Bacterial Biosynthesis of 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase and Indole-3-Acetic Acid (IAA) as Endophytic Preferential Selection Traits by Rice Plant Seedlings

In this study, bacteria were isolated from the rhizosphere and inside the roots and nodules of berseem clover plants grown in the field in Iran. Two hundred isolates were obtained from the rhizosphere (120 isolates), interior roots (57 isolates), and nodules (23 isolates) of clover plants grown in rotation with rice plants. Production of chitinase, pectinase, cellulase, siderophore, salicylic acid, hydrogen cyanide, indole acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase, solubilization of phosphate, antifungal activity against various rice plant pathogen fungi, N₂ fixation, and colonization assay on rice seedlings by these strains was evaluated and compared (endophytic isolates vs. rhizosphere bacteria). The results showed both the number and the ability of plant growth-promoting (PGP) traits were different between endophytic and rhizosphere isolates. A higher percentage of endophytic isolates were positive for production of IAA, ACC deaminase, and siderophore than rhizosphere isolates. Therefore, it is suggested that clover plant may shape its own associated microbial community and act as filters for endophyte communities, and rhizosphere isolates with different (PGP) traits. We also studied the PGP effect of the most promising endophytic and rhizosphere isolates on rice seedlings. A significant relationship among IAA and ACC deaminase production, the size of root colonization, and plant growth (root elongation) in comparison with siderophore production and phosphate solubilization for the isolates was observed. The best bacterial isolates (one endophytic isolate and one rhizosphere isolate), based on their ability to promote rice growth and colonize rice roots, were identified. Based on 16S rDNA sequence analysis, the endophytic isolate CEN7 and the rhizosphere isolate CEN8 were closely related to Pseudomonas putida and Pseudomonas fluorescens, respectively. It seems that PGP trait production (such as IAA, ACC deaminase) may be required for endophytic and rhizosphere competence as compared to other PGP traits in rice seedlings under constant flooded conditions. The study also shows that the presence of diverse rhizobacteria with effective growth-promoting traits associated with clover plants may be used for sustainable crop management under field conditions.     

Expression of an Exogenous 1-Aminocyclopropane-1-Carboxylate Deaminase Gene in Sinorhizobium meliloti Increases Its Ability To Nodulate Alfalfa

1-Aminocyclopropane-1-carboxylate (ACC) deaminase has been found in various plant growth-promoting rhizobacteria, including rhizobia. This enzyme degrades ACC, the immediate precursor of ethylene, and thus decreases the biosynthesis of ethylene in higher plants. The ACC deaminase of Rhizobium leguminosarum bv. viciae 128C53K was previously reported to be able to enhance nodulation of peas. The ACC deaminase structural gene (acdS) and its upstream regulatory gene, a leucine-responsive regulatory protein (LRP)-like gene (lrpL), from R. leguminosarum bv. viciae 128C53K were introduced into Sinorhizobium meliloti, which does not produce this enzyme, in two different ways: through a plasmid vector and by in situ transposon replacement. The resulting ACC deaminase-producing S. meliloti strains showed 35 to 40% greater efficiency in nodulating Medicago sativa (alfalfa), likely by reducing ethylene production in the host plants. Furthermore, the ACC deaminase-producing S. meliloti strain was more competitive in nodulation than the wild-type strain. We postulate that the increased competitiveness might be related to utilization of ACC as a nutrient within the infection threads.     

Isolation of ACC deaminase-producing habitat-adapted symbiotic bacteria associated with halophyte Limonium sinense (Girard) Kuntze and evaluating their plant growth-promoting activity under salt stress

Background and aims

Many plant growth-promoting endophytes (PGPE) possessing 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity can reduce the level of stress ethylene and assist their host plants cope with various biotic and abiotic stresses. However, information about the endophytic bacteria colonizing in the coastal halophytes is still very scarce. This study aims at isolating efficient ACC deaminase-producing plant growth-promoting (PGP) bacterial strains from the inner tissues of a traditional Chinese folk medicine Limonium sinense (Girard) Kuntze, a halophyte which has high economic and medicinal values grown in the coastal saline soils. Their PGP activity and effects on host seed germination and seedling growth under salinity stress were also evaluated.

