吉貝素在堇蘭不等大子葉發育中扮演的角色

在苦苣苔科 (Gesneriaceae) 中，子葉不等大為其特有的現象。在種子萌芽後，其中一片子葉基部的分生組織 (basal meristem) 會持續不斷生長成大子葉，另一小子葉則逐漸停止生長、最後凋亡。在前人研究中，施加吉貝素 (GA3) 會抑制basal meristem的活力，使菫蘭不等大子葉的現象消失，形成二片小子葉的型態，這和一般植物中維持頂端分生組織活力需要有低濃度的吉貝素，似乎有著相似的模式。在本研究中，我們藉由吉貝素處理後與正常型堇蘭的形態觀察差異為基礎，進行RT-PCR、RNA原位雜交的試驗，嘗試解開吉貝素在大小子葉中所扮演的角色。我們分離出在菫蘭 (Streptocarpus rexii) 中降解吉貝素的酵素基因SrGA2ox以及合成吉貝素的酵素基因SrGA20ox；在堇蘭幼苗形成不等大子葉的時期，SrGA2ox表現在basal meristem以及groove meristem (形成新葉phyllomorph及長成花序的分生組織) 的位置，證實basal meristem的形成是需要SrGA2ox的表現降解吉貝素，以低濃度的吉貝素來維持basal meristem的活力，而產生持續不斷生長的大子葉；SrGA20ox則表現在小子葉和大子葉的尖端，推測可能參與細胞分裂區外的細胞生長部份。此外，我們也檢測菫蘭的胚胎時期，吉貝素降解基因SrGA2ox表現在二子葉及子葉的近軸面，意味著二片子葉皆具有分生組織的能力，所以大小子葉現象是種子萌芽後才受到環境調控形成；而吉貝素合成基因SrGA20ox在胚胎時期集中表現在shoot meristem (兩片子葉中間) 的位置和子葉的遠軸面，可能因累積了高濃度的吉貝素在將來莖頂分生組織位置，使植株無法形成正常頂芽現象。因為細胞分裂素（CK）處理會使大小子葉現象消失但相反的形成兩片大子葉現象，未來可繼續研究細胞分裂素在堇蘭子葉生長過程中所扮演的角色，試著去解開吉貝素及細胞分裂素與苦苣苔科植物不等大子葉如何發育的謎團。

關鍵字

子葉不等大； Streptocarpus rexii ； basal meristem ； groove meristem ； GA2-oxidase ； GA20-oxidase

並列摘要

Anisocotyly is an unorthodox phenomenon among angiosperms, found only in the family members of Gesneriaceae. The rosulate species Streptocarpus rexii exhibits an anisocotylous growth pattern as one of its cotyledons (macrocotyledon) grows continuously via sustained basal meristem (BM) activity at the proximal end. Previous findings suggest that exogenous gibberellin (GA3) can inhibit anisocotyly via suppression of the formation of the BM, causing two equal-sized, small microcotyledons. This study is therefore aimed to shed light on the role of gibberellin (GA) in anisocotyly by revealing the expression pattern of GA catabolism and biosynthesis genes, SrGA2ox and SrGA20ox, to determine how they regulate the unusual basal meristem activity that gives rise to anisocotyly in S. rexii. RT-PCR and RNA in situ hybridization demonstrated that in the macrocotyledon, SrGA2ox shows restricted expression in the basal meristem and also in a shoot apical meristem (SAM) equivalent groove meristem (GM), where further phyllomorph and inflorescence are produced. On the contrary, SrGA20ox transcripts, distributed in the microcotyledon and the distal end of the macrocotyledon, never distributed in the meristem area. These results show agreement with that of model plants, which also suggest that a low concentration of GA is necessary for maintaining meristem activity. Moreover, this mutually exclusive expression pattern between SrGA2ox and SrGA20ox was established since the embryo stage. SrGA2ox was expressed on the adaxial side of both cotyledons, whereas SrGA20ox was distributed in the shoot meristem (SM) and abaxial side of the cotyledons during the embryo stage. The expression of SrGA20ox in the SM may explain the lack of normal shoot initiation in S. rexii seedlings. Because it is also known that exogenous cytokinin (CK) can induce both cotyledons into macrocotyledons, further investigation on the expression profiles of CK genes and their interaction with GA genes is important to fully uncover the developmental mechanism of anisocotyly.

並列關鍵字

Anisocotyly ； Streptocarpus rexii ； basal meristem ； groove meristem ； GA20-oxidase ； GA2-oxidase

參考文獻

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國際替代計量

吉貝素在堇蘭不等大子葉發育中扮演的角色

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