Nodulation and effective nitrogen fixation of Macroptilium atropurpureum (siratro) by Burkholderia tuberum, a nodulating and plant growth promoting beta-proteobacterium, are influenced by environmental factors

Background and aims

Burkholderia tuberum STM678^T was isolated from a South African legume, but did not renodulate this plant. Until a reliable host is found, studies on this and other interesting beta-rhizobia cannot advance. We investigated B. tuberum STM678^T’s ability to induce Fix⁺ nodules on a small-seeded, easy-to-propagate legume (Macroptilium atropurpureum). Previous studies demonstrated that B. tuberum elicited either Fix^- or Fix⁺ nodules on siratro, but the reasons for this difference were unexplored.

Methods

Experiments to promote effective siratro nodule formation under different environmental conditions were performed. B. tuberum STM678^T’s ability to withstand high temperatures and desiccation was checked as well as its potential for promoting plant growth via mechanisms in addition to nitrogen fixation, e.g., phosphate solubilization and siderophore production. Potential genes for these activities were found in the sequenced genomes.

Results

Higher temperatures and reduced watering resulted in reliable, effective nodulation on siratro. Burkholderia spp. solubilize phosphate and produce siderophores. Genes encoding proteins potentially involved in these growth-promoting activities were detected and are described.

Conclusions

Siratro is an excellent model plant for B. tuberum STM678^T. We identified genes that might be involved in the ability of diazotrophic Burkholderia species to survive harsh conditions, solubilize phosphate, and produce siderophores.     

Comparison among bacterial communities present in arenized and adjacent areas subjected to different soil management regimes

Aims

The aims of this work were to characterize the soil bacterial communities in an arenized area in southern Brazil subjected to different management regimes through cultivation-dependent and cultivation-independent methods and to evaluate the potential of selected plant growth-promoting (PGP) bacteria to improve the growth of native Lupinus albescens plants.

Methods

Bulk soil samples from an arenized site and rhizospheric soil and roots of L. albescens grown in this arenized site as well as samples from soils of the same region outside of the arenized area and rhizospheric soil and roots of L. albescens grown in non-arenized sites were evaluated. Phosphate solubilization, indolic compound and siderophore production abilities of the isolates were screened and compared. Some isolates were selected for in vivo plant growth promotion in greenhouse experiment.

Results

The samples from the arenized area presented less microbial biomass and less diverse bacterial communities compared with those from non-arenized areas. The PGP characteristics produced by the bacterial isolates showed differences among arenized and non arenized areas. A growth chamber experiment with L. albescens showed that phosphate-insoluble conditions coupled with bacterial inoculation resulted in the best PGP effect.

Conclusions

Culture-dependent and culture-independent methods showed converging results regarding diversity indices and the rhizospheric environments increased bacterial diversity and biomass when compared to bulk soils. The PGP traits analyzed in this work were affected by environmental conditions.     

Screening of plant growth promoting Rhizobacteria isolated from sunflower (Helianthus annuus L.)

Background and Aims

This study was aimed at assessing the diversity of putatively diazotrophic rhizobacteria associated with sunflower (Helianthus annuus L.) cropped in the south of Brazil, and to examine key plant growth promotion (PGP) characteristics of the isolates for the purposes of increasing plant productivity.

Methods

299 strains were isolated from the roots and rhizosphere of sunflower cultivated in five different areas using N-free media. 16S rDNA PCR-RFLP and 16S rRNA partial sequencing were used for identification and the Shannon index was used to evaluate bacterial diversity. Production of siderophores and indolic compounds (ICs), as well phosphate solubilization activities of each isolate were also evaluated in vitro. On the basis of multiple PGP activities, eight isolates were selected and tested for their N-fixation ability, and their capacity as potential PGPR on sunflower plants was also assessed.

