Journal of Mushroom Science and Production 한국버섯학회지 ⓒThe Korean Society of Mushroom Science J. Mushroom Sci. Prod. 11(2):53-62(2013) 53 Optimization of Indole-3-Acetic production by phosphate solubilization bacteria isolated from waste mushroom bed of Agaricus bisporus Buddhi Charana Walpola 1 , Jae-Geun Noh 1 , Chan Kyem Kim 1 , Ki-Cheon Kyung 2 , Won-Sik Kong 3 and Min-Ho Yoon 1 * 1 Department of Bio-Environmental Chemistry, College of Agriculture and Life Sciences, Chungnam National University, Daejeon 305-764, Korea 2 Taean Lily Experiment Station, Chungcheongnam-do Agricultural Research and Extention Services. Taean 357-952, Korea 3 Mushroom Research Division, National Institute of Horticultural & Herbal Science, RDA, Eumseong 369-873, Korea. (Received June 5, 2013. Accepted June 18, 2013) ABSTRACT  A total of 35 phosphate solubilizing bacterial strains were isolated from waste mushroom bed of Agar- icus bisporus in Buyeo-Gun, Chungnam and screened for the production of indole acetic acid (IAA). The best IAA producing strain was identified as Pantoea rodasii using 16S rRNA analysis. In addition to the IAA production, this strain could act as an efficient phosphate solubilizer (1100 μg ml -1 after 5 days of incubation) also. The selected strain was cultured under different conditions in order to assess the optimum conditions for maximum IAA production. The nutrient broth (NB) medium was recorded as the best medium, where the maximum IAA production (229 μg ml -1 ) was recorded at the start of stationary phase (12 hours after inoculation) of the bacteria growth. The performance of the strain was found to be maximum at the temperature of 30 o C followed by 25 o C. IAA production was found to be increased with increasing tryptophan concentration (from 0.1 to 0.6%), however beyond this limit, a slight reduc- tion in IAA production was observed. The strains’ ability to produce IAA was further confirmed by extraction of crude IAA and subsequent TLC analysis. A specific spot from the extracted IAA preparation was found corresponding with the standard spot of IAA with same R f value. The results of HPLC analysis conducted in identifying and quan- tifying the IAA production more precisely, are in agreement with the results of the assessment done with colorimetric method. As revealed by the results of the pot experiment, the isolated strain could significantly enhance the growth (as measured by shoot and root growth) of mung bean plants compared to that of non-inoculated plants. Therefore it can be concluded that the present strain, Pantoea rodasii has great potential to be used as bio-inoculants. KEYWORDS  Pantoea rodasii, indole acetic acid, bio-inoculants, phosphate solubilizer Introduction Microorganisms can act directly or indirectly resulting in beneficial or detrimental effects on the soil environment. Soil microorganisms such as bac- teria, fungi and algae, which are capable of produc- ing physiologically active quantities of auxins may exert pronounced effects on plant growth and devel- opment. The most common, best characterized and physiologically most active auxin is known to be indole acetic acid (IAA), which is mainly produced by tryptophan dependent pathway (Datta and Basu, 2000; Ahmad et al., 2005; Gosh and Basu, 2006; Mandal et al., 2007). Some microorganisms are known to produce auxin in the rhizosphere using a substrate of plant exudates which contain amino acid L-tryptophan (Ahmad et al., 2005) As stated by Ahmad et al. (2008), both rapid response (e.g. increased cell elongation) as well as long-term response (e.g. cell division and differenti- ation) is distinguished when IAA stimulates plant growth. Furthermore, IAA enhances lateral root for- mation which in turn could facilitate high root sur- face area for better absorption of nutrients (Compant et al., 2010). Therefore, microbes through the pro- duction of IAA could have definite effect on growth of the host plant. It has been reported that micro- bial IAA promotes root growth either directly by stimulating plant cell elongation or cell division or indirectly by its influence on the 1-aminocyclopro- pane-1-carboxylate (ACC) deaminase activity (Pat- ten and Glick, 2002). It has also reported that IAA *Corresponding author: mhyoon@cnu.ac.kr