Methods

A total of 126 isolates were obtained from the surface sterilized roots, stems and leaves of L. sinense (Girard) Kuntze. They were initially selected for their ability to produce ACC deaminase as well as other PGP properties such as production of indole-3-acetic acid (IAA), N₂-fixation, and phosphate-solubilizing activities and subsequently identified by the 16S rRNA gene sequencing. For selected strains, seed germination, seedling growth, and flavonoids production in axenically growth L. sinense (Girard) Kuntze seedlings at different NaCl concentrations (0–500 mM) were quantified.

Results

Thirteen isolates possessing ACC deaminase activity were obtained. The 16S rRNA gene sequencing analysis showed them to belong to eight genera: Bacillus, Pseudomonas, Klebsiella, Serratia, Arthrobacter, Streptomyces, Isoptericola, and Microbacterium. Inoculation with four of the selected ACC deaminase-producing strains not only stimulated the growth of the host plant but also influenced the flavonoids accumulation. All four strains could colonize and can be re-isolated from the host plant interior tissues.

Conclusions

These results demonstrate that ACC deaminase-producing habitat-adapted symbiotic bacteria isolated from halophyte could enhance plant growth under saline stress conditions and the PGPE strains could be appropriate as bioinoculants to enhance soil fertility and protect the plants against salt stress.     

Bacterial biosynthesis of 1-aminocyclopropane-1-caboxylate (ACC) deaminase,a useful trait to elongation and endophytic colonization of the roots of rice under constant flooded conditions

This study was conducted to investigate the role of 1-aminocyclopropane-1-carboxylate (ACC) deaminase in Pseudomonas fluorescens strain REN₁ and its ability to reduce ethylene levels produced during stress, endophytically colonize and promote the elongation of the roots of rice seedlings under gnotobiotic conditions. We isolated 80 bacteria from inside roots of rice plants grown in the farmers’ fields in Guilan, Iran. All of the isolates were characterized for plant growth promoting (PGP) traits. In addition, the colonization assay of these isolates on rice seedlings was carried out to screen for competent endophytes. The best bacterial isolate, based on ACC deaminase production, was identified and used for further study. 16S rDNA sequence analysis revealed that the endophyte was closely related to Pseudomonas fluorescens. The results of this study showed ACC deaminase containing P. fluorescens REN1 increased in vitro root elongation and endophytically colonized the root of rice seedlings significantly, as compared to control under constant flooded conditions. The trait of low amount of indole-3-acetic acid (IAA) production (<15 μg mL⁻¹) and the high production of ACC deaminase by bacteria may be main factors in colonizing rice seedling roots compared to other PGP traits (siderophore production and phosphate solubilization) in this study. Endophytic IAA and ACC deaminase-producing bacteria may be preferential selections by rice seedlings. Therefore, it may be suggested that the utilization of ACC as a nutrient gives the isolates advantages in more endophytic colonization and increase of root length of rice seedlings.     

Native bacterial endophytes promote host growth in a species-specific manner; phytohormone manipulations do not result in common growth responses

Background

All plants in nature harbor a diverse community of endophytic bacteria which can positively affect host plant growth. Changes in plant growth frequently reflect alterations in phytohormone homoeostasis by plant-growth-promoting (PGP) rhizobacteria which can decrease ethylene (ET) levels enzymatically by 1-aminocyclopropane-1-carboxylate (ACC) deaminase or produce indole acetic acid (IAA). Whether these common PGP mechanisms work similarly for different plant species has not been rigorously tested.

Methodology/ Principal Findings

We isolated bacterial endophytes from field-grown Solanum nigrum; characterized PGP traits (ACC deaminase activity, IAA production, phosphate solubilization and seedling colonization); and determined their effects on their host, S. nigrum, as well as on another Solanaceous native plant, Nicotiana attenuata. In S. nigrum, a majority of isolates that promoted root growth were associated with ACC deaminase activity and IAA production. However, in N. attenuata, IAA but not ACC deaminase activity was associated with root growth. Inoculating N. attenuata and S. nigrum with known PGP bacteria from a culture collection (DSMZ) reinforced the conclusion that the PGP effects are not highly conserved.

Conclusions/ Significance

We conclude that natural endophytic bacteria with PGP traits do not have general and predictable effects on the growth and fitness of all host plants, although the underlying mechanisms are conserved.     

