Pseudomonas induces salinity tolerance in cotton (Gossypium hirsutum) and resistance to Fusarium root rot through the modulation of indole-3-acetic acid

Abiotic stresses cause changes in the balance of phytohormones in plants and result in inhibited root growth and an increase in the susceptibility of plants to root rot disease. The aim of this work was to ascertain whether microbial indole-3-acetic acid (IAA) plays a role in the regulation of root growth and microbially mediated control of root rot of cotton caused by Fusarium solani. Seed germination and seedling growth were improved by both NaCl and Mg₂SO₄ (100 mM) solutions when treated with root-associated bacterial strains Pseudomonas putida R4 and Pseudomonas chlororaphis R5, which are able to produce IAA. These bacterial strains were also able to reduce the infection rate of cotton root rot (from 70 to 39%) caused by F. solani under gnotobiotic conditions. The application of a low concentration of IAA (0.01 and 0.001 μg/ml) stimulated plant growth and reduced disease incidence caused by F. solani (from 70 to 41–56%, respectively). Shoot and root growth and dry matter increased significantly and disease incidence was reduced by bacterial inoculants in natural saline soil. These results suggest that bacterial IAA plays a major role in salt stress tolerance and may be involved in induced resistance against root rot disease of cotton.     

Actinomycetes isolated from medicinal plant rhizosphere soils: diversity and screening of antifungal compounds,indole-3-acetic acid and siderophore production

A total of 445 actinomycete isolates were obtained from 16 medicinal plant rhizosphere soils. Morphological and chemotaxonomic studies indicated that 89% of the isolates belonged to the genus Streptomyces, 11% were non-Streptomycetes: Actinomadura sp., Microbispora sp., Micromonospora sp., Nocardia sp, Nonomurea sp. and three isolates were unclassified. The highest number and diversity of actinomycetes were isolated from Curcuma mangga rhizosphere soil. Twenty-three Streptomyces isolates showed activity against at least one of the five phytopathogenic fungi: Alternaria brassicicola, Collectotrichum gloeosporioides, Fusarium oxysporum, Penicillium digitatum and Sclerotium rolfsii. Thirty-six actinomycete isolates showed abilities to produce indole-3-acetic acid (IAA) and 75 isolates produced siderophores on chrome azurol S (CAS) agar. Streptomyces CMU-PA101 and Streptomyces CMU-SK126 had high ability to produced antifungal compounds, IAA and siderophores.     

Involvement of indole-3-acetic acid in the circadian growth of the first internode of Arabidopsis

The extension rate of the first inflorescence node of Arabidopsis was measured during light/dark or continuous light exposure and was found to exhibit oscillations which showed a circadian rhythmicity. Decapitation induced a strong inhibition of stem extension. Subsequent application of IAA restored growth and the associated extension–rate oscillations. In addition, IAA treatments, after decapitation, re-established the circadian rhythmicity visible in the intact plants during free run. This indicates that the upper zone of the inflorescence has a major influence on the extension rate of floral stems and implies a role for auxin. Application of N-(1-naphthyl)phthalamic acid, an IAA transport inhibitor, to an intact floral stem inhibited growth and the rhythmicity in the extension rate oscillations, indicating that IAA polar transport may play a role in the dynamics of stem elongation. Furthermore, IAA-aspartate application, after decapitation, did not restore growth and rhythmicity. Nevertheless, biochemical analysis of IAA and IAA-aspartate demonstrated circadian fluctuations of the endogenous levels of both compounds. These observations suggest that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of Arabidopsis floral stems. Received: 21 September 1998 / Accepted: 23 January 1999     

An <Emphasis Type="Italic">ipdC</Emphasis> gene knock-out of <Emphasis Type="Italic">Azospirillum brasilense</Emphasis> strain SM and its implications on indole-3-acetic acid biosynthesis and plant growth promotion

The indole-3-pyruvate decarboxylase gene (ipdC), coding for a key enzyme of the indole-3-pyruvic acid pathway of IAA biosynthesis in Azospirillum brasilense SM was functionally disrupted in a site-specific manner. This disruption was brought about by group II intron-based Targetron gene knock-out system as other conventional methods were unsuccessful in generating an IAA-attenuated mutant. Intron insertion was targeted to position 568 on the sense strand of ipdC, resulting in the knock-out strain, SMIT568s10 which showed a significant (∼50%) decrease in the levels of indole-3-acetic acid, indole-3-acetaldehyde and tryptophol compared to the wild type strain SM. In addition, a significant decrease in indole-3-pyruvate decarboxylase enzyme activity by ∼50% was identified confirming a functional knock-out. Consequently, a reduction in the plant growth promoting response of strain SMIT568s10 was observed in terms of root length and lateral root proliferation as well as the total dry weight of the treated plants. Residual indole-3-pyruvate decarboxylase enzyme activity, and indole-3-acetic acid, tryptophol and indole-3-acetaldehyde formed along with the plant growth promoting response by strain SMIT568s10 in comparison with an untreated set suggest the presence of more than one copy of ipdC in the A. brasilense SM genome.     

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.     

In vitro analyses are not reliable predictors of the plant growth promotion capability of bacteria; a Pseudomonas fluorescens strain that promotes the growth and yield of wheat

Aims: In this study, we set out to identify bacteria that can be used to promote the growth of cereals, while concurrently investigating the merits of using a range of such tests to preselect bacteria for glasshouse studies. Methods and Results: A panel of 15 strains isolated from the rhizosphere and phyllosphere of cereals was tested for the ability to improve the germination of wheat seeds and for production of a range of factors associated with plant growth promotion. In parallel, all bacteria were tested for their ability to improve biomass and grain yield when applied as a soil amendment in glasshouse trials. Conclusions: There was no significant correlation between growth promotion potential in the glasshouse and the results of either the phenotypic or the germination tests. Glasshouse tests identified that only one strain, Pseudomonas fluorescens strain MKB37, gave a significant increase in head weight and grain yield. Significance and Impact of the Study: While this study has identified a candidate for further field tests, it has also highlighted the fact that the modes of action for plant growth‐promoting bacteria (PGPB) are still not fully understood, and that there is no efficient and effective screening method for identifying PGPB by laboratory tests.