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Biological soil fertility restoration techniques within the smallholder agroecosystems, in combination with other agronomic management practices, would provide the much-needed solutions for revitalizing the declining global food production (Raimi et al., 2017). Beneficial soil microbiota such as plant growth promoting microorganisms (PGPMs), comprising of specific groups of bacteria and fungi, provide essential agroecosystem services that support plant growth (Rouphael and Colla, 2020) and ameliorates soil productivity (Santos et al., 2019). PGPMs maintain key agroecological cycles fundamental for soil nutrient enrichment, crop nutrient improvement, plant tolerance to biotic and abiotic stresses, biocontrol of pests and diseases, and water uptake enhancement (Lobo et al., 2019; Goswami and Deka, 2020). They are actively involved in healthy plant development and growth through secretion of hormonal growth regulators, and resistance induction against phytopathogens (Dakora et al., 2015). Besides, versatile PGPMs could be used to bioremediate polluted fields and increase the land available for production as in the case of heavy metals polluted soils (Gouda et al., 2018). These agroecosystem services are primarily important in supporting crop production in smallholder agroecosystems, which are characteristically defined by limited resource inputs.

PGPMs promote plant growth and productivity through various direct and indirect approaches. Several direct mechanisms have been established through previous studies and can be broadly classified into phytostimulants (Babalola and Glick, 2012), biofertilizers (Kalayu, 2019), rhizomediators, or stress regulators (Stamenković et al., 2018). Indirect mechanisms mainly occur in form of biocontrol of phytopathogens through competition for nutrients, enzymatic lysis, antibiosis (Köhl et al., 2019), secretion of volatile organic compounds (VOCs) (Sun and Tang, 2013), and triggering of antioxidative defense mechanism (Sandhya et al., 2010; Malik et al., 2020) and induced systemic resistance (ISR) response in the host plant (Heil and Bostock, 2002). PGPM biofertilizers promote plant growth by enhancing nutrient availability to the plants and the most studied pathways include N fixation (Ahemad and Kibret, 2014; Fukami et al., 2018b), P and K solubilization (Sharma et al., 2013; Soumare et al., 2020), S oxidation, Fe and C sequestration (Kannahi and Senbagam, 2014; Velivelli et al., 2014). PGPMs enhance the availability of P, K, Zn, Se, and Fe in the soil through biochemical processes such as solubilization, chelation, mineralization, oxidation and reduction reactions (Ahmed and Holmström, 2014; Velivelli et al., 2014; Rouphael and Colla, 2020). PGPMs are also known to secrete phytohormones such as auxins (Lin and Xu, 2013; Azizoglu, 2019), cytokinin, abscisic acid, ethylene, brassinosteroids, jasmonic acid, salicylic acid, strigolactones, and gibberellins (Goswami and Deka, 2020; Saad et al., 2020) that act as plant growth stimulators and stress controllers.

The PGPMs functionality and vigor, however, depend on intrinsic soil properties, environmental and agronomic management factors. Nutrient availability, soil pH, water, temperature, crop genotype, and cultural management are some of the key drivers determining the survival and function of PGPMs in the soil (Gouda et al., 2018; Gupta et al., 2019). For maximum farm benefits to be realized in a highly heterogenous smallholder systems (Njeru et al., 2020), tradeoffs in balancing the highlighted determinants have to be considered and appropriate farm management practices are ought to be carried out. For instance, the choice of crop species and diversity is critical in stimulating specific plant-microbe interactions and consequently the intended output (Orrell and Bennett, 2013; Saad et al., 2020). The authors suggest the use of multiple cropping systems that promote synergy and minimize the yield gap between the potential and realized production, a phenomenon that is commonly seen in smallholder systems. Additional methods efficient in augmenting indigenous soil PGPMs can be integrated to enhance microbial bio-functionality. These include the use of organic amendments such as farmyard manure and vermicompost (Mosa et al., 2018; Koskey et al., 2020), biotechnological approaches such as plant breeding (Bakker et al., 2012), crop management practices such as agroforestry, rotation, intercropping, cover cropping, and practicing reduced soil disturbance (Ventorino et al., 2012; Hontoria et al., 2019; Elagib and Al-Saidi, 2020).

