Plant growth-promoting bacteria (PGPB) are excellent biocontrol agents and stimulators of plant growth, nutrition, and production. Therefore, these plant-associated bacteria are considered an excellent alternative to reduce or eliminate the use of toxic agrochemicals. In this work, we review the current state of the beneficial mechanisms (direct and indirect), including the production of antibiotic compounds and enzymes, facilitation of resource acquisition, or production of stimulating phytohormones/metabolites. Some aspects of the formulation technology and bioinoculant efficiency of diverse PGPBs (e.g., rhizobacteria, phyllobacteria and endophytic bacteria) in the field are also discussed. However, the commercialization and application of these biological agents in agriculture occur mainly in developed countries, limiting their success in developing regions. The possible causes of the delay in the application of bioinoculants for sustainable agriculture and the plausible solutions are also discussed in this study. Finally, the use of PGPBs is currently a priority for sustainable production in agriculture.

The excessive use of agrochemicals in the field to increase production and counteract the negative effects caused by biotic and abiotic factors has led to a deterioration in soil fertility, plus an increment in negative impacts on the environment and human health. Therefore, the application of beneficial microorganisms as bioinoculants is an eco-friendly alternative to agrochemicals. Plant growth-promoting bacteria and fungi have been effective in promoting plant growth and production, as well as reducing the action of pathogens in multiple crops. However, successful application of such beneficial microorganisms in the agricultural field has faced several difficulties, such as survival, colonization efficiency and short periods of shelf storage. Therefore, it is essential to explore novel ways to encapsulate, formulate and apply bioinoculants. To obtain the expected quality in bioencapsulated products, it is essential to determine the type of polymer, capsule size, encapsulation technique and use the correct chemical and physical cofactors involved in the production process. Thus, this review highlights the various formulation types and application techniques, as well as discussing the multiple advantages of using microbial encapsulates to have better results in agricultural production.

El género Bacillusproduce compuestos volátiles, hormonas vegetales, polisacáridos y enzimas relacionadas con el metabolismo de los fenilpropanoides, lo que representa un alto potencial para la promoción del crecimiento vegetal. En este estudio se analizaron los efectos de compuestos difusibles y volátiles producidos por cuatro endófitos bacterianos de Bacillus(Bacillussp. E25, B. toyonensisCOPE52, B. thuringiensisUM96 y Bacillussp. CR71) sobre la promoción del crecimiento de plántulas de zarzamora(Rubus fruticosus L.), cultivares Tupy, Kiowa y UM-13,mediante cultivo in vitro.Los resultados demostraron que la emisión de compuestos difusibles y orgánicos volátiles por la cepa COPE52 promovieron el aumento de la longitud y peso seco de la raíz, número de raíces y la concentración de clorofila en plántulas del cultivar Tupy. La emisión de compuestos difusibles de la cepa COPE52 indujo el aumento del peso seco de raíz y aéreo, y longitud de la raíz en el cultivar Kiowa, mientras que los compuestos volátiles emitidos porla cepa E25 promovieron incrementos en la mayoría de las variables analizadas en esta misma variedad. Los compuestos difusibles y volátiles producidos por la cepa CR71 tuvieron un mayor efecto sobre el número, longitud y peso seco de raíz en las plántulasdel cultivar UM-13, mientras que la concentración de clorofila aumentó con los compuestos producidos por la cepa E-25. En conclusión, las bacterias endófitas de Bacillusspp. promovieron diferencialmente el crecimiento de plántulas de los cultivares de zarzamora, en función del tipo de cepa inoculada y del modo de acción de los compuestos producidos.

The traditional milpa system is a polyculture originating in Mesoamerica, whose core is maize (Zea mays L.), associated with squash (Cucurbita spp.) and beans (Phaseolus vulgaris L.). In recent years, milpa-type crops have decreased owing to climate change, rapid population growth, and the excessive use of agrochemicals; therefore, the application of plant growth-promoting rhizobacteria (PGPR) to counteract these negative effects has been little explored. In this study, a maize crop in a milpa system was fertilized with the PGPR Pseudomonas fluorescens UM270, and the endophytic root microbiome (endobiome) of maize was assessed by 16S rRNA and internal transcribed spacer regions (ITS) sequencing. The results showed that UM270 the rhizosphere inoculation of P. fluorescens UM270 did not increase alpha diversity in either monoculture or the milpa, but it did alter the endophytic microbiome of maize plant roots by stimulating the presence of bacterial operational taxonomic units (OTUs) of the genera Burkholderia and Pseudomonas (in a monoculture), whereas in the milpa system, the PGPR stimulated a greater endophytic diversity and the presence of genera such as Burkholderia, Variovorax, and N-fixing rhizobia genera, including Rhizobium, Mesorhizobium and Bradyrhizobium. No clear association was found between fungal diversity and the presence of strain UM270, but beneficial fungi such as Rizophagus irregularis and Exophiala pisciphila were detected in the milpa system. In addition, network analysis revealed unique interactions with species like Stenotrophomonas sp., Burkholderia xenovorans, and Sphingobium yanoikuyae, which would potentially be playing a beneficial role with the plant. To the best of our knowledge, this is the first study in which the root microbiome of maize growing under a milpa model was assessed by bio-inoculation with PGPRs.