Ana B. Hervás

Antagonistic activity of Bacteria from the chickpea rhizosphere againstFusarium Oxysporum f. sp.Ciceris

The antagonistic activity againstin vitro growth ofFusarium oxysporum f. sp.ciceris was determined for 74 bacterial isolates obtained from the rhizosphere of chickpeas grown in two field soils with different histories of Fusarium wilt, and for seven isolates ofPseudomonas spp. from culture collections. Twenty-four isolates ofBacillus spp. andPseudomonas chlororaphis 30-84 showed a strong antagonism against three races (0, 1 and 5) ofF. o. ciceris tested. Three selectedBacillus isolates andP. chlororaphis 30-84 were further tested against 30 isolates of races 0, 1 and 5 ofF. o. ciceris, races 0, 1 and 2 ofF. o. melonis, F. o. phaseoli and nonpathogenicF. oxysporum. Bacillus isolates differed in their antagonistic activity and were less inhibitory to mycelial growth ofF. o. ciceris than to that of other fungal isolates. Furthermore, the extent of growth inhibition ofF. o. ciceris was influenced both by bacterial isolates and by race of the pathogen. Cell-free culture filtrates of fourBacillus isolates inhibited conidial germination and hyphal growth ofF. o. ciceris and nonpathogenicF. oxysporum. Joint seed+soil treatment with some selected antagonisticBacillus spp. isolates suppressed disease caused by the highly virulentF. o. ciceris race-5 in cv. ICCV 4 and cv. PV 61 chickpeas. However, the degree of protection was influenced by the host genotype and the inoculum concentration of the pathogen.

Transcriptome Analysis of Pseudomonas putida in Response to Nitrogen Availability

This work describes a regulatory network of Pseudomonas putida controlled in response to nitrogen availability. We define NtrC as the master nitrogen regulator and suggest that it not only activates pathways for the assimilation of alternative nitrogen sources but also represses carbon catabolism under nitrogen-limited conditions, possibly to prevent excessive carbon and energy flow in the cell.

Influence of Temperature and Inoculum Density of Fusarium oxysporum f. sp. ciceris on Suppression of Fusarium Wilt of Chickpea by Rhizosphere Bacteria.

The effects of temperature and inoculum density of Fusarium oxysporum f. sp. ciceris race 5 on suppression of Fusarium wilt in chickpea (Cicer arietinum) cv. PV 61 by seed and soil treatments with rhizobacteria isolated from the chickpea rhizosphere were studied in a model system. Disease development over a range of temperatures (20, 25, and 30 degrees C) and inoculum densities (25 to 1,000 chlamydospores per gram of soil) was described by the Gompertz model. The Gompertz relative rate of disease progress and final amount of disease increased exponentially and monomolecularly, respectively, with increasing inoculum densities. Disease development was greater at 25 degrees C compared with 20 and 30 degrees C. At 20 and 30 degrees C, disease development was greater at 250 to 1,000 chlamydospores per gram of soil compared with 25 to 100 chlamydospores per gram of soil. At 25 degrees C, increasing inoculum densities of the pathogen did not influence disease. Nineteen Bacillus, Paenibacillus, Pseudomonas, and Stenotrophomonas spp. out of 23 bacterial isolates tested inhibited F. oxysporum f. sp. ciceris in vitro. Pseudomonas fluorescens RGAF 19 and RG 26, which did not inhibit the pathogen, showed the greatest Fusarium wilt suppression. Disease was suppressed only at 20 or 30 degrees C and at inoculum densities below 250 chlamydospores per gram of soil. Bacterial treatments increased the time to initial symptoms, reduced the Gompertz relative rate of disease progress, and reduced the overall amount of disease developed.