The increasing demand among smallholder farmers to cut input costs and the need for sustainable nutrient management practices is driving the growing adoption and use of microbial-rich fertilizers in smallholder setups (Raimi et al., 2017). Commercial microbial inoculants (commonly used as biofertilizers or bioenhancers) containing single species or multiple strains of rhizobia, Pseudomonas spp., Azotobacter spp., Bacillus spp., Trichoderma spp., Aspergillus spp., and Glomus spp. (Figure 1) have been largely used in smallholder agroecosystems for crop production (Koskey et al., 2017; Bargaz et al., 2018; Adeyemi et al., 2019). Previous field researches carried out in different agroecosystems around the world have reported varying levels of successes on the use of PGPMs to support crop performance quantitatively and qualitatively (Pellegrino et al., 2012; Mishra et al., 2019; Saad et al., 2020). However, little has been done focusing on the use of PGPMs to address various challenges facing smallholder agroecosystems in the context of changing climatic conditions. Therefore, this review explores different available strategies involving the use of beneficial microorganisms as biofertilizers, within the unique context of smallholder agroecosystems, to promote sustainable maintenance of plant and soil health, and enhance agroecosystem resilience against unpredictable climatic perturbations.

Eradicating hunger, poverty, and food insecurity while ensuring sustainable use of natural resources for agriculture, as highlighted in the United Nations Sustainable Development Goals (SDGs), is paramount in a world faced with a myriad of economic, social, political, and environmental challenges (Pérez-Escamilla, 2017). Globally, ~700 million people have no access to sufficient food while about 2 billion people face nutritional deficiencies, of which about 50% of the food insecure individuals are from Asia, 35% from Africa, and 10% from Latin America (FAO, 2018). This is likely to increase due to the current global health and economic crisis due to the Coronavirus disease 2019 (Covid-19) pandemic. Smallholder agroecosystems, predominantly found in developing countries, are considered critical food security resources that will support food production for the increasing human population in the coming years. Currently, it is estimated that smallholder agroecosystems account for more than 50% of the food produced globally (Herrero et al., 2017). In Africa, they contribute about 75% of the total crop production and 50% of the animal products (Nyambo et al., 2019) and, thus, are significantly involved in rural poverty reduction, economic development, and food security. However, compared to large-scale profit-driven systems, smallholder agroecosystems have limited land size, stringent financial resources, low market sharing, and product range, thus, are faced with more risks and vulnerabilities (Kuivanen et al., 2016; Herrero et al., 2017).

The productivity of smallholder agroecosystems largely depends on the services naturally provided by the ecosystem such as soil fertility, nutrient cycling, water availability, pest control, and pollination (Altieri et al., 2012). Farmers' decision and selection of their appropriate agronomic management practices affect the extent of agroecosystem functioning. External pressures such as poverty, unreliable climatic conditions, and farmer's need for immediate satisfaction exert pressure on land use and cause negative impacts on the ecosystem (IFAD and UNEP, 2013). Agricultural intensification coupled with the use of harmful agrochemical inputs has negatively impacted on smallholder agroecosystems (Bationo et al., 2012). Their long-term sustainability in the face of new challenges such as the shrinking per capita arable lands, emerging diseases, dwindling world economies, and unpredictable climate change, is on the balance. Notwithstanding their benefits, economic and policy marginalization, low investment support, and the increasing land fragmentation of small farms threaten their contribution to global food security, leaving many farmers vulnerable (IFAD and UNEP, 2013). The rising environmental awareness, depletion of natural resources, and human health nutritional concerns have led to a paradigm shift among the farmers from over-dependence on agrochemical inputs to the use of ecofriendly biological agents for agricultural production (Herrero et al., 2017; Alori and Babalola, 2018). To increase the use and adoption of biological agents in smallholder agroecosystems, more robust integrated pathways that encourage food production based on local innovations, practices, and resources should be established. Tapsoba et al. (2020) emphasize that smallholder farmers should be involved in re-designing agricultural production. This way, farmers are likely to integrate new techniques into their current farm management practices to meet their agrosociological and economic needs. The authors warn that some agricultural initiatives may remain localized and isolated despite evidence of success in other agroecosystems. It is, therefore, necessary to have the right network of stakeholders on a territorial scale to support agricultural initiatives and their implementation.

