Plant Growth Promoting Rhizobacteria, Arbuscular Mycorrhizal Fungi and Their Synergistic Interactions to Counteract the Negative Effects of Saline Soil on Agriculture: Key Macromolecules and Mechanisms.

Soil saltiness is a noteworthy issue as it results in loss of profitability and development of agrarian harvests and decline in soil health. Microorganisms associated with plants contribute to their growth promotion and salinity tolerance by employing a multitude of macromolecules and pathways. Plant growth promoting rhizobacteria (PGPR) have an immediate impact on improving profitability based on higher crop yield. Some PGPR produce 1-aminocyclopropane-1-carboxylic (ACC) deaminase (EC 4.1.99.4), which controls ethylene production by diverting ACC into α-ketobutyrate and ammonia. ACC deaminase enhances germination rate and growth parameters of root and shoot in different harvests with and without salt stress. Arbuscular mycorrhizal fungi (AMF) show a symbiotic relationship with plants, which helps in efficient uptake of mineral nutrients and water by the plants and also provide protection to the plants against pathogens and various abiotic stresses. The dual inoculation of PGPR and AMF enhances nutrient uptake and productivity of several crops compared to a single inoculation in both normal and stressed environments. Positively interacting PGPR + AMF combination is an efficient and cost-effective recipe for improving plant tolerance against salinity stress, which can be an extremely useful approach for sustainable agriculture.

Rhizobacteria-mediated resistance against the blackeye cowpea mosaic strain of bean common mosaic virus in cowpea (Vigna unguiculata).

BACKGROUND: The present study investigated the effect of seven Bacillus-species plant-growth-promoting rhizobacteria (PGPR) seed treatments on the induction of disease resistance in cowpea against mosaic disease caused by the blackeye cowpea mosaic strain of bean common mosaic virus (BCMV). RESULTS: Initially, although all PGPR strains recorded significant enhancement of seed germination and seedling vigour, GBO3 and T4 strains were very promising. In general, all strains gave reduced BCMV incidence compared with the non-bacterised control, both under screen-house and under field conditions. Cowpea seeds treated with Bacillus pumilus (T4) and Bacillus subtilis (GBO3) strains offered protection of 42 and 41% against BCMV under screen-house conditions. Under field conditions, strain GBO3 offered 34% protection against BCMV. The protection offered by PGPR strains against BCMV was evaluated by indirect enzyme-linked immunosorbent assay (ELISA), with lowest immunoreactive values recorded in cowpea seeds treated with strains GBO3 and T4 in comparison with the non-bacterised control. In addition, it was observed that strain combination worked better in inducing resistance than individual strains. Cowpea seeds treated with a combination of strains GBO3 + T4 registered the highest protection against BCMV. CONCLUSION: PGPR strains were effective in protecting cowpea plants against BCMV under both screen-house and field conditions by inducing resistance against the virus. Thus, it is proposed that PGPR strains, particularly GBO3, could be potential inducers against BCMV and growth enhancers in cowpea.

Control of Fusarium verticillioides, cause of ear rot of maize, by Pseudomonas fluorescens.

BACKGROUND: Maize is one of the staple food crops grown in India. Fusarium verticillioides (Sacc.) Nirenberg is the most important fungal pathogen of maize, associated with diseases such as ear rot and kernel rot. Apart from the disease, it is capable of producing fumonisins, which have elicited considerable attention over the past decade owing to their association with animal disease syndromes. Hence, the present study was conducted to evaluate ecofriendly approaches by using a maize rhizosphere isolate of Pseudomonas fluorescens (Trev.) Mig. and its formulation to control ear rot disease and fumonisin accumulation, and also to study the capacity to promote growth and yield of maize. In vitro assays were conducted to test the efficacy of P. fluorescens as a seed treatment on seed germination, seedling vigour and also the incidence of F. verticillioides in different maize cultivars. The field trials included both seed treatment and foliar spray. For all the experiments, P. fluorescens was formulated using corn starch, wheat bran and talc powder. In each case there were three different treatments of P. fluorescens, a non-treated control and chemical control. RESULTS: Pure culture and the formulations, in comparison with the control, increased plant growth and vigour as measured by seed germination, seedling vigour, plant height, 1000 seed weight and yield. P. fluorescens pure culture used as seed treatment and as spray treatment enhanced the growth parameters and reduced the incidence of F. verticillioides and the level of fumonisins to a maximum extent compared with the other treatments. CONCLUSION: The study demonstrates the potential role of P. fluorescens and its formulations in ear rot disease management. The biocontrol potential of this isolate is more suited for fumonisin reduction in maize kernels intended for human and animal feed.

Bacterial volatiles induce systemic resistance in Arabidopsis.

Plant growth-promoting rhizobacteria, in association with plant roots, can trigger induced systemic resistance (ISR). Considering that low-molecular weight volatile hormone analogues such as methyl jasmonate and methyl salicylate can trigger defense responses in plants, we examined whether volatile organic compounds (VOCs) associated with rhizobacteria can initiate ISR. In Arabidopsis seedlings exposed to bacterial volatile blends from Bacillus subtilis GB03 and Bacillus amyloliquefaciens IN937a, disease severity by the bacterial pathogen Erwinia carotovora subsp. carotovora was significantly reduced compared with seedlings not exposed to bacterial volatiles before pathogen inoculation. Exposure to VOCs from rhizobacteria for as little as 4 d was sufficient to activate ISR in Arabidopsis seedlings. Chemical analysis of the bacterial volatile emissions revealed the release of a series of low-molecular weight hydrocarbons including the growth promoting VOC (2R,3R)-(-)-butanediol. Exogenous application of racemic mixture of (RR) and (SS) isomers of 2,3-butanediol was found to trigger ISR and transgenic lines of B. subtilis that emitted reduced levels of 2,3-butanediol and acetoin conferred reduced Arabidopsis protection to pathogen infection compared with seedlings exposed to VOCs from wild-type bacterial lines. Using transgenic and mutant lines of Arabidopsis, we provide evidence that the signaling pathway activated by volatiles from GB03 is dependent on ethylene, albeit independent of the salicylic acid or jasmonic acid signaling pathways. This study provides new insight into the role of bacteria VOCs as initiators of defense responses in plants.

Bacterial volatiles promote growth in Arabidopsis.

Several chemical changes in soil are associated with plant growth-promoting rhizobacteria (PGPR). Some bacterial strains directly regulate plant physiology by mimicking synthesis of plant hormones, whereas others increase mineral and nitrogen availability in the soil as a way to augment growth. Identification of bacterial chemical messengers that trigger growth promotion has been limited in part by the understanding of how plants respond to external stimuli. With an increasing appreciation of how volatile organic compounds signal plants and serve in plant defense, investigations into the role of volatile components in plant-bacterial systems now can follow. Here, we present chemical and plant-growth data showing that some PGPR release a blend of volatile components that promote growth of Arabidopsis thaliana. In particular, the volatile components 2,3-butanediol and acetoin were released exclusively from two bacterial strains that trigger the greatest level of growth promotion. Furthermore, pharmacological applications of 2,3-butanediol enhanced plant growth whereas bacterial mutants blocked in 2,3-butanediol and acetoin synthesis were devoid in this growth-promotion capacity. The demonstration that PGPR strains release different volatile blends and that plant growth is stimulated by differences in these volatile blends establishes an additional function for volatile organic compounds as signaling molecules mediating plant-microbe interactions.