Results

All except three Gram-positive strains (phylum Actinobacteria) belonged to the Gram-negative Proteobacteria subgroups [Gamma (167), Beta (78), and Alpha (50)] and the family Flavobacteriaceae (1)]. Shannon indexes ranged from 0.96 to 2.13 between the five sampling sites. Enterobacter and Burkholderia were the predominant genera isolated from roots and rhizosphere, respectively. Producers of siderophores and ICs were widely found amongst the isolates, but only 19.8% of them solubilized phosphate. About 8% of the isolates exhibited all three PGP traits, and these mostly belonged to the genus Burkholderia. Four isolates were able to stimulate the growth of sunflower plants under gnotobiotic conditions.

Conclusions

Enterobacter and Burkholderia were the dominant rhizospheric bacterial genera associated with sunflower plants. Inoculation with isolates belonging to the genera Achromobacter, Chryseobacterium, Azospirillum, and Burkholderia had a stimulatory effect on plant growth.     

Arthrobacter agilis UMCV2 induces iron acquisition in Medicago truncatula (strategy I plant) in vitro via dimethylhexadecylamine emission

Background and aims

Iron is an essential nutrient for plant growth. Although abundant in soil, iron is poorly available. Therefore, plants have evolved mechanisms for iron mobilization and uptake from the rhizospheric environment. In this study, we examined the physiological responses to iron deficiency in Medicago truncatula plants exposed to volatile organic compounds (VOCs) produced by Arthrobacter agilis UMCV2.

Methods

The VOC profiles of the plant and bacterium were determined separately and during interaction assays using gas chromatography. M. truncatula plants exposed to A. agilis VOCs and pure dimethylhexadecylamine were transferred to conditions of iron deficiency, and parameters associated with iron nutritional status were measured.

Results

The relative abundance of the bacterial VOC dimethylhexadecylamine increased 12-fold when in co-cultures of A. agilis and M. truncatula, compared to axenic cultures. Plants exposed to bacterial VOCs or dimethylhexadecylamine exhibited a higher rhizosphere acidification capacity, enhanced ferric reductase activity, higher biomass generation, and elevated chlorophyll and iron content relative to controls.

Conclusions

The VOCs emitted by A. agilis UMCV2 induce iron acquisition mechanisms in vitro in the Strategy I plant M. truncatula. Dimethylhexadecylamine is the signal molecule responsible for producing the beneficial effects.     

The effect of plant growth-promoting rhizobacteria on the growth of rice (Oryza sativa L.) cropped in southern Brazilian fields

Background and Aims

Several strains of rhizobacteria may be found in the rhizospheric soil, on the root surface or in association with rice plants. These bacteria are able to colonize plant root systems and promote plant growth and crop yield through a variety of mechanisms. The objectives of this study were to isolate, identify, and characterize putative plant growth-promoting rhizobacteria (PGPR) associated with rice cropped in different areas of southern Brazil.

Methods

Bacterial strains were selectively isolated based on their growth on three selective semi-solid nitrogen-free media. Bacteria were identified at the genus level by PCR-RFLP 16S rRNA gene analysis and partial sequencing methodologies. Bacterial isolates were evaluated for their ability to produce indolic compounds and siderophores and to solubilize phosphate. In vitro biological nitrogen fixation and the ability to produce 1-aminocyclopropane-1-carboxylate deaminase were evaluated for each bacterial isolate used in the inoculation experiments.

Results

In total, 336 bacterial strains were isolated representing 31 different bacterial genera. Strains belonging to the genera Agrobacterium, Burkholderia, Enterobacter, and Pseudomonas were the most prominent isolates. Siderophore and indolic compounds producers were widely found among isolates, but 101 isolates were able to solubilize phosphate. Under gnotobiotic conditions, eight isolates were able to stimulate the growth of rice plants. Five of these eight isolates were also field tested in rice plants subjected to different nitrogen fertilization rates.

Conclusions

The results showed that the condition of half-fertilization plus separate inoculation with the isolates AC32 (Herbaspirillum sp.), AG15 (Burkholderia sp.), CA21 (Pseudacidovorax sp.), and UR51 (Azospirillum sp.) achieved rice growth similar to those achieved by full-fertilization without inoculation, thus highlighting the potential of these strains for formulating new bioinoculants for rice crops.     

Carbon mineralization in soil of roots from twenty crop species,as affected by their chemical composition and botanical family

Background and aims

Plant growth-promoting rhizobacteria (PGPR) have been widely studied for agricultural applications. One aim of this study was to isolate cadmium (Cd)-tolerant bacteria from nodules of Glycine max (L.) Merr. grown in heavy metal-contaminated soil in southwest of China. The plant growth-promoting (PGP) traits and the effects of the isolate on plant growth and Cd uptake by legume and non-legume plants in Cd-polluted soil were investigated.

Methods

Cd-tolerant bacteria were isolated by selective media. The isolates were identified by 16S rRNA gene and phylogenetic analysis. The PGR traits of the isolates were evaluated in vitro. Cd in soil and plant samples was determined by ICP-MS.

Results

One of the most Cd-tolerant bacteria simultaneously exhibited several PGP traits. Inoculation with the PGPR strain had positive impacts on contents of photosynthesis pigments and mineral nutrients (Fe or Mg) in plant leaves. The shoot dry weights of Lolium multiflorum Lam. increased significantly compared to uninoculated control. Furthermore, inoculation with the PGPR strain increased the Cd concentrations in root of L. multiflorum Lam. and extractable Cd concentrations in the rhizosphere, while the Cd concentrations in root and shoot of G. max (L.) Merr. significantly decreased.

Conclusions

This study indicates that inoculation with Cd-tolerant PGPR can alleviate Cd toxicity to the plants, increase Cd accumulation in L. multiflorum Lam. by enhancing Cd availability in soils and plant biomass, but decrease Cd accumulation in G. max (L.) Merr. by increasing Fe availability, thus highlighting new insight into the exploration of PGPR on Cd-contaminated soil.     

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.

     

Phosphate-solubilizing soil yeast Meyerozyma guilliermondii CC1 improves maize (Zea mays L.) productivity and minimizes requisite chemical fertilization

Aim

Phosphate-solubilizing yeasts have been under-exploited in eco-friendly maize cultivation. In this regard, soil yeasts Meyerozyma guilliermondii CC1, Rhodotorula mucilaginosa CC2 and M. caribbica CC3 were investigated for their plant growth-promoting (PGP) activities.

Methods

Soil yeasts were isolated and characterized. Maize (Zea mays L. cv. Tainong No.1) and Chinese cabbage (Brassica rapa L. cv. Pekinensis) were used for seed bioassay. Growth-promoting effects of yeasts under greenhouse conditions were evaluated using maize and lettuce (Lactuca sativa L. cvs. Capitata and Taiwan sword leaf). Ultimately, M. guilliermondii CC1 was tested on field-grown maize; treatments included full-dose chemical fertilizers (CF), yeast (CC1), half-dose chemical fertilizers (½CF), CC1?+?½CF and control. Nutrient uptake, growth, and yield of maize and rhizospheric soil microbes were estimated.

Results

Strain M. guilliermondii CC1 exhibited better seed vigor index in maize and Chinese cabbage. CC1?+?½CF significantly improved the dry-weights, and nutrient uptakes of maize and sword leaf lettuce under greenhouse conditions. In field, CC1?+?½CF application exerted a pronounced effect on growth of maize, cob yield, nutrient-uptake and rhizospheric soil microbial counts.

Conclusion

Our results validated superior biochemical potency and PGP traits of M. guilliermondii CC1 that reduced requisite chemical fertilizer application without affecting the optimal productivity in maize.     

Rhizobacterial inoculants can improve nickel phytoextraction by the hyperaccumulator Alyssum pintodasilvae

Aim

Rhizobacteria can influence plant growth and metal accumulation. The aim of this study was to evaluate the effect of rhizobacterial inoculants on the Ni phytoextraction efficiency of the Ni-hyperaccumulator Alyssum pintodasilvae.

Method

In a preliminary screening 15 metal-tolerant bacterial strains were tested for their plant growth promoting (PGP) capacity or effect on Ni bioaccumulation. Strains were selected for their Ni tolerance, plant growth promoting traits and Ni solubilizing capacity. In a re-inoculation experiment five of the previously screened bacterial isolates were used to inoculate A. pintodasilvae in two contrasting Ni-rich soils (a serpentine (SP) soil and a sewage sludge-affected agricultural (LF) soil).

