1. Introduction

The global environment is changing continuously and the incidence of global warming caused by extreme climatic events is also on the rise, consequently disturbing the world ecosystems, including agro-ecosystems [1]. Such extreme changes in climate can affect the quality and quantity of crops severely by inducing various environmental stresses to crops, threatening food security worldwide [2]. An increase in global temperature, atmospheric CO₂ level, tropospheric O₃, and acid rains can cause multifarious chronic stresses to plants, reducing their capability to respond in case of pathogen attacks [3]. Among these stresses, drought, water scarcity, and soil salinization are the most problematic and complicated factors of agricultural losses resulting from human-induced climate changes [4]. Fluctuations in temperature and rainfall variations are key indicators of environmental stresses [5]. Elevated temperatures lead to an amplification of the rates of respiration and evapotranspiration in crops, a higher infestation of pests, shifts in weed flora patterns, and reduction in crop duration [6]. Water scarcity is also considered one of the prime global issues that have direct effects on agricultural systems and according to climate projections, its severity will increase in the future [7]. Water scarcity piercingly influences a crop’s gaseous exchange capacity, causing the closure of stomata [8]. This leads to the impairment of the evapotranspiration and photosynthetic activities of plants, affecting overall biomass production [9]. Impaired evapotranspiration reductions also affect the nutrient uptake ability of plants [8]. In semi-arid and arid climatic zones where rainfalls are already less intense and sporadic, the damages caused by drought stress can be exacerbated due to excessive accumulation of salts in soil [10].

Furthermore, the liberal use of inorganic fertilizers and pesticides to increase crop productivity and meet the food requirement of the ever-growing human population, which is projected to reach 9.7 billion by 2050, has severely affected the health of agro-ecosystems and human beings. Confrontational challenges of improving agriculture production with limited arable land rely on sustainable technologies. Several technical advances have been suggested in the past three decades to increase the productivity of agricultural production processes by reducing toxic agrochemical substances such as pesticides and fertilizers. An emerging technology tackling these critical problems includes the creation of novel plant biostimulants and successful methods for their application [11,12,13,14,15]. Plant biostimulants differ from other agricultural inputs such as fertilizers and plant protection products because they utilize different mechanisms and work regardless of the presence of nutrients in the products. They also do not take any direct action against pests or diseases and therefore complement the use of fertilizers and plant protection products. According to the latest European Regulation (EU 2019/1009), a biostimulant is an EU fertilizer that seeks to promote processes for plant feeding, regardless of the product’s nutrient quality, solely to boost the following plant or plant rhizosphere characteristics: (i) increased nutrient utilization efficiency, (ii) abiotic stress alleviation/tolerance, (iii) quality traits, and (iv) soil or rhizosphere supply of stored nutrients [16,17]. Over the past decade, microbiome research has changed our understanding of the complexity and composition of microbial communities. The intense interest of industry and academics in biostimulants based on live microbes has increased due to the reason that the growth and development of a plant can be improved under field conditions more effortlessly than other biostimulants [18,19]. Biostimulants are not nutrients, but encourage the utilization of nutrients or help foster plant growth or plants’ resistance/tolerance to various types of stresses [9,20]. Beneficial plant fungi and bacteria can be considered the most promising microbial biostimulants [21]. The recent trend has underscored the fact that plants are not autonomous agents in their environments but are associated with bacterial and fungal microorganisms, and that many external and internal microbial interactions respond to biotic and abiotic stresses [22,23]. Therefore, biostimulants are gradually being incorporated into production systems to alter physiological processes in plants to maximize productivity [24].

Bacterial plant biostimulants (BPBs) comprise a major category of plant biostimulants. Plant growth-promoting rhizobacteria (PGPR) that colonize the plant rhizosphere are the most prominent group in this category [24]. These PGPR improve plant growth, control plant pathogens, improve nutrient and mineral uptake in plants, and increase plants’ resistance to various types of biotic stresses and tolerance towards abiotic stresses (Figure 1). The representative beneficial groups of PGPR-based BPBs include nitrogen-fixing Rhizobium, Azotobacter spp., Azospirillum spp., Pseudomonas spp., and Bacillus spp. [25,26].

Figure 1. The beneficial influence of PGPR on crop plants.

2. Global Market for PGPR-Based Biostimulants

Biostimulants are emerging as an essential component in sustainable agricultural practices. Instances of environmental hazards and soil contamination from injudicious and excessive application of chemical-based products on crops have been a key issue for the industry in recent times. The global biostimulants market size was estimated at USD 1.74 billion in 2016, and projected to expand at a Compound Annual Growth Rate (CAGR) of 10.2% from 2017 to 2025. A rising focus on enhanced productivity, coupled with rapid soil degradation, is likely to drive the market over the forecast period. The global biostimulants market size was estimated at USD 2.30 billion in 2019 and is expected to reach USD 2.53 billion in 2020. The global biostimulants market is expected to grow at a compound annual growth rate of 10.2% from 2017 to 2025 to reach USD 4.14 billion by 2025 [27]. Although not all biostimulants are biological in nature [28], the bacteria are ancestral companions of a plant in all conditions. Moreover, according to the currently available literature, less than 25% of the commercial products of biostimulants are microbial based [9]. Table 1 provides a list of some popular PGPR-based commercial biostimulants [29,30,31]. Although some formulations contain fungal associations, the preparations are mainly based on PGPR.

Table 1. Examples of commercial PGPR-based plant biostimulants [29,30,31].

This entry is adapted from the peer-reviewed paper 10.3390/su13052856