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Understanding how biotic interactions can improve plant tolerance to drought is a challenging prospect for agronomy and ecology. Plant growth-promoting rhizobacteria (PGPR) are promising candidates but the phenotypic changes induced by PGPR under drought remain to be elucidated. We investigated the effects of Phyllobacterium brassicacearum STM196 strain, a PGPR isolated from the rhizosphere of oilseed rape, on two accessions of Arabidopsis thaliana with contrasting flowering time. We measured multiple morphophysiological traits related to plant growth and development in order to quantify the added value of the bacteria to drought-response strategies of Arabidopsis in soil conditions. A delay in reproductive development induced by the bacteria resulted in a gain of biomass that was independent of the accession and the watering regime. Coordinated changes in transpiration, ABA content, photosynthesis and development resulted in higher water-use efficiency and a better tolerance to drought of inoculated plants. Our findings give new insights into the ecophysiological bases by which PGPR can confer stress tolerance to plants. Rhizobacteria-induced delay in flowering time could represent a valuable strategy for increasing biomass yield, whereas rhizobacteria-induced improvement of water use is of particular interest in multiple scenarios of water availability.

There has been many recent studies on the use of microbial antagonists to control diseases incited by soilborne and airborne plant pathogenic bacteria and fungi, in an attempt to replace existing methods of chemical control and avoid extensive use of fungicides, which often lead to resistance in plant pathogens. In agriculture, plant growth-promoting and biocontrol microorganisms have emerged as safe alternatives to chemical pesticides. Streptomyces spp. and their metabolites may have great potential as excellent agents for controlling various fungal and bacterial phytopathogens. Streptomycetes belong to the rhizosoil microbial communities and are efficient colonizers of plant tissues, from roots to the aerial parts. They are active producers of antibiotics and volatile organic compounds, both in soil and in planta, and this feature is helpful for identifying active antagonists of plant pathogens and can be used in several cropping systems as biocontrol agents. Additionally, their ability to promote plant growth has been demonstrated in a number of crops, thus inspiring the wide application of streptomycetes as biofertilizers to increase plant productivity. The present review highlights Streptomyces spp.-mediated functional traits, such as enhancement of plant growth and biocontrol of phytopathogens.

Background Our knowledge of plant beneficial bacteria in the rhizosphere is rapidly expanding due to intense interest in utilizing these types of microbes in agriculture. Laboratory and field studies consistently document the growth, health and protective benefits conferred to plants by applying plant growth promoting rhizobacteria (PGPR). PGPR exert their influence on other species, including plants, in the rhizosphere by producing a wide array of extracellular molecules for communication and defense. Scope The types of PGPR molecular products are characteristically diverse, and the mechanisms by which they are acting on the plant are only beginning to be understood. While plants may contribute to shape their microbiome, it is these bacterial products which induce beneficial responses in plants. PGPR extracellular products can directly stimulate plant genetic and molecular pathways, leading to increases in plant growth and induction of plant resistance and tolerance. This review will discuss known PGPR-derived molecules, and how these products are implicated in inducing plant beneficial outcomes through complex plant response mechanisms. Conclusions In order to move PGPR research to the next level, it will be important to describe and document the genetic and molecular mechanisms employed in these interactions. In this way, we will be able to restructure and harness these mechanisms in a way that allows for broad-based applications in agriculture. A greater depth of understanding of how these PGPR molecules are acting on the plant will allow more effective development of rhizobacterial applications in the field.

