BABA誘發免疫反應對抗細菌性病原菌之作用機制

Abstract

BABA (beta-aminobutyric acid)是一種無法用於合成蛋白質的胺基酸，但是卻被發現能有效促進植物對病原菌的抵抗性。植物經BABA處理後，一旦感懶細菌性斑點病病原菌 (Pseudomonas syringae pv. tomato DC3000, Pst DC3000)，水楊酸 (salicylic acid, SA) 訊息傳導途徑中的PR1 基因將被快速誘發而表現。本論文報導由BABA所引起的病原菌抵抗性，並非完全依賴於水楊酸訊息傳導途徑；BABA能誘發植物的早期免疫反應 (plant early defense response)。一經細菌Pst DC3000感染，BABA誘發諸多免疫基因表現，且這些基因分別隸屬於不同訊息傳導途徑，例如，MAPK (mitogen-activated protein kinases) ，乙烯 (ethylene, ET)，以及其他訊息途徑。同時，BABA能有效誘發細菌感染時所引起的胝質累積(callose deposition)。除此之外，如將水楊酸的突變株sid2以RNA干擾技術(RNA interference)造成PR2基因靜默(gene silencing)，不會改變BABA對該突變株的作用，顯示PR2的存在與否對於BABA的作用沒有影響。為深入瞭解BABA如何調節阿拉伯芥的免疫作用，金洛仁博士的實驗室成員們測試七十餘株突變株對於BABA的敏感性與細菌Pst DC3000的感病性。本論文發現某一LRR家族蛋白激酶 (leucine-rich repeat protein kinase, LRR-PK)的突變株，不僅易受細菌感染、胝質的累積量減少，且對於BABA的敏感度降低。該LRR-PK表現於細胞膜。且於植物遭受細菌感染，或者感受到病原相關分子模式(pathogen-associated molecular patterns, PAMPs)時，LRR-PK基因能被快速誘導表現。初步證據顯示此基因參與阿拉伯芥早期的免疫反應。除此之外，本研究亦發現四磷酸雙腺苷水解酶(Ap4A hydrolase)對於阿拉伯芥抵抗腐生性病原菌(necrotrophic pathogen)具有重要功能。該基因的突變株對於真菌性灰黴病病原菌(Botrytis cinerea)和細菌性軟腐病病原菌(Erwinia carotovora subsp. carotovora)較為感病。

The non-protein amino acid beta-aminobutyric acid (BABA)has been known for years to be an effective inducer of resistance against pathogen infection. When BABA-pretreated Arabidopsis plants are challenged with the pathogenic bacteria Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), a strong potentiation of PR1 expression is observed. PR1 is the marker gene for salicylic acid (SA)-related defense response. Here, we report that BABA-mediated bacterial resistance acts in a partly SA-independent manner to prime the early Arabidopsis defense response. Upon Pst DC3000 infection, mitogen-activated protein kinase (MAPK) cascade genes, ethylene (ET) and other signaling pathways were potentiated by BABA pretreatment in both Col-0 wild type and in the SA biosynthetic sid2 mutant. Callose deposition in response to Pst DC3000 inoculation was also primed by BABA. In addition, RNAi-mediated PR2 gene silencing in sid2 mutant demonstrated BABA-induced resistance (BABA-IR) to Pst DC3000 similar to sid2 indicating that PR2 is not critical for BABA-induced resistance in sid2. To further investigate how BABA regulates Arabidopsis immunity, members of Zimmerli’s laboratory tested over 70 T-DNA knock-out mutant lines for their sensitivity towards BABA and Pst DC3000 infection. Two T-DNA knock-out lines of a leucine-rich repeat protein kinase (LRR-PK) demonstrated enhanced bacterial susceptibility, reduced callose deposition, as well as impaired BABA sensitivity. This LRR-PK localizes to the plasma membrane. Upon Pst DC3000 infection and PAMPs treatment, this LRR-PK gene was rapidly induced. Hence, our preliminary data suggest that this gene is involved in Arabidopsis early defense responses. In addition, one gene was found important for plant resistance to necrotrophic pathogens. Mutation of AtNUDX25, one Ap4A hydrolase, led to enhanced susceptibility to Botrytis cinerea and Erwinia carotovora subsp. carotovora strain WPP14 (Ecc WPP14).