俄氏草花序轉換中花序特性與成花定性基因之研究

Abstract

部分被子植物已分化的花芽或花序分生組織，會逆轉回營養芽或營養性莖頂，這種現象前人統稱為「成花反轉」。一般推測，成花反轉現象能保障該類植物具有多年性生活史、或具有假性胎生（如產生珠芽等無性繁殖體）繁殖策略。臺灣原生之苦苣苔科植物俄氏草（Titanotrichum oldhamii）於其一年為週期之生活史中具獨特的成花反轉（花序轉換）現象：成花花序於花期後轉變為珠芽枝，即由重複產生側生的珠芽（枝）分生組織取代花芽分生組織以進行假性胎生。為檢視俄氏草花序轉換的可能機制，本研究中釣取與花序及成花相關的四個候選調節基因：協調花芽形成的花芽分生組織特性基因ToFLO、概與其拮抗之花序分生組織特性基因ToCEN、莖頂分生組織幹細胞特性基因ToROA，及花芽分化末期抑制幹細胞活性的花器特性基因ToPLE，並相互比較其在不同發育時期基因表現與形態上的關聯。 各候選基因中唯獨花序分生組織ToCEN的表現趨勢吻合發育上的轉換：原先未在分化花芽中表現之ToCEN卻於將成為珠芽枝的分生組織中持續表現，此即暗示俄氏草花序反轉與分生組織額外獲賦予營養型生長之花序特性有關；另外花芽分生組織特性基因ToFLO仍可表現在側生之珠芽（枝）分生組織中，推測其表現下降時非花序轉換發生時之必要條件，而ToFLO表現亦非步向花芽分化之充分條件，故尚無證據支持俄氏草花序轉換係與花芽分生組織失去成花特性有關。本研究初步顯示俄氏草花序轉換概與花序分生組織特性基因ToCEN表現相關，預計可再進行功能性分析以釐清ToCEN所扮演的角色，及明瞭俄氏草如何藉此機制調節花序轉換。

In certain angiosperms, vegetative growth, such as leaves and asexual propagules formation on inflorescence, may reappear after floral commitment. This phenomenon is named “floral reversion”. Floral reversion is probably essential for pseudovivpary plants’ survival for its life history, i.e. sexual reproduction can be replaced by asexual propagation, such as bulbils. Titanotrichum oldhamii (Gesneriaceae), one of pseudoviviparous plant, is native in Taiwan. Its flowering inflorescence can reverse to the bulbiliferous shoot after flowering. What molecular mechanism controls the floral reversion in Titanotrichum is of particular interest. Four candidate genes including floral meristem identity gene ToFLO, inflorescence meristem identity gene ToCEN, floral organ identity gene ToPLE and shoot apical meristem stem cell identity gene ToROA were isolated in this study. Expression patterns of these genes were compared among developmental stages: before and during the inflorescence transition. Putative inflorescence meristem identity gene ToCEN is the only candidate gene whose expression was alternated when inflorescence transition occurred. During the floral differentiation, ToCEN is not expressed in the floral meristems but in meristems that are potentially transferred to the bulbiliferous shoot. It is known that overexpression of TFL1/CEN homolog in Arabidopsis and rice promotes vegetative growth and branching of inflorescence. Thus, inflorescence transition in Titanotrichum may be associated with ToCEN in obtaining vegetative meristem identity. In contrast to ToCEN, the floral meristem identity gene ToFLO could somehow be expressed in bulbiliferous meristem. This result might imply that down-regulation of ToFLO is not necessary for inflorescence transition, and ToFLO is not sufficient to provide floral differentiation as well. Thus, there is not enough evidence to support that inflorescence transition is due to loss of floral meristem identity. In conclusion, I propose the inflorescence meristem identity gene ToCEN is a good candidate for studying the inflorescence transition in Titanotricum. Further functional study would be helpful to ensure the role of ToCEN in inflorescence transition.