Key messageA detailed study of the response of wheat plants, inoculated with drought-tolerant PGPR is studied which would be beneficial to achieve genetic improvement of wheat for drought tolerance.Drought stress, a major challenge under current climatic conditions, adversely affects wheat productivity. In the current study, we observed the response of wheat plants, inoculated with drought-tolerant plant growth-promoting rhizobacteria (PGPR) Bacillus megaterium (MU2) and Bacillus licheniformis (MU8) under induced drought stress. In vitro study of 90 rhizobacteria exhibited 38 isolates showed one or more plant growth-promoting properties, such as solubilization of phosphorus, potassium, and exopolysaccharide production. Four strains revealing the best activities were tested for their drought-tolerance ability by growing them on varying water potentials (− 0.05 to − 0.73 MPa). Among them, two bacterial strains Bacillus megaterium and Bacillus licheniformis showed the best drought-tolerance potential, ACC deaminase activities, IAA production, and antagonistic activities against plant pathogens. Additionally, these strains when exposed to drought stress (− 0.73 MPa) revealed the induction of three new polypeptides (18 kDa, 35 kDa, 30 kDa) in Bacillus megaterium. We determined that 106 cells/mL of Bacillus megaterium and Bacillus licheniformis were enough to induce drought tolerance in wheat under drought stress. These drought-tolerant strains increased the germination index (11–46%), promptness index (16–50%), seedling vigor index (11–151%), fresh weight (35–192%), and dry weight (58–226%) of wheat under irrigated and drought stress. Moreover, these strains efficiently colonized the wheat roots and increased plant biomass, relative water content, photosynthetic pigments, and osmolytes. Upon exposure to drought stress, Bacillus megaterium inoculated wheat plants exhibited improved tolerance by enhancing 59% relative water content, 260, 174 and 70% chlorophyll a, b and carotenoid, 136% protein content, 117% proline content and 57% decline in MDA content. Further, activities of defense-related antioxidant enzymes were also upregulated. Our results revealed that drought tolerance was more evident in Bacillus megaterium as compared to Bacillus licheniformis. These strains could be effective bioenhancer and biofertilizer for wheat cultivation in arid and semi-arid regions. However, a detailed study at the molecular level to deduce the mechanism by which these strains alleviate drought stress in wheat plants needs to be explored.

Beneficial microbes are applied to the soil and plant tissues directly or through seed inoculation, whereas soil application is preferred when there is risk of inhibitors or antagonistic microbes on the plant tissues. Insufficient survival of the microorganisms, hindrance in application of fungicides to the seeds and exposure to heat and sunlight in subsequent seed storage in conventional inoculation methods force to explore appropriate and efficient bacterial application method. Seed priming, where seeds are hydrated to activate metabolism without actual germination followed by drying, increases the germination, stand establishment and stress tolerance in different crops. Seed priming with living bacterial inoculum is termed as biopriming that involves the application of plant growth promoting rhizobacteria. It increases speed and uniformity of germination; also ensures rapid, uniform and high establishment of crops; and hence improves harvest quality and yield. Seed biopriming allows the bacteria to enter/adhere the seeds and also acclimatization of bacteria in the prevalent conditions. This review focuses on methods used for biopriming, and also the role in improving crop productivity and stress tolerance along with prospects of this technology. The comparison of methods being followed is also reviewed proposing biopriming as a promising technique for application of beneficial microbes to the seeds. The article reviews the potential of seed priming with plant growth promoting bacteria over conventional methods of bacterial application to the soil in improving plant productivity. Graphical Abstract Figure. The article reviews the potential of seed priming with plant growth promoting bacteria over conventional methods of bacterial application to the soil in improving plant productivity.

Background and aims Plant growth-promoting rhizobacteria (PGPR) have important roles in improving plant growth and alleviating stress induced damage under drought stress conditions. The aims of this study are: 1) to isolate and identify drought-tolerant PGPR from rhizosphere soil of jujube, a drought-tolerant plant grown in semi-arid and arid regions; and 2) to evaluate the effects of inoculation with the isolated strains on the growth and physiological responses of jujube seedlings under drought stress conditions. Methods Rhizosphere bacteria with 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity were isolated from the rhizosphere of jujube and were identified by 16S rDNA sequencing analysis. Then, the isolates were screened for drought tolerance and plant growth promoting activities. The growth and physiological changes of jujube under drought stress, including the plant height, shoot and root dry matter, malondialdehyde (MDA), indoleacetic acid (IAA), abscisic acid (ABA) content, superoxide dismutase (SOD), and peroxidase (POD) activity, were also detected. Results Eight ACC deaminase-producing bacterial strains were isolated and identified as Pseudomonas , Bacillus , and Serratia . Under drought stress conditions, Pseudomonas lini and Serratia plymuthica significantly increased plant height, shoot and root dry matter, and relative water content. Moreover, malondialdehyde and ABA levels were decreased, and antioxidant enzyme activities were increased. In addition, they also enhanced soil aggregate stability. The best effect was observed with mixed inoculation. Conclusions Strains of Pseudomonas lini and Serratia plymuthica played an important role in enhancing tolerance of jujube seedling and can be considered as promising bioinoculants of jujube.

