Arabidopsis thaliana root and root exudate metabolism is altered by the growth-promoting bacterium Kosakonia radicincitans DSM 16656T

Plant growth-promoting bacteria (PGPB) affect host physiological processes in various ways. This study aims at elucidating the dependence of bacterial-induced growth promotion on the plant genotype and characterizing plant metabolic adaptations to PGPB. Eighteen Arabidopsis thaliana accessions were inoculated with the PGPB strain Kosakonia radicincitans DSM 16656(T). Colonisation pattern was assessed by enhanced green fluorescent protein (eGFP)-tagged K. radicincitans in three A. thaliana accessions differing in their growth response. Metabolic impact of bacterial colonisation was determined for the best responding accession by profiling distinct classes of plant secondary metabolites and root exudates. Inoculation of 18 A. thaliana accessions resulted in a wide range of growth responses, from repression to enhancement. Testing the bacterial colonisation of three accessions did not reveal a differential pattern. Profiling of plant secondary metabolites showed a differential accumulation of glucosinolates, phenylpropanoids and carotenoids in roots. Analysis of root exudates demonstrated that primary and secondary metabolites were predominantly differentially depleted by bacterial inoculation. The plant genotype controls the bacterial growth promoting traits. Levels of lutein and beta-carotene were elevated in inoculated roots. Supplementing a bacterial suspension with beta-carotene increased bacterial growth, while this was not the case when lutein was applied, indicating that beta-carotene could be a positive regulator of plant growth promotion

How plants might recognize rhizospheric bacterial volatiles

In contrast to animals, plants possess neither olfactory organs nor a central nervous system. However, they do perceive and systemically react to volatile stimuli. Such function serves in monitoring the immediate and remote environments and translates into optimized responses to biotic and abiotic stresses. While the ecological relevance of volatile-mediated plant–plant and plant–insect interactions is today unquestioned, both above- and below-ground plant–microbe communication through VOCs has only gained attention recently. The common metabolic origins that yield the vast chemical diversity of plant and microbes allow for a substantial overlap between plant and microbial volatile species. Hence, it remains unclear if plants recognize and/or distinguish plant-like from foreign cues. The identities of the cellular components ensuring such recognition are even more obscure. Easy-to-score plant outputs in response to microbial VOCs elicitation, like plant growth promotion and innate immunity stimulation, will be instrumental to pinpointing VOCs-sensing proteins. Several major phytohormones have a gaseous nature and dedicated perception machineries that could serve as a basis to envisage how volatile semiochemicals might be sensed by plants. If volatile-mediated communication represents an ancestral cellular feature, VOCs perception and signalling might rely on basal protein families and define a universal chemical language

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Not AvailableSymbiotic (Rhizobia, Frankia, and VAM) or free-living (Azotobacter, and Clostridium) association of plant growth-promoting rhizobacteria (PGPR) and fungi (PGPF) is essential for plant and soil health. Nitrogen (N), phosphorus (P) and potassium (K) as major and iron (Fe) and zinc (Zn) as the minor elements are key to plant health. They are important constituents of plant genetic material (N, P) and chlorophyll content (N, Fe) and important for enzymatic activities (Fe, Zn) and are involved in many biochemical and physiological activities. The ‘microbiome’ around the rhizosphere is specific to plant type and involved in nutrient cycling through various processes such as fixation (N), solubilization, mineralization (P, K) and uptake, with the help of various organic acids (gluconic acid, oxalic acid, and tartaric acid), siderophore activity (Fe uptake) and enzymatic actions (nitrogenase, phytases, and acid phosphatases). Phytohormones essential to plant growth and development are produced by microbes themselves or induce their production via other hormones or communication chemicals, viz., volatile organic compounds (VOCs) like 2-pentylfuran, 2,3-butanediol and acetonin. PGPR (Pseudomonas, Trichoderma and Streptomyces) helps the host plant to fight against various abiotic and biotic stresses by the release of bactericidal and fungicidal enzymes, metabolite accumulation and induced systemic resistance (ISR), systemic acquired resistance (SAR) by phytohormones (jasmonic acid, salicylic acid, and ethylene) and VOCs. Attributing to so many benefits, microbes are increasingly becoming part of sustainable agriculture where PGPR (Rhizobium and Pseudomonas) and fungi (Aspergillus, Trichoderma and VAM) are being used as biofertilizers either single strained or in consortia approach, where the latter is found to be more beneficial for plant and soil health.Not Availabl