Fig. 3. The three major types of seed coating: film coating (a), entrusting (b) and pelleting (c).

Pelleting

In the seed pelleting process, seeds are coated with inert materials (such as vermiculite, kaolin, calcium peroxide, perlite, talc, and diatomaceous earth) so that the initial size or shape of seeds is not clear (Fig. 3c). Ultimately, seed pelleting cab changes thin seeds into larger and spherical-shaped ones, which could help cultivate very small with low vigor seeds (Afzal et al. 2020, Pedrini et al. 2017).
According to these characteristics of the natural coating of seeds, mainly different agents are used for seed coating, such as protectants, micronutrients (Williams et al. 2016), microorganisms (bacteria and fungi) (Rocha et al. 2019b). The uptake and translocation of agent compounds into seeds can be performed through imbibing seeds in water or emerging radicle and root systems. Overall, the applications of agents are effective in improving seed germination rate, establishment, and increasing the yield of crops (Ma 2019, Pedrini et al. 2017).

Formulation process of seed coating

Three basic components, namely the elected microorganism, an appropriate carrier (solid or liquid), and various additives can be applied to create an efficient formulation of PBM (Rocha et al. 2019a). Various factors such as incorrect formulation of the inoculant and limited shelf life may preclude the application of seed coating (Ahmed and Kumar 2020). The formulation acts a significant role in the inoculation process as it can determine the bioagent potential (Jambhulkar et al. 2016). Nowadays, formulation development by industries is essential to commercialize biocontrol technologies. The expansion of optimal formulations with appropriate carriers for the utilization of microbial inoculants contributes significantly to the control and management of pathogens and seed-borne diseases in crops (Aeron et al. 2011). There are several types of formulations such as wettable powder, liquid, and granular used in soils or spray applications (Knowles 2006).

Shelf life of the microbial coating

An essential commercial issue for seed coating is microbial survival (Bashan et al. 2014). Several factors such as coating type, inoculants (e.g., strain, type, purity, sterile or not, moisture status, and age), coating carrier (e.g., silica, carboxymethyl cellulose, and biochar), drying process (e.g., polymer, final moisture status, time, and temperature), storage condition (e.g., temperature, humidity, water status, polymer, and contaminants) can affect the survival of microorganisms (Ma 2019). It has been reported that the changes in physiology and morphology of cells during inoculants can influence the survival of microorganisms physiological and morphological changes of cells during inoculants can influence the survival of microorganisms (Gemell et al. 2005). In structural biopolymers, water activity and its solvent properties can inhibit the survival of microorganisms during desiccation (Mnasri et al. 2017). Also, at different relative humidity, polymers influence water available to microorganisms by moisture sorption (Deaker et al. 2007). One of the most important factors influencing rhizobia survival on seeds is desiccation (Deaker et al. 2012). During inoculation and inoculated seed storage, the expansion and rate of desiccation depend on the ambient relative humidity. For instance, some studies have reported that relative humidity and water activity are effective in the survival of rhizobia (Mugnier and Jung 1985). Many evidence demonstrated that the survival of microbes is improved when the difference in water status between intracellular and extracellular is reduced (McInnes and Date 2005). Low relative humidity storage of the environment may increase the survival of freeze-dried cells or decrease the survival of completely hydrated survival cells (Kosanke et al. 1992, Ma 2019). For the survival of cells, rehydration is important to improve the cell viability of microbes by decreasing water influx via cell membranes (Deaker et al. 2012, Ma 2019). The polymeric adhesives or coating materials include pigments, nutrients, and protection agents of seeds that can be applied to enhance the survival of microbes on seeds (Deaker et al. 2012, Deaker et al. 2007). Furthermore, polymers can increase the ability to protect cells of microbes against different environmental stresses (Ma 2019). It has been indicated that drying seeds of Trifolium subterraneum , Trifolium repens , and Medicago sativa after coating can enhance microbial survival for a long time (Deaker et al. 2012).

Delivery methods

Innovative seed coating technology can provide the delivery of many kinds of materials that are effective in the enhancement of seedling establishment and plant growth (Jambhulkar et al. 2016). Some studies have shown that several bacteria, includingPseudomonads fluorescent , Pantoea sp.,Bacillus cereu, and the fungus Trichoderma harzianumplayed important role in controlling a range of soil-borne diseases (Moussa et al. 2013). Seed bio-priming is a proceeding of coating seeds with fungal or bacterial agents in which biological and physiological treatments are used to control the disease (El-Mougy and Abdel-Kader 2008). Coating rice seeds with two biological agents Pseudomonas andBacillus could protect rice against Xanthomonas oryzae and increase seed quality and germination (Palupi et al. 2017). UsingPseudomonas fluresences SP700s bacteria as the coating factor not only increased emergence percentage and yield of rice but also reduced dirty panicle disease incidence and severity (Prathuangwong et al. 2013). TheTrichoderma atroviride inoculated corn seeds had the highest percentage of germination (Gravel et al. 2007). The pathogens of seed-born and soil-born can form a host-parasite relationship through the root. In this regard, PBM can protect the rhizosphere zone against soil-borne diseases. It was demonstrated that the inoculation Trichoderma harzianum in soil was more effective in controlling Armillaria root rot inCamellia sinensis (Mutai 2015). The inoculation of the combinations of Pseudomonas fluresencesand Bacillus subtilis could prevent the growth of pathogens on the wheat roots (Moussa et al. 2013). A study investigated the influence of Bacillus subtilis andPseudomonas fluorescent on the germination indices and seedling growth of Cuminum cyminum under salinity conditions. Results from this study demonstrated that bacterial inoculation improved the germination and seedling characteristics in both optimal conditions and salinity stress. (Piri et al. 2020). However, the co-inoculation of Bacillus subtilis andPseudomonas fluorescent caused a decreasing in plant growth and yield. According to some results of research (Ma et al. 2019), inoculation of singleRhizophagus irregularis or dual Pseudomonas libanensis +Rhizophagus irregularis under greenhouse did not affect cowpea seed yield, however, application of P. libanensis increased plant growth performance. Similarly, co-inoculation of Trichoderma sp.,B. bassiana , Metarhizium anisopliae , and AM fungi had no effect on seed germination of Lactuca sativa(Diniz et al. 2006). A reserch showed that coating of Triticum turgidum seeds with P. fluorescens was more effective on growth parameters than B. subtilis and F. graminearum(Moussa et al. 2013). A study on the evaluation of biological control of wheat root in field conditions reported that using P. fluorescents was the most effective treatment compared to other treatments (Hue et al. 2009).

Application of microbial seed coating in the agricultural system

For billions of years, it has been proved that microorganisms had an intense influence on the whole planet (Akinola and Babalola 2020). Nowadays, the enormous diversity of microbes and their ability on the earth have been known (Pretscher et al. 2018). For instance, bacteria and fungi can manage agricultural sustainability in the world (Akinola and Babalola 2020). It has been confirmed that for developing a sustainable strategy, the application of microbial seed coating in crop production systems can increase crop production, improve resource use efficiency, and protect plants against phytopathogens (Colla et al. 2015).

