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.