探討沼澤紅假單胞菌提升阿拉伯芥耐受鹽害與高溫複合逆境之作用機制

Abstract

在氣候變遷的影響下，土壤鹽害與高溫逆境日益頻繁地同時發生，對作物生長、光合作用及氧化還原恆定造成超越單一逆境之協同性損害。植物根際促生菌（PGPR）為提升作物抗逆性提供了一種永續策略，然而PGPR在複合逆境下介導保護作用之機制仍不甚明瞭。本論文探討沼澤紅假單胞菌（Rhodopseudomonas palustris）菌株 PS3、TPN1及 YSC3是否能提升阿拉伯芥（Arabidopsis thaliana）對鹽害、高溫及鹽熱複合逆境之耐受性，並解析介導此保護作用之細菌性狀與植物訊號傳遞途徑。 本研究首先評估細菌產生之5-胺基乙醯丙酸（5-ALA）對鹽逆境耐受性之貢獻，比較三株ALA產量不同之菌株與外源ALA處理在50 mM NaCl條件下之效果。TPN1賦予最強之保護作用，改善離子恆定（ NHX1、HKT1）、葉綠素生合成（ HEMA1、CHLH）、抗氧化防禦及根系構型。ALA產量較低之菌株亦賦予相當之保護效果，顯示ALA產量不足以完全解釋紫色非硫細菌介導之鹽害耐受性，而胞外多醣（EPS）及吲哚乙酸（IAA）之產生具有重要貢獻。 進一步將分析延伸至高溫及鹽熱複合逆境，並利用fin219-2（JAR1）突變體解析茉莉酸訊號途徑。菌株效力排序在高溫逆境下發生逆轉：先前被歸類為促生無效之 YSC3，因其優越之細菌耐熱性及在45°C下維持EPS與IAA產生之能力，提供最強之保護作用。LC-MS/MS及RT-qPCR分析顯示，保護作用主要透過非JA-Ile依賴性途徑運作，特別是HSFA2–HSP101分子伴護蛋白軸及 CAT2/APX1抗氧化網絡，此等途徑在fin219-2中仍維持顯著之菌株依賴性誘導能力，而典型 COI1–JAZ–MYC2訊號傳遞僅具部分貢獻。 綜合而言，本研究結果支持一項模型：R. palustris透過協調調控多種細菌性狀與植物訊號傳遞網絡來增強植物逆境耐受性，而特定機制之相對重要性取決於所遭受逆境之類型與組合。

As climate change progresses, soil salinity and heat stress increasingly co-occur, causing synergistic damage to crop development, photosynthetic capacity, and oxidative-reductive equilibrium that exceeds the impact of either stress alone. Plant growth-enhancing rhizobacteria (PGPR) offer a sustainable strategy for improving crop stress tolerance, yet the underlying mechanisms of PGPR-mediated protection under combined stress remain insufficiently understood. This thesis examined whether Rhodopseudomonas palustris strains (PS3, TPN1, YSC3) can improve Arabidopsis thaliana resilience to salt, heat, and combined salt-heat stress, and identified the microbial characteristics and plant signaling routes responsible for this protective effect. The contribution of bacterially produced 5-aminolevulinic acid (5-ALA) in salt stress tolerance was initially evaluated by contrasting three strains differing in ALA output alongside exogenous ALA application under 50 mM NaCl. TPN1 provided the most robust protection, strengthening ionic equilibrium (NHX1, HKT1), chlorophyll biosynthesis (HEMA1, CHLH), antioxidant defense, and root architecture. Isolates with lower ALA output still provided comparable advantages, confirming that ALA biosynthesis alone is insufficient to account for nPNSB-driven salt tolerance and that EPS and IAA synthesis play significant roles. The analysis was subsequently broadened to include heat and combined salt-heat stress, with jasmonate signaling dissected using the fin219-2 (JAR1) mutant. The order of strain effectiveness reversed under heat stress conditions: YSC3, previously categorized as growth-promotion-inactive, delivered the strongest protection attributable to superior bacterial thermotolerance and sustained EPS and IAA biosynthesis at 45 °C. LC-MS/MS and RT-qPCR data indicated that protection functions primarily through JA-Ile-independent routes, particularly the HSFA2–HSP101 chaperone axis and CAT2/APX1 antioxidant networks, which maintained considerable responsiveness in the fin219-2 background, whereas the canonical COI1–JAZ–MYC2 signaling made a partial contribution. Together, these findings support a model in which R. palustris enhances plant stress tolerance through synchronized regulation of diverse microbial characteristics and plant signaling cascades, where the relative significance of particular pathways varies according to the type and combination of stresses experienced.