Results

Plant growth was greater in serpentine soil (where it grows naturally) than in the LF soil, probably due to Cd phytotoxicity. Rhizobacterial inoculants influenced plant growth and Ni uptake and accumulation, but the effect of the strains was dependent upon soil type. The increase in plant biomass and/or Ni accumulation significantly promoted shoot Ni removal.

Conclusion

One strain (Arthrobacter nicotinovorans SA40) was able to promote plant growth and phytoextraction of Ni in both soil types and could be a useful candidate for future field-based trials.     

A soil-free root observation system for the study of root-microorganism interactions in maize

Background and aims

The root surface of a plant usually exceeds the leaf area and is constantly exposed to a variety of soil-borne microorganisms. Root pathogens and pests, as well as belowground interactions with beneficial microbes, can significantly influence a plants' performance. Unfortunately, the analysis of these interactions is often limited because of the arduous task of accessing roots growing in soil. Here, we present a soil-free root observation system (SF-ROBS) designed to grow maize (Zea mays) plants and to study root interactions with either beneficial or pathogenic microbes.

Methods

The SF-ROBS consists of pouches lined with wet filter paper supplying nutrient solution.

Results

The aspect of maize grown in the SF-ROBS was similar to soil-grown maize; the plant growth was similar for the shoot but different for the roots (biomass and length increased in the SF-ROBS). SF-ROBS-grown roots were successfully inoculated with the hemi-biotrophic maize fungal pathogen Colletotrichum graminicola and the beneficial rhizobacteria Pseudomonas putida KT2440. Thus, the SF-ROBS is a system suitable to study two major belowground phenomena, namely root fungal defense reactions and interactions of roots with beneficial soil-borne bacteria.

Conclusions

This system contributes to a better understanding of belowground plant microbe interactions in maize and most likely also in other crops.     

Monoclonal antibodies against the iron regulated outer membrane Proteins of Acinetobacter baumanniiare bactericidal

Background

Iron is an important nutrient required by all forms of life.In the case of human hosts,the free iron availability is 10^-18M,which is far less than what is needed for the survival of the invading bacterial pathogen.To survive in such conditions, bacteria express new proteins in their outer membrane and also secrete iron chelators called siderophores.

Results/ Discussion

Acinetobacter baumanniiATCC 19606, a nosocomial pathogen which grows under iron restricted conditions, expresses four new outer membrane proteins,with molecular weight ranging from 77 kDa to 88 kDa, that are called Iron Regulated Outer Membrane Proteins (IROMPs). We studied the functional and immunological properties of IROMPs expressed by A.baumaniiATCC 19606.The bands corresponding to IROMPs were eluted from SDS-PAGE and were used to immunize BALB/c mice for the production of monoclonal antibodies. Hybridomas secreting specific antibodies against these IROMPs were selected after screening by ELISA and their reactivity was confirmed by Western Blot. The antibodies then generated belonged to IgM isotype and showed bactericidical and opsonising activities against A.baumanii in vitro.These antibodies also blocked siderophore mediated iron uptake via IROMPs in bacteria.

Conclusion

This proves that iron uptake via IROMPs,which is mediated through siderophores,may have an important role in the survival of A.baumaniiinside the host,and helps establishing the infection.     

Variation in cadmium accumulation and translocation among peanut cultivars as affected by iron deficiency

Purpose

The current study aimed to test the hypothesis that the variations in shoot Cd accumulation among peanut cultivars was ascribed to the difference in capacity of competition with Fe transport, xylem loading and transpiration.

Methods

A hydroponics experiment was conducted to determine the plant biomass, gas exchange, and Cd accumulation in Fe-sufficient or -deficient plants of 12 peanut cultivars, at low Cd level (0.2 μM CdCl₂).

