Rice (Oryza sativa) is a staple food in Thailand and, in addition, feeds around one half of the world's population. Therefore, diseases of rice are of special concern. Rice is destroyed by 2 main pathogens, Fusarium oxysporum and Pyricularia oryzae the causative agents of root rot and blast in rice respectively. These pathogens result in low grain yield in Thailand and other Southeast Asian countries. Soil samples were taken from paddy fields in Northern Thailand and bacteria were isolated using the soil dilution plate method on Nutrient agar. Isolation yielded 216 bacterial isolates which were subsequently tested for their siderophore production and effectiveness in inhibiting mycelial growth in vitro of the rice pathogenic fungi; Alternaria sp., Fusarium oxysporum, Pyricularia oryzae and Sclerotium sp., the causal agent of leaf spot, root rot, blast and stem rot in rice. It was found that 23% of the bacteria isolated produced siderophore on solid plating medium and liquid medium, In dual culture technique, the siderophore producing rhizobacteria showed a strong antagonistic effect against the Alternaria (35.4%), Fusarium oxysporum (37.5%), Pyricularia oryzae (31.2%) and Sclerotium sp. (10.4%) strains tested. Streptomyces sp. strain A 130 and Pseudomonas sp. strain MW 2.6 in particular showed a significant higher antagonistic effect against Alternaria sp. while Ochrobactrum anthropi D 5.2 exhibited a good antagonistic effect against F. oxysporum. Bacillus firmus D 4.1 inhibited P. oryzae and Kocuria rhizophila 4(2.

A total of 216 bacterial strains were isolated from rice rhizospheric soils in Northern Thailand. The bacterial strains were initially tested for solubilization of inorganic phosphate, indole acetic acid (IAA) production, selected strains were then tested for optimized conditions for IAA production and whether these caused stimulatory effects on bean and maize seedling growth. It was found that all strains had solubilized inorganic phosphate (P), but only 18.05% produced IAA. The best IAA producer was identified by biochemical testing and 16S rDNA sequence analysis as Klebsiella SN 1.1. In addition to being the best IAA producer, this strain was a high P-solubilizer and produced the highest amount of IAA (291.97 ± 0.19 ppm) in culture media supplemented with L-tryptophan. The maximum production of IAA was achieved after 9 days of incubation. The culture requirements were optimized for maximum IAA production. The tested of IAA production by selected isolates was studied in a medium with 0, 0.1, 0.2, 0.5, 0.7, and 0.9% (v/v) L-tryptophan. Low levels (12.6 ppm) of IAA production was recorded without tryptophan addition. Production of IAA in Klebsiella SN 1.1 increased with an increase to 0.2% (v/v) tryptophan concentration. The production of IAA was further confirmed by extraction of crude IAA from this isolate and subsequent Thin Layer Chromatography (TLC) analysis. A specific spot from the extracted IAA production was found to correspond with a standard spot of IAA with the same R (f) value. The Klebsiella strain SN 1.1 also demonstrated stimulatory effects on bean seedlings in vivo.

Plant growth-promoting rhizobacteria (PGPR) are known to influence plant growth by various direct or indirect mechanisms. A total of 216 phosphate-solubilizing bacterial isolates were isolated from different rice rhizospheric soil in Northern Thailand. These isolate were screened in vitro for their plant growth-promoting activities such as solubilization of inorganic phosphate, ammonia (NH 3 ), catalase and cell wall-degrading enzyme activity. It was found that 100% solubilized inorganic phosphate, 77.77% produced NH 3 and most of the isolates were positive for catalase. In addition, some strains also produced cell wall-degrading enzymes such as protease (7%), chitinase (1%), cellulase (3%) and b-glucanase (3%), as evidenced by phenotypic biochemical test and quantitative assay using spectrophotometry. The isolates could exhibit more than two or three plant growth-promoting (PGP) traits, which may promote plant growth directly or indirectly or synergistically. Part of this study focused on the effect of NaCl, temperature, and pH on a specific the bacterial isolate Acinetobacter CR 1.8. Strain CR 1.8 was able to grow on up to 25% NaCl, between 25 and 55°C, and at pH 5-9. Maximum solubilization of tricalcium phosphate and aluminium phosphate was obtained at neutral pH, and 37°C. Strain CR 1.8 had protease activity but no cellulase, b-glucanase and cellulase activities.

Rhizosphere bacteria can positively influence plant growth by direct and indirect mechanisms. A total of 112 bacterial strains were isolated from the rhizosphere of rice and tested for plant beneficial activities such as siderophore production, cell-wall-degrading enzyme production, hydrogen cyanide (HCN) production and antifungal activity against rice blast disease fungus. The actinomycetes count was 3.8 × 106 CFU/g soil. Streptomyces strains PC 12, D 4.1, D 4.3 and W1 showed strong growth inhibition of blast disease fungus, Pyricularia sp. (87.3%, 82.2%, 80.0% and 80.5%) in vitro. Greenhouse experiments revealed that rice plants treated with Streptomyces strain PC 12 recorded maximum plant height, root length and root dry weight compared to the control. Taxonomic characterization of this strain on the basis of 16S rRNA gene sequence led to its identification as Streptomyces palmae PC 12. Streptomyces palmae PC 12 may be used as biofertilizer to enhance the growth and productivity of commercially important rice cultivar RD6 and the biocontrol of blast disease fungus.

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