Quorum sensing activity of the plant growth-promoting rhizobacterium <Emphasis Type="Italic">Serratia glossinae</Emphasis> GS2 isolated from the sesame (<Emphasis Type="Italic">Sesamum indicum</Emphasis> L.) rhizosphere

Plant growth-promoting rhizobacteria (PGPR) affect plant growth through various mechanisms, such as indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, and biofilm formation. The aim of the study reported here was to isolate and characterize rhizobacteria that produce quorum-sensing signal molecules and other PGPR-related molecules. A biofilm-forming bacterium, GS2, was isolated from the rhizosphere of a sesame plant and subsequently found to produce two quorum-sensing signal molecules that were identified as N-hexanoyl-L-homoserine lactone (m/z 200) and N-octanoyl-L-homoserine lactone (m/z 228) by liquid chromatography–tandem mass spectrometry analysis. The strain was also found to produce IAA (17.2 μg mL^?1), gibberellins (113.7 μg mL^?1), and ACC deaminase (9.7 μM α-ketobutyrate mg^?1 protein h^?1). The strain was identified as Serratia glossinae based on a comparison of 16S rRNA gene sequences. Inoculation of the strain promoted growth of a gibberellin-deficient rice dwarf mutant (Waito-C). Different growth attributes, including shoot and root elongation, chlorophyll content, and plant weight could be attributed to the PGPR characteristics of strain GS2. These results suggest that S. glossinae strain GS2 can serve as a microbial agent that improves plant growth.     

Rhizobium leguminosarum Biovar viciae 1-Aminocyclopropane-1-Carboxylate Deaminase Promotes Nodulation of Pea Plants

Ethylene inhibits nodulation in various legumes. In order to investigate strategies employed by Rhizobium to regulate nodulation, the 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene was isolated and characterized from one of the ACC deaminase-producing rhizobia, Rhizobium leguminosarum bv. viciae 128C53K. ACC deaminase degrades ACC, the immediate precursor of ethylene in higher plants. Through the action of this enzyme, ACC deaminase-containing bacteria can reduce ethylene biosynthesis in plants. Insertion mutants with mutations in the rhizobial ACC deaminase gene (acdS) and its regulatory gene, a leucine-responsive regulatory protein-like gene (lrpL), were constructed and tested to determine their abilities to nodulate Pisum sativum L. cv. Sparkle (pea). Both mutants, neither of which synthesized ACC deaminase, showed decreased nodulation efficiency compared to that of the parental strain. Our results suggest that ACC deaminase in R. leguminosarum bv. viciae 128C53K enhances the nodulation of P. sativum L. cv. Sparkle, likely by modulating ethylene levels in the plant roots during the early stages of nodule development. ACC deaminase might be the second described strategy utilized by Rhizobium to promote nodulation by adjusting ethylene levels in legumes.     

Improved growth performance of the mangrove <Emphasis Type="Italic">Avicennia marina</Emphasis> seedlings using a 1-aminocyclopropane-1-carboxylic acid deaminase-producing isolate of <Emphasis Type="Italic">Pseudoalteromonas maricaloris</Emphasis>

Out of 62 bacterial isolates obtained from the mangrove Avicennia marina rhizosphere that grows along the Abu Dhabi coast, United Arab Emirates (UAE), an isolate of Pseudoalteromonas maricaloris (Wild type strain) (WT) produced relatively high levels of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase in vitro. Application of this WT strain under greenhouse conditions to A. marina seedlings significantly (P < 0.05), reduced endogenous levels of ACC in the roots and shoots, and significantly (P < 0.05) increased the levels of in planta endogenous plant growth regulators (PGRs) including indole-3-acetic acid (IAA), indole-3-pyruvic acid (IPYA), putrescine (Put), spermidine (Spd) and spermine (Spm) in roots and shoots compared with control mangrove seedlings. WT application has also significantly (P < 0.05) increased photosynthetic pigment contents, photosynthetic carbon assimilation, plant water use efficiency and promoted mangrove seedlings growth characteristics including increased dry weight and length of roots and shoots, total leaf area and the number of the side branches compared with control mangrove seedlings. In comparison, an ACC deaminase non-producing mutant strain (NPM) failed to reduce endogenous levels of ACC in the roots and shoots and also failed to increase endogenous PGRs and photosynthetic pigments and did not promote seedling growth. Both WT and NPM strains were incapable of producing in vitro detectable levels of IAA, IPYA, Gibberellic acid (GA₃), zeatin (Z), Put, Spd and Spm in the culture filtrates. This study demonstrated for the first time the ability of ACC deaminase-producing bacteria to promote mangrove growth under greenhouse conditions. P. maricaloris has potential as biological inoculants to promote the growth of mangrove seedlings in afforestation programs in nutrient impoverished sediments in hyper-saline coastal areas in the UAE.     