Plants emit a plethora of volatile organic compounds in response to biotic and abiotic stresses. These compounds act as infochemicals for ecological communication in the phytobiome. This study reviews the role of microbe-induced plant volatiles (MIPVs) in plant–microbe interactions. MIPVs are affected by the taxonomic position of the microbe, the identity of the plant and the type of interaction. Plants also emit exclusive blends of volatiles in response to nonhost and host interactions, as well as to beneficial microbes and necrotrophic/biotrophic pathogens. These MIPVs directly inhibit pathogen growth and indirectly promote resistance/susceptibility to subsequent plant pathogen attack. Viruses and phloem-limiting bacteria modify plant volatiles to attract insect vectors. Susceptible plants can respond to MIPVs from resistant plants and become resistant. Recent advances in our understanding of the molecular mechanisms of MIPV synthesis in plants and how plant pathogen effectors manipulate their biosynthesis are discussed. This knowledge will help broaden our understanding of plant–microbe interactions and should facilitate the development of new emerging techniques for sustainable plant disease management.

A pot study was performed to examine the effect of plant growth-promoting rhizobacteria (PGPR) including Azospirillum lipoferum or Azotobacter chroococcum on growth criteria (leaf area and seedlings fresh and dry weight), pigments [chlorophylls (Chl a and b) and carotenoids], osmolytes (soluble sugars, soluble proteins and proline), nutrient uptake, antioxidant enzyme activities, and oxidative stress in maize plants under normal and salt-affected soils. The results showed that salt stress induced a reduction in growth traits, pigments, soluble proteins, K+, and K+/Na+ ratio. On the other side, it increased soluble sugars, proline, Na+, malondialdehyde (MDA), and the activity of peroxidase (POD) and catalase (CAT). Meanwhile, salt stress did not significantly change the activity of ascorbate peroxidase (APX) in maize plants. The inoculation using Azospirillum lipoferum or Azotobacter chroococcum significantly enhanced growth parameters, pigments, K+, osmolytes, K+/Na+ ratio, and the activity of CAT, POD, and APX of the salt-affected maize plants as well as uninoculated control plants. In addition, the results showed that both types of bacteria have attributed to lower MDA and Na+ in maize plants. Interestingly, Azospirillum lipoferum has affected more compared to Azotobacter chroococcum in control and salt-stressed plants. We, therefore, have observed in this study that microbial inoculation significantly improved plant physiological activities and that adding bacteria such as Azospiroillum or Azotobacter to the soil could mitigate the negative effects of salt stress on maize plants.

A variety of soil factors are known to increase nutrient availability and plant productivity. The most influential might be the organisms comprising the soil microbial community of the rhizosphere, which is the soil surrounding the roots of plants where complex interactions occur between the roots, soil, and microorganisms. Root exudates act as substrates and signaling molecules for microbes creating a complex and interwoven relationship between plants and the microbiome. While individual microorganisms such as endophytes, symbionts, pathogens, and plant growth promoting rhizobacteria are increasingly featured in the literature, the larger community of soil microorganisms, or soil microbiome, may have more far-reaching effects. Each microorganism functions in coordination with the overall soil microbiome to influence plant health and crop productivity. Increasing evidence indicates that plants can shape the soil microbiome through the secretion of root exudates. The molecular communication fluctuates according to the plant development stage, proximity to neighboring species, management techniques, and many other factors. This review seeks to summarize the current knowledge on this topic.

The most common, devastating problem in agriculture is plant (pathogenic) diseases and abiotic conditions which have a profound effect on growth and yield of the plant resulting in heavy losses. In order to prevent losses, different chemicals are used indiscriminately, which in turn lead to environmental pollution due to their persistence and toxicity yet employed to meet consumer demand. To fight ever increasing demand and indiscriminate use of chemical agents along with their devastating after effects in agriculture, we need less invasive, eco-friendly and most importantly sustainable practices. Plant growth promoting rhizobacteria (PGPR) influence different physiological activities of the plant through various mechanisms (metabolites, antibiotics, Induced Systemic Resistance and enzymes) and impart protection from pathogens as well as environmental stress factors. But, current applications are limited in this regard as mechanisms involved, field applications variance and lack of farmer awareness contributing majorly. Current review tries to provide comprehensive knowledge on the PGPR’s applications as plant protectant against pathogens & abiotic factors leading to sustainable agricultural practices.