甘藷塊根形成基因IbMADS1之生理功能分析

Abstract

甘薯（Ipomoea batats (L.) Lam.）塊根膨大過程中的營養元素需求、形態變化、內在植物激素消長等方面，長久以來已被廣泛研究，但是其基因調控機理至今仍不明確；為此，本研究欲著眼於分子層面來探究甘薯的塊根形成機制。本論文針對從甘薯塊根篩選出的MADS-box基因，IbMADS1（Ipomoea batatas MADS-box gene 1）進行分析，分別探討該基因在甘薯各部位的表現方式、受植物荷爾蒙調節的影響，以及甘薯屬中野生親緣種間的基因表現歧異性，再進一步從轉殖植物的外表形態了解此基因可能的生理功能。 RNA表現模式分析中，IbMADS1基因與植物的營養生長有關，且在甘薯塊根膨大初期大量表現，同時會受外加細胞分裂素及茉莉酸的誘導而提高表現量。分析親源基因在種間表現之歧異性，發現IbMADS1基因或其同源基因雖然在甘薯塊根大量表現，但在不會結薯的墨西哥牽牛（Ipomoea trifida）與濱牽牛（Ipomoea leucantha）這二個甘薯近緣野生種的根部均不表現，推測IbMADS1基因經功能性演化後與甘薯結薯有所關聯。 為了解IbMADS1基因之功能，我們觀察大量表現IbMADS1的馬鈴薯轉殖植株之外表形態變化，得到轉殖植株根部出現區域性膨大情形。利用解剖切片觀察並比較轉殖馬鈴薯與野生型甘薯的根部塊狀膨大區域，可以發現兩者在膨大初期，中柱內含的細胞數皆增多，尤其是木質部細胞出現不正常增生情形，進而擴展中柱區域；因此，我們可以初步推論在甘薯塊根發育初期，IbMADS1藉著作用於根部分生組織，尤其是中柱的原始形成層，從而促進細胞分裂，並參與甘薯的初期結薯。

Tuberization is an important stage during plant growth and development, especially in tuber crops. As an example, sweet potato (Ipomoea batatas) is characterized with fleshy tuberous roots for nutrient storage. Factors directing the induction of early tuberization have been investigated for a long time in sweet potato. At a glance of the molecular mechanism, one gene encoding 218 amino acids was identified from cDNA-AFLP differential study and then named as Ipomoea batatas MADS-box 1 gene (IbMADS1) based on earlier studies. From sequence alignment and phylogenic analysis, IbMADS1 exhibited a typical type II-MIKC structure, including two conserve domains, MADS-box and K-box. Besides, IbMADS1 was grouped together with other orphan group MADS-box genes, such as AGL17 (Agamous-like 17) in Arabidopsis. After further dissecting the IbMADS1 expression pattern in sweet potato, RNA transcripts were specifically detectable in roots, and the increase of IbMADS1 transcripts was correlated with the early-stage formation of tuberous roots. Southern and Northern analyses were also performed to detect IbMADS1 homologues among tuber-deficient Ipomoea relatives of sweet potato to investigate the evolutionary relationship in tuber formation. In functional study, overexpression assay was conducted in potato (Solanum tuberosum). Interestingly, tuberous swelling along roots was observed in some transgenic potato lines in addition to stem-derived tuber formation. Conclusively, we propose a hypothesis to illustrate the mechanism by which IbMADS1 participates in the initiation of tuberization in sweet potato.