The impact of inoculated plant growth-promoting rhizobacteria (PGPR) on its host physiology and nutrition depends on inoculum level. Whether the impact of the inoculated PGPR on the indigenous ...rhizosphere microbiota also varies with the PGPR inoculum level is unclear. Here, we tested this issue using the PGPR
CRT1-maize model system, where the initial seed inoculation is known to enhance maize growth and germination, and impacts the maize rhizomicrobiota, including microbial functional groups modulating plant growth.
CRT1 was added to the seeds at standard (10
cells.seed
) or reduced (10
cells.seed
) inoculation levels, in three fields. The effect of the two PGPR formulations was assessed on maize growth and on the
(nitrogen fixation),
(ACC deaminase activity) and
(2,4-diacetylphloroglucinol production) microbial functional groups. The size of the three functional groups was monitored by qPCR at the six-leaf stage and the flowering stage, and the diversity of the
and
functional groups (as well as the bacterial community) were estimated by MiSeq metabarcoding at the six-leaf stage. The results showed that the benefits of the reduced inoculant formulation were significant in two out of three fields, but different (often lower) than those of the standard formulation. The effects of formulations on the size of the three functional groups differed, and depended on field site and functional group. The reduced formulation had an impact on the diversity of
and
groups at one site, whereas the standard formulation had an impact at the two other sites. Inoculation significantly impacted the total bacterial community in the three fields, but only with the reduced formulation. In conclusion, the reduced inoculant formulation impacted the indigenous rhizosphere microbiota differently, but not less efficiently, than the standard formulation.
Pseudomonas inoculants may lose colony‐forming ability in soil, but soil properties involved are poorly documented. Here, we tested the hypothesis that soil acidity could reduce persistence and cell ...culturability of Pseudomonas protegens CHA0. At 1 week in vitro, strain CHA0 was found as culturable cells at pH 7, whereas most cells at pH 4 and all cells at pH 3 were noncultured. In 21 natural soils of contrasted pH, cell culturability loss of P. protegens CHA0 took place in all six very acidic soils (pH < 5.0) and in three of five acidic soils (5.0 < pH < 6.5), whereas it was negligible in the neutral and alkaline soils at 2 weeks and 2 months. No correlation was found between total cell counts of P. protegens CHA0 and soil composition data, whereas colony counts of the strain correlated with soil pH. Maintenance of cell culturability in soils coincided with a reduction in inoculant cell size. Some of the noncultured CHA0 cells were nutrient responsive in Kogure's viability test, both in vitro and in soil. Thus, this shows for the first time that the sole intrinsic soil composition factor triggering cell culturability loss in P. protegens CHA0 is soil acidity.
The soil property that can lead to deterioration of the routine ability of Pseudomonas bacteria to grow on laboratory medium is soil acidity, whereas growth ability is retained in non‐acidic soils.
Crop varieties differ in their ability to interact with Plant Growth-Promoting Rhizobacteria (PGPR), but the genetic basis for these differences is unknown. This issue was addressed with the PGPR
...Sp245, using 187 wheat accessions. We screened the accessions based on the seedling colonization by the PGPR and the expression of the phenylpyruvate decarboxylase gene
(for synthesis of the auxin indole-3-acetic acid), using
fusions. Then, the effects of the PGPR on the selected accessions stimulating Sp245 (or not) were compared in soil under stress. Finally, a genome-wide association approach was implemented to identify the quantitative trait loci (QTL) associated with PGPR interaction. Overall, the ancient genotypes were more effective than the modern genotypes for
root colonization and
expression. In non-sterile soil,
Sp245 improved wheat performance for three of the four PGPR-stimulating genotypes and none of the four non-PGPR-stimulating genotypes. The genome-wide association did not identify any region for root colonization but revealed 22 regions spread on 11 wheat chromosomes for
expression and/or
induction rate. This is the first QTL study focusing on molecular interaction with PGPR bacteria. The molecular markers identified provide the possibility to improve the capacity of modern wheat genotypes to interact with Sp245, as well as, potentially, other
strains.
Fluorescent pseudomonads from bean root and rhizosphere in Iran were investigated for biocontrol of the fungal pathogen Rhizoctonia solani. Phylogenetic analysis of concatenated 16S rRNA, gyrB and ...rpoD sequences for 33 Pseudomonas isolates showed that 15 belonged to four clusters within the ‘P. fluorescens’ group, i.e. one corresponding to P. thivervalensis, two others including P. moraviensis or P. baetica, and the last one without closely-related established species. The 18 other isolates belonged to five clusters within the ‘P. putida’ group, one including P. mosselii and P. entomophila, another including strains currently described as P. putida, and three without closely-related species described. Ten isolates were selected based on in vitro inhibition of R. solani. Cellulase activity was identified in three pseudomonads, chitinase activity in two pseudomonads, extracellular protease activity in nine pseudomonads and hydrogen cyanide production in two pseudomonads. Genes coding for production of phenazine, pyoluteorin, pyrrolnitrin and 2,4-diacetylphloroglucinol were not found, whereas the 1-aminocyclopropane-1-carboxylate deamination gene acdS was present in three pseudomonads. The antagonistic acdS+ strain VKh13 from the ‘P. putida’ group effectively protected soil-grown bean from R. solani AG 4-HGI. Results show that pseudomonads from uncharacterized taxa were readily obtained from Iranian soils and displayed biocontrol potential against R. solani.
