Evidence for horizontal transfer of 1-aminocyclopropane-1-carboxylate deaminase genes

ACC (1-aminocyclopropane-1-carboxylate) deaminase activity, a widespread trait in Burkholderia species, and its growth-promoting effect on tomato plants

The genus Burkholderia includes pathogens of plants and animals and some human opportunistic pathogens, such as the Burkholderia cepacia complex (Bcc), but most species are nonpathogenic, plant associated, and rhizospheric or endophytic. Since rhizobacteria expressing ACC (1-aminocyclopropane-1-carboxylate) deaminase may enhance plant growth by lowering plant ethylene levels, in this work we investigated the presence of ACC deaminase activity and the acdS gene in 45 strains, most of which are plant associated, representing 20 well-known Burkholderia species. The results demonstrated that ACC deaminase activity is a widespread feature in the genus Burkholderia, since 18 species exhibited ACC deaminase activities in the range from 2 to 15 mumol of alpha-ketobutyrate/h/mg protein, which suggests that these species may be able to modulate ethylene levels and enhance plant growth. In these 18 Burkholderia species the acdS gene sequences were highly conserved (76 to 99% identity). Phylogenetic analysis of acdS gene sequences in Burkholderia showed tight clustering of the Bcc species, which were clearly distinct from diazotrophic plant-associated Burkholderia species. In addition, an acdS knockout mutant of the N(2)-fixing bacterium Burkholderia unamae MTl-641(T) and a transcriptional acdSp-gusA fusion constructed in this strain showed that ACC deaminase could play an important role in promotion of the growth of tomato plants. The widespread ACC deaminase activity in Burkholderia species and the common association of these species with plants suggest that this genus could be a major contributor to plant growth under natural conditions.

Proteomic analysis of the response of the plant growth-promoting bacterium Pseudomonas putida UW4 to nickel stress

Background

Plant growth-promoting bacteria can alleviate the inhibitory effects of various heavy metals on plant growth, via decreasing levels of stress-induced ethylene. However, little has been done to detect any mechanisms specific for heavy metal resistance of this kind of bacteria. Here, we investigate the response of the wild-type plant growth-promoting bacterium Pseudomonas putida UW4 to nickel stress using proteomic approaches. The mutant strain P. putida UW4/AcdS^-, lacking a functional 1-aminocyclopropane-1-carboxylic acid deaminase gene, was also assessed for its response to nickel stress.

Results

Two dimensional difference in-gel electrophoresis (DIGE) was used to detect significantly up- or down- regulated proteins (p < 0.05, | ratio | > 1.5) in P. putida in response to the presence of 2 mM Ni. Out of a total number of 1,702 proteins detected on the analytical gels for P. putida UW4, the expression levels of 82 (4.82%) proteins increased significantly while the expression of 81 (4.76%) proteins decreased significantly. Of 1,575 proteins detected on the analytical gels for P. putida UW4/AcdS^-, the expression levels of 74 (4.70%) proteins increased and 51 (3.24%) proteins decreased significantly. Thirty-five proteins whose expression was altered were successfully identified by mass spectrometry and sequence comparisons with related species. Nineteen of the identified proteins were detected as differentially expressed in both wild-type and mutant expression profiles.

Conclusion

Functional assessment of proteins with significantly altered expression levels revealed several mechanisms thought to be involved in bacterial heavy metal detoxification, including general stress adaptation, anti-oxidative stress and heavy metal efflux proteins. This information may contribute to the development of plant growth-promoting bacteria mediated phytoremediation processes.

Ethylene levels are regulated by a plant encoded 1-aminocyclopropane-1-carboxylic acid deaminase

Control of the levels of the plant hormone ethylene is crucial in the regulation of many developmental processes and stress responses. Ethylene production can be controlled by altering endogenous levels of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor to ethylene or by altering its conversion to ethylene. ACC is known to be irreversibly broken down by bacterial or fungal ACC deaminases (ACDs). Sequence analysis revealed two putative ACD genes encoded for in the genome of Arabidopsis thaliana (A. thaliana) and we detected ACD activity in plant extracts. Expression of one of these A. thaliana genes (AtACD1) in bacteria indicated that it had ACD activity. Moreover, transgenic plants harboring antisense constructs of the gene decreased ACD activity to 70% of wild-type (WT) levels, displayed an increased sensitivity to ACC and produced significantly more ethylene. Taken together, these results show that AtACD1 can act as a regulator of ACC levels in A. thaliana.

1-aminocyclopropane-1-carboxylate (ACC) deaminase genes in rhizobia from southern Saskatchewan

A collection of 233 rhizobia strains from 30 different sites across Saskatchewan, Canada was assayed for 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, with 27 of the strains displaying activity. When all 27 strains were characterized based on 16S rRNA gene sequences, it was noted that 26 strains are close to Rhizobium leguminosarum and one strain is close to Rhizobium gallicum. Polymerase chain reaction (PCR) was used to rapidly isolate ACC deaminase structural genes from the above-mentioned 27 strains; 17 of them have 99% identities with the previously characterized ACC deaminase structural gene (acdS) from R. leguminosarum bv. viciae 128C53K, whereas the other ten strains are 84% identical (864-866/1,020 bp) compared to the acdS from strain 128C53K. Southern hybridization showed that each strain has only one ACC deaminase gene. Using inverse PCR, the region upstream of the ACC deaminase structural genes was characterized for all 27 strains, and 17 of these strains were shown to encode a leucine-responsive regulatory protein. The results are discussed in the context of a previously proposed model for the regulation of bacterial ACC deaminase in R. leguminosarum 128C53K.

Effect of a nickel-tolerant ACC deaminase-producing Pseudomonas strain on growth of nontransformed and transgenic canola plants

Four bacterial strains were isolated from soils at nickel-contaminated sites based on their ability to utilize 1-aminocyclopropane-1-carboxylate (ACC) as a sole source of nitrogen. The four isolates were all identified as Pseudomonas putida Biovar B, and subsequent testing revealed that they all exhibited traits previously associated with plant growth promotion (i.e., indoleacetic acid and siderophore production and ACC deaminase activity). These four strains were also tolerant of nickel concentrations of up to 13.2 mM in the culture medium. The strain, HS-2, selected for further characterization, was used in pot experiments to inoculate both nontransformed and transgenic canola plants (expressing a bacterial ACC deaminase gene in its roots). Plants inoculated with the HS-2 strain produced an increase in plant biomass as well as in nickel (Ni) uptake by shoots and roots. The results suggest that this strain is a potential candidate to be used as an inoculant in both phytoremediation protocols and in plant growth promotion.

Transcriptional regulation of ACC deaminase gene expression inPseudomonas putidaUW4

One of the major mechanisms that plant growth-promoting bacteria use to facilitate plant growth is through the lowering of plant ethylene levels by the bacterial enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Many of the bacterial ACC deaminase genes (acdS) that have been examined to date are under the transcriptional control of a leucine-responsive regulatory protein, Lrp, encoded by acdR and referred to here as AcdR. The work presented here is focused on how AcdR and the newly discovered AcdB protein from Pseudomonas putida UW4 are involved in the regulation of acdS expression. First, the results of gel retardation experiments showed that AcdR binds to the acdS regulatory region, and this binding activity in vitro is not affected by the addition of 2 mmol·L–1ACC but can be eliminated by addition of 20 μg·mL–1leucine. Second, a potential regulatory protein, AcdB, involved in the regulation of acdS expression, was identified through both yeast 2-hybrid screen and coimmunoprecipitation based on its ability to bind to AcdR; subsequently, its binding to the acdS regulatory region in the presence of ACC was shown by gel retardation experiments. The data are interpreted in terms of a model in which AcdR and AcdB co-regulate the expression of the acdS gene.

ACC deaminase from Pseudomonas fluorescens mediated saline resistance in groundnut (Arachis hypogea) plants

AIM

To study the effect of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase from Pseudomonas fluorescens against saline stress under in vitro and field conditions in groundnut (Arachis hypogea) plants.

METHODS AND RESULTS

Four plant growth-promoting rhizobacteria (PGPR) strains were used in this study to evaluate their efficacy in groundnut plants against saline stress under in vitro. Among the four PGPR strains used, Ps. fluorescens strain TDK1 showed greater performance in improving the plant growth parameters of groundnut seedlings in vitro. PCR amplification using Pseudomonas-specific 16S-23S rRNA internal transcribed spacers (ITS) primers revealed that all the four strains belonged to the group of fluorescent pseudomonads. ITS region of Ps. fluorescens strain TDK1 was cloned and sequenced. ACC deaminase activity using biochemical and molecular (PCR) analysis revealed that among all the four strains, Ps. fluorescens strain TDK1 showed greater amount of ACC deaminase activity and positive reaction to PCR amplification. ACC deaminase gene from Ps. fluorescens strain TDK1 was isolated, cloned and sequenced. Pseudomonas bioformulations were developed and they were tested in groundnut plants under saline-affected soils. The results indicated the superior performance by Ps. fluorescens strain TDK1 possessing ACC deaminase activity in improving yield parameters in groundnut plants despite salinity.

CONCLUSIONS

Pseudomonas fluorescens strain TDK1 possessing ACC deaminase activity enhanced the saline resistance in groundnut plants, which in turn resulted in increased yield when compared with the groundnuts treated with Pseudomonas strains not having ACC deaminase activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The promising role of ACC deaminase from Ps. fluorescens strain TDK1 in alleviating saline stress has been concluded in groundnut plants. This study will be useful for exploiting the activity of ACC deaminase from microbial strains against various biotic and abiotic stresses wherever ACC accumulated as precursor for ethylene biosynthesis.

1-Aminocyclopropane-1-carboxylate deaminase from Pseudomonas putida UW4 facilitates the growth of canola in the presence of salt

The growth of canola plants treated with either wild-type Pseudomonas putida UW4 or a 1-aminocyclopropane-1-carboxylate (ACC) deaminase minus mutant of this strain was monitored in the presence of inhibitory levels of salt, i.e., 1.0 mol/L at 10 °C and 150 mmol/L at 20 °C. This strain is psychrotolerant with a maximal growth rate of approximately 30 °C and the ability to proliferate at 4 °C. Although plant growth was inhibited dramatically by the addition of 1.0 mol/L salt at 10 °C and only slightly by 150 mmol/L salt at 20 °C under both sets of conditions, the addition of the wild type but not the mutant strain of P. putida UW4 significantly improved plant growth. This result confirms the previous suggestion that bacterial strains that contain ACC deaminase confer salt tolerance to plants by lowering salt-induced ethylene synthesis.

Tolerance of transgenic canola plants (Brassica napus) amended with plant growth-promoting bacteria to flooding stress at a metal-contaminated field site

The growth of transgenic canola (Brassica napus) expressing a gene for the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase was compared to non-transformed canola exposed to flooding and elevated soil Ni concentration, in situ. In addition, the ability of the plant growth-promoting bacterium Pseudomonas putida UW4, which also expresses ACC deaminase, to facilitate the growth of non-transformed and transgenic canola under the above mentioned conditions was examined. Transgenic canola and/or canola treated with P. putida UW4 had greater shoot biomass compared to non-transformed canola under low flood-stress conditions. Under high flood-stress conditions, shoot biomass was reduced and Ni accumulation was increased in all instances relative to low flood-stress conditions. This is the first field study to document the increase in plant tolerance utilizing transgenic plants and plant growth-promoting bacteria exposed to multiple stressors.

Reaction mechanisms of the bacterial enzyme 1-aminocyclopropane-1-carboxylate deaminase

The enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase promotes plant growth by sequestering and cleaving plant-produced ACC thereby lowering the level of ethylene in the plant. Decreased ethylene levels allow the plant to be more resistant to a wide variety of environmental stresses. Here the biochemical reaction mechanisms involved in ACC deaminase activity are critically reviewed.

Genetic modulation of ethylene biosynthesis and signaling in plants

With the isolation and characterization of the key enzymes and proteins, and the corresponding genes, involved in ethylene biosynthesis and sensing it has become possible to manipulate plant ethylene levels and thereby alter a wide range of physiological processes. The phytohormone ethylene is an essential signaling molecule that affects a large number of physiological processes; plants deprived of ethylene do not grow and develop normally. In a search for flexible on-off ethylene control, scientists have used inducible organ- and tissue-specific promoters to drive expression of different transgenes. Here, the various strategies that have been used to genetically engineer plants with decreased ethylene biosynthesis and sensitivity are reviewed and discussed.

Growth of canola (Brassica napus) in the presence of plant growth-promoting bacteria and either copper or polycyclic aromatic hydrocarbons

Growth of canola (Brassica napus) seeds treated with plant growth-promoting bacteria in copper-contaminated and polycyclic aromatic hydrocarbon (PAH)-contaminated soils was monitored. Pseudomonas asplenii AC, isolated from PAH-contaminated soil, was transformed to express a bacterial gene encoding 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and both native and transformed bacteria were tested for growth promotion. Inoculation of seeds, grown in the presence of copper or creosote, with either native or transformed P. asplenii AC significantly increased root and shoot biomass. Native and transformed P. asplenii AC and transformed P. asplenii AC encapsulated in alginate were equally effective at promoting plant growth in copper-contaminated soils. In creosote-contaminated soils the native bacterium was the least effective, and the transformed encapsulated bacterium was the most effective in growth promotion.Key words: plant growth-promoting bacteria, phytoremediation, copper, polycyclic aromatic hydrocarbons, Brassica napus, ethylene, alginate encapsulation.