Abstract
Indole-3-acetic acid (IAA) is an imperative phytohormone for plant growth and development. Ectomycorrhizal fungi (ECM) are able to produce IAA. However, only a few studies on IAA biosynthesis pathways in ECM fungi have been reported. This study aimed to investigate the IAA biosynthesis pathway of six ECM cultures including Astraeus odoratus, Gyrodon suthepensis, Phlebopus portentosus, Pisolithus albus, Pisolithus orientalis and Scleroderma suthepense. The results showed that all ECM fungi produced IAA in liquid medium that had been supplemented with L-tryptophan. Notably, fungal IAA levels vary for different fungal species. The detection of indole-3-lactic acid and indole-3-ethanol in the crude culture extracts of all ECM fungi indicated an enzymatic reduction of indole-3-pyruvic acid and indole-3-acetaldehyde, respectively in the IAA biosynthesis via the indole-3-pyruvic acid pathway. Moreover, the tryptophan aminotransferase activity confirmed that all ECM fungi synthesize IAA through the indole-3-pyruvic acid pathway. Additionally, the elongation of rice and oat coleoptiles was stimulated by crude culture extract. This is the first report of the biosynthesis pathway of IAA in the tested ECM fungi.
Highlights
The most active phytohormone in the auxin class is indole-3-acetic acid (IAA), which is widely produced in plants and plays an important role in plant physiology including cell division and elongation, tissue differentiation, root initiation and phototropic response [1, 2]
The results indicated that all Ectomycorrhizal fungi (ECM) fungi were positive for IAA production, as was indicated by the formation of a pink to red color by a reaction with Salkowski’s reagent
Our previous report (Kumla et al 2014) and this present study found that A. odoratus, G. suthepensis, Ph. portentosus, Pi. albus, Pi. orientalis and Sc. suthepense produced IAA levels ranging from 12.84 to 54.56 μg/mL under cultivation in liquid medium that was supplemented with L-Trp
Summary
The most active phytohormone in the auxin class is indole-3-acetic acid (IAA), which is widely produced in plants and plays an important role in plant physiology including cell division and elongation, tissue differentiation, root initiation and phototropic response [1, 2]. Plants and microorganisms (bacteria and fungi) have been reported as IAA producers [3,4,5,6]. The role of microbial IAA in plant-microbe interaction including ectomycorrhizal, endophytic, pathogenic, phyllosphere and rhizospheric microbes has gained increasing amounts of attention from researchers [6,7,8,9]. The intermediate indole compounds are a major component for the identification of the differential IAA biosynthetic pathway in microorganisms, while at least five different pathways have been described including the indole-3-acetamide (IAM), indole-.
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