New developments in a decades-old quest on flowering

I have long worked on plant development, but I have recently switched fields to focus on plant immunity and joined the Nobori group at The Sainsbury Laboratory. My interest in plant development remains, so I do my best to stay connected with the literature. For my first post on The Node, it seems fitting to write about plants.

Plants dominate the earth. It is estimated that they make up the majority of Earth’s living biomass, and among them flowering plants (angiosperms) account for around 90% of all plant species¹.

Land plants have been around for at least 450 million years, according to fossil records². Angiosperms, however, appeared much later, emerging suddenly and in remarkable diversity during the early Cretaceous period, around 130 million years ago³. This rapid rise and diversification puzzled Charles Darwin, who famously called the explosive expansion of flowering plants “an abominable mystery”⁴.

Flowering is a crucial step in plant development, and its timing depends on a variety of factors to maximise reproductive success. The manipulation of flowering time has also been central to crop domestication, since humans rely on plants for survival. Yet, despite its importance, the molecular basis of flowering has only begun to unfold over the past few decades, thanks to advances in modern molecular biology.

An important early figure in this field is Mikhail Chailakhyan, who carried out his PhD during a turbulent era for science in the Soviet Union in the 1930s (beautifully summarized by Marc Somssich⁵). Chailakhyan noticed that the plants that he worked on flowered faster under short days than long days and discovered that simply exposing the leaves to a specific light regime was enough to trigger flowering. In a series of clever experiments, including grafting the main stem of a long-day plant onto the leaves of a short-day plant, the long-day stem would flower under short-day conditions. This led him to propose that leaves produce a mobile signal that travels from leaves to the shoot and initiates flowering. Believing it to be a hormone, he named the mysterious substance “florigen” (flower-former)⁶.

Figure 1: Plant scientist in the greenhouse, by scientist-artist Hsuan Pai.

In 2007, many years after being suggested by Chailakhyan, multiple independent studies including one from the group of George Coupland showed that florigen is Flowering locus T (FT), a mobile protein that moves from the leaves to the inflorescence, where it then induces the transition to flowering^{7, 8, 9, 10}.

Recently, George Coupland’s group at the Max Planck Institute in Cologne published two new studies on florigen and its partners. The findings, published in Nature and Development, reveal that once FT reaches inflorescence, it creates the florigen activation complex (FAC), which assembles directly on DