Bacteria associated with orchid roots and microbial production of auxin

Abstract

Associative bacteria of terrestrial (Paphiopedilum appletonianum) and epiphytic (Pholidota articulata) tropical orchids were investigated. Microbial community of epiphytic plant differed from that of the terrestrial one. Streptomyces, Bacillus, Pseudomonas, Burkholderia, Erwinia and Nocardia strains populated Paphiopedilum roots, whereas Pseudomonas, Flavobacterium, Stenotrophomonas, Pantoea, Chryseobacterium, Bacillus, Agrobacterium, Erwinia, Burkholderia and Paracoccus strains colonized Pholidota roots. Endophytic bacteria populations were represented with less diversity: Streptomyces, Bacillus, Erwinia and Pseudomonas genera were isolated from P. appletonianum, and Pseudomonas, Bacillus, and Flavobacterium genera were isolated from Ph. articulata. Microorganisms produced indole-3-acetic acid (IAA). Variations in its biosynthesis among the strains of the same genus were also observed. The highest auxin level was detected during the stationary growth phase. Biological activity of microbial IAA was proved by treatment of kidney bean cuttings with bacterial supernatants, revealing considerable stimulation of root formation and growth.

Role of Auxin in Orchid Development

“…76 Researchers have also documented that auxin is not only involved in initiation, but also enhanced root growth. 77,78 Further, orchid roots are typically inhabited by fungal 79 and bacterial 80,81 symbionts that synthesize IAA. In epiphytes, auxin-producing rhizobacteria are located in the protective velamen layer.…”

“…81 When provided with L-tryptophan, which is a common root exudate and an auxin precursor, these endophytic isolates generate copious amounts of IAA. 80 Researchers have clearly demonstrated that the IAA produced by these microorganisms is biologically active, promoting both root growth and seed germination.…”

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

“…The root tissue was harvested during the summer. Roots were cut and disinfected according to the protocol described by Tsavkelova et al (2007a).…”

“…An aliquot (50 μL) of each dilution was transferred to Czapek agar medium (Tsavkelova et al, 2007a), supplemented with a filtered cyclohexamide antifungal solution (1.0 mg mL -1 ).…”

“…The latter has lignified cells and can serve as a reserve source of water and nutrients and can function as protection (mechanical protection and solar radiation reflection), especially of roots of epiphytic orchids (Arditti, 2008). The velamen is permeable to gas and also serves as an important and specific niche inhabited by associative microorganisms (Tsavkelova et al, 2007a).…”