NtrC-Dependent Regulatory Network for Nitrogen Assimilation in Pseudomonas putida

Pseudomonas putida KT2440 is a model strain for studying bacterial biodegradation processes. However, very little is known about nitrogen regulation in this strain. Here, we show that the nitrogen regulatory NtrC proteins from P. putida and Escherichia coli are functionally equivalent and that substitutions leading to partially active forms of enterobacterial NtrC provoke the same phenotypes in P. putida NtrC. P. putida has only a single P(II)-like protein, encoded by glnK, whose expression is nitrogen regulated. Two contiguous NtrC binding sites located upstream of the sigma(N)-dependent glnK promoter have been identified by footprinting analysis. In vitro experiments with purified proteins demonstrated that glnK transcription was directly activated by NtrC and that open complex formation at this promoter required integration host factor. Transcription of genes orthologous to enterobacterial codB, dppA, and ureD genes, whose transcription is dependent on sigma(70) and which are activated by Nac in E. coli, has also been analyzed for P. putida. Whereas dppA does not appear to be regulated by nitrogen via NtrC, the codB and ureD genes have sigma(N)-dependent promoters and their nitrogen regulation was exerted directly by NtrC, thus avoiding the need for Nac, which is missing in this bacterial species. Based upon these results, we propose a simplified nitrogen regulatory network in P. putida (compared to that in enterobacteria), which involves an indirect-feedback autoregulation of glnK using NtrC as an intermediary.

Distinct roles for NtrC and GlnK in nitrogen regulation of the Pseudomonas sp. strain ADP cyanuric acid utilization operon

The Pseudomonas sp. strain ADP atzDEF operon encodes the enzymes involved in cyanuric acid mineralization, the final stage of the s-triazine herbicide atrazine degradative pathway. We have previously shown that atzDEF is under nitrogen control in both its natural host and Pseudomonas putida KT2442. Expression of atzDEF requires the divergently encoded LysR-type transcriptional regulator AtzR. Here, we take advantage of the poor induction of atzDEF in Escherichia coli to identify Pseudomonas factors involved in nitrogen control of atzDEF expression. Simultaneous production of P. putida NtrC and GlnK, along with AtzR, restored the normal atzDEF regulatory pattern. Gene expression analysis in E. coli and P. putida indicated that NtrC activates atzR expression, while the role of GlnK is to promote AtzR activation of atzDEF under nitrogen limitation. Activation of atzDEF in a mutant background deficient in GlnK uridylylation suggests that post-translational modification is not strictly required for transduction of the nitrogen limitation signal to AtzR. The present data and our previous results are integrated in a regulatory circuit that describes all the known responses of the atzDEF operon.

Regulation of glutamate dehydrogenase expression in Pseudomonas putida results from its direct repression by NtrC under nitrogen-limiting conditions

Nitrogen‐regulated genes in enterobacteria are positively controlled by the transcriptional activator of σN‐dependent promoters NtrC, either directly or indirectly, through the dual regulator Nac. Similar to enterobacteria, gdhA encoding glutamate dehydrogenase from Pseudomonas putida is one of the few genes that is induced by excess nitrogen. In P. putida, the binding of NtrC to the gdhA promoter region and in vitro transcription suggest that, unlike its enterobacterial homologue that is repressed by Nac, gdhA is directly repressed by NtrC. Footprinting analyses demonstrated that NtrC binds to four distinct sites in the gdhA promoter. NtrC dimers bind cooperatively, and those bound closer to the promoter interact with the dimers bound further upstream, thus producing a proposed repressor loop in the DNA. The formation of the higher‐order complex and the repressor loop appears to be important for repression but not absolutely essential. Both the phosphorylated and the non‐phosphorylated forms of NtrC efficiently repressed gdhA transcription in vitro and in vivo. Therefore, NtrC repression of gdhA under nitrogen‐limiting conditions does not depend on the phosphorylation of the regulator; rather, it relies on an increase in the repressor concentration under these conditions.

Glutamate Dehydrogenases: Enzymology, Physiological Role and Biotechnological Relevance

© 2012 Santero et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Glutamate Dehydrogenases: Enzymology, Physiological Role and Biotechnological Relevance

REGULATION OF THE ATRAZINE DEGRADATIVE PATHWAY IN Pseudomonas

9 paginas, 2 figuras. NATO Advanced Research Workshop on Bioredediation of Soils Contaminated with Aromatic Compounds (2004. Tartu, Estonia). Editado por Hermann J. Heipieper.