Plant Growth-promoting Characteristics of Yeasts Isolated from Rhizosphere Soil and Efficacy for Growth Promoting of Selected Plants

Abstract

Chemical fertilizers are the most common method for increasing agricultural production in Thailand, but they are not considered a long-term solution due to health and environmental hazards. For this reason, researchers have been interested in plant growthpromoting microorganisms as an alternative way to replace chemical fertilizers. However, plant growth promotion involving yeasts in soil has not yet been extensively investigated. Therefore, this study aimed to isolate and identify yeast strains obtained from rhizosphere soils of the Assam tea plant (Camellia sinensis var. assamica) in northern Thailand, as well as to determine their ability to promote plant growth and select effective yeasts for biofertilizer development. A total of 41 yeast strains were obtained and identified using the D1/D2 domain of the large subunit ribosomal RNA gene. Seventeen strains of the genera Aureobasidium, Galactomyces, Kazachstania, Saturnispora, Schwanniomyces, and Wickerhamomyces were identified as belonging to the phylum Ascomycota, and twenty-four strains of the genera Apiotrichum, Curvibasidium, Papiliotrema, Rhodosporidiobolus, and Trichosporon belonged to the phylum Basidiomycota. Moreover, two new Wickerhamomyces species, Wickerhamomyces lannaensis and Wickerhamomyces nanensis were introduced base on phenotypic (morphological, biochemical and physiological characteristics) and molecular analysis. Among 41 yeast strains, all the yeast strains had a positive ammonia production. Twenty-seven strains could produce IAA in a range of 2.12 to 37.32 mg/L, and only 8 strains showed positive siderophore production. Two yeast strains, Papiliotrema laurentii SDBR-CMU-S1-02 and Wickerhamomyces anomalus SDBR-CMU-S1-06), are able to solubilize the different phosphate insoluble forms. Papiliotrema laurentii could solubilize both Ca3(PO4)2 and AlPO4 forms, while W. anomalus could solubilize only Ca3(PO4)2 form. Moreover, P. laurentii and W. anomalus had ability to produce acid and alkaline phosphatase, respectively. Thirty-two strains could produce extracellular enzymes (amylase, cellulase, lipase, pectinase, protease, and xylanase) depended on the strain of the isolated yeast species. The two effective yeast strains (P. laurentii SDBR-CMU-S1-02 and W. anomalus SDBR-CMU-S1-06) were found to promote seed germination and seedling development of plants. The result revealed that two yeast species were increased radicle length, coefficient of velocity (CV), mean germination time (MGT), and germination index (GI) of Chinese kale, cucumber, lettuce, rice, and tomato. Moreover, two yeast species increased weight, high, inorganic phosphate (Pi) content, the number of leaves, and chlorophyll in seedlings of Chinese kale, corn, eggplant, lettuce, and tomato in greenhouse conditions. The develop the biofertilizer, two yeast strains were immobilization in 2% (v/v) sodium alginate. The encapsulation yield (EY) was evaluated using the exponential values of cell counts, The EY of P. laurentii and W. anomalus were 52.64 % and 63.90 %. The viability of yeast in alginate beads was found that after storage at 4 °C P. laurentii and W. anomalus decrease to less than 50 % survival at 170 and 70 days. For viability, storage at 25 °C resulted that P. laurentii and W. anomalus decrease to less than 50 % survival at 110 and 40 days, respectively. The shelf life of immobilized P. laurentii and W. anomalus showed the longest shelf life at 4 °C, with 612 and 285 days, respectively. To determine the efficiency of applying yeast inoculum in alginate beads and cell suspension on growth and yield of Chinese kale and lettuce plants in greenhouse experiment. It was found that the use of plant growth-promoting yeast as biofertilizers by cell suspension inoculum was suitable for Chinese kale and lettuce. Both selected yeasts (P. laurentii and W. anomalus) increased the yield and growth of Chinese kale and lettuce, however the cell suspension of W. anomalus had the most significant stimulating effect. In addition, the growth of Chinese kale was highest when grown in soil adding Ca3(PO4)2, while lettuce growth was highest when grown in soil without Ca3(PO4)2.