Protection against pathogen and salt stress by four plant growth-promoting rhizobacteria isolated from Pinus sp. on Arabidopsis thaliana

The ability of four plant growth-promoting rhizobacteria, isolated in a previous study, to induce systemic resistance on Arabidopsis thaliana Col 0 against biotic and abiotic stress was evaluated. All the bacteria enhanced protection against the foliar pathogen Pseudomonas syringae DC3000 and increased plant tolerance to salt stress (NaCl 60 mM). Bacillus sp. strain L81 and Arthrobacter oxidans strain BB1 performed best with a decrease in the disease index of 61.2 and 52.3%, respectively, and a reduction in the mortality due to salt stress of 72.4 and 57.8%, respectively. Additionally, significant differences were found in growth and photosynthesis, again, L81 and BB1 performed best either in normal or under stress conditions. In order to elucidate the pathway elicited by these two strains to induce systemic resistance, experiments with the transgenic line of Arabidopsis thaliana NahG (defective in salicylic acid [SA]) and with the jar1 mutant (defective in jasmonic acid) were carried out. Results showed that the SA-dependent pathway was involved in the defense response induced by strains L81 and BB1. Results from quantitative reverse transcription-polymerase chain reaction analysis of the PR1 gene, related to the SA-dependent pathway and the PDF1.2 gene related to the SA-independent pathway, showed an increased expression of PR1 in BB1-treated plants, confirming involvement of the SA-dependent pathway in the defensive response.

Systemic disease protection elicited by plant growth promoting rhizobacteria strains: relationship between metabolic responses, systemic disease protection, and biotic elicitors

A study of plant defensive systemic responses induced by three plant growth promoting rhizobacteria (PGPR) on Arabidopsis thaliana Col 0 against Pseudomonas syringae pv. tomato DC3000 at the biochemical and transcriptional levels is reported in this paper. All three strains decreased disease severity when applied to A. thaliana prior to pathogen inoculation. At the biochemical level, each of the three strains induced ethylene (ET) when incubated with 1-amino-cyclopropane-1-carboxylic acid, and salicylic acid (SA) production in the plant. Plants treated with each of the three strains were also reduced in salicylic acid production after pathogen challenge compared to untreated controls. This effect was more marked in plants treated with Chryseobacterium balustinum AUR9, the strain most effective in decreasing disease severity. The expression level of PR1, a transcriptional marker of the SA-dependent pathway in C. balustinum AUR9-treated plants, is fourfold that of controls while the expression of PDF1.2, a transcriptional marker for the SA-independent pathway, is not induced. C. balustinum cell wall lipopolysaccharides, being putative bacterial elicitor molecules, are able to reproduce this systemic induction effect at low doses. From these observations, we hypothesize that certain PGPR strains are capable of stimulating different systemic responses in host plants. With C. balustinum AUR9, the SA-dependent pathway is stimulated first, as indicated by increases in SA levels and PR1 expression, followed by induction of the SA-independent pathway, as indicated by the increases in ET concentrations. The effects of both pathways combined with respect to disease suppression appear to be additive.

Effect of heavy metals on soil denitrification and CO2 production after short term incubation

The toxicity of three heavy metals, Cd, Zn and Cu, has been tested in a Mediterranean soil. The soil was incubated (108h) with mixed solutions of those metals before evaluating denitrification and CO2 production, both by gas chromatography. These activities were used as biological indicators of heavy metal toxicity, and compared to non-treated control soil samples. Statistical analyses showed no significant differences in CO2 production between treated and non-treated control soils. The lowest levels of respiration were observed in soils treated with the largest amounts of Zn and Cd. Denitrification increased significantly in soils treated with solutions containing 100 micrograms/ml of Cu and 1000 micrograms/ml of Cd or Zn.

[Density and activity of microorganisms of the carbon cycle under the canopy of Myrica gale L]

Plants, especially actinorhizal, regulate edaphic microflora through various ways, modifying thus nutrients recycling. Myrica gale effect on microorganisms in the carbon cycle is studied in this work by comparing soil samples collected under the canopy in summer and control samples. The results indicate that under M. gale C-organic and N-total concentration and anaerobic cellulolytic, hemicellulolytic and amilolytic density increase, and pH, C/N ratio and aerobic cellulolytic microorganisms density decrease. Microbial activity in soil is also modified.