A quantitative analysis of root distortion from contrasting wheat cropping systems

Aims

Bacterial ACC deaminase is one of the key tools to ameliorate plant stress by lowering ethylene level in plants. The effects of ACC deaminase-producing bacteria on the volatile profiles in plants have not been examined to date. To address this, we performed metabolic profiling of volatiles in carrots following inoculation of the bacteria producing ACC deaminase.

Methods

We isolated ACC deaminase-producing bacteria from the inner part of the fruits and vegetables grown on organic farms by culturing on ACC-containing media, and screened them with PCR for the acdS gene, mungbean growth assay, and in vitro ACC deaminase activity. The isolated endophytes were evaluated for their ability to alter volatile profiles in carrots.

Results

Eleven bacterial strains possessing the activity to cleave ACC were selected among the 60 isolates grown on the medium containing ACC as a sole N source. Three of them that belonged to Pseudomonas could reduce the levels of (E)-2-hexenal and the other green leaf volatiles (GLVs) and terpenoids in the carrot leaves following inoculation of the seeds.

Conclusions

The isolated endophytes with ACC deaminase activity could alter the composition of volatiles in plants, probably through lowering ethylene level in the plant.

     

Expression of an exogenous 1-aminocyclopropane-1-carboxylate deaminase gene in Sinorhizobium meliloti increases its ability to nodulate alfalfa

1-Aminocyclopropane-1-carboxylate (ACC) deaminase has been found in various plant growth-promoting rhizobacteria, including rhizobia. This enzyme degrades ACC, the immediate precursor of ethylene, and thus decreases the biosynthesis of ethylene in higher plants. The ACC deaminase of Rhizobium leguminosarum bv. viciae 128C53K was previously reported to be able to enhance nodulation of peas. The ACC deaminase structural gene (acdS) and its upstream regulatory gene, a leucine-responsive regulatory protein (LRP)-like gene (lrpL), from R. leguminosarum bv. viciae 128C53K were introduced into Sinorhizobium meliloti, which does not produce this enzyme, in two different ways: through a plasmid vector and by in situ transposon replacement. The resulting ACC deaminase-producing S. meliloti strains showed 35 to 40% greater efficiency in nodulating Medicago sativa (alfalfa), likely by reducing ethylene production in the host plants. Furthermore, the ACC deaminase-producing S. meliloti strain was more competitive in nodulation than the wild-type strain. We postulate that the increased competitiveness might be related to utilization of ACC as a nutrient within the infection threads.     

Rhizosphere bacteria affect growth and metal uptake of heavy metal accumulating willows

A variety of plants growing on metalliferous soils accumulate metals in their harvestable parts and have the potential to be used for phytoremediation of heavy metal polluted land. There is increasing evidence that rhizosphere bacteria contribute to the metal extraction process, but the mechanisms of this plant–microbe interaction are not yet understood. In this study ten rhizosphere isolates obtained from heavy metal accumulating willows affiliating with Pseudomonas, Janthinobacterium, Serratia, Flavobacterium, Streptomyces and Agromyces were analysed for their effect on plant growth, Zn and Cd uptake. In plate assays Zn, Cd and Pb resistances and the ability of the bacteria to produce indole-3-acetic acid (IAA), 1-amino-cyclopropane-1-carboxylic acid deaminase (ACC deaminase) and siderophores were determined. The isolates showed resistance to high Zn concentrations, indicating an adaptation to high concentrations of mobile Zn in the rhizosphere of Salix caprea. Four siderophore producers, two IAA producers and one strain producing both siderophores and IAA were identified. None of the analysed strains produced ACC deaminase. Metal mobilization by bacterial metabolites was assessed by extracting Zn and Cd from soil with supernatants of liquid cultures. Strain Agromyces AR33 almost doubled Zn and Cd extractability, probably by the relase of Zn and Cd specific ligands. The remaining strains, immobilized both metals. When Salix caprea plantlets were grown in γ-sterilized, Zn/Cd/Pb contaminated soil and inoculated with the Zn resistant isolates, Streptomyces AR17 enhanced Zn and Cd uptake. Agromyces AR33 tendentiously promoted plant growth and thereby increased the total amount of Zn and Cd extracted from soil. The IAA producing strains did not affect plant growth, and the siderophore producers did not enhance Zn and Cd accumulation. Apparently other mechanisms than the production of IAA, ACC deaminase and siderophores were involved in the observed plant–microbe interactions.     

Molecular diversity of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing PGPR from wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) rhizosphere

Aims

The present study was planned to investigate the diversity of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing bacteria from the rhizosphere of wheat plants and subsequent evaluation of selected PGPR on growth enhancement of wheat seedlings under drought and saline conditions.

Methods

ACC deaminase producing plant growth promoting rhizobacteria (PGPR) were isolated from the rhizosphere of wheat and identified using 16S rRNA gene sequence analysis. Isolates were evaluated for various direct and indirect plant growth promoting (PGP) traits. Plant inoculation experiment was conducted using isolates IG 19 and IG 22 in wheat to assess their plant growth promotion potential under salinity and drought stress.

Results

Thirty-eight ACC deaminase producing PGPR were isolated which belonged to 12 distinct genera and falling into four phyla γ-proteobacteria, β-proteobacteria, Flavobacteria and Firmicutes. Klebsiella sp. was the most abundant genera and followed by Enterobacter sp. The isolates exhibited ACC deaminase activities ranging from 0.106–0.980 μM α- ketobutyrate μg protein^?1 h^?1. The isolates showed multiple PGP traits such as IAA production, phosphate, zinc, potassium solubilization and siderophore production. Enterobacter cloacae (IG 19) and Citrobacter sp. (IG 22) inoculated wheat seedlings showed notable increases in fresh and dry biomass under non-stress as well as under stressed condition.

Conclusion

To the best of our knowledge this is the first report of presence of ACC deaminase activity and other PGP traits from the genus Citrobacter and Empedobacter. Our finding revealed that the γ-proteobacteria group dominated the wheat rhizosphere. Plant inoculation with PGPR could be a sustainable approach to alleviate abiotic stresses in wheat plants. These native PGPR isolates could be used as potential biofertilizers for sustainable agriculture.

     

Detection and Characterization of ACC Deaminase in Plant Growth Promoting Rhizobacteria

The enzyme 1-aminocyclopropane-1-carboxylate deaminase converts ACC, the precursor of the plant hormone ethylene to α-ketobutyrate and ammonium. The enzyme has been identified in few soil bacteria, and is proposed to play a key role in plant growth promotion. In this study, the isolates of plant growth promoting rhizobacteria were screened for ACC deaminase activity based on their ability to grow on ACC as a sole nitrogen source. The selected isolates showed the presence of other plant growth promoting characteristics such as IAA production, phosphate solubilization and siderophore production. The role of ACC deaminase in lowering ethylene production under cadmium stress condition was also studied by measuring in vitro ethylene evolution by wheat seedlings treated with ACC deaminase positive isolates. Nucleic acid hybridization confirmed the presence of ACC deaminase gene (acdS) in the bacterial isolates.     

Regulation of Ethylene Biosynthesis Under Salt Stress in Red Pepper (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.) by 1-Aminocyclopropane-1-Carboxylic Acid (ACC) Deaminase-producing Halotolerant Bacteria

The present study was carried out to understand the mechanism of salt stress amelioration in red pepper plants by inoculation of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing halotolerant bacteria. In general, ethylene production, ACC concentration, ACC synthase (ACS), and ACC oxidase (ACO) enzyme activities increased with increasing levels of salt stress. Treatment with halotolerant bacteria reduced ethylene production by 47–64%, ACC concentration by 47–55% and ACO activity by 18–19% in salt-stressed (150 mmol NaCl) red pepper seedlings compared to uninoculated controls. ACS activity was lower in red pepper seedlings treated with Bacillus aryabhattai RS341 but higher in seedlings treated with Brevibacterium epidermidis RS15 (44%) and Micrococcus yunnanensis RS222 (23%) under salt-stressed conditions as compared to uninoculated controls. A significant increase was recorded in red pepper plant growth under salt stress when treated with ACC deaminase-producing halotolerant bacteria as compared to uninoculated controls. The results of this study collectively suggest that salt stress enhanced ethylene production by increasing enzyme activities of the ethylene biosynthetic pathway. Inoculation with ACC deaminase-producing halotolerant bacteria plays an important role in ethylene metabolism, particularly by reducing the ACC concentration, although a direct effect on reducing ACO activity was also observed. It is suggested that growth promotion in inoculated red pepper plants under inhibitory levels of salt stress is due to ACC deaminase activity present in the halotolerant bacteria.     

