Alleviation of heavy metals toxicity by the application of plant growth promoting rhizobacteria and effects on wheat grown in saline sodic field

The aim of the study was to determine tolerance of plant growth promoting rhizobacteria (PGPR) in different concentrations of Cu, Cr, Co, Cd, Ni, Mn, and Pb and to evaluate the PGPR-modulated bioavailability of different heavy metals in the rhizosphere soil and wheat tissues, grown in saline sodic soil. Bacillus cereus and Pseudomonas moraviensis were isolated from Cenchrus ciliaris L. growing in the Khewra salt range. Seven-day-old cultures of PGPR were applied on wheat as single inoculum, co-inoculation and carrier-based biofertilizer (using maize straw and sugarcane husk as carrier). At 100 ppm of Cr and Cu, the survival rates of rhizobacteria were decreased by 40%. Single inoculation of PGPR decreased 50% of Co, Ni, Cr and Mn concentrations in the rhizosphere soil. Co-inoculation of PGPR and biofertilizer treatment further augmented the decreases by 15% in Co, Ni, Cr and Mn over single inoculation except Pb and Co where decreases were 40% and 77%, respectively. The maximum decrease in biological concentration factor (BCF) was observed for Cd, Co, Cr, and Mn. P. moraviensis inoculation decreases the biological accumulation coefficient (BAC) as well as translocation factor (TF) for Cd, Cr, Cu Mn, and Ni. The PGPR inoculation minimized the deleterious effects of heavy metals, and the addition of carriers further assisted the PGPR.     

Isolation and Characterization of Mutants of the Plant Growth-Promoting Rhizobacterium Pseudomonas putida GR12-2 That Overproduce Indoleacetic Acid

Following transposon Tn5 mutagenesis of the plant growth-promoting rhizobacterium Pseudomonas putida GR12-2, mutants that were able to grow in the presence of the tryptophan analog 5-fluorotryptophan were selected. Seven of the 50 5-fluorotryptophan-resistant mutants overproduced the phytohormone indoleacetic acid (IAA). Of these seven mutants, the highest level of IAA was observed with strain P. putida GR12-2/aux1, which produced four times the amount of indoleacetic acid synthesized by the wild-type strain. Strain P. putida GR12-2/aux1, in contrast to the wild type, lost the ability to stimulate the elongation of the roots of canola seedlings under gnotobiotic conditions. The growth rate, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminase activity of mutant strain P. putida GR12-2/aux1 were identical to those of the wild-type strain. The role of IAA in the mechanism of plant growth stimulation by P. putida GR12-2 and other plant growth-promoting rhizobacteria is discussed.     

Differential IAA dose response relations of the axr1 and axr2 mutants of Arabidopsis

In comparison to wild type Arabidopsis thaliana, the auxin resistant mutants axr1 and axr2 exhibit reduced inhibition of root elongation in response to auxins. Several auxin-regulated physiological processes are also altered in the mutant plants. When wild-type, axr1 and axr2 seedlings were grown in darkness on media containing indoleacetic acid (IAA), promotion of root growth was observed at low concentrations of IAA (10^?11 to 10^?7M) in 5-day-old axr2 seedlings, but not in axr1 or wild-type seedlings. In axr1 there was little or no measurable root growth response over the same concentration range. In wild type, root growth was inhibited at concentrations greater than 10^?10M and no detectable root growth response was observed at lower concentrations. In addition, production of lateral roots in response to IAA increased in axr2 seedlings and decreased in axr1 seedlings relative to wild type. Promotion of root elongation and initiation of lateral roots in axr2 seedlings in response to auxin indicate that axr2 seedlings are able to perceive and respond to IAA.     

Physiological characterization and electron microscopic investigation of cyanobacteria associated with wheat rhizosphere

Physiological attributes of a set of cyanobacterial strains, isolated from the rhizosphere of wheat (var. HD 2687), identified as belonging to the genera Calothrix (n = 3), Westiellopsis (1), Hapalosiphon (2) and Nostoc (2), were axenized and evaluated. The concentrated culture filtrates of three cyanobacterial strains — C. ghosei, H. intricatus and Nostoc sp. were able to enhance germination percentage, radicle and coleoptile length in inhibition experiments with wheat seeds. Indole-3-acetic acid (IAA) production was recorded in light and dark (+0.5 % glucose) incubated cultures. Incubation in the presence of tryptophan significantly enhanced IAA production. Acetylene-reducing activity was higher in light incubated cultures of Nostoc sp. followed by C. ghosei, while in the dark, C. ghosei recorded highest values. TLC of the filtrates revealed the presence of several amino acids such as histidine, and auxin-like compounds. Co-culturing with selected strains recorded significant enhancement in plant chlorophyll. Root sections of wheat seedlings co-cultured with C. ghosei revealed the presence of short filaments inside the root hairs and cortical region. Such strains can be promising candidates for developing plant growth promoting associations for wheat crop, besides serving as model systems for understanding the metabolic interactions of cyanobacteria with host plant, such as wheat.     

三种蓝莓根际细菌的分离及其对蓝莓苗生长发育的影响

【目的】从3种蓝莓根际土壤中分离细菌,探究蓝莓根际土壤细菌多样性,并筛选具有产酸、促生长、抑菌性能的菌株,为蓝莓专用微生物肥料的研究提供优质菌株资源和理论基础。【方法】选用5种培养基分离3种蓝莓根际土壤细菌,并进行16S rRNA基因测序和系统发育分析。筛选产酸、产吲哚-3-乙酸（indole-3-acetic acid,IAA）和铁载体、固氮、溶磷和抑制灰葡萄孢生长的菌株,挑选最适菌株制备菌剂进行蓝莓苗盆栽实验验证促生能力,并检测菌剂对蓝莓元素吸收和根际土壤肥力的影响。【结果】从3种蓝莓根际土壤分离得到124株细菌,挑选70株代表性菌株进行16S rRNA基因测序,分布于3个门21个属,其中芽孢杆菌属（Bacillus）、假单胞菌属（Pseudomonas）、链霉菌属（Streptomyces）和红球菌属（Rhodococcus）为优势分离菌群。代表性菌株中,21.4%的菌株能产酸,21.4%的菌株产吲哚-3-乙酸,47.1%的菌株具有固氮潜力,65.7%的菌株具有解磷能力,14.3%的菌株能产铁载体。少量菌株同时具有产酸、产IAA、固氮、解磷和抑菌等能力。选取具有产酸和多种促生特征的菌株绿针假单胞菌CSM-70和双鱼假单胞菌CSM-129进行盆栽蓝莓苗处理,发现2株菌均能显著促进蓝莓苗的生长发育并调控根际土壤pH,其中菌株CSM-70处理还显著促进了蓝莓叶片氮、磷元素的吸收,提升了土壤速效钾、碱解氮的含量。【结论】蓝莓根际细菌多样性高且蕴藏着丰富的促生长菌株,绿针假单胞菌CSM-70和双鱼假单胞菌CSM-129能够促进蓝莓苗生长、调控根际土壤pH和肥力,并促进植株养分吸收,具有蓝莓专用微生物菌剂研制与应用的潜力。     

In vitro morphogenic characteristics of phytochrome mutants in Nicotiana plumbaginifolia are modified and correlated to high indole-3-acetic acid levels

The involvement of indole-3-acetic acid (IAA) in the integration of the light signal perceived by phytochrome during the morphogenesis of plants was investigated in Nicotiana plumbaginifolia Viviani. The chromophore mutant pew1, deficient in all the phytochrome types, and the aurea-like mutant pew2, which appears to be specifically deficient in phytochromes expressed in darkness, were analysed for IAA-related morphogenic effects such as rooting, shooting and callus formation. We observed, in the absence of exogenously applied hormones, abundant root formation by the pew2 mutant. The pew1 mutant exhibited callus formation in the presence of gibberellins and cytokinins when the wild type did not. The previously described lethality of the double mutant pew1–pew2 was shown to be hormone-dependent since, in the light, exogenously applied auxin and cytokinin (0.1 mg·1?1 each) led to plant regeneration from calli and subsequent normal development. These observations suggested an increase in the auxin/cytokinin ratio as a consequence of the phytochrome mutations. We correlated these morphogenic characteristics with high IAA levels in the mutants. The difference in IAA accumulation in the two mutants indicates that among the different phytochromes expressed by N. plumbaginifolia, the light-expressed isoforms play a major role in the control of IAA levels.     

INVOLVEMENT OF INDOLE‐3‐ACETIC ACID PRODUCED BY THE GROWTH‐PROMOTING BACTERIUM AZOSPIRILLUM SPP. IN PROMOTING GROWTH OF CHLORELLA VULGARIS1

Involvement of indole‐3‐acetic acid (IAA), produced by the microalgae‐growth‐promoting bacteria Azospirillum brasilens and A. lipoferum, in promoting growth of the microalga Chlorella vulgaris Beij. was studied. Four wildtype strains of Azospirillum and their IAA‐deficient mutants were co‐immobilized with C. vulgaris in alginate beads. Cultures were grown in synthetic growth medium supplemented with tryptophan. Growth promotion of microalgae and production of exogenous IAA by Azospirillum spp. were monitored. All wildtype Azospirillum spp. produced significant but varying amounts of IAA, while their mutant forms produced significantly less. The results demonstrated a significant growth promotion in Chlorella cultures when immobilized with the four wildtype strains of Azospirillum, while very low or no enhanced growth was induced by the four IAA‐deficient mutants, compared to when C. vulgaris is immobilized alone. A complementation experiment, where an IAA‐attenuated mutant (A. brasilense SpM7918) was supplemented with IAA produced by its parental wildtype strain (A. brasilense Sp6), restored growth promotion in the microalgae‐mutant culture.     

Assimilation of exogenous 2′-14C-indole-3-acetic acid and 3′-14C-tryptophan exposed to the roots of three wheat varieties

This study was conducted to determine if plants can assimilate indole-3-acetic acid (IAA) from rooting media and if exogenous L-tryptophan (L-TRP) can be assimilated and converted by plants into auxins. The addition of 2-¹⁴C-IAA (3.7 kBq plant^-1) to wheat (Triticum aestivum L.) seedlings of three varieties grown in nutrient solution resulted in the uptake (avg.=7.6%) of labelled IAA. Most of the label IAA was recovered in the shoot (avg.=7.2%) with little accumulation in the root (avg.=0.43%). A portion of the assimilated IAA-label in the plant was identified by co-chromatography and UV spectral confirmation as IAA-glycine and IAA-aspartic acid conjugates. Little of the assimilated IAA label was found as free IAA in the wheat plants. These same assimilation patterns were observed when 2-¹⁴C-IAA was added to wheat plants grown in sterile and nonsterile soil. In contrast, the wheat varieties assimilated considerably less (avg.=1.3%) of the added microbial IAA precursor, 3-¹⁴C-L-TRP (3.7 kBq plant^-1) and thus much lower amounts of IAA conjugates were detected. Glasshouse soil experiments revealed that 2 out of 3 wheat varieties had increased growth rates and increased yields when L-TRP (10^-5 and 10^-7 M) was added to the root zone. It is surmised that this positive response is a result of microbial auxin production within the rhizosphere upon the addition of the precursor, L-TRP. The amino acid composition of the root exudates plays a critical role in microbial production of auxins in the rhizosphere. This study showed that wheat roots can assimilate IAA from their rooting media, which will supplement the endogenous IAA levels in the shoot tissue and may positively influence plant growth and subsequent yield.     

N-methyl-N′-nitro-N-nitrosoguanidine and UV induced mutants of the dinoflagellate Crypthecodinium cohnii*

SYNOPSIS. Mutant strains were chemically induced by treatment with N-methyl-N′-nitro-N-nitrosoguanidine (NTG) and UV irradiation. UV and NTG mutation rates were obtained that were both consistent with the organism being haploid. Three types of mutants were produced: (a) strains deficient in both β- and γ-carotene, the only carotenoids found in the wild type; phenotypes include albinos (translucent, dull white, “snow white”) and cream-colored on agar as compared to the yellow-orange color of wild type colonies; (b) strains requiring adenine, guanine or cytosine in addition to the minimal medium for growth; (c) mutants that grow at a rate less than 40% of the wild type in minimal medium.     

Production of indole-3-acetic acid, aromatic amino acid aminotransferase activities and plant growth promotion by Pantoea agglomerans rhizosphere isolates

The production of auxins, such as indole-3-acetic acid (IAA), by rhizobacteria has been associated with plant growth promotion, especially root initiation and elongation. Six indole-producing bacteria isolated from the rhizosphere of legumes grown in Saskatchewan soils and identified as Pantoea agglomerans spp. were examined for their ability to promote the growth of canola, lentil and pea under gnotobiotic conditions and for tryptophan (Trp)-dependent IAA production. Five of the isolates enhanced root length, root weight or shoot weight by 15–37% in at least one of the plant species, but isolates 3–117 and 5–51 were most consistent in enhancing plant growth across the three species. Indole concentrations in the rhizosphere of plants grown under gnotobiotic conditions increased in the presence of the rhizosphere isolates and when Trp was added 3 days prior to plant harvest. Isolates 3–117, 5–51 and 5–105 were most effective in increasing rhizosphere indole concentrations. Colony hybridization confirmed that all of the isolates possessed the ipdC gene which codes for a key enzyme in the Trp-dependent IAA synthetic pathway. The activity of amino acid aminotransferase (AAT), catalyzing the first step in the Trp-dependent synthetic pathway, was examined in the presence of Trp and other aromatic amino acids. All of the isolates accumulated Trp internally and released different amounts of IAA. The production of IAA from the isolates was greatest in the presence of Trp, ranging from 2.78 to 16.34 μg mg protein⁻¹ in the presence of 250 μg of Trp ml⁻¹. The specific activity of AAT was correlated with the concentration of IAA produced in the presence of Trp but not when tyrosine (Tyr), phenylalanine (Phe) or aspartate (Asp) was used as a sole nitrogen source. Isolate 3–117, which produced significant concentrations of IAA in the presence and absence of Trp, was able to use aromatic amino acids as sole sources of nitrogen and was most consistent in enhancing the growth of canola, lentil and pea may have potential for development as a plant growth-promoting inoculant. Responsible Editor: Peter A. H. Bakker.     

Effects of antibiotics in soil on the population dynamics of transposon Tn5 carrying Pseudomonas fluorescens

The effects of kanamycin and streptomycin added to soil on the survival of transposon Tn5 modified Pseudomonas fluorescens strain R2f were investigated. Kanamycin in high (180 g g^-1 dry soil) or low (18 g g^-1) concentration or streptomycin in low concentration in Ede loamy sand soil had no noticeable effect on inoculant population dynamics in soil and wheat rhizosphere, whereas streptomycin in high concentration had a consistent significant stimulatory effect, in particular in the wheat rhizosphere. Streptomycin exerted its effect by selecting P. fluorescens with Tn5 insertion whilst suppressing the unmodified sensitive parent strain, as evidenced by comparing the behaviour of these two strains in separate and mixed inoculation studies.Soil textural type influenced the effect of streptomycin on the Tn5 carrying inoculant; the effect was consistently detected in rhizosphere and rhizoplane samples of wheat grown in Ede loamy sand after 7 and 14 days incubation, whereas it was only apparent after 7 days in rhizoplane or rhizosphere (and bulk soil) samples of wheat grown in two silt loam soils. Modification of soil pH by the addition of CaCO₃ or bentonite clay resulted in an enhancement of the selective effect of streptomycin by CaCO₃ and its abolishment by bentonite clay.The addition to soil of malic acid or wheat root exudate, but not of glucose, enhanced the streptomycin selective effect on the Tn5-modified P. fluorescens strain. Neither the streptomycin producer Streptomyces griseus nor two non-inhibiting mutants obtained following UV irradiation affected the dynamics of P. fluorescens (chr::Tn5) in soil and wheat rhizosphere.The effect of streptomycin in soil on inoculant Tn5 carrying bacteria depends on conditions such as soil type, the presence of (wheat) root exudates and the type of available substrate.     

Selection of Tn5::lacZ mutants isogenic to wild type Azospirillum brasilense strains capable of growing at sub-optimal temperature

Azospirillum brasilense strains, CDJA and A40, capable of growing at sub-optimal temperature were tagged with stable chromogenic marker Tn5-lacZ. Mutants were screened for plant growth promoting activities at 20, 22, 25, 30 and 37 °C. Mutants MC48 and MA3 were found to fix nitrogen upto 85% and produced indole acetic acid (IAA) and siderophore in isogenic manner to their respective wild type strains, CDJA and A40, at sub-optimal temperatures. Co-inoculation of mutants with their respective parent (1:1 ratio) to the wheat revealed that colonization potential of the mutants was affected greatly. Tn5-lacZ tagged mutants MC48 and MA3 were found isogenic to their respective wild type Azospirillum strain, with regards to plant growth promoting activities and root colonization ability. This revised version was published online in July 2006 with corrections to the Cover Date.     

Upscaling from Rhizosphere to Whole Root System: Modelling the Effects of Phospholipid Surfactants on Water and Nutrient Uptake

While the rhizosphere presents a different chemical, physical and biological environment to bulk soil, most experimental and modelling investigations of plant growth and productivity are based on bulk soil parameters. In this study, water and nutrient acquisition by wheat (Triticum aestivum L.) roots was investigated using rhizosphere- and root-system-scale modelling. The physical and chemical properties of rhizosphere soil could be influenced by phospholipid surfactants in the root mucilage. Two models were compared: a 2-dimensional (2D) Finite Element Method rhizosphere model, and a 3-dimensional (3D) root architecture model, ROOTMAP. ROOTMAP was parameterised to reproduce the results of the detailed 2D model, and was modified to include a rhizosphere soil volume. Lecithin (a phospholipid surfactant) could be exuded into the rhizosphere soil volume, decreasing soil water content and hydraulic conductivity at any given soil water potential, and decreasing phosphate adsorption to soil particles. The rhizosphere-scale modelling (5 × 5 mm² soil area, 10 mm root length, uptake over 12 h) predicted a reduction in water uptake (up to 16% at 30 kPa) and an increase in phosphate uptake (up to 4%) with lecithin exudation into the rhizosphere, but little effect on nitrate uptake, with only a small reduction in dry soil (1.6% at 200 kPa). The 3D root model reproduced the water (y = 1.013x, R² = 0.996), nitrate (y = 1x, R² = 1) and phosphate (y = 0.978x, R² = 0.998) uptake predictions of the rhizosphere model, providing confidence that a whole root system model could reproduce the dynamics simulated by a Finite Element Method rhizosphere model. The 3D root architecture model was then used to scale-up the rhizosphere dynamics, simulating the effect of lecithin exudation on water, nitrate and phosphate acquisition by a wheat root system, growing over 41 d. When applied to growing and responsive roots, lecithin exudation increased P acquisition by up to 13% in nutrient-rich, and 49% in relatively nutrient-poor soil. A comparison of wheat (Triticum aestivum L.) and lupin (Lupinus angustifolius L.) root architectures, suggested an interaction between the P acquisition benefit of rhizosphere lecithin and root architecture, with the more highly-branched wheat root structure acquiring relatively more P in the presence of lecithin than the sparsely-branched lupin root system.     

Chemotaxis of Pseudomonas syringae subsp. savastanoi Virulence Mutants

Several mutants of Pseudomonas syringae subsp. savastanoi were tested for their ability to sense and respond to a chemotactic gradient in low concentrations of yeast extract. The mutants were deficient in one or both of the genes coding for the synthesis of the plant hormones indole-3-acetic acid (IAA) and isopentenyl adenosine. Mutations which resulted in the loss of IAA production were due to the loss of the entire plasmid containing the iaa operon or to an 18-kb deletion of the iaa region. Additional mutants tested were deficient in their ability to produce isopentenyl adenosine as a result of the loss of the ptz-bearing plasmid. In all cases, strains which had lost the ability to produce IAA exhibited enhanced motility of up to 2.5 times that of the wild type (IAA⁺) in medium containing 0.01% yeast extract. No differences in motility were observed on medium containing lower concentrations of yeast extract. The presence or absence of the cytokinin plasmid and the presence or absence of inorganic nitrogen in the medium had no effect on the relative mobility of the strains.     

Potential of indole-3-acetic acid-producing rhizobacteria to resist Pb toxicity in polluted soil

Heavy metal toxicity in industrial polluted area is imparting serious consequences on crops. Two Pb-tolerant bacteria were isolated from maize growing in a dumping site of Attock Oil Refinery, Rawalpindi. The oil-polluted field had a higher geo-accumulation index (I_geo) and pollution load index (PLI) value for Pb than standard. The soil accumulated higher Cd, Zn, and Mn contents too. Maize leaves and roots accumulated higher Pb and Zn and exhibited biological concentration factor (BCF), biological accumulating factor (BAC), and translocation factor (TF) Zn. Two bacterial strains (Exiguobacterium aurantiacum and Bacillus firmus) were isolated from maize rhizosphere growing in an oil-polluted field and applied as bio-inoculants on maize in a greenhouse experiment for 80 days. Both bio-inoculants were tolerant to Pb at 500 ppm and had the potential to produce indole-3-acetic acid (IAA) in the presence or absence of Pb. Results revealed that single inoculation of bio-inoculants decreased Pb contents in the soil, leaves, and roots of maize by 30% over the control. Growth and physiological attributes of maize were also improved by 25% in a single application of bio-inoculants. Application of Pb with bio-inoculants decreased the efficiency of PGPR, and there were only 10–15% increases in growth and physiological attributes over single inoculation. Bio-inoculants exhibited the best results in the presence of IAA and Pb application by intensive root growth (60% better than control), reducing Pb toxicity (38%) and increasing growth and physiological attributes by 10–15% over single inoculation of bio-inoculants. Application of bio-inoculants with IAA may decrease the deleterious effects of Pb toxicity in oil-polluted agriculture fields.     

An alternative explanation for plant growth promotion by bacteria of the genus Azospirillum

Experiments were performed to identify the substances that are excreted by the soil bacterium Azospirillum brasilense Sp7 and that were reported to stimulate the formation of lateral roots and of root hairs of grasses. Azospirillum forms indole-3 acetic acid (IAA) but only in the late stationary growth phase or when tryptophan is present in the medium, but not in continuous cultures or in the logarithmic growth phase of batch cultures. Formation of IAA by Azospirillum requires aerobic conditions. Nitrite can replace IAA in several phytohormone assay, and is even more active than IAA in a test with wheat root segments in which the increase of wet weight is determined. Higher amounts of nitrite are necessary for activity in other classical auxin assays. Nitrite shows 40–60% of the activity of IAA in the straight-growth test of Avena coleoptiles and in the formation of C₂H₄ by pea epicotyl segments. Like IAA, nitrite is inactive in promoting C₂H₄ formation by ripe apple tissues. Since nitrite alone can hardly exert phytohormonal effects, it is postulated that nitrite reacts with a substance in the cells and that a product formed by this reaction functions as auxin. Such a substance could be ascorbate. Exogenously added ascorbate enhances the rate of nitrite-dependent C₂H₄ formation by pea epicotyl sections and the nitrite-dependent increase in the wet weight of wheat root segments. Nitrite is formed by nitrate respiration of Azospirillum. The findings that nitrite can have phytohormonal effects offers an alternative explanation of the promotion of the growth of roots and the enhancement of mineral uptake of grasses by Azospirillum. Indole-acetic acid completely and nitrite partly substitute for an inoculation with Azospirillum in an assay where the increase of the dry weight of intact wheat roots is determined after an incubation for 10 d. Nitrite and IAA are, therefore, possibly the only factors causing an enhancement of the growth of roots of grasses.Abbreviations HPLC high-performance liquid chromatography - IAA indole-3-acetic acid     

Biosynthesis of phytohormones from novel rhizobacterial isolates and their in vitro plant growth-promoting efficacy

The health of the plant and soil fertility is dependent on the plant–microbes interaction in the rhizosphere. Microbial life tends to endure various rhizosphere plant–microbe interactions. Phytohormones such as auxins, cytokinins, gibberellic acid, ethylene and abscisic acid are termed as the classical group of hormones. Out of the 70 rhizobacterial strains isolated from the Coleus rhizosphere, three different rhizobacterial strains Pseudomonas stutzeri MTP40, Stenotrophomonas maltophilia MTP42 and Pseudomonas putida MTP50 having plant growth-promoting attributes were isolated and characterized for its phytohormone-producing ability. The phytohormones such as indole 3-acetic acid (IAA), gibberellic acid and cytokinin (kinetin and 6-benzyladenosine) were affirmed in culture supernatant of the above isolates. IAA was detected in all the three isolates, where in highest production was found in S. maltophilia MTP42 (240?µg/mL) followed by P. stutzeri MTP40 (250?µg/mL) and P. putida MTP50 (233?µg/mL). Gibberellic acid production was found maximum in MTP40 (34?µg/mL), followed by MTP42 (31?µg/mL) and MTP50 (27?µg/mL). The cytokinin production from the isolates, namely, MTP40, MTP42 and MTP50 were 13, 11 and 7.5?µg/mL, respectively. The isolates showing the production of plant growth enhancing phytohormones can be commercialized as potent bioformulations.     

Indole Glucosinolates and Camalexin do not Influence the Development of the Clubroot Disease in Arabidopsis thaliana

Root galls of Brassicaceae caused by Plasmodiophora brassicae are dependent on increased auxin and cytokinin formation. In this study we investigated whether indole glucosinolates are involved in indole‐3‐acetic acid (IAA) biosynthesis in root galls, by using a genetic approach. The cytochrome P450 enzymes, CYP79B2 and CYP79B3, convert tryptophan to indole‐3‐acetaldoxime (IAOx), which is a precursor for indole glucosinolates and the phytoalexin camalexin in Arabidopsis thaliana. Root galls of the Arabidopsis ecotypes Wassilewskija (WS) and Columbia (Col) accumulated camalexin, WS at levels up to 320 μg/g dry weight. By contrast, camalexin was absent in root galls of cyp79b2/b3 double mutants. Infection rate and disease index as a measure of club development in mutant and wild‐type plants of the two ecotypes were investigated and no differences were found in gall formation. This demonstrates that camalexin is an ineffective inhibitor of P. brassicae and indole glucosinolates are not the source of elevated levels of IAA in galls, because free IAA levels in mutant galls were comparable with those in wild type.     

IAA-overproducer mutants of Hebeloma cylindrosporum Romagnesi mycorrhizal with Pinus pinaster (Ait.) Sol. and P. sylvestris L. in hydroponic culture

Indole-3-acetic acid (IAA) is thought to play a role in the regulation of ectomycorrhiza development, and vigorous mycorrhiza formers such as Pisolithus and Laccaria have previously been shown to accumulate large amounts of IAA in the culture medium in vitro, particularly in the presence of tryptophan. Recently, 5-fluoroindole-resistant and IAA-overproducing mutant strains of Hebeloma cylindrosporum Romagnesi have been developed and described by Durand et al. (1992). We have used some of these and corresponding wild-type strains as mycobionts on seedlings of Pinus pinaster (Ait.) Sol. and P. sylvestris L. in semi-hydroponic culture in an attempt to study IAA effects independent of species-specific differences. However, no significant differences between strains were found in host growth rate, shoot carbohydrate concentration, root morphology, root IAA concentration or mycorrhizal biomass. Since previous work showed a stimulation by these and other mutants and strains on mycorrhiza formation in Petri dish and test tube cultures, we assume that a semi-hydroponic culture system prevents the build up of tryptophan of fungal origin, which is most likely a precondition for enhanced IAA production.     

Rhizosphere Properties of Poaceae Genotypes Under P-limiting Conditions

Plant genotypes differ in P efficiency, i.e. their capacity to grow in soil with low P availability. Plant properties such as root and root hair length, release of P mineralising and mobilising compounds by the roots and P requirement for optimal growth are known to influence P efficiency. In order to improve the understanding of the role of rhizosphere properties in plant P uptake, we grew three Poaceae genotypes [two wheat (Triticum aestivum L.) genotypes (the P-efficient Goldmark and the P-inefficient Janz), and the Australian native grass Austrostipa densiflora L.] to maturity in an acidic loamy sand with low P availability. Addition of 120 mg P as FePO₄ kg⁻¹ (P120) improved the growth of all three genotypes. In both P0 and P120, growth and P uptake were smaller in Janz than in Goldmark. During the vegetative phase, growth and P uptake of Austrostipa were smaller than in Goldmark in P0 but greater in P120. These differences can be explained by plant properties such as root growth, specific P uptake, mobilisation of inorganic and organic P by root exudates and P utilisation efficiency. In P120, P availability in the rhizosphere was least in Janz and greatest in Austrostipa. Microbial biomass P in the rhizosphere was least in Janz. Acid phosphatase activity was greatest in the rhizosphere of Austrostipa and least in Janz. Plant growth and P uptake were positively correlated with microbial P, acid phosphatase activity and resin P in the rhizosphere, suggesting that microorganisms contribute to uptake of P by plants in this soil. Microbial community composition in the rhizosphere [analysed by fatty acid methylester (FAME) analysis and denaturing gradient gel electrophoresis (DGGE)] differed among genotypes, changed during plant development and was affected by P addition to the soil. Genotype-specific microbial community composition in the rhizosphere may have contributed to the observed differential capacity of plants to grow at low P availability.