Bacteria from many genera, including Alcaligenes, Agrobacterium, Azospirillum, Azotobacter, Arthrobacter, Bacillus, Bradyrhizobium, Burkholderia, Caulobacter, Chromobacterium, Enterobacter, Erwinia, Flavobacterium, Herbaspirillum, Klebsiella, Mesorhizobium, Micrococcus, Pseudomonas, Rhizobium, Rhodococcus, and Serratia, can enhance plant growth . PGPRs can be divided intosymbiotic bacteria and free-living rhizobacteria based on their interaction with plants. Symbiotic bacteria live within plant tissues and exchange metabolites with the host directly. However, free-living rhizobacteria live outside of plant tissues and promote plant growth . Direct mechanisms include biofertilization fxation, production of plant hormones and siderophores , phosphorus solubilization, etc., root growth stimulation, rhizoremediation, and the control of plant stress . Additionally, PGPRs produce metabolites, such as indole-3-acetic acid , 1-aminocyclopropane-1-carboxylate-deaminase , phosphate solubilizing enzyme , and SDs . They can also indirectly enhance plant growth by reducing the detrimental effects of phytopathogens through induced systemic resistance and the production of antimicrobial compounds, such as bacteriocin, zwittermicin, fengycin, chitinase, and cell wall-degrading enzymes . Although several studies have indicated that PGPRs can be successfully used in soilless agriculture , black plastic pots for plants it remains unknown whether they will adapt to a different environment from their natural habitat when used in this manner .
In this review, we focus on the future of PGPRs in soilless agriculture, placing their use into perspective with other views to discuss and evaluate the recent advances in the biotechnological applications of these rhizobacteria.Soilless agriculture is a method based on the cultivation of plants in substrates other than the soil and dates back to ancient times. The cultivation of plants in pots placed on the soil surface has been attempted at various times throughout the ages. For instance, Egyptians utilized this method of plant cultivation 4000 years ago, and the murals found in the temple of Deir el-Bahari are known as the first documented evidence of plants being grown in pots . The use of smart applications in agriculture has recently become widespread. Many technological tools have also been implemented, such as satellite controls, global system for mobile communication operators, sensors, mini weather stations, drones for aerial monitoring, and unmanned aerial vehicles. The techniques involving these applications and tools are termed “innovation techniques”. Thanks to these techniques, agricultural productivity is predicted to increase. Additionally, the adoption of these technologies by farmers is expected to contribute to improving their quality of life and reducing the labor force . The use of technological applications in agriculture has allowed the practical creation of controlled cultivation environments in controlled-environment agriculture . Many factors, such as light, humidity, soil, ventilation, irrigation, and fertilization, can be controlled with automation in greenhouses where CEA is performed, which provides the maximum benefit to plants . Greenhouses are structures built with the aim to optimize agricultural production per unit area, and this benefit can be maximized with soilless agricultural systems.
Soilless agriculture is a type of new-generation growing system that is based on providing a requisite amount of water and nutrients for plant life . In soilless agriculture, hydroponics involves the direct provision of nutrient solutions in a liquid environment, whereas solid culture involves the growth of plant roots in peat, perlite, vermiculite, coco peat, rockwool, sand, sawdust, or pumice enriched with nutrient solutions. The hydroponics system provides the opportunity to produce throughout the year without being dependent on the soil. It has been determined that the substrate used in soilless agriculture affects plant growth, fruit quality, and yield . Nihad et al. investigated yield and quality parameters in strawberries using five different substrates and concluded that the coco peat, used as a potentially eco-friendly substrate in soilless strawberry culture, had a more positive effect on leaf physiology, fruit yield, and quality than the tuf. It has also been highlighted that berry fruits can be cultivated in soilless culture. This is especially true for strawberries and blueberries, which need environments where the nutrient contents can be controlled, and hydroponic systems are thus suitable for their cultivation . Moreover, another study showed that the addition of shredded corn stalk to a growing medium containing perlite and pumice can increase the fruit quality and yield of tomato . However, tomato plants cultivated in coconut substrate showed higher vegetative growth than those cultivated in perlite medium . Kılıc et al. reported that a medium containing coconut shells was the most suitable in terms of fruit quality of tomato, while perlite was more advantageous in terms of productivity.
