農桿菌新知與宿主植物的免疫攻防戰

The arms race between Agrobacterium and host plants

農桿菌（Agrobacterium tumefaciens）自二十世紀初被發現造成冠瘿狀腫瘤後，因其獨特的病徵令許多科學家想探究農桿菌的秘密，科學界就此展開對農桿菌的研究旅程，歷經多年研究農桿菌的致病機制逐漸明朗，並且已普遍作為轉殖的工具之一。農桿菌的基因體中包含一個腫瘤誘導質體（tumor-inducing plasmid，Ti質體），其上帶有一段轉移DNA（transfer DNA，T-DNA）可送入植物細胞中，使感染植株產生腫瘤。而目前已知有五個致病蛋白質（VirD2、VirD3、VirE2、VirE3及VirF）會透過由VirB1-11和VirD4組成的第四型分泌系統進入植物細胞中，同時藉由致病蛋白質與植物蛋白質產生的交互作用，使得T-DNA可在植物細胞中運送並嵌入植物染色體中，導致植株過度生合成植物生長素與細胞分裂素因而產生腫瘤。此外，T-DNA上亦帶有冠瘿鹼的編碼基因，也得以在受感染的植物中表現，作為農桿菌可利用的碳氮源。然而當植物受到病原菌的感染時，植株細胞膜上的受器會偵測病原菌分泌的化學分子，除了透過MAPK訊息傳遞啟動防禦反應相關基因的表現之外，植物賀爾蒙水楊酸對於植株產生系統性抗病也有重要的功能，不僅可提升植株的抗病性，還能有效地抑制農桿菌致病基因的表現，降低農桿菌的致病機率。農桿菌利用致病蛋白質啟動植物的防禦反應，順勢讓致病蛋白質帶著T-DNA進入植物細胞核並嵌入染色體，以獲得所需資源生存。藉由了解植物與農桿菌之間的交互關係，可使未來進一步利用農桿菌建立重要作物的轉殖系統時更為順利。

"The fascinating journey of Agrobacterium tumefaciens research started from the discovery of causative agent of crown gall disease at the early times of the 20th century. Since then, the infection mechanism of A. tumefaciens has gradually become understood and the Agrobacterium-mediated transformation method has been widely used as one of the important and popular tools to generate transgenic plants. The A. tumefaciens genome contains a tumor-inducing (Ti) plasmid with a specific DNA fragment (transfer DNA, T-DNA) which is processed and transferred into plant cells to cause tumors. The T-DNA and several virulence (Vir) proteins, including VirD2, VirD3, VirE2, VirE3, and VirF, are exported into plant cells by a type IV secretion system (T4SS) which is composed of VirB1-11 and VirD4 proteins. After entering the plant cell, these virulence proteins can interact with different plant proteins to subvert plant defenses and mediate T-DNA nuclear import and integration. Expression of the oncogenes in the native T-DNA leads to the overproduction of plant hormones auxin and cytokinin, resulting in tumors. Another set of transferred genes causes the production of a bacterial carbon/nitrogen source, called opines, in the infected plant cell. Host plant cells have evolved to detect pathogen-associated molecular patterns (PAMPs) by pattern-recognized receptors (PRRs) of plants and activate the host immune responses. These responses include initiation of the mitogen-activated protein kinases (MAPKs) signaling cascade, defense gene expressions, and alternations of phytohormone homeostasis, which may interfere A. tumefaciens vir gene expressions and reduce bacterial infection. The A. tumefaciens utilizes virulence proteins to turn on plant defense responses to its own advantage and obtains nutrients from the infected plant cells. Further understanding the molecular mechanism of a continuing battle between A. tumefaciens and the infected plants shall provide more useful information on generating transgenic plants of economically important crops in the near future."