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.