「葉序」とは、茎のまわりの葉や花などの側生器官の規則的な配置のことで、植物の形を決める主要なパターニングの一つである。葉の配置を上から眺めるとその並びにはフィボナッチ数が頻繁に現れることから、植物学者だけでなく、様々な分野の理論家の興味の対象となっていたが、その規則的な配置の制御の分子実態は長らく不明であった。

この論文の著者らは、以前に植物ホルモンのオーキシンが茎頂メリステムから側生器官の形成を誘導することを見出していた。オーキシンはPIN-FORMED1 (PIN1)とよばれる排出キャリアにより、細胞間を極性輸送される。この論文で著者らは、SAMにおけるPIN1の局在解析や、オーキシンのマイクロアプリケーションと遺伝学的解析を組み合わせ、PIN1を介したオーキシン極性輸送が単なる器官形成のトリガーでなく、葉序を自律的に生み出すメカニズムの主要な要素であることを示している。

今回の授業では、まず論文に出てくる実験結果を解釈し、著者らが提唱したオーキシン極性輸送による葉序制御モデルの妥当性やこの論文からどのような疑問が新たに生まれるかについて議論したい。

Phyllotaxis is a regular arrangement of lateral organs such as leaves and flowers around the plant stem. In addition to the importance of phyllotaxis to determine the plant architecture, Fibonacci numbers that often appears in the spatial arrangements of leaves fascinated many researchers not only plant biologists but also mathematicians and theoreticians. However, the molecular basis that regulates phyllotaxis has long been unknown.

A plant hormone, auxin, is known to be transported by its efflux carrier, PIN-FORMED1 (PIN1), from a cell to a neighboring cell in a polar manor. The authors of this paper previously found that auxin triggers lateral organ initiation from the shoot apical meristem. In this paper, they studied spatial expression patterns of PIN1 in the shoot meristems and performed auxin micro-applications in mutants in auxin pathways. They proposed an instructive role of auxin polar transport in the regulation of phyllotaxis.

In the class, we aim to interpret their results, then discuss about the reasonability of their model and new questions emerged from this study.

Shoot apical meristem (SAM): A dome-shaped undifferentiated tissue located at the tip of plant stems. The SAM produces leaves and flowers during vegetative and reproductive phase, respectively.

Indole-3-acetic acid (IAA): One of naturally-occurring auxin compounds. Transport of IAA is controlled by its efflux carrier families (including PIN1) and influx carriers (including AUX1).

PIN-FORMED1 (PIN1): One of auxin efflux carriers. PIN1 is localized on one side of the cell membrane and is considered to be a driving force of the auxin polar transport. pin1 mutants fail to produce flowers from shoot meristems and show “pin-shaped” naked stems.

MONOPTEROS (MP): A transcription factor essential for auxin signaling in shoot meristems. In the presence of auxin, MP triggers downstream gene expression. mp mutants show a pin-shaped phenotype similar to pin1.

PINOID (PID): A protein kinase that enhances polar auxin transport. pid mutants also show a pin-shaped phenotype.

2,4-D: A synthetic auxin compound. 2,4-D is a poor substrate for PIN1 and therefore, it diffuses among cells.

AUXIN RESISTANT1 (AUX1): One of auxin influx carriers.