Leaf-opposed tendrils in the grape family, Vitaceae
ブドウのシュートを観察すると、葉と対生の位置に蔓（ツル：Tendril）が付いている（Gerrath and Posluszny 2007, 邑田と前田 2022）。このツルのつき方は、他の被子植物では見られず、ブドウ科のツルを作る全ての種に共通であることから(Wen 2007)、ブドウ科の共有派生形質だと考えられる。
When observing grape shoots, tendrils are attached opposite the leaves (Gerrath and Posluszny 2007, 邑田と前田 2022). This attachment pattern of tendrils is not observed in other angiosperms and is common to all tendril-producing species in the grape family (Vitaceae, Wen 2007). Therefore, it is considered a synapomorphy of the Vitaceae.
Leaf-opposed tendrils of the Vitaceae are formed from axillary meristems as monopodial branching
ブドウ科の4属（ノブドウ属 Ampelopsis、セイシカズラ属 Cissus、ツタ属 Parthenocissus、ブドウ属 Vitis）10種における茎頂表面の落射照明光学顕微鏡や走査電子顕微鏡による観察や、茎頂切片の光学顕微鏡観察から（Bugnon 1964, Millington 1966, Tucker and Hoefert 1968, Shah and Dave 1970, Gerrath and Posluszny 1988, Gerrath and Posluszny 1989a, Gerrath and Posluszny 1989b, Gerrath and Posluszny 1994, Wilson and Posluszny 2003, Gerrath and Posluszny 2007, Timmons et al. 2007） 、もともとの茎頂分裂組織は維持され、腋芽が蔓原基となっていることがわかった（総説としてGerrath and Posluszny 2007)。このような成長様式を単軸分枝と呼び、シロイヌナズナの栄養成長期に見られる分枝様式と同じである。シロイヌナズナの茎は伸長しないが、タバコなどの被子植物の茎は、腋芽と次の葉の間の部分が伸長する。ブドウ科で変わっている点は、腋芽とその蓋葉（腋芽に伴っている葉）原基（図のL2)の間の組織（黄色い部分）が伸長することで、次の葉原基（L1)と対生の位置に蔓が配置する点である。したがって、葉に対生する蔓というブドウ科に特有の体制は、茎が偏差成長することによって進化したと考えられる。
From epi-illumination light microscopy and scanning electron microscopy of the shoot apical meristem and light microscopy of the sections in ten species across four genera of the Vitaceae (Ampelopsis, Cissus, Parthenocissus, and Vitis) (Bugnon 1964, Millington 1966, Tucker and Hoefert 1968, Shah and Dave 1970, Gerrath and Posluszny 1988, Gerrath and Posluszny 1989a, Gerrath and Posluszny 1989b, Gerrath and Posluszny 1994, Wilson and Posluszny 2003, Gerrath and Posluszny 2007, Timmons et al. 2007), it has been found that the original shoot apical meristem is maintained, and the axillary meristem forms the tendril primordium (reviewed in Gerrath and Posluszny 2007). This type of growth is called monopodial branching, which is the same branching pattern observed during the vegetative growth phase of Arabidopsis thaliana. While the stem of Arabidopsis does not elongate, in other angiosperms like tobacco, the region between the axillary meristem and the next leaf primordium elongates. The unique characteristic of the Vitaceae is that the tissue (yellow part in the figure) between the axillary meristem (Tendril primordium) and the primordium of subtending leaf (L2 in the figure) elongates, positioning the tendril opposite the next leaf primordium (L1). Therefore, it is considered that the tendril arrangement opposite the leaves, which is a unique trait of the Vitaceae, evolved through anisotrophic stem growth.
Tendrils of Rhoicissus digitata appear to be formed sympodially
一方、Rhoicissus digitataでは、蔓原基ができるころの茎頂で、蔓原基の方が茎頂原基よりも大きいことから、もともとの茎頂分裂組織が蔓原基へと変化し、図のL1の腋芽が新しい茎頂分裂組織を形成している（仮軸分枝と呼ぶ）のではないかと報告された（Gerrath et al. 1998, Gerrath and Posluszny 2007)。また、同属のR. rhomboideaでは、茎頂分裂組織と蔓原基の大きさがほぼ同じである（Gerrath et al. 1998, Gerrath and Posluszny 2007)。シロイヌナズナやイネの腋芽発生初期と茎頂では、発現する遺伝子が異なっており（総説として、Wang and Jiao 2018, Nicolas and Laufs 2022）、遺伝子の発現様式を発生段階を追って調べれば、どこが茎頂でどこが腋芽か、茎頂から腋芽への変化がおきているかなどを調べることが可能である。従来の外部形態や切片による細胞形態による観察に、遺伝子発現様式を加えた研究を行うことで、ブドウ科の蔓がどのように形成されるかがよりはっきりと推定できるだろう。
On the other hand, in Rhoicissus digitata, it has been reported that, at the shoot apex when the tendril primordium forms, the tendril primordium is larger than the shoot apical meristem, suggesting that the original shoot apical meristem transforms into the tendril primordium, and the axillary meristem of L1 forms a new shoot apical meristem (referred to as sympodial branching) (Gerrath et al. 1998, Gerrath and Posluszny 2007). In the related species R. rhomboidea, the shoot apical meristem and the tendril primordium are almost the same size (Gerrath et al. 1998, Gerrath and Posluszny 2007). In Arabidopsis thaliana and rice, different sets of genes are expressed at the shoot apical meristem and the initial stage of axillary meristem formation (reviewd in Wang and Jiao 2018, Nicolas and Laufs 2022), and by investigating the gene expression patterns over developmental stages, it is possible to determine the boundary between the shoot apex and the axillary bud and to investigate whether a transformation from shoot apical meristem to axillary meristem is occurring. By combining studies of traditional external morphology and cell morphology in sections with gene expression patterns, it would become clearer how tendrils in the Vitaceae are formed.
References:
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