Enhancement of plant growth and yield

Standardization of size, weight, shape, and uniformity of seeds in seed coating can enhance plantability in the field and crop growth and yield (Ma 2019). While morphological characteristics of the seeds are improved by seed coating, however, seed coating may be an obstacle to germination and emergency (Moussa et al. 2013, Piri et al. 2020). A study has pointed out that delayed germination of D. carota(Conceição and Vieira 2008), and Z. mays (Nascimento et al. 2009) caused by seed coating is due to coating combinations on imbibition of water and available oxygen. To increase the longevity of coated seeds and microbial functionality in situ , the application of an effective formulation plays a role in the expansion of commercial coated seeds (Ma 2019, Rocha et al. 2019a). Application of polymeric adhesives (such as polyvinylpyrrolidone, xanthan gum, methylcellulose) could maintain water activity levels optimal in coating formulations to improve the viability of inoculants (Deaker et al. 2007). To achieve food security and sustainable agriculture, seed quality such as germination, vigor, and mister content is important. Therefore, the microbial seed coating is the seed’s primary defense from unfavorable environmental conditions and pathogens, thus improving seed viability and vigor (Palupi et al. 2017). The impact of PBM on plant growth has been reported for numerous crops grown in greenhouse and field environments (Gravel et al. 2007).
Using PBM in seed coating can enhance the percentage of germination, seedling indices, and subsequent plant growth in both optimum and stress conditions (Ma 2019, Rocha et al. 2019a). It has been reported that the yield and macro and microelements, antioxidant activity, total phenolic, caffeoylquinic acids, and flavonoids increased in propagated Cynara cardunculus seeds coated byRhizophagus intraradices , Funneliformis mosseae, andTrichoderma atrovirid (Rouphael and Colla 2020). It has been determined that the use ofPseudomonas fluorescence bacteria and Trichoderma harzianum in the coating of C. cyminum seeds improved seedling emergence rate and seedling growth, antioxidant activity under drought stress in greenhouse conditions (Piri et al. 2019). Similarly, it has been shown that seed coating of T. turgidum with Rhizophagus irregularis BEG140 using silicon dioxide resulted in an enhancement in shoot dry weight, seed weight and nutrition (K and Zn) contents under low fertilization. Some entomopathogenic fungi associated with plant roots can protect the host plants against disease and insect pests (Oliveira et al. 2016). For instance, the seed coated with entomopathogenic fungi such as Metarhiziumand Beauveria protected Z. mays against Costelytra giveni and Fusarium graminearum and improved germination and growth (Rivas-Franco et al. 2019). Seed coating through the formulation of T. harzianum, T. viride andT. atroviride enhanced plant growth and germination of Z. mays var. saccharata, T. aestivum , and Beta vulgaris ) (Rezaloo et al. 2020).

Alleviation of abiotic stress

The use of PBM as biocontrol agents is an attractive management strategy for both the conventional and organic farming industry that can meliorate plant growth and performance under optimal and stressful conditions and also defend plants across a diversity of soil and seed pathogens (Lazarovits and Subbarao 2010). Besides, several factors have an effective role in the success of microbial seed coating for biocontrol purposes including cultivation practices, dosage, timing, and method of PBM application (Singh et al. 2016). Environmental stresses such as biotic stresses (e.g., drought, salinity, extreme temperatures, and nutrient deficiency, etc.) and abiotic stress (living organisms such as bacteria, viruses, parasitic nematodes, insects, weeds, and other indigenous) are environmental factors that may limit worldwide crop production (Ma 2019, Pedrini et al. 2017). It has been suggested that several PBM (bacterial and fungal strains) such asPaenibacillus alvei and Bacillus amiloliquefaciens in potato, Pseudomonas sp. in potato and strawberry, andTalaromyces flavus in tomato were successfully protected plants against Verticillium dahliae(Lopisso et al. 2017). The bacteria and fungi application had affirmative agents on plant growth against drought stress and facilitated plant growth and development by supplying mineral nutrients and phytohormones (Aalipour et al. 2020). In a greenhouse, research was found that seed coating of the combination of microbial strains, polymers with several doses of trace and macro-micro-nutrients with Z. mays , G. max ,Brassica napus , T. turgidum , Hordeum vulgare , andLens culinaris under water-stressed conditions helped to fix plant cell membranes and decreased the damages from drying cycles, and eventually enhanced crop productivity under water stress (Islam and Vujanovic 2017). Seed coating of Vigna unguiculata with Bacillus sp. could improve the growth and production, and nutrients of crops and decreased usage of the chemical fertilizers in arid agriculture (Nain et al. 2012). The use of a combination of genus Pseudomonas , Azotobacter ,Azospirillium and Rhizobium as biofertilizers in coating materials of cotton seed enhanced the growth, relative water content, and contents of chlorophyll and ionic (K+/Na+) under both salinity and normal conditions, but decreased shoot growth and leaf gas exchange under salinity stress (Amjad et al. 2015). In an experiment performed under salinity stress conditions, coating maize seeds with Bacillus and Pseudomonas , andPseudomonas produced more IAA and ACC deaminase, different hydrolytic enzymes, and antifungal activity against two fungal pathogens compared to non-salinity stress (Mukhtar et al. 2020). Co-inoculants of AMF and PGPB onto seeds of soybean in the laboratory and under greenhouse conditions improved the germination, seedling growth, and potassium uptake under drought and salinity stress (Bakhshandeh et al. 2020). In the greenhouse experiment, the growth and photosynthetic state of T. turgidum were promoted by seed coating with PGPB Paraburkholderia phytofrmans under water-nutrient stress (Ben-Jabeur et al. 2021).

