Orchid-associated bacteria produce indole-3-acetic acid, promote seed germination, and increase their microbial yield in response to exogenous auxin

Abstract

Germination of orchid seeds is a complex process. In this paper we focus on interactions between the host-plant and its bacterial partners via indole-3-acetic acid (IAA). Originally isolated from the roots of the epiphytic orchid Dendrobium moschatum, the strains of Rhizobium, Microbacterium, Sphingomonas, and Mycobacterium genera were among the most active IAA producers. Addition of exogenous tryptophan significantly enhanced auxin formation both in mineral and complex media. The presence of IAA and indole-3-acetaldehyde was confirmed by HPLC. Indole-3-pyruvic and indole-3-lactic acids were also detected in supernatants of culture filtrates of Sphingomonas sp., Rhizobium sp., and Microbacterium sp., while indole-3-acetamide was identified only in Mycobacterium sp. Some concentration- and strain-dependent effects of exogenous IAA on bacterial development were also established. Treatment of the cultures with 10 and 100 microg/ml of auxin resulted in an increase in microbial yield. None of the investigated strains was able to utilize IAA as a source of carbon and energy. Furthermore, inoculation of D. moschatum seeds with Sphingomonas sp. and Mycobacterium sp. resulted in considerable enhancement of orchid seeds germination. This growth-promoting activity was observed in the absence of any plant growth stimulators or mycorrhizal fungi, usually required for orchid germination.

Bactérias produtoras de auxinas isoladas de raízes de Cattleya walkeriana, orquídea Brasileira ameaçada de extinção, e sua função na aclimatização

“…cultures and also found great differences in the amount of IAA measured between the two techniques (more than 50-fold); HPLC detected IAA levels undetectable in the colorimetric assay. This discrepancy may be attributed to the fact that several indole compounds present in the liquid culture supernatants are more reactive to IAA alone when in Salkowski's solution (Tsavkelova et al, 2007b).…”

“…The four rhizobacterial isolates producing most indolic compounds were grown as described above, by the method modified by Tsavkelova et al (2007b). HPLC was performed by injection of a 5 μL aliquot into a column of a reverse phase C18 HRC-ODS (Shimadzu, Kyoto, Japan) HPLC apparatus connected to a SCL 10A VP (Shimadzu) and a UV absorbance detector set to measure a visible signal at 254 nm.…”

Mechanisms and Practical Considerations Involved in Plant Growth Promotion by Rhizobacteria

“…Inoculation with IAA producing PGPR has been used to stimulate seed germination, to accelerate root growth and modify the architecture of the root system, and to increase the root biomass. In recent studies, Tsavkelova et al (2007) have extended beyond individual strains as inoculants and reported an increase in the germination of orchid seeds (Dendrobium moschatum) inoculated with Sphingomonas sp. and IAA producing Mycobacterium sp.…”

Temperature, pH and carbon source affect drastically indole acetic acid production of plant growth promoting yeasts

“…The auxin group is a class of plant hormones known to stimulate responses in plants both quickly and in the long term. Indole acetic acid is a type of auxin produced by many groups of microorganisms, including bacteria (Ahmed and Hasnain, 2010;Tsavkelova et al, 2007), filamentous fungi (Manici et al, 2014;Hasan, 2002;Sirrenberg et al, 2007), and yeasts (Nassar et al, 2005).…”