Results

Peanut varied among cultivars in morpho-physiological response to Cd stress as well as Cd accumulation, translocation and distribution. Qishan 208 and Xvhua 13 showed a higher capacity for accumulating Cd in their shoots. Fe deficiency increased the concentration and amount of Cd in plant organs, but decreased TF _{root to shoot} and TF _{root to stem}, while TF _{stem to leaf} remained unaffected. Fe deficiency-induced increase rates of Cd concentration and total Cd amount in roots and leaves were negatively correlated with the values in Fe-sufficient plants. Transpiration rate was positively correlated with leaf Cd concentration, TF _{root to shoot}, TF _{root to stem} and TF _{stem to leaf}.

Conclusions

The difference in shoot Cd concentration among peanut cultivars was mainly ascribed to the difference in Fe transport system, xylem loading capacity and transpiration.     

Biofortification of wheat with zinc through zinc fertilization in seven countries

Background and aims

Phosphorus (P) is a commonly limiting nutrient for plant growth in natural environments. Many legumes capable of N₂-fixation require more P than non-legumes do. Some legume crops can use sparingly soluble forms of P such as iron phosphate much better than other species, but reports on the ability of woody legumes to access iron phosphate are rare.

Methods

Plants of four Acacia species (Acacia stipuligera F. Muell., A. ancistrocarpa Maiden & Blakely, A. stellaticeps Kodela, Tindale & D. Keith and A. robeorum Maslin), native to the Great Sandy Desert in north-western Australia, were grown in a glasshouse in river sand with different levels of iron phosphate, between 0 and 16?μg P g^?1 sand. Plant growth, tissue P concentrations, and pH and carboxylates in the rhizosphere were measured.

Results

Growth of A. stipuligera and A. ancistrocarpa was not responsive to increased P supply; in contrast, A. stellaticeps and A. robeorum produced significantly more root and shoot dry mass at 8 and 16?μg P g^?1 sand than at 0?μg P g^?1 sand; differences in root mass ratio were significant between species but not between P treatments. A. robeorum was the only species colonised by mycorrhizal fungi, and the colonisation percentage decreased with increasing P supply. In all species, P-uptake rates and tissue P concentrations were significantly higher at greater P supply. Rhizosphere pH and the amount of carboxylates in the rhizosphere decreased with increasing P supply.

Conclusions

Net P uptake increased with increasing P supply, showing that the present Acacia species can access P from iron phosphate. However, due to their inherently slow growth rate, enhanced P supply did not increase growth of two of the four studied species. The ability of the Acacia species to access P from iron phosphate is presumably related with carboxylate exudation and rhizosphere acidification.     

Biological nitrogen fixation and phosphate solubilization by bacteria isolated from tropical soils

Introduction

In addition to fixing atmospheric nitrogen, some bacterial isolates can also solubilize insoluble phosphates, further contributing to plant growth.

Aims

The objectives of this study were the following: isolate, select, and identify nodulating bacteria in the cowpea that are efficient not only in biological nitrogen fixation (BNF) but also in the solubilization of insoluble inorganic phosphates; identify and quantify the organic acids produced; and establish the relationship between those acids and the solubilizing capacity.

Methods

The bacteria were captured from two soils containing high concentrations of insoluble phosphorus from the cities of Lavras and Patos de Minas, using the cowpea [Vigna unguiculata (L.) Walp.] as bait. We obtained 78 strains, which were characterized according to their cultural attributes in culture medium 79 with the strains UFLA 03-84, INPA 03-11B, and BR3267 (approved by the Ministry of Livestock and Supply Agriculture—MAPA, as inoculants for the cowpea) and Burkholderia cepacia (LMG1222^T), which was used as a positive control for phosphate solubilization. Strains that were selected for their efficiency in both processes were identified by 16S rDNA sequence analysis. We evaluated the symbiotic efficiency (BNF) in a greenhouse and the solubilization efficiency of CaHPO₄, Al(H₂PO₄)₃, and FePO₄.2H₂O in solid and liquid GELP media. Strains that excelled at the solubilization of these phosphate sources were also evaluated for the production of the following organic acids: oxalic, citric, gluconic, lactic, succinic, and propionic.

Results

The presence of Acinetobacter, Bacillus, Firmicutes, Microbacterium, Paenibacillus, and Rhizobium was detected by 16S rDNA sequencing and analysis. Bacterial strains obtained from cowpea nodules varied greatly in the efficiency of their BNF and phosphate solubilization processes, especially in the strains UFLA 03-09, UFLA 03-10, UFLA 03-12, and UFLA 03-13, which were more efficient in both processes. More strains were able to solubilize insoluble inorganic calcium and iron phosphates in liquid medium than in solid medium. The production of organic acids was related to the solubilization of CaHPO₄ and FePO₄.2H₂O for some strains, and the type and concentration of the acid influenced this process.

Conclusions

These are the first results obtained with bacterial isolates from tropical soils in which the production of organic acids was detected and quantified to examine the solubilization of insoluble inorganic phosphates.     

Morpho-physiological parameters affecting iron deficiency chlorosis in soybean (Glycine max L.)

Background and aims

Iron deficiency chlorosis (IDC) leads to severe leaf chlorosis, low photosynthetic rates, and yield reductions of several million metric tonnes each year. In order to devise breeding and genetic transformation programs that aim at generating high-yielding and IDC-tolerant soybean lines, it is necessary to better understand the mechanisms that enable tolerant plants to survive under Fe-limiting conditions.

Methods

An in silico analysis in the USDA soybean collection allowed the identification of a set of novel efficient and inefficient soybean cultivars which can be used in future studies concerning IDC response. Plants were grown in iron deficient and iron sufficient conditions using a bicarbonate system and several IDC-related aspects were studied.

Results

A new set of efficient and inefficient soybean lines were identified in silico, and their tolerance to IDC was confirmed under laboratorial conditions. New plant traits that are highly correlated to IDC scoring were identified: a negative correlation was found between SPAD values and stem weight, weight of the unifoliolates and iron concentration of the first unifoliolates was found; higher SPAD values were correlated with the amount of iron in the first trifoliate leaves. Our data also show that having higher concentrations of iron in the seeds provides increased resistance to IDC. No correlation was found between root iron reductase activity and chlorosis.

Conclusions

Soybean differential chlorosis susceptibility between different accessions is linked to specific morpho-physiological parameters such as unifoliolate leaf size, stem weigh, concentration of iron in the seeds, and tissue iron partitioning.     

GmPAP4, a novel purple acid phosphatase gene isolated from soybean (Glycine max), enhanced extracellular phytate utilization in Arabidopsis thaliana

Key message

GmPAP4 , a novel plant PAP gene in soybean, has phytase activity. Over-expressing GmPAP4 can enhance Arabidopsis growth when phytate is the sole P source in culture.

Abstract

Phosphorus (P) is an important macronutrient for plant growth and development. However, most of the total P in soils is fixed into organic phosphate (Po). Purple acid phosphatase (PAP) can hydrolyze Po in the soil to liberate inorganic phosphate and enhance plant P utilization. We isolated a novel PAP gene, GmPAP4, from soybean (Glycine max). It had an open reading frame of 1,329 bp, encoding 442 amino acid residues. Sequence alignment and phylogenetics analysis indicated that GmPAP4 was similar to other plant PAPs with large molecular masses. Quantitative real-time PCR analysis showed that the induced expression of GmPAP4 was greater in P-efficient genotype Zhonghuang15 (ZH15) than in P-inefficient genotype Niumaohuang (NMH) during the periods of flowering (28–35 days post phytate stress; DPP) and pod formation (49–63 DPP). Moreover, peak expression, at 63 DPP, was about 3-fold higher in ‘ZH15’ than in ‘NMH’. Sub-cellular localization showed that GmPAP4 might be on plasma membrane or in cytoplasm. Over-expressing GmPAP4 in Arabidopsis resulted in significant rises in P acquisition and utilization compared with the wild-type (WT). Under phytate condition, transgenic Arabidopsis plants showed increases of approximately 132.7 % in dry weight and 162.6 % in shoot P content compared with the WT. Furthermore, when phytate was added as the sole P source in cultures, the activity of acid phosphatase was significantly higher in transgenic plants. Therefore, GmPAP4 is a novel PAP gene that functions in plant’s utilization of organic phosphate especially under phytate condition.     

Soil microbial biomass and the fate of phosphorus during long-term ecosystem development

Background

Soil phosphorus availability declines during long-term ecosystem development on stable land surfaces due to a gradual loss of phosphorus in runoff and transformation of primary mineral phosphate into secondary minerals and organic compounds. These changes have been linked to a reduction in plant biomass as ecosystems age, but the implications for belowground organisms remain unknown.

Methods

We constructed a phosphorus budget for the well-studied 120,000 year temperate rainforest chronosequence at Franz Josef, New Zealand. The budget included the amounts of phosphorus in plant biomass, soil microbial biomass, and other soil pools.

Results

Soil microbes contained 68–78 % of the total biomass phosphorus (i.e. plant plus microbial) for the majority of the 120,000 year chronosequence. In contrast, plant phosphorus was a relatively small pool that occurred predominantly in wood. This points to the central role of the microbial biomass in determining phosphorus availability as ecosystems mature, yet also indicates the likelihood of strong competition between plants and saprotrophic microbes for soil phosphorus.

Conclusions

This novel perspective on terrestrial biogeochemistry challenges our understanding of phosphorus cycling by identifying soil microbes as the major biological phosphorus pool during long-term ecosystem development.     

Signalling mechanisms involved in the response of two varieties of Humulus lupulus L. to soil drying: I. changes in xylem sap pH and the concentrations of abscisic acid and anions

Background and aims

Soil drying leads to the generation of chemical signals in plants that regulate water use via control of the stomatal aperture. The aim of our work was to identify the presence and identity of potential chemical signals, their dynamics, and their relationship with transpiration rate during soil drying in hop (Humulus lupulus (L.)) plants.

Methods

We used pressure chamber technique for measurement of shoot water potential and collection of shoot xylem sap. We analyzed concentrations of abscisic acid (ABA), nitrate, phosphate, sulphate and malate in sap and also the rate of whole plant transpiration.

Results

Transpiration rate decreased prior to changes in shoot water potential. The concentration of ABA in xylem sap continuously increased from early to later stages of water stress, whereas in leaves it increased only at later stages. Shoot sap pH increased simultaneously with the decrease of transpiration rate. Xylem sap alkalization was in some cases accompanied by a decrease in nitrate concentration and an increase in malate concentration. Concentration of sulphate increased in xylem sap during drying and sulphate in combination with a higher ABA concentration enhanced stomatal closure.

Conclusions

Several early chemical signals appear in sap of hop plants during soil drying and their impact on transpiration may vary according to the stage of soil drying.     

Screening of plant growth-promoting traits in arsenic-resistant bacteria isolated from the rhizosphere of soybean plants from Argentinean agricultural soil

Aims

The purpose of this study was to investigate plant-growth promoting traits in native and arsenic (As) highly-resistant bacterial strains isolated from the rhizosphere of soybean (Glycine max) plants grown in an Argentinean agricultural field.

Methods

Determination of MICs (Minimum inhibitory concentration) was carried out on solid media supplemented with arsenite (As 3+) or arsenate (As 5+). Morphological, cultural, physiological, biochemical and molecular characterization, and in vitro determination of plant growth promoting (PGP) properties of As resistant isolates were carried out. Arsenic in soil samples was determined by ICP-OES while residual arsenic on post-removal culture medium and accumulation in cells were estimated by GF-AAS after wet acid digestion.

Results

Isolated strains included γ-proteobacteria such as Enterobacter sp. and Pseudomonas sp., and actinobacteria as Rhodococcus sp. All bacterial strains grew in presence of very high arsenite -over 24?mM- and arsenate –over 400?mM- concentrations. Pseudomonas sp. strains presented simultaneously several in vitro PGP traits, although Rhodococcus erythropolis AW3 did not display PGP traits. However, R. erythropolis AW3 was the most As resistant strain and removed and accumulated the greatest amounts of the metalloid.

Conclusion

The presence of As resistant and plant-growth promoting bacterial strains in the rhizosphere of Glycine max, in arsenic containing agricultural soil, suggest that they could potentially play an important role in plant-growth promotion in stressed conditions. These strains were able to remove and accumulate As from liquid media, thus they could be beneficial for sustainable crop production.