植物成長を促進する生物学的ストレス耐性イネ菌を用いたイネ作物の干ばつストレスの軽減

Mitigation of drought stress in rice crop with plant growth-promoting abiotic stress-tolerant rice phyllosphere bacteria.

• Devarajan Arun K
• Kuttalingam G Sabarinathan
• Muthukrishnan Gomathy
• Rengasamy Kannan
• Dananjeyan Balachandar

抄録

効果的な干ばつ緩和・生育促進性フィロスフィア細菌を探索するために、干ばつに強いイネ品種、Mattaikar、Nootripattu、Anna R(4)、PMK3 の葉面から合計 44 個の細菌分離株を分離し、生育培地を温度、塩分濃度、浸透圧ストレスに曝すことにより、生物学的ストレス耐性をスクリーニングした。その結果、異なる環境下で生育・増殖が認められたのは8株のみであった。これらの分離株を16SリボソームDNA遺伝子配列を用いて同定し、NCBIデータベースに登録した。さらに、これらの菌株をポリエチレングリコール（PEG6000）を用いて3つの異なる浸透圧ストレス条件でIAA産生、GA産生、ACCデアミナーゼ活性、エキソ多糖類産生などの植物成長促進（PGP）形質のスクリーニングを行った。さらに、ミネラルの可溶化を通常の条件で測定した。Bacillus endophyticus PB3、Bacillus altitudinis PB46、およびBacillus megaterium PB50は、多面的なPGP形質を有することが明らかになった。また、水耕栽培を用いてイネ種子に接種し、浸透圧ストレス（25% PEG 6000）条件下と非ストレス条件下で、個々の株の植物生育改善性能を検討した。さらに、ポット実験(-1.2MPa)を用いて、イネの繁殖期に細菌を葉面散布することにより、イネにおける細菌株の干ばつ緩和能力を評価した。最後に，種子接種と葉面散布の結果，B. megaterium PB50の散布により，浸透圧ストレス下での植物の生育が有意に改善され，気孔閉鎖による物理的な干ばつから植物を保護し，カロテノイド，全可溶性糖および全タンパク質含量が改善された。PB50 の代謝物をガスクロマトグラフィー質量分析法を用いてストレス条件と非ストレス条件の両方で分析した。

In the search of effective drought-alleviating and growth-promoting phyllosphere bacteria, a total of 44 bacterial isolates were isolated from the leaf surface of drought-tolerant rice varieties, Mattaikar, Nootripattu, Anna R(4), and PMK3, and screened for their abiotic stress tolerance by subjecting their growth medium to temperature, salinity, and osmotic stress. Only eight isolates were found to grow and proliferate under different abiotic stress conditions. These isolates were identified using 16S ribosomal DNA gene sequence and submitted to the NCBI database. All the bacterial isolates were identified as Bacillus sp., except PB24, which was identified as Staphylococcus sp., and these isolates were further screened for plant growth-promoting (PGP) traits such as IAA production, GA production, ACC deaminase activity, and exopolysaccharide production under three different osmotic stress conditions adjusted using polyethylene glycol (PEG 6000). Additionally, mineral solubilization was measured under the normal condition. Bacillus endophyticus PB3, Bacillus altitudinis PB46, and Bacillus megaterium PB50 were found to have multifarious PGP traits. Consecutively, the performance of an individual strain to improve the plant growth was investigated under the osmotic stress (25% PEG 6000) and nonstress condition by inoculating them into rice seeds using hydroponics culture. Furthermore, the drought-alleviating potency of bacterial strains was assessed in the rice plants using pot experiment (-1.2 MPa) through bacterial foliar application during the reproductive stage. Finally, as a result of seed inoculation and foliar spray, the application of B. megaterium PB50 significantly improved the plant growth under osmotic stress, protected plants from physical drought through stomatal closure, and improved carotenoid, total soluble sugars, and total protein content. Metabolites of PB50 were profiled under both stress and nonstress conditions using gas chromatography-mass spectroscopy.

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