Soil microbial communities mediate nearly every biogeochemical process occurring on earth crust controlling the functionality of an ecosystem (Escalas et al., 2019). Their ubiquitous nature, diversity richness, and ability to establish multiple interactions with higher organisms and among themselves, make them the best candidates in delivery of essential agroecosystem services (Brussaard et al., 2007). Understanding the importance of having a specific functional species or group of PGPMs here referred to as “functional identity” or “functional diversity,” respectively (definitions adapted from Barberi, 2015) is not well-established in smallholder agroecosystems. This drives the rising research demand for soil biodiversity in the quest for the delivery of beneficial agroecosystem services. In the previous decades, most of the commercial inoculants contained a single microbial species or strain targeting a specific crop genotype (Kaminsky et al., 2019). Economically, this no longer favors the smallholder agroecosystems that are nowadays characterized by a wide range of crop production.

Currently, through research, various microbial species and strain combinations have been produced targeting a broad range of crop species depending on the market requirement and species compatibility. For instance, SumaGrow® bioinoculant (Table 1) produced by Bio Soil Enhancers Inc. (USA) contains a consortium of polyfunctional PGPMs comprising of N fixers, P mobilizers and solubilizers, micronutrient mobilizers, growth hormones, and organic humic acid, and enhance growth and yield in a wide range of crops including vegetables, cereals, legumes, trees, and fruits (Preininger et al., 2018). Co-inoculation or mixed inoculation of diverse multifunctional microbial groups as a single inoculant could maximize the chances of strain functional performance (Escalas et al., 2019) in particular the functional biodiversity effect when functional identity effect is suppressed. For instance, a mixture of PGPMs that enhance P solubilization (e.g., Bacillus spp.), phytopathogenic biocontrol (e.g., Pseudomonas spp.), BNF (e.g., Rhizobium spp.), and phytohormone production (e.g., Azospirillum spp.) could have synergistic or additive functional biodiversity effect on crops (Hungria et al., 2015; Rashid et al., 2016). However, the performance of co-inoculation is not always the case as its efficiency is affected by several factors including strain compatibility, concentration ratios, inoculation methods, plant genotypes, soil factors, and environmental conditions at the time of application (Kaminsky et al., 2019). Considering all these factors and the changing climatic conditions, the performance of the current inoculants may not be guaranteed in the near future. Thus, research should be done to deepen the understanding of complex interactions associated with mixing various inoculants to come up with new formulations for use with different crops. Efficient delivery methods of inoculant application in the context of the changing climate should also be investigated.

Studies have shown that there are significant benefits of using PGPM inoculants as biofertilizers or biostimulants in crop production (Chavoshi et al., 2018; Aliyu et al., 2019; Santos et al., 2019). A global meta-analysis of biofertilizer efficiency in enhancing crop nutrients and yields showed an average of 16 % increase in yield of all crops, with legumes showing a superior response to inoculation (Schütz et al., 2018). The beneficial effects of bioinoculation could be more in nutrient-limited soil conditions which reflects the case of SSA smallholder agroecosystems. According to Singh et al. (2017), Rhizobium biofertilizer inoculum when applied in semiarid and arid environments can supplement nutrient requirement in legumes and hence improving crop yield. Chavoshi et al. (2018) reported a maximum biomass accumulation of 7,985 kg ha⁻¹ in red beans and a higher water use efficiency after inoculation with Bio-P and Bio-K fertilizers in limited water resources. In desert areas of the Sahel region, double inoculation of indigenous rhizobia and AMF isolates improved the survival rate and growth of Vachellia seyal (acacia) plantations (Fofana et al., 2020). The authors attribute the improved plant growth to the adaptation of the native bioinoculants to the pedoclimatic conditions of the region and the microbial synergism in delivering plant nutrients under water and heat stress conditions. The fact that PGPM inoculants mitigate water stress in crops and enhance crop health and productivity ensures that there is consistency in yields for smallholder farmers.

The use of effective PGPMs enhances leaf photosynthesis, seed quality traits, and yield of legumes such as Phaseolus vulgaris L.) (Iriti et al., 2019). High-quality agricultural products are easily marketable and fetch high prices, therefore, bringing more fortune to smallholder farmers resulting in improved livelihoods. The choice and selection of superior native strains over exotic strains is often encouraged (Aliyu et al., 2019). For instance, the application of native strains of Bacillus spp. as PGPM in the cultivation of cumin (Cuminum cyminum Linn.) increased the seed oil content and yields compared to uninoculated control (Mishra et al., 2019). Studies have revealed that it is economical for smallholder farmers to apply polyfunctional microbial inoculants with multiple plant growth-promoting traits such as P solubilization, N fixation, and biocontrol compared to the use of single-trait inoculants (Reddy and Saravanan, 2013). However, some studies favor the promotion of functional identity over functional diversity when dealing with specific crop genotypes and environments (Njeru et al., 2017). Further studies to elucidate this disparity should be done in the context of smallholder agroecosystems paying attention to proper management of microbial inoculants as this could significantly affect microbial functioning, abundance, and effectiveness.

The global demand for healthy food and fiber is expected to rise by 2050 to 70% (Singh and Trivedi, 2017). The increase in food demand needs to be satisfied from the existing arable land, which is already under pressure from the rising human population, harsh climatic conditions, and the decline in soil fertility and water availability. In addition, there is a need to safeguard farm produce from emerging and re-emerging pests and diseases. Despite the success of chemically associated conventional farming practices in increasing agricultural productivity, their future reliability is on the balance due to various health concerns arising from food contamination and disease resistance (Alori and Babalola, 2018). Harnessing natural resources such as plant-associated microbiome (PAM) could be one of the most effective strategies to improve agricultural productivity and future food security in a more sustainable way. The PAM technology has a better potential to minimize environmental hazards and increase food quality and quantity while lessening resource inputs compared to the conventional farm practices. Additional plant-microbiome discoveries and technological improvements are emerging and embracing a shift in the paradigm toward next-generation microbial applications could lead to better food production systems (Nezhad et al., 2015). In situ plant-microbiome engineering, high throughput sequencing, and plant breeding will be integral to enhancing the understanding and development of efficient microbial inoculants. Evidence shows that more attention has been given to rhizosphere and root microbiota that play a key role in plant productivity (Goswami and Deka, 2020; Nuzzo et al., 2020; Sousa et al., 2020), while other plant sections such as leaf, stem, seeds, and flowers remains largely unexplored. Yet, they could be playing important roles in plant defense system and response against abiotic and biotic stress (Singh and Trivedi, 2017).

Modern farming encourages the integration of bio-inoculants with other farm management practices and this has been shown to have complementary and synergistic effects in improving growth and yield quality characteristics in apple fruit farming (Mosa et al., 2018). The positive association between agronomic management practices and ecosystem functioning could be exploited to enhance soil fertility amelioration and plant productivity (Costanzo and Bàrberi, 2014). One possible approach is to increase the above-ground litter and below-ground root traits that host and provide energy to the PGPMs responsible for decomposition and nutrient recycling (Bakker et al., 2012). This approach encourages farmers to utilize agronomic practices that increase genetic, species, and habitat diversity within the field scale hence increasing farm's overall productivity and agroecosystem resilience (Moonen and Bàrberi, 2008; Mburu et al., 2016). Conserving a high diversity of indigenous microbial functional communities in the soil ensures continuous maintenance of critical soil functions amidst the changing climatic conditions. This could provide production-related insurance to the farmers on ecosystem productivity and stability upon any ecological perturbations (Yachi and Loreau, 1999; Shanafelt et al., 2015).

