Soil inoculation

The method of soil inoculation is the direct transmission of PBM to the soil via drenching, soil incorporation, and microcapsules (Romeiro 2007). Inoculation ofBrachiaria brizantha seeds by Burkholderia pyrrocinia andPseudomonas fluorescent was not successful, in contrast, the soil inoculation with drenching improved plant growth and seedlings emergence (Lopes et al. 2021). Soil inoculation with Pseudomonas sp. resulted in better nodulation and growth than seed inoculation of Cicer arietinum(Bhattacharjya and Chandra 2013). Recently, it has been found that soil inoculation with PGPB improved the growth, the productivity of nutrient and water uptaking by roots ofRanunculus asiaticus (Domenico 2020). It has been shown that direct soil inoculation with PGPB and AMF boosts growth, yield, and nutrient uptake (Saia et al. 2015). Soil inoculation with Pseudomonas aeruginosa , Corynebacterium agropyri , andEnterobacter gergoviae was more significant on the disease suppression of aerobic rice compared to Bacillus amyloliquefaciens , Trichoderma harzianum and Trichoderma virens (Ng et al. 2016). A study suggested that nutrient availability increased after soil inoculation ofProvidencia rettgeri, Acinetobacter calcoaceticus andSerratia plymuthica (Li et al. 2020). Soil inoculation using Bacillus subtilis has been reported to decrease the toxicity of chromium in Triticum aestivum (Seleiman et al. 2020).

Root inoculation

In this method, the roots immerse in a microbial solution (Romeiro 2007). After microbial inoculation, the seedlings are grown at a proper substratum for their development. In this way, this method provides plant size standardization and also causes the direct relationship between roots and inoculants to improve root colonization (Ahemad and Kibret 2014). The inoculation of Burkholderia phytofirmans with Vitis viniferaroots plant’s low-temperature tolerance, altered carbohydrate metabolism, and improved plant growth and yield (Fernandez et al. 2012). Root inoculation of Oryza sativa with Rhizobia was more efficient in improving plant length compared with seed inoculation (Ullah et al. 2017). One study found that root inoculation with Pseudomonas fluorescens caused an increase in induced systemic resistance in leaves of Arabidopsis thaliana (Löser et al. 2021). The inoculation of Pseudomonas putida with roots of Z. mayscaused the reduction of leaf necrosis (Planchamp et al. 2015).

Seed inoculation

To decrease the use of chemical seed treatment, the method of seed inoculation with PBM is a better alternative. In this method, seeds immerse in the microbial solution of known concentration. During the germination process, the seed releases carbohydrates and amino acids in the exudates. In turn, microorganisms use the released seed exudates as the nutritional source in soils and then colonize plant roots (Ammor et al. 2008). It has been reported that the inoculation of Burkholderia phytofirmans withRyegrass seeds enhanced plant growth, hydrocarbon degradation, and phytoremediation (Afzal et al. 2013). Association of PBM with plant roots caused the modulated phytohormones levels. Compared with seedling inoculation, seed inoculation with PGPB and AMF has been more effective, stimulating the growth and wood production of Schizolobium parahyba var. amazonicum(Cely et al. 2016). While the growing root tips have not been activated, inoculum stays dormant in the soil (Lopes et al. 2018). In a study, inoculation of wheat seeds of Streptomyces , Aspergillus, Bacillus with seeds of T. aestivum caused the increased grain yield (Barnett et al. 2019). Under cold stress, the inoculation of Glycine max seeds with Bacillus megaterium, Trichoderma longibrachiatum and Trichoderma simmonsii was more efficient in increasing germination indices and seedling growth (Bakhshandeh et al. 2020).

Mechanisms PBM to survive in diverse conditions

Microorganisms can induce several mechanisms to cope with stressful conditions and improve the growth of host plants. Some microbes survive under low and high temperatures, drought, salinity, acid and alkaline conditions (Lopes et al. 2021) through modification of cell walls, metabolic responses, and gene expression (Sharma et al. 2012). Some microorganisms (e.g., Bacillus sp., Azospirillum sp., andPseudomonas sp.) can secrete volatile organic compounds (VOC) (such as alkyl sulfides, indole, and terpenes). The signal interactions between plants and microbes can be achieved through the distribution of VOC in soil pores (Hashem et al. 2019). Microbes can accumulate amino acids and avoid dehydration and death against low soil moisture (Venturi and Keel 2016). AMF increased soil organic carbon and changed the microbial population in the rhizosphere, thus causing the modification of the rhizosphere (Zhang et al. 2019) The pigments produced by Bacillus and Serratia can clear radiation and stop DNA damage against high light (Zion et al. 2006). Microorganisms such as Azospirillum sp., Pseudomonas sp., andBacillus sp. significantly influenced soil micronutrient accessibility through reduction of solubilization, chelation and oxidation, and altered the pH of their surrounding soils (Souza et al. 2015).

Influence of abiotic factors on PBM

The abiotic factors can induce stress in the metabolism of plants and modified the compositions of root exudates. This can affect the microbiome in the rhizosphere and the interactions between plants and microbes. In this way, the benefits of PBM can be declined by abiotic factors (Fig. 2).

Soil

Soil pH is an important factor in influencing the solubility of various metallic ions and the accessibility of nutrients, as well as the physical properties of the soil. One of the problems with agricultural productivity in the world is high or low pH. Soil salinity can limit plant growth and thus crop productivity. Hence, these conditions reduce the nutrient deficiency and yield and cause ion toxification, osmotic and oxidative stress (Dutta and Bora 2019). Salinity stress influences crop production by declining the levels of mineral availability and growth regulators, and persuading ions interceded toxicity, osmotic stress, and ROS production, which conclusively causes the blockage of seed germination, seedling growth, the onset of flowering and fruit (Salwan et al. 2019). The conditions of soil nutrition also influence PBM efficiency. It has been evaluated that inoculation of PBM was more effective on growth in nutrient-poor conditions (Strigul and Kravchenko 2006). Inoculations of Pseudomonas sp., Bacillus sp., andMycobacterium sp. caused enhanced plant growth in soils with a nutrient deficit (Mathimaran et al. 2021). In addition, heavy metal contamination in soils can inhibit the beneficial effects of inoculants on plant growth and agricultural productivity (Mathimaran et al. 2021). However, Pseudomonas aeruginosa , Alcaligenes feacalis , andBacillus subtilis can serve as an effective remedial approach to increase plant tolerance against heavy metals (Aka and Babalola 2016). Another research revealed that Klebsiella variicola and Azospirillum sp. caused the improved growth and tolerance of Glycin max(Kim et al. 2017) and Z. mays(Czarnes et al. 2020) under flooding stress.