Indole-3-acetic acid (IAA) is an important phytohormone with the capacity to control plant development in both beneficial and deleterious ways. The ability to synthesize IAA is an attribute that many bacteria including both plant growth-promoters and phytopathogens possess. There are three main pathways through which IAA is synthesized; the indole-3-pyruvic acid, indole-3-acetamide and indole-3-acetonitrile pathways. This chapter reviews the factors that effect the production of this phytohormone, the role of IAA in bacterial physiology and in plant–microbe interactions including phytostimulation and phytopathogenesis.

Indole-3-acetic acid in plant–microbe interactions

* Abstract Indole acetic acid (IAA) production is a major property of rhizosphere bacteria that stimulate and facilitate plant growth. The present work deals with isolation, characterization and identification of indole acetic acid producing bacteria from the rhizospheric soil. Out of ten Indole acetic acid producing isolates, five were selected as efficient producers. Optimization of indole acetic acid production was carried out at different cultural conditions of pH and temperature with varying media components such as carbon and nitrogen source, tryptophan concentration. Partial purification of IAA was done and purity was confirmed with Thin layer chromatography. Subsequently, effect on plant growth was tested by pot assay. In conclusion the study suggests the IAA producing bacteria as efficient biofertilizer inoculants to promote plant growth.

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Isolation and characterization of indole acetic acid (IAA) producing bacteria from rhizospheric soil and its effect on plant growth

There has been many recent studies on the use of microbial antagonists to control diseases incited by soilborne and airborne plant pathogenic bacteria and fungi, in an attempt to replace existing methods of chemical control and avoid extensive use of fungicides, which often lead to resistance in plant pathogens. In agriculture, plant growth-promoting and biocontrol microorganisms have emerged as safe alternatives to chemical pesticides. Streptomyces spp. and their metabolites may have great potential as excellent agents for controlling various fungal and bacterial phytopathogens. Streptomycetes belong to the rhizosoil microbial communities and are efficient colonizers of plant tissues, from roots to the aerial parts. They are active producers of antibiotics and volatile organic compounds, both in soil and in planta, and this feature is helpful for identifying active antagonists of plant pathogens and can be used in several cropping systems as biocontrol agents. Additionally, their ability to promote plant growth has been demonstrated in a number of crops, thus inspiring the wide application of streptomycetes as biofertilizers to increase plant productivity. The present review highlights Streptomyces spp.-mediated functional traits, such as enhancement of plant growth and biocontrol of phytopathogens.

https://www.mdpi.com/1422-0067/19/4/952/pdf

Plant Growth Promoting and Biocontrol Activity of Streptomyces spp. as Endophytes.

Biological control and plant growth promotion by plant beneficial microbes has been viewed as an alternative to the use of chemical pesticides and fertilizers. Bacteria and fungi that are naturally associated with plants and have a beneficial effect on plant growth by the alleviation of biotic and abiotic stresses were isolated and developed into biocontrol (BCA) and plant growth-promoting agents (PGPA). Actinobacteria are a group of important plant-associated spore-forming bacteria, which have been studied for their biocontrol, plant growth promotion, and interaction with plants. This review summarizes the effects of actinobacteria as BCA, PGPA, and its beneficial associations with plants.

Effects of actinobacteria on plant disease suppression and growth promotion.

Drought tolerant endophytic actinobacteria Streptomyces
 coelicolor DE07, S. olivaceus DE10 and Streptomyces geysiriensis DE27 were isolated from cultivated plants of arid and drought affected regions of Rajasthan, India. These isolates exhibited plant growth promotion traits and intrinsic water stress tolerance from −0.05 to −0.73 MPa. Maximum auxin production was observed in majority of actinobacterial cultures in the logarithmic to stationary phase of growth. Significant enhancement of wheat seedling vigour was recorded by the inoculation of these endophytic actinobacteria. S. olivaceus DE10 recorded maximum accumulation of indole 3-acetic acid (84.34 μg mg−1 protein). Culture and cell-free extract of the endophytes was applied on to wheat seeds to assess the effect on growth in water-stressed soil. Maximum yield was recorded with the inoculation of S. olivaceus DE10 culture (492.77 kg ha−1) and cell-free extract (262.31 kg ha−1). Co-inoculation of S. olivaceus DE10 + S. geysiriensis DE27 recorded highest yield of 550.09 kg ha−1 while their cell-free extract yielded 524.92 kg ha−1. Overall, wheat seeds treated with cultures showed better plant growth and yield in comparison to control. Direct coating of cultures on seeds yielded better performance than cell-free extract coated on seeds and co-inoculation of cultures or cell-free extract proved better than single culture inoculations. Production of phytohormones, plant growth promotion traits combined with water stress tolerance potential in these endophytic actinobacteria played a cumulative synergistic role that supported enhanced plant growth promotion of wheat in the stressed soil.

Drought-tolerant endophytic actinobacteria promote growth of wheat ( Triticum aestivum ) under water stress conditions

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 Sclerotiumrolfsii. 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.

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

A collection of Streptomyces spp. isolated from the rhizosphere soils of 14 Thai medicinal plants were found to produce, the plant growth hormone indole-3-acetic acid (IAA) in a yeast malt extract medium supplemented with 2 mg/mL L-tryptophan. Streptomyces CMU-H009 recovered from soil associated with lemongrass (Cymbopogon citratus) was very effective in producing IAA. Based on the 16S rDNA sequence analysis, this strain was most closely related to Streptomyces viridis NBRC 13373T (99.3% similarity). The IAA production was maximums (300 mg/mL) when the strain CMU-H009 was cultivated in a yeast malt extract broth amended with 2 mg/mL L-tryptophan a pH 7.0, and incubated at 30 o C with shaking at 125 rpm for 3 d. The culture filtrate from the strain CMU-H009 stimulated a significant increase in the germination and root elongation of maize (Zea mays) and cow pea (Bruguiera parviflora) seeds. The IAA from this strain was extracted, purified and identified by thin layer chromatography.

Indole-3-acetic acid production by Streptomyces sp. isolated from some Thai medicinal plant rhizosphere soils

The actinomycetes (445 strains), isolated from 16 Thai medicinal plant rhizosphere soils were examined for their L-asparaginase activity. It was found that thirty strains showed enzyme activity. The range of enzyme production was 0.03-1.50 mol ammonia/ml/hour. Strain CMU-H002 isolated from lemongrass (Cymbopogon citratus) rhizosphere soil showed highest enzyme activity. Based on the 16S rDNA sequence analysis, this strain was most closely related to Amycolatopsis kerataniphila subsp. kerataniphila DSM 44409 T (98.6% similarity) and Amycolatopsis keratiniphila subsp. nogabecina DMS 44586 T (98.7% similarity). The L-asparaginase activity was maximums (3.05 mol ammonia/ml/hour) when strain CMU-H002 was cultivated in asparagine dextrose salts broth amended with soluble starch (0.2%) and yeast extract (1.5%), pH 7.0 and incubated at 30  C with shaking at 125 rpm for 7 days.

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L-Asparaginase production by actinomycetes isolated from some Thai medicinal plant rhizosphere soils

A collection of Streptomyces isolates from the rhizosphere of sixteen medicinal plants was assessed for in vitro antagonistic activity against six plant pathogenic fungi; Alternaria brassicicola, A. porri, Colletotrichum gloeosporioides, Fusarium oxysporum, Penicillium digitatum and Sclerotium rolfsii. Twenty-seven isolates showed activity against at least one of them. Streptomyces spectabilis CMU-PA101 recovered from soil associated with pandanus palm (Pandanus amaryllifolius) was very effective in producing bioactive metabolites against the six test fungi used in the screening test. The culture filtrate of strain CMU-PA101 was extracted using various solvents (chloroform, n-butanol, diethyl ether, ethyl acetate, and n-hexane). The n-butanol extract demonstrated the highest activity against the test organisms, with a minimum inhibitory concentration (MIC) of 0.781 to 6.250 mg/ml. The culture filtrate of this strain was effective against A. porri on Allium ascalonicum L (shallot) in greenhouse conditions.

Sutthinan Khamna

Papers

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

Indole-3-acetic acid production by Streptomyces sp. isolated from some Thai medicinal plant rhizosphere soils

L-Asparaginase production by actinomycetes isolated from some Thai medicinal plant rhizosphere soils

Antifungal activity of Streptomyces spp. isolated from rhizosphere of Thai medicinal plants.