Multiple cropping systems coupled with rotational practices is demonstrated to sequester more carbon to the soil (Hontoria et al., 2019). Obligate PGPMs, that would not survive without a plant host, utilize carbon as the sole energy source (Ventorino et al., 2012). Therefore, farmers should critically choose cropping systems that favor carbon sequestration in order to conserve the functioning of beneficial obligate microbial communities. However, for maximum benefits to be achieved in multiple cropping systems, choosing complementary plant genotypes known to host multiple beneficial PGPMs would be ideal. For instance, intercropping of cereals with legumes such as lentils, faba bean, and chickpea, which are considered (Faucon et al., 2017; Lazzaro et al., 2018). Cereal-legume intercropping system creates a microclimate that regulates heat stress, moisture, wind stress, weed and pest infestation (Lazzaro et al., 2018). The use of PGPMs can enhance the interspecific plant-microbial interactions in intercropping system (Figure 3). The addition of organic amendments such as vermicompost, manure, and biochar that are rich in PGPMs and nutrients would help to sustain high energy demanding soil processes and promotes microbial decomposition and nutrient recycling (Cobb et al., 2018; Nyamwange et al., 2018; Koskey et al., 2020). Non-chemical weeding and pest control methods promote the build-up of beneficial plant-soil biodiversity which are considered the main drivers of an ecosystem (Marzaioli et al., 2010). On the contrary, intensive cultivation, a common practice in smallholder agroecosystems, reduces soil biodiversity, organic matter, and increase CN ratio, therefore, reducing the overall microbial functionality (Ventorino et al., 2012). Therefore, a change in cultivation practices to more sustainable agroecological practices in smallholder systems would be inevitable if food security is to be realized.

Plants are considered naturally as selective agents that continuously shape rhizospheric soil microbiome and rhizosphere engineering is slowly emerging as a new research field to potentially address crop production (Haldar and Sengupta, 2015). AMF are important ecosystem promoters and are recognized as key elements in low-input agroecosystems; however, their structural composition, diversity and performance are highly influenced by plant genotype (López-García et al., 2017). Certain plant taxonomic families are known to be poor AMF hosts while others such as legumes are known to be excellent AMF hosts. A few plant taxa are non-host. Currently, plants are selectively bred to produce root exudates that enhance rhizosphere plant-microbiome interactions and possibly promote agroecosystem sustainability and productivity (Bakker et al., 2012). To embrace this strategy and apply in smallholder agroecosystems, it will require farmers no infrastructural changes other than the selection of their preferred crop cultivars or species bred to enhance root exudation. However, more knowledge in this area should be generated to deepen the understanding of their mechanisms, pros and cons, cost-benefit analysis, and their applicability to resource-strained smallholder farmers. Enhancing farmers' knowledge of agroecosystem functionality would optimize soil fertility restoration successes, agroecosystem sustainability, and crop productivity.

Currently, there is a rising global market demand for microbial inoculants that can be used as biofertilizers or biostimulants in crop production (Lobo et al., 2019). Bioinoculants contain one or more PGPMs (bacteria, algae, or fungi) packaged in a carrier material. A carrier material refers to the delivery “vehicle” that is packaged to transfer the bioinoculum to the plant rhizosphere. It determines the form (either liquid or solid), shelf life, and the application or delivery methods of the microbial inoculant (Reddy and Saravanan, 2013). According to Soumare et al. (2020), a good microbial inoculum should be packaged in a carrier material that provides an optimum microenvironment (pH, water, and carbon content) for microorganisms, maintain longer shelf life and microbial viability without the need for a special storage facility. Besides, the carrier material should be cost-effective, readily available, eco-friendly, acquiescent to nutrient supplement, and not harmful to the user (Alori and Babalola, 2018). It is interesting to note that very few studies have focused on the selection and development of carrier material and their effect on bio-inoculum as most studies emphasize on the performance of microbial strains (Herrmann and Lesueur, 2013). Encapsulation of bioinoculants is a newly emerging technique that utilizes polymer beads to enclose one or more microbial species. This technique allows the incorporation of other organic bio-effectors such as humic acid and strigolactones, protects the microbial life from desiccation, and allows slow release of the components into the soil (Gouda et al., 2018). Despite its biotechnological potentials as bioinoculant carriers, nano- and micro-encapsulation methods have not been exploited commercially particularly by entities targeting smallholder systems (Herrmann and Lesueur, 2013). Therefore, with advancing technologies, more studies that could lead to the development of versatile carrier inoculants suitable for use in smallholder agroecosystems should be done.

