Rhodopseudomonas palustris strains PS3 and YSC3 are purple non-sulfur phototrophic bacteria isolated from Taiwanese paddy soils. PS3 has beneficial effects on plant growth and enhances the uptake efficiency of applied fertilizer nutrients. In contrast, YSC3 has no significant effect on plant growth. The genomic structures of PS3 and YSC3 are similar; each contains one circular chromosome that is 5,269,926 or 5,371,816 bp in size, with 4,799 or 4,907 protein-coding genes, respectively. In this study, a large class of genes involved in chemotaxis and motility was identified in both strains, and genes associated with plant growth promotion, such as nitrogen fixation-, IAA synthesis- and ACC deamination-associated genes, were also identified. We noticed that the growth rate, the amount of biofilm formation, and the relative expression levels of several chemotaxis-associated genes were significantly higher for PS3 than for YSC3 upon treatment with root exudates. These results indicate that PS3 responds better to the presence of plant hosts, which may contribute to the successful interactions of PS3 with plant hosts. Moreover, these findings indicate that the existence of gene clusters associated with plant growth promotion is required but not sufficient for a bacterium to exhibit phenotypes associated with plant growth promotion.

Biofertilizers can help improve soil quality, promote crop growth, and sustain soil health. The photosynthetic bacterium Rhodopseudomonas palustris strain PS3 (hereafter, PS3), which was isolated from Taiwanese paddy soil, can not only exert beneficial effects on plant growth but also enhance the efficiency of nutrient uptake from applied fertilizer. To produce this elite microbial isolate for practical use, product development and formulation are needed to permit the maintenance of the high quality of the inoculant during storage. The aim of this study was to select a suitable formulation that improves the survival and maintains the beneficial effects of the PS3 inoculant. Six additives (alginate, polyethylene glycol [PEG], polyvinylpyrrolidone-40 [PVP], glycerol, glucose, and horticultural oil) were used in liquid-based formulations, and their capacities for maintaining PS3 cell viability during storage in low, medium, and high temperature ranges were evaluated. Horticultural oil (0.5 %) was chosen as a potential additive because it could maintain a relatively high population and conferred greater microbial vitality under various storage conditions. Furthermore, the growth-promoting effects exerted on Chinese cabbage by the formulated inoculants were significantly greater than those of the unformulated treatments. The fresh and dry weights of the shoots were significantly increased, by 10-27 and 22-40 %, respectively. Horticultural oil is considered a safe, low-cost, and easy-to-process material, and this formulation would facilitate the practical use of strain PS3 in agriculture.

Rhodopseudomonas palustris PS3 is one of the purple phototrophic non-sulfur bacteria (PNSB), which have plant growth-promoting effects on various plants. To expand the scale of PS3 fermentation in a time-and cost-effective fashion, the purpose of this work was to evaluate the use of low-cost materials as culture media and to optimize the culture conditions via response surface methodology. Corn steep liquor (CSL) and molasses were identified as potential materials to replace the nitrogen and carbon sources, respectively, in the conventional growth medium. The optimum culture conditions identified through central composite design were CSL, 39.41 mL/L; molasses, 32.35 g/L; temperature, 37.9˚C; pH, 7.0; and DO 30%. Under the optimized conditions, the biomass yield reached 2.18 ± 0.01 g/ L at 24 hours, which was 7.8-fold higher than that under the original medium (0.28 ± 0.01 g/ L). The correlation between the predicted and experimental values of the model was over 98%, which verified the validity of the response models. Furthermore, we verified the effectiveness of the R. palustris PS3 inoculant grown under the newly developed culture conditions for plant growth promotion. This study provides a potential strategy for improving the fermentation of R. palustris PS3 in low-cost media for large-scale industrial production.

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