種傳病毒之特性、檢測與管理

Abstract

根據報告已知的植物病毒中共有108 種具有種子傳播(seed transmission) 的特性。這些病毒 可以藉由極少量的帶毒種子在下一代之植株族群中建立初次感染源，然後再經由媒介生物的二 次傳染形成病害的快速流行，而危及作物的生產。在近代的農業歷史經驗中不乏由種傳病毒所 導致嚴重損失的例證。種傳特性對於寄主範圍狹窄或缺乏媒介生物的病毒種類而言尤其重要， 乃維護其族群能否持續繁衍的關鍵特性。近年來世界各國在植物種源交換上日趨頻繁，種傳特 性也是造成病毒透過帶毒種子廣泛散佈的主要原因。為防止種傳病毒病害的流行，發展帶毒種 子檢測技術，確定不同批號種子之帶毒比率(種傳率)，進而預估其可能造成作物損失之潛力與 作物生產能夠忍受之種傳率，藉以擬定適當之因應方法，乃近年來農業先進國家針對此類病毒 病害所採取之普識對策。目前已知除了少數性質極為穩定之病毒，如蕃茄嵌紋病毒(To m a t o mosaic viru s, ToMV) 可以在不感染種子胚(embryo) 組織而僅存在於種皮(seed coat) 或其他種子 組織下尚能造成種傳現象外，絕大多數的種傳病毒都能入侵並感染胚組織，但通常一個感病植 株僅有少數種子之胚組織會遭到感染而具有傳毒能力。影響種傳率高低之因素，包括寄主品系 (cultivars) 與年齡( a g e )、病毒系統(virus strains) 及感染時機(infection stage)。多數種傳病毒均 於胚珠受精前即已感染胚珠或花粉，僅有少數是在胚珠受精後才入侵其細胞，因此植株如果在 開花後才遭受病毒感染，其種子通常不具傳毒能力。目前最廣為應用之帶毒種子檢測技術為兼 具敏感、專一與經濟性之抗血清檢測法，包括酵素連結抗體免疫吸附法( e n z y m e - l i n k e d immunosorbent assay, ELISA) 及組織點漬免疫分析法(tissue-blot immunoassay, TBIA)。另外敏感 度更為優異但效率與成本稍差的反轉錄聚合酵素連鎖反應(reverse transcription polymerase chain reaction, RT-PCR) 也於近來被證實可用以偵測種子組織中所存在之微量病毒。晚近更有結合抗 體與RT-PCR 之免疫捕捉聚合酵素連鎖反應( i m m u n o c a p t u r e - RT-PCR, IC-RT-PCR) 被應用在種傳 病毒之偵測。但不管應用何種偵測方法與流程，其關鍵在於必須能偵測出存在於種子胚組織細 胞中具感染活力之病毒顆粒，而不會受到種子中其他組織所存在不具傳染能力之病毒訊息所干 擾。因此在發展特定種傳病毒之檢測流程時，必須先釐清病毒存在之位置進而確定種子萃取之 方法，以避免獲致錯誤之結論。種傳率之正確檢測可以作為擬定或評估種傳忍受極限( s e e d transmission tolerance limit) 之參考，也是近年來先進國家因應種傳病毒病害所積極發展之種子 驗證制度(seed certification) 之關鍵依據。 There have been more than 108 plant viruses documented in the literatures that have seed transmission ability. Infected seeds germinate and become infected seedlings which serve as virus inoculum and subsequently result in an epidemic if there is co-existence of efficient viral vectors. During last century there have been numerous reports of severe losses in crops resulted by seed-borne viruses. Seed transmission is especially crucial for those viruses with restricted host range and those without vectors to survive, disseminate or maintain their population. Viruses with seed transmission abilities are especially capable of long distance transmission. Most literatures agree that seedtransmitted viruses are carried within the embryo of infected seeds. However, there are exceptions, such as tobamoviruses, which are carried as contaminant on the seed surface and can infect seedlings during germination and early growth. In this paper, the mechanisms of virus invasion to the embryonic tissues will be discussed. To control seed-transmitted virus diseases, it is essential to develop sensitive and efficient seed testing techniques to determine seed transmission rate in diff e r e n t seed lots. During last decade, there have been numerous techniques developed especially for seed testing. However, the enzyme-linked immunosorbent assay (ELISA) has been recognized as the most suitable seed testing technique and is currently the most widely used one in seed industry. Besides detection technique, sampling the right parts of seed tissue for virus indexing is crucial for obtaining correct seed transmission rate. In this paper, the existence of viruses in different parts of seed tissue in relation to the decision of sampling method is discussed. Finally, various control or management strategies, including the application of seed transmission tolerance limit (STTL), against seedtransmitted viruses will also be summarized and discussed.