Abstract
Members of the A ctinobacteria are among the most important litter decomposers in soil. The site of a waterlogged deciduous forest with acidic soil was explored for actinobacteria because ...seasonality of litter inputs, temperature, and precipitation provided contrasting environmental conditions, particularly variation of organic matter quantity and quality. We hypothesized that these factors, which are known to influence decomposition, were also likely to affect actinobacterial community composition. The relationship between the actinobacterial community, soil moisture and organic matter content was assessed in two soil horizons in the summer and winter seasons using a 16S rRNA taxonomic microarray and cloning-sequencing of 16S rRNA genes. Both approaches showed that the community differed significantly between horizons and seasons, paralleling the changes in soil moisture and organic matter content. The microarray analysis further indicated that the actinobacterial community of the upper horizon was characterized by high incidence of the genus M ycobacterium. In both horizons and seasons, the actinobacterial clone libraries were dominated (by 80%) by sequences of a separate clade sharing an ancestral node with S treptosporangineae. This relatedness is supported also by some common adaptations, for example, to soil acidity and periodic oxygen deprivation or dryness.
The phytostimulatory alphaproteobacterium Azospirillum lipoferum 4B exhibits the plant-beneficial gene acdS, which enables deamination of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid ...(ACC). Here, we show that acdS is in the vicinity of acdR, a homolog to leucine-responsive regulator lrp, in A. lipoferum 4B and most other acdS⁺Proteobacteria. Unlike in Beta- and Gammaproteobacteria, acdS (and acdR) is preferentially located on symbiotic islands and plasmids in Alphaproteobacteria. In A. lipoferum 4B, acdS was mapped on a 750-kb plasmid that is lost during phenotypic variation, whereas other phytobeneficial genes such as nifH (associative nitrogen fixation) are maintained. In Proteobacteria, the phylogenies of acdR and acdS were largely but not totally congruent, despite physical proximity of the genes, regardless of whether DNA or deduced protein sequences were used. Potential Lrp, cAMP receptor protein (CRP) and fumarate-nitrate reduction regulator (FNR) binding sites were evidenced in the acdS promoter regions of strain 4B and most of 46 other acdS⁺Proteobacteria. Indeed, transcriptional and enzymatic analyses done in vitro pointed to the involvement of Lrp- and FNR-like transcriptional up-regulation of ACC deaminase activity in A. lipoferum 4B. This is the first synteny, phylogenetic, and functional analysis of factors modulating acdS expression in Azospirillum plant growth-promoting rhizobacterium.
Pseudomonas strains producing 2,4-diacetylphloroglucinol (DAPG) can protect plants from soilborne phytopathogens and are considered the primary reason for suppressiveness of morainic Swiss soils to ...Thielaviopsis basicola-mediated black root-rot disease of tobacco, even though they also occur nearby in conducive sandstone soils. The underlying molecular mechanisms accounting for this discrepancy are not understood. In this study, we assessed the hypothesis that the presence of iron-rich vermiculite clay (dominant in suppressive soils) instead of illite (dominant in neighboring conducive soils) translates into higher levels of iron bioavailability and transcription of Pseudomonas DAPG synthetic genes in the tobacco rhizosphere. Rhizosphere monitoring of reporter gene systems pvd-inaZ and phlA-gfp in Pseudomonas protegens indicated that the level of iron bioavailability and the number of cells expressing phl genes (DAPG synthesis), respectively, were higher in vermiculitic than in illitic artificial soils. This was in accordance with the effect of iron on phlA-gfp expression in vitro and, indeed, iron addition to the illitic soil increased the number of cells expressing phlA-gfp. Similar findings were made in the presence of the pathogen T. basicola. Altogether, results substantiate the hypothesis that iron-releasing minerals may confer disease suppressiveness by modulating iron bioavailability in the rhizosphere and expression of biocontrol-relevant genes in antagonistic P. protegens.
Functional type III secretion system (T3SS) genes are needed for effective biocontrol of Pythium damping-off of cucumber by Pseudomonas fluorescens KD, but whether biocontrol Pseudomonas strains with ...T3SS genes display overall a higher plant-protecting activity is unknown. The assessment of 198 biocontrol fluorescent pseudomonads originating from 60 soils worldwide indicated that 32% harbour the ATPase-encoding T3SS gene hrcN, which was most often found in tomato isolates. The hrcN+ biocontrol strains (and especially those also producing 2,4-diacetylphloroglucinol and displaying 1-aminocyclopropane-1-carboxylate deaminase activity) displayed higher plant-protecting ability in comparison with hrcN− biocontrol strains, both in the Pythium/cucumber and Fusarium/cucumber pathosystems.
PDF