The fact that the majority of microbial inoculants in SSA are imported (Babalola and Glick, 2012) raises a question if they are tailored to match the varying smallholder farmers' agroecosystems, shelf-life needs, local storage, and soil conditions. A short inoculant shelf-life constraints inoculant supply chain and significantly reduces inoculant reliability, viability, and availability (Deaker et al., 2012). Inoculant viability determines the success of its use and continuous adoption by the farmers, who are in most cases production-oriented rather than agroecology conservationists. Therefore, effective bioinoculants should be able to competitively and successfully establish themselves in the soil within the shortest time possible amidst the presence of already established native microbes. Various studies have tried to unveil the fate of microbial inoculum and their effect on the native microbial communities upon their introduction into the soil (Sharma et al., 2012). For instance, Nuzzo et al. (2020) demonstrated that some PGPM formulations have no impact on plant growth but significantly affect the diversity and structure of native microbial communities. On the contrary, PGPM inoculation improved plant growth but had no influence on species diversity and richness of native microbes in the host plant roots (Piromyou et al., 2011). These inconsistencies are likely to be common in smallholder agroecosystems and, therefore, calls for selection and development of microbial strains that interact well with native microbial communities.

Soil conditions such as pH, presence of organic matter, water availability, and other physicochemical properties affect the infectivity of microbial inoculants (Njeru et al., 2020; Saad et al., 2020). Some PGPM inoculants can confer resilience against such harsh local environmental conditions (Agami et al., 2016). This explains why microbial inoculants are recommended even in soils with less water and where most nutrients are immobile (Sindhu et al., 2020). In some instances, seed companies and researchers have solved this challenge by producing crop cultivars that customarily favor colonization of specific PGPMs under a wide range of soil conditions (Faye et al., 2020). Through this approach, Gitonga et al. (2021) investigated how organic and conventional smallholder farming systems and soybean cultivars (promiscuous vs. non-promiscuous) affect native Bradyrhizobium spp. diversity. Similarly, Sinong et al. (2021) found P-solubilizing microbial isolates that could be exploited to enhance the growth of two rice cultivars under low-input cultivation than conventional practices. The authors aimed at optimizing smallholder agroecosystem at the farm level that potentially harbor diverse soil microbiota that enhances crop productivity.

In other cases, seed manufacturers have partnered with farmers to establish ‘custom seed inoculation’ where on farmer's request, seeds are inoculated with specific PGPMs by the manufacturer prior to packaging and delivery to the farmers for planting (Deaker et al., 2012). These two approaches can be easily replicated in smallholder agroecosystems, but more research and partnerships on bioprospecting for effective PGPMs compatible with various local crop cultivars should be initiated. Researchers should keep in mind the need for new PGPM inoculants adapted to the current and incoming stressful climatic conditions as the future performance of the inoculants currently in the market may not be guaranteed.

Notwithstanding their importance in upscaling agriculture, the use of PGPM inoculants in smallholder settings is largely unaccountable (Oruru and Njeru, 2016) and more research should focus on quantifying their use, adoption, and their overall effect on soil, crops, and farmers' livelihood. Farmers are used to the application of ‘blanket’ solutions in solving their day-to-day field challenges and the adoption of particular techniques providing specific solutions in certain locations should be encouraged. Industrially, there is a challenge in the production of bioinoculants that could be used for a broad range of crops grown in geographically and climatically diverse territories (Santos et al., 2019). PGPMs, unlike the broad-spectrum agrochemicals, are highly selective in their use and only target specific plant hosts. Their viability is short and the cost of maintaining PGPMs during storage is very high especially in rural setups where electricity and modern storage facilities are limiting resources (Tabassum et al., 2017). Thus, the search for innovative microbial solutions based on farmers' needs should be done with geographical considerations, increasing episodes of climatic and environmental stresses.

The risk of toxicity arising from inoculant contamination is high if proper quality control standards and storage measures are not taken into consideration. Raimi et al. (2019) reported 67% of the South African biofertilizers, analyzed through sequencing, showed high levels of toxins and contaminants affecting the quality of the inoculants, therefore, jeopardizing their potential benefits. The intentional movement of bioinoculants containing various exotic microbial species or strains to new agroecosystems is growing, but the possible negative ecological consequence of their introduction is poorly understood (Schwartz et al., 2006). This may lead to unintended negative invasion and the establishment of microbes in new agroecosystems. Non-sterile and contaminated inoculum can result in the emergence of phytopathogens that are parasitic to the native PGPMs and plants and can potentially cause significant crop losses. Controlling the cases of invasive species would be costly to the farmers and thus, the need for inoculation particularly with imported inoculants should first be carefully evaluated and where possible, the use of high-quality local indigenous species should be recommended. In some of the African countries where economic and technological policies have been put in place to support the use of biofertilizers, there is hardly any evidence of the successful implementation of the approaches. This failure is linked to financial misappropriation, policy management and lack of investment interest among the stakeholders (Abdullah and Samah, 2013). Additionally, unreliable climatic conditions, variation in soil factors and poor agronomic management practices need to be addressed especially in the SSA region where the impact of bio-inoculation would be profound if fully adopted (Ngetich et al., 2012).

The increasing demand for safe food and better nutrition, advancing research technologies, and interest in sustainable agriculture has further renewed global interest on PGPM bioinoculants. For instance, by 2019, China registered more than 800 inoculant related patents while India surpassed 100 patents (Santos et al., 2019). Therefore, it is expected that, through advanced research innovations, more bioinoculants will be produced in the following years. The effects of climate change present a major challenge to industrial bioinoculant producers, and research on PGPMs that are more effective under a broader range of stressful conditions and induce plant tolerance would increase. The challenges on microbial shelf life, storage and viability losses should be addressed and new technologies of seed coating that deliver stable formulations able to withstand harsh storage conditions should be developed (Bargaz et al., 2018). Plant breeders, seed producers, and farmers could overcome these challenges through the initiation of “tailor-made” products that could address specific challenges. However, further scientific research and economic benefit analysis in this area should be done.

More knowledge and deeper understanding are needed on how agronomic practices under changing climatic conditions affect the composition, abundance, and bio-functionality of PGPMs in delivering multiple agroecosystem services. Farmers need to know how PGPM communities are managed at spatial and temporal scale to promote synergies, effectiveness, and avoid trade-offs. Fostering proximity to smallholder farmers in redesigning agroecosystems and policy making should be prioritized. This can be achieved by involving them in-field research demonstrations, data collection, reporting, and policy recommendation drafting (IFAD and UNEP, 2013). These approaches will enhance farmers' knowledge and technological capacity in the use of bio-inoculants, therefore, ensuring a continuous adoption of techniques that take into account their local ecological conditions and knowledge.

The use of PGPMs as biostimulants, biofertilizers, or biocontrol agents by smallholder farmers has substantially grown owing to their impressive performance, economic benefits, and environmental safety associated with their use. They provide beneficial agroecosystem services such as soil nutrient amelioration, crop nutrient, and yield improvement, plant tolerance to biotic and abiotic stresses, biocontrol of pests and diseases, and water uptake. The adoption of PGPMs in smallholder agroecosystems is on the rise with the increasing number of patents and new inoculants being observed in developing countries. Multisectoral research on the use of PGPMs involving smallholder farmers is encouraged and its output should capture the aims of both the “productionist” and “agroecologist” paradigms. More knowledge on the effects of climate change and agronomic practices on the bio-functionality of PGPMs in delivering multiple agroecosystem services should be generated. Robust technologies are needed to enhance the PGPM production and most importantly to improve the effectiveness, stability, and reliability of the products under environmentally stressing conditions. Putting into consideration the collective experience, needs, and indigenous knowledge of smallholder farmers, PGPM inoculation could be a key pillar in addressing SDG 2 goal of ending hunger, promoting food security, and environmental sustainability.

GK drafted the outline structure and prepared the manuscript with contributions from SM and RA. EN and JM provided technical guidelines and reviews during manuscript preparation. All authors approved the final draft of the manuscript for submission.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.