PNAS：甘薯——天然的转基因作物

2015年4月23日讯/生物谷BIOON/–“反转”（反对转基因）人士不接受转基因作物的其中一个原因是，转基因作物是人工改造的，不是自然界本身就存在的。但是，PNAS近日发表的最新研究表明，甘薯（sweet potato，又称红薯，地瓜）是一种天然的转基因作物。在科学家研究的291个样品中，都发现了农杆菌T-DNA的序列。科学家认为在甘薯进化的过程中，曾被农杆菌侵染，而且被转入的农杆菌T-DNA在后续的自然选择中保留了下来。

农杆菌是一种天然的植物病原菌，可以通过植物的创伤部位侵染植物，然后将自身的“转移DNA（T-DNA）”[transfer DNA (T-DNA)]转入并整合到植物的基因组中。转入的农杆菌T-DNA基因可以在植物体内转录表达，合成农杆菌所需的营养物质，并使植物长出肿瘤——“冠瘿瘤”（Crown gall）。科学家们正是利用农杆菌的这种特性，把农杆菌改造成了构建转基因作物的工具。

科学家利用DNA杂交（southern blot），定量PCR（quantitative PCR），染色体步移(genome walking)，BAC库（bacterial artificial chromosome library）筛选和测序等方法分析了甘薯栽培种“Huachano”的291个样品。所有的检测样品都发现了农杆菌的序列，并发现其中4个基因与农杆菌Acs, C-prot, iaaH, and iaaM四个基因同源。这4个农杆菌同源的基因，在甘薯的茎，叶和块根中都有表达。不仅如此，这一段农杆菌DNA片段的插入，还破坏了甘薯自身一个F盒基因（F-box gene）的表达和正常功能。

人类种植甘薯的历史，可以追逐到8000-10000年前。这次的研究结果证明，在甘薯的进化过程中，在农杆菌和甘薯祖先之间发生了水平基因转移[Horizontal gene transfer (HGT)]。这些农杆菌基因的转入，可能影响了甘薯的性状，而自然选择又保留了这些性状。科学家认为这个新发现证明了，在作物进化过程中，转基因可以自然的发生。这或许会改变人们对转基因作物的观点，不再认为转基因是一件“不正常”的事儿了。（生物谷 Bioon.com）

本文系生物谷原创编译整理，欢迎转载！转载请注明来源并附原文链接。谢谢！更多资讯请下载生物谷资讯APP。

PNAS Early Edition

原文链接：www.pnas.org/cgi/doi/10.1073/pnas.1419685112

The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop

Tina Kyndt, Dora Quispea, Hong Zhai, Robert Jarret, Marc Ghislain, Qingchang Liu, Godelieve Gheysen, and Jan F. Kreuze

Abstract

Agrobacterium rhizogenes and Agrobacterium tumefaciens are plant pathogenic bacteria capable of transferring DNA fragments [transfer DNA (T-DNA)] bearing functional genes into the host plant genome. This naturally occurring mechanism has been adapted by plant biotechnologists to develop genetically modified crops that today are grown on more than 10% of the world’s arable land, although their use can result in considerable controversy. While assembling small interfering RNAs, or siRNAs, of sweet potato plants for metagenomic analysis, sequences homologous to T-DNA sequences from Agrobacterium spp. were discovered. Simple and quantitative PCR, Southern blotting, genome walking, and bacterial artificial chromosome library screening and sequencing unambiguously demonstrated that two different T-DNA regions (IbT-DNA1 and IbT-DNA2) are present in the cultivated sweet potato (Ipomoea batatas [L.] Lam.) genome and that these foreign genes are expressed at detectable levels in different tissues of the sweet potato plant. IbT-DNA1 was found to contain four open reading frames (ORFs) homologous to the tryptophan-2-monooxygenase (iaaM), indole-3-acetamide hydrolase (iaaH), C-protein (C-prot), and agrocinopine synthase (Acs) genes of Agrobacterium spp. IbT-DNA1 was detected in all 291 cultigens examined, but not in close wild relatives. IbT-DNA2 contained at least five ORFs with significant homology to the ORF14, ORF17n, rooting locus (Rol)B/RolC, ORF13, and ORF18/ORF17n genes of A. rhizogenes. IbT-DNA2 was detected in 45 of 217 genotypes that included both cultivated and wild species. Our finding, that sweet potato is naturally transgenic while being a widely and traditionally consumed food crop, could affect the current consumer distrust of the safety of transgenic food crops.