Characterization of plant-growth promoting diazotrophic bacteria isolated from field grown Chinese cabbage under different fertilization conditions

Diazotrophic bacteria isolated from the rhizosphere of Chinese cabbage were assessed for other plant growth promoting characteristics viz., production of IAA, ethylene, ACC deaminase, phosphate solubilization, and gnotobiotic root elongation. Their effect on inoculation to Chinese cabbage was also observed under growth chamber conditions. A total of 19 strains that showed higher nitrogenase activity identified by 16S rRNA gene sequence analysis were found to be the members of the genera Pseudomonas and Agrobacterium belonging to α- and γ-Proteobacteria groups. These strains were also efficient in producing IAA and ACC deaminase though they produced low levels of ethylene and no phosphate solubilization. In addition, inoculation of selected diazotrophic bacterial strains significantly increased seedling length, dry weight, and total nitrogen when compared to uninoculated control. The colonization of crop plants by diazotrophic bacteria can be affected by many biotic and abiotic factors, and further studies are oriented towards investigating the factors that could influence the establishment of a selected bacterial community.     

Bacteria with ACC deaminase can promote plant growth and help to feed the world

To feed all of the world's people, it is necessary to sustainably increase agricultural productivity. One way to do this is through the increased use of plant growth-promoting bacteria; recently, scientists have developed a more profound understanding of the mechanisms employed by these bacteria to facilitate plant growth. Here, it is argued that the ability of plant growth-promoting bacteria that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase to lower plant ethylene levels, often a result of various stresses, is a key component in the efficacious functioning of these bacteria. The optimal functioning of these bacteria includes the synergistic interaction between ACC deaminase and both plant and bacterial auxin, indole-3-acetic acid (IAA). These bacteria not only directly promote plant growth, they also protect plants against flooding, drought, salt, flower wilting, metals, organic contaminants, and both bacterial and fungal pathogens. While a considerable amount of both basic and applied work remains to be done before ACC deaminase-producing plant growth-promoting bacteria become a mainstay of plant agriculture, the evidence indicates that with the expected shift from chemicals to soil bacteria, the world is on the verge of a major paradigm shift in plant agriculture.     

Evidence for the involvement of ACC deaminase from Pseudomonas putida UW4 in the biocontrol of pine wilt disease caused by Bursaphelenchus xylophilus

Pine wilt disease, caused by the nematode Bursaphelenchus xylophilus, is responsible for devastation of pine forests worldwide. Until now, there are no effective ways of dealing with this serious threat. The use of 1-aminocyclopropane-1-carboxylate (ACC) deaminase (encoded by the acdS gene)-producing plant growth-promoting bacteria has been shown to be a useful strategy to reduce the damage due to biotic and abiotic stresses. Pinus pinaster seedlings inoculated with the ACC deaminase-producing bacterium Pseudomonas putida strain UW4 showed an increased root and shoot development and reduction of B. xylophilus induced symptoms. In contrast, a P. putida UW4 acdS mutant was unable to promote pine seedling growth or to decrease B. xylophilus induced symptoms. This is the first report on the use of ACC deaminase-producing bacteria as a potential biological control agent for a tree disease, thus suggesting that the inoculation of pine seedlings grown in a tree nursery might constitute a novel strategy to obtain B. xylophilus resistant pine trees.     