Biological control

Microbial inoculation to soils in the plant ecosystem can help decrease disease damage (Heydari and Pessarakli 2010). The biological potential of Bacillus thuringiensis ,Rhizobium meliloti , Aspergillus niger , andTrichoderma harzianum has been evaluated through seed coating with gum arabic, glucose, sugar, and molasses in the suppression of root rot fungi (e.g., Rhizoctonia solani and Fusarium sp.) onHelianthus annuus and Abelmoschus esculentus . For instance, seed dressing of microbial antagonists e.g., B. thuringiensis , R. meliloti and T. harzianumimproved the microbial efficiency in the control of root rot fungi on crop plants (Meena et al. 2015). Also, it has been reported that the growth parameters such as shoot and root length, shoot and root weight considerably boosted in A.esculentus and H. annuus plants when seeds were treated with microorganisms, whereas no considerable varieties were perceived in the germination of seed treated by sugar, molasses, glucose, and gum Arabic (Dawar et al. 2008). The research was carried out to appraise the impact of seed coating with biological agents on the seed quality of rice. In this study, isolates ofPseudomonas and Bacillus subtilis were tested againstXanthomonas oryzae pv. Oryzae. Results showed that treatments of biological control boosted seed vigor, and reduced infection of Xanthomonas oryzae pv. Oryzae in the seed (Palupi and Riyanto 2020). To reduce aflatoxin contamination in corn kernels, the biocontrol techniques were performed via film coating. The findings demonstrated that seeds coated with conventional pesticides such as insecticide (e.g., imidacloprid), fungicide (e.g., metalaxyl-M), and spores of non-aflatoxigenicAspergillus flavus NRRL 30797 reduced aflatoxin contamination of kernels (Accinelli et al. 2018). Lately, it has been found that seed coating and soil drenching with three biocontrol bacterial strains (e.g., strains (e.g., Providencia vermicola and Pseudomonas fluorescens ) boosted cucumber yield and decreased nematode infestation (Panpatte et al. 2021). A biological investigation demonstrated that coating the seeds with the formulation of hydrogel, Trichoderma harzianum , and Burkholderia gladioli could protect Phaseolus vulgaris against common phytopathogens and improve seed germination (Elshafie et al. 2020). Coating seeds ofTriticum durum with sixty-two rhizosphere and endophytic bacterial strains caused the blockage of growth and germinationFusarium culmorum (Mnasri et al. 2017). It has been proved that biological agents used in rice seed coating could improve the seed quality, seedling growth and decrease the blast disease to 0% (Palupi and Riyanto 2020). Seed coating with entomopathogenic fungi Metarhizium sp., and Beauveria sp. protected seedlings of Z. mays against herbivorous insects by enhancing salicylic acid, and jasmonic acid contents (Rivas-Franco et al. 2019).

Ecological restoration by a beneficial microorganism

Restoration of ecology is a process that helps the recovery of degraded, damaged, or destroyed ecosystems. It is well known that PBM and their interactions with plants play an important part in the confirmation of ecological vegetation and sustaining physical structures in soils and nutrient cycling (Chen et al. 2020). Seed coating with PBM can reduce challenges regarding soil moisture variables, low soil nutrients, pathogens in the environment (Gornish et al. 2019). For instance, the inoculation of Aspergillus sp. and Streptomyces sp. via seed coating improved the emergence of seedlings and survival ofLolium multiflorum and Astragalus sinicus on degraded rangeland in the Qinghai–Tibetan Plateau (Liu et al. 2010). The use of combinationP. libanensis and R. irregularis in seed coating of cowpea not only enhanced the production of crops but also improved soil fertility and seedlings’ tolerance against environmental stresses (Ma 2019). Indeed, the application of PBM can be a suitable tool for the sustainable production of crops and enhancement of yield and ecological restoration under different environmental conditions.