Hydroponic systems involve the direct delivery of the nutrient solution to the plant root zone, and thus water and fertilizer can be used more efficiently than in other systems . Crops grown in hydroponic systems had 20–25% higher yields than those grown under conventional agriculture. Furthermore, due to the controlled environmental conditions, the impact of climatic changes can be balanced with the help of these systems, resulting in a lack of negative effects on annual crop production . Additionally, problems caused by abiotic stress can be more easily overcome due to the abundant oxygen available in the system . Although it has many advantages, such as the elimination of pesticides, reduced exposure to stress factors and high productivity, hydroponic systems have some disadvantages, such as high expenses, a lack of information and a shortage of qualified personnel . Consequently, scientists should engage in training programs to provide more information about the potential of soilless agriculture for farmers. Despite some disadvantages, advances in soilless agriculture will likely continue in the future because of environmental pollution, growing populations and decreases in soil fertility.Soil is not an essential factor for the growth and development of plants. However, it provides all the macro- and micronutrients necessary for plant life. Traditional agriculture has certain disadvantages, such as the waste of irrigation water, the requirement of a large land area, and the use of large amounts of chemical fertilizers. Thus, soilless agriculture has garnered attention because of its elimination of these disadvantages. Thanks to the presence of a closed-loop system, soilless agriculture involves a fixed recycled water supply, retains 85–90% of the applied irrigation water, and provides better efficiency than traditional production . Europe has been at the forefront of the application of advanced techniques in soilless horticulture, and countries, such as France, Spain, and the Netherlands, have large areas over which greenhouse cultivation is implemented. Furthermore, advances in smart technologies have supported the development of soilless agriculture in Europe. National government statistics indicate that the adoption of soilless agriculture has been the highest in European countries. Hence, the market is highly improved in this region . Nutrients, pH, oxygen, carbon dioxide, light and temperature can be easily adjusted and controlled in soilless agriculture, thus positively affecting the yield of plants, and deleterious elements present above certain dosages could be limited within safe dosages. In soilless agriculture, the surrounding environmental and root temperature and supply to roots can be controlled by oxygen. It is well known that water resources are decreasing worldwide. People should thus be very careful when using water, and soilless agriculture is important in protecting our water resources. Vegetables produced with soilless agriculture can be of high quality and need little washing. In soilless agriculture, irrigation water is accurately controlled in extremely lower amounts than in traditional agriculture . In addition to the routine use of soilless agriculture in crop production, it is used in basic abiotic stress-based and plant genetic studies. In addition, it plays an important role in the development of new plant varieties suitable for the ever-changing climate characteristics resulting from global climate change. The agricultural and physiological effects of salt stress on tomato, drainage pot which is one of the most significant horticultural crops in the world, have been studied using different soilless agriculture practices . These studies have shown that efficiency decreases in the following order: “deep flow technique > perlite > nutrient film technique” . Chemutai et al. investigated the effects of nitrogen, phosphorus, and potassium and plant tea manure with selected soilless growth media , sawdust+coffee husks , charcoal dust+coffee husks , and charcoal dust + saw dust + coffee husks on Amaranthus cruentus to identify alternative growth media for its production. Based on the results, the mixture of charcoal dust and dry coffee husks along with the application of either NPK or plant tea manure was identified as the best alternative growth media. The LECA + coco peat mixture was reported to have the lightest weight in terms of field capacity, sufficient ventilation, and the best water-holding capacity. Moreover, it has been determined that LECA +organic matter substrate is the best mixture candidate in terms of physical properties .An emerging method in agriculture is the use of soilless culture, which refers to any method of growing plants without soil as a rooting medium. This technique provides significant advantages over traditional methods by decoupling plant growth from soil-associated problems, such as soil-borne pests and diseases, decreased arability, salinity, and low soil quality .
A plant biostimulator is defined by current European Union legislation on fertilizers as “any substance or microorganism, in the form in which it is supplied to the user, applied to plants, seeds or the root environment with the intention to stimulate natural processes of plants benefiting nutrient use efficiency and/or tolerance to abiotic stress, regardless of its nutrient content, or any combination of such substances and/or microorganisms intended for this use” . To this end, we focus on microbial plant growth-promoting inoculants, which have been studied extensively in recent decades. These PGPR-based biostimulants are the main components of the biofertilizers used in agriculture . PGPRs release extracellular enzymes for the degradation of the cell wall of fungi that can also result in the suppression of phytopathogenic fungi. They can be excellent candidates for providing long-term induced resistance in plants . Furthermore, heavy metal-resistant PGPRs have been identified that have proven their potential to promote plant growth under heavy metal stress possessing . The resistance mechanisms include some processes, such as intracellular bioaccumulation/biosorption, extracellular complexation with polysaccharides/siderophores and enzymatic metal transformation . These bacteria can assist heavy metal hyperaccumulator plants by accelerating their uptake, on the other hand, reduce the heavy metal uptake in non-hyper-accumulator and can minimize heavy metal accumulation in edible parts of the plants . Recently, Awan et al. reported that two wheat varieties under heavy metal stress accumulated more heavy metal in the roots and shoots, resulting in severe oxidative stress, evident by an increase in malondialdehyde content. Additionally, they also observed that these varieties under stress altered antioxidant enzymes, such as catalase, ascorbate peroxidase and superoxide dismutase. However, the inoculation of two wheat varieties with Bacillus siamensis enhanced plant growth, reduced oxidative stress, and improved the activities of antioxidant enzymes in both varieties. As a result, B. siamensis reduced the metal toxicity in wheat varieties through the augmentation of the antioxidant defense system. Similarly, Ullah et al. found that endophytic Serratia sp.UI01 and Enterobacter aerogenes UI02 strains isolated from Solanum nigrum improved plant growth and reduced oxidative stress in Brassica juncea exposed to heavy metal stress. They also observed that antioxidant enzymes and metabolites against reactive oxygen species including peroxidase, catalase, alcohol dehydrogenase, polyphenol oxidase, superoxide dismutase, reduced glutathione, malondialdehyde and favonoid were significantly relieved by inoculation of IU01 and IU02 strains in the plant. The key characteristics required for microbes to be considered PGPRs include the production of phytohormones, such as cytokinins, auxins, gibberellins, ethylene, and abscisic acid . These phytohormones facilitate plant cell enlargement and division and the extension of roots and influence the hormonal balance of plants. In addition to the production of phytohormones, free nitrogen fxation, phosphate solubilization, and SD production are important characteristics of PGPRs . Nitrogen serves as a critical component in the synthesis of proteins, cellular enzymes, RNA, DNA, and chlorophyll, which in turn enable plant growth . Additionally, phosphorus is the second-most crucial element after nitrogen, but its uptake by plants is limited because it exists in an insoluble form; rhizospheric bacteria are able to solubilize phosphate, thereby making it available for plant uptake . Similarly, bacteria present in the rhizosphere also release organic compounds for the chelation of Fe3+ . The use of PGPRs in soilless agriculture has been studied and examined by various research groups using different crops. Van Peer et al. grew cucumber, lettuce, and tomato plants hydroponically and used Pseudomonas sp. strain WCS417r as the PGP bacteria.