Bacteria associated with yellow lupine grown on a metal‐contaminated soil: in vitro screening and in vivo evaluation for their potential to enhance Cd phytoextraction

In order to stimulate selection for plant‐associated bacteria with the potential to improve Cd phytoextraction, yellow lupine plants were grown on a metal‐contaminated field soil. It was hypothesised that growing these plants on this contaminated soil, which is a source of bacteria possessing different traits to cope with Cd, could enhance colonisation of lupine with potential plant‐associated bacteria that could then be inoculated in Cd‐exposed plants to reduce Cd phytotoxicity and enhance Cd uptake. All cultivable bacteria from rhizosphere, root and stem were isolated and genotypically and phenotypically characterised. Many of the rhizobacteria and root endophytes produce siderophores, organic acids, indole‐3‐acetic acid (IAA) and aminocyclopropane‐1‐carboxylate (ACC) deaminase, as well as being resistant to Cd and Zn. Most of the stem endophytes could produce organic acids (73.8%) and IAA (74.3%), however, only a minor fraction (up to 0.7%) were Cd or Zn resistant or could produce siderophores or ACC deaminase. A siderophore‐ and ACC deaminase‐producing, highly Cd‐resistant Rhizobium sp. from the rhizosphere, a siderophore‐, organic acid‐, IAA‐ and ACC deaminase‐producing highly Cd‐resistant Pseudomonas sp. colonising the roots, a highly Cd‐ and Zn‐resistant organic acid and IAA‐producing Clavibacter sp. present in the stem, and a consortium composed of these three strains were inoculated into non‐exposed and Cd‐exposed yellow lupine plants. Although all selected strains possessed promising in vitro characteristics to improve Cd phytoextraction, inoculation of none of the strains (i) reduced Cd phytotoxicity nor (ii) strongly affected plant Cd uptake. This work highlights that in vitro characterisation of bacteria is not sufficient to predict the in vivo behaviour of bacteria in interaction with their host plants.     

海州香薷（Elsholtzia splendens）根际铜抗性细菌的筛选及生物多样性

摘要：【目的】重金属耐性植物海州香薷根际铜抗性细菌的筛选及生物多样性研究将有助于了解微生物－超富集植物相互关系和植物修复机理、开发微生物－香薷重金属修复新技术。【方法】采用稀释平板涂布法从海州香薷根际筛选铜抗性菌株,测定菌株溶磷和产生吲哚乙酸、铁载体、1-氨基环丙烷-1-羧酸（ACC）脱氨酶的特性,采用16S rDNA限制性酶切多态性分析（amplified rDNA restriction analysis, ARDRA）研究铜抗性细菌的遗传多样性,根据16S rDNA相似性对产ACC脱氨酶的菌株进行了     

旱区盐生植物根际促生菌的分离鉴定及其干旱、盐胁迫下促生特性北大核心

【目的】从在干旱、高盐碱生境下生长的盐生杂类草根际土壤中分离具有耐盐和促生性能的根际微生物,并研究其促生特性,为改良旱区土壤盐碱化提供优质菌种资源和理论基础。【方法】通过选择培养基筛选具有耐盐、解磷和解钾能力的菌株,再检测菌株产生长激素(indole-3-acetic acid,IAA)、产1-氨基环丙烷-1-羧酸(1-aminocyclopropane-1-carboxylate,ACC)脱氨酶、产铁载体以及产胞外多糖的能力,选择性状优良者通过拮抗实验组建复合菌剂。并采用菌液侵染萝卜和玉米种子验证菌株对在盐胁迫下种子发芽率和植株在干旱与盐双重胁迫下生长的影响。最后通过16S rRNA基因测序进行分子生物学鉴定。【结果】得到3株具有良好耐盐促生能力的根际微生物yl923、hs032和hy127,菌株yl923兼具解磷(46.29 mg/L)、解钾(58.07 mg/L)、产IAA(29.23 mg/L)、产ACC脱氨酶(13.83 U/mg)和产铁载体(SU=0.43)能力,菌株hs032具有最强产IAA(61.18mg/L)和产铁载体(SU=0.23)能力,菌株hy127具有最强产ACC脱氨酶(15.29U/mg)能力。经16SrRNA基因序列分析后分别将yl923和hs032鉴定为枯草芽孢杆菌(Bacillus subtilis),hy127鉴定为巨大普里斯特氏菌(Priestia megaterium)。3株菌互不拮抗可组建复合菌剂,2%混合菌液可提高种子在盐胁迫下种子发芽率(77%),对干旱和盐胁迫下玉米的根长、株高、干重和叶绿素也都有显著的提高(P<0.05),并且可以显著地降低玉米体内丙二醛(malondialdehyde,MDA)含量(60%)。【结论】菌株yl923、hs032和hy127具有优秀的耐盐促生性能,组合成的混合菌剂能在干旱和盐胁迫下促进植物的生长,具有改良旱区盐渍化土壤的潜力。