The quest for enhancing agricultural yields due to increased pressure on food production has inevitably led to the indiscriminate use of chemical fertilizers and other agrochemicals. Biofertilizers are emerging as a suitable alternative to counteract the adverse environmental impacts exerted by synthetic agrochemicals. Biofertilizers facilitate the overall growth and yield of crops in an eco-friendly manner. They contain living or dormant microbes, which are applied to the soil or used for treating crop seeds. One of the foremost candidates in this respect is rhizobacteria. Plant growth promoting rhizobacteria (PGPR) are an important cluster of beneficial, root-colonizing bacteria thriving in the plant rhizosphere and bulk soil. They exhibit synergistic and antagonistic interactions with the soil microbiota and engage in an array of activities of ecological significance. They promote plant growth by facilitating biotic and abiotic stress tolerance and support the nutrition of host plants. Due to their active growth endorsing activities, PGPRs are considered an eco-friendly alternative to hazardous chemical fertilizers. The use of PGPRs as biofertilizers is a biological approach toward the sustainable intensification of agriculture. However, their application for increasing agricultural yields has several pros and cons. Application of potential biofertilizers that perform well in the laboratory and greenhouse conditions often fails to deliver the expected effects on plant development in field settings. Here we review the different types of PGPR-based biofertilizers, discuss the challenges faced in the widespread adoption of biofertilizers, and deliberate the prospects of using biofertilizers to promote sustainable agriculture.

Acidobacteria represents an underrepresented soil bacterial phylum whose members are pervasive and copiously distributed across nearly all ecosystems. Acidobacterial sequences are abundant in soils and represent a significant fraction of soil microbial community. Being recalcitrant and difficult-to-cultivate under laboratory conditions, holistic, polyphasic approaches are required to study these refractive bacteria extensively. Acidobacteria possesses an inventory of genes involved in diverse metabolic pathways, as evidenced by their pan-genomic profiles. Because of their preponderance and ubiquity in the soil, speculations have been made regarding their dynamic roles in vital ecological processes viz., regulation of biogeochemical cycles, decomposition of biopolymers, exopolysaccharide secretion, and plant growth promotion. These bacteria are expected to have genes that might help in survival and competitive colonization in the rhizosphere, leading to the establishment of beneficial relationships with plants. Exploration of these genetic attributes and more in-depth insights into the belowground mechanics and dynamics would lead to a better understanding of the functions and ecological significance of this enigmatic phylum in the soil-plant environment. This review is an effort to provide a recent update into the diversity of genes in Acidobacteria useful for characterization, understanding ecological roles, and future biotechnological perspectives.

The rhizosphere offers a quintessential habitat for the microbial communities and facilitates a variety of plant-microbe interactions. Members of the genus Bacillus constitute an important group of plant growth promoting rhizobacteria (PGPR), which improve growth and yield of crops. In a total of 60 bacterial isolates from the tomato rhizosphere, 7 isolates were selected based on distinct morphological characteristics and designated as tomato rhizosphere (TRS) isolates with a number suffixed viz., TRS-1, 2, 3, 4, 5, 7, and TRS-8. All the seven isolates were Gram positive, with in vitro plant growth promoting (PGP) traits like phosphate and zinc solubilization, and also produced indoleacetic acid (IAA), phytase, siderophore, hydrogen cyanide (HCN), and 1-aminocyclopropane-1-carboxylate (ACC) deaminase, besides being antagonistic to other microbes and formed biofilm. The seven isolates belonged to the genus Bacillus as per the 16S rDNA sequence analysis. Phylogenetic tree grouped the isolates into four groups, while BOX-PCR fingerprinting allowed further differentiation of the seven isolates. The PGP activity of the isolates was measured on tomato seedlings in plant tissue culture and greenhouse assays. A significant increase in root colonization was observed over 15 days with all the isolates. Greenhouse experiments with these isolates indicated an overall increase in the growth of tomato plants, over 60 days. Isolates TRS-7 and TRS-8 were best plant growth promoters among the seven isolates, with a potential as inoculants to increase tomato productivity.

Pigeon pea (Cajanus cajan L. Millsp. ) is a legume crop resilient to climate change due to its tolerance to drought. It is grown by millions of resource-poor farmers in semiarid and tropical subregions of Asia and Africa and is a major contributor to their nutritional food security. Pigeon pea is the sixth most important legume in the world, with India contributing more than 70% of the total production and harbouring a wide variety of cultivars. Nevertheless, the low yield of pigeon pea grown under dry land conditions and its yield instability need to be improved. This may be done by enhancing crop nodulation and, hence, biological nitrogen fixation (BNF) by supplying effective symbiotic rhizobia through the application of “elite” inoculants. Therefore, the main aim in this study was the isolation and genomic analysis of effective rhizobial strains potentially adapted to drought conditions. Accordingly, pigeon pea endosymbionts were isolated from different soil types in Southern, Central, and Northern India. After functional characterisation of the isolated strains in terms of their ability to nodulate and promote the growth of pigeon pea, 19 were selected for full genome sequencing, along with eight commercial inoculant strains obtained from the ICRISAT culture collection. The phylogenomic analysis [Average nucleotide identity MUMmer (ANIm)] revealed that the pigeon pea endosymbionts were members of the genera Bradyrhizobium and Ensifer. Based on nodC phylogeny and nod cluster synteny, Bradyrhizobium yuanmingense was revealed as the most common endosymbiont, harbouring nod genes similar to those of Bradyrhizobium cajani and Bradyrhizobium zhanjiangense. This symbiont type (e.g., strain BRP05 from Madhya Pradesh) also outperformed all other strains tested on pigeon pea, with the notable exception of an Ensifer alkalisoli strain from North India (NBAIM29). The results provide the basis for the development of pigeon pea inoculants to increase the yield of this legume through the use of effective nitrogen-fixing rhizobia, tailored for the different agroclimatic regions of India.