as salinity, water deficit, heavy metals in soil, and low or high temperatures.

The role of rhizosphere microorganisms in alleviating environmental stresses
Stress factors affect the growth and development of plants in agricultural and horticultural production. Light, water, and minerals are the factors regulating their growth, development and reproduction (Lata et al. 2018). However, when the access to them is disturbed, plants undergo physiological and morphological modifications to adapt to sudden changes (Shukla et al. 2012).
Abiotic stresses that affect the plant production efficiency include drought, salinity, hot and cold stress, as well as light stress. When listing the factors negatively affecting yielding, one cannot ignore the lack of nutrient availability in the soil, content of heavy metals, and the presence of plant pathogens (Lata and Gond 2019).
Plant growth under stress conditions can be enhanced by the use of stress-resistant rhizosphere microorganisms such as PGPR, PGPF and AMF (Nadeem et al. 2014). According to Spence and Bais (2015), these microorganisms enhance the plant development through, for example, regulation of the hormonal and nutritional balance, production of plant growth regulators, and induction of resistance to pathogens.

The role of rhizosphere microorganisms in alleviating the drought stress
The drought-induced stress is one of the most serious world problems, which reduces the crop production. Almost 30% of the Earth's soils are exposed to this stress (Calvo-Polanco et al. 2016). This stress has multidimensional influence on plants, from the phenological and morphological levels down to the molecular level (Anjum et al. 2011).
According to Lata and Prasad (2011) and Naveed et al. (2014), the water deficit causes many negative changes in plants such as decrease of chlorophyll concentration, disorders of photosynthetic apparatus, inhibition of photosynthesis and transpiration, increase in ethylene production and decrease in relative water content. The limited water content causes a decrease in the size of cells in tissues, disrupts membrane integrity, inhibits production of ROS in plants, and promotes leaf senescence (Tiwari et al. 2015, Kaur andAsthir 2016).
The rhizosphere microorganisms stimulate the growth of plants during drought stress by inducing various mechanisms such as production of plant growth regulators (IAA, cytokinins and ABA), production of bacterial exopolysaccharides (EPS), and synthesis of ACC deaminase (Farooq et al. 2009, Porcel et al. 2014. Plant Growth Promoting Rhizobacteria have the ability to produce phytohormones that stimulate cell division and plant growth under the water deficit conditions (Kumar and Verma 2018). According to Goswami et al. (2015), IAA regulates differentiation of vascular tissues, stimulates cell division, and root and shoot growth under stress. Abscisic acid (ABA) alleviates the stress caused by water deficit through transcription and regulation of xylem transport to the aerial parts of plants   Julkowska and Testerink 2015). Salinity is mainly caused by Na + , Ca 2+ , K + and also Cland NO 3-(Shrivastava and Kumar 2015). It reduces the microbiological activity of the soil, which is caused by ion toxicity and osmotic stress, which affect the reduction in growth of plant.
There have been many studies confirming that the inoculation with rhizosphere microorganisms alleviates the negative effects of salinity on various plants. PGPM can stimulate the growth of the plants that are exposed to salinity, by direct and indirect mechanisms. Rhizosphere bacteria reduce the effects of excessive soil salinity, also by producing the so-called biofilm (biological membrane) on the roots (Kasim et al. 2016).
Both PGPR and AMF help plants adapt to salinity, increasing the availability of nutrients, improving water uptake, increasing the efficiency of CO 2 assimilation, and the synthesis of osmoregulators and phytohormones (auxins, cytokinins,  Table 2. The role of rhizosphere microorganisms in alleviating temperature stress (heat stress, cold stress) The constantly changing climate contributes to increasing the risk of temperature stress, a significant threat to the crop productivity worldwide     apparatus and starch metabolism in plant cells.
The rhizosphere microorganisms induce the processes by which plants are able to inhibit or eliminate the effects of cold stress. These processes include: production of ACC deaminase to minimize the synthesis of ethylene caused by low temperature, increased the nitrogen fixation processes for the plants exposed to frost, synthesis of plant growth regulators (ABA, GA, IAA), activation of antioxidant enzymes, release of iron chelators (siderophores), and increasing the nutrients uptake (Kushwaha et al. 2020). According to Turan et al. (2013), the inoculation with PGPR such as Azospirillum brasilense, Bacillus megaterium, Bacillus subtilis and Raoultella terrigena minimized the adverse effects of low temperature on barley and wheat seedlings.     Table 6.

The role of rhizosphere microorganisms in pathogen elimination
According to Etesami Table 7. Table 5. Responses of plants exposed to heavy metal accumulation in the soil to inoculation with different rhizosphere microorganisms

CONCLUSIONS
The plant growth promoting microorganisms are important to the rhizosphere and can improve the growth and development of plants. PGPM can support the human activity in protecting plants from stress factors in agricultural and horticultural crops. Furthermore, they contribute to the availability of nutrients and protection against soil pathogens, and have an significant role in phytoremediation and soil fertility improvement. This issue is extremely important and requires further research on the possibilities of using microorganisms in global plant production in different ecosystems. The extension of the research should be based on a thorough analysis of the plant-microorganism-stress factor-soil interactions. Understanding the interrelationships between these factors is important for improving the rational application of PGPM in plant crops.