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sp. DMKU-RP206 was isolated from rice leaves in Thailand and identified by the 16S rRNA gene and multilocus sequence (, , and genes) analysis. The bacterium was assessed on plant growth-promoting traits including indole-3-acetic acid (IAA) production. Phosphate solubilization, ammonia production, and antagonism to fungal plant pathogens, as well as siderophore production, were shown by this bacterium. However, only IAA production was focused on. The production of IAA by sp. DMKU-RP206 was optimized by statistical methods. A Box-Behnken design was used for the investigation of interactions among the basic influencing factors and for the optimization of IAA production. The results showed that l-tryptophan had a significant importance in terms of IAA production. sp. DMKU-RP206 produced a higher amount of IAA than previously reported for the genus. 0.85% of lactose as a carbon source, 1.3% of yeast extract as a nitrogen source, 1.1% of l-tryptophan as a precursor, 0.4% of NaCl, an initial pH of 5.8, an incubation temperature at 30 °C, and a shaking speed of 200 rpm were found to be the optimum conditions for IAA production. In addition, IAA production was performed to scale up IAA production, and the highest amount, 5561.7 mg l, was obtained. This study reported a 13.4-fold improvement in IAA production by sp. DMKU-RP206.

Microorganisms produce plant growth regulators, such as auxins, cytokinins and gibberellins, to promote plant growth. Auxins are a group of compounds with an indole ring that have a positive effect on plant growth. Indole-3-acetic acid (IAA) is a plant growth hormone classified as an indole derivative of the auxin family. IAA biosynthesis pathways have been reported and widely studied in several groups of bacteria. Only a few studies on IAA biosynthesis pathways have been conducted in yeast. This study aimed to investigate IAA biosynthesis pathways in a basidiomycetous yeast (Rhodosporidium paludigenum DMKU-RP301). Investigations were performed both with and without a tryptophan supplement. Indole compound intermediates were detected by gas chromatography-mass spectrometry. Indole-3-lactic acid and indole-3-ethanol were found as a result of the enzymatic reduction of indole-3-pyruvic acid and indole-3-acetaldehyde, in IAA biosynthesis via an indole-3-pyruvic acid pathway. In addition, we also found indole-3-pyruvic acid in culture supernatants determined by high-performance liquid chromatography. Identification of tryptophan aminotransferase activity supports indole-3-pyruvic acid-routed IAA biosynthesis in R. paludigenum DMKU-RP301. We hence concluded that R. paludigenum DMKU-RP301 produces IAA through an indole-3-pyruvic acid pathway.

Indole 3-acetic acid (IAA) is the principal hormone which regulates various developmental and physiological processes in plants. IAA production is considered as a key trait for supporting plant growth. Hence, in this study, production of indole-3-acetic acid (IAA) by a basidiomycetous red yeast Rhodosporidium paludigenum DMKU-RP301 (AB920314) was investigated and improved by the optimization of the culture medium and culture conditions using one factor at a time (OFAT) and response surface methodology (RSM). The study considered the effects of incubation time, carbon and nitrogen sources, growth factor, tryptophan, temperature, shaking speed, NaCl and pH, on the production of IAA. The results showed that all the factors studied, except NaCl, affected IAA production by R. paludigenum DMKU-RP301. Maximum IAA production of 1,623.9 mg/l was obtained as a result of the studies using RSM. The optimal medium and growth conditions observed in this study resulted in an increase of IAA production by a factor of up to 5.0 compared to the unoptimized condition, i.e. when yeast extract peptone dextrose (YPD) broth supplemented with 0.1% l-tryptophan was used as the production medium. The production of IAA was then scaled up in a 2-l stirred tank fermenter, and the maximum IAA of 1,627.1 mg/l was obtained. This experiment indicated that the obtained optimal medium and condition (pH and temperature) from shaking flask production can be used for the production of IAA in a larger size production. In addition, the present research is the first to report on the optimization of IAA production by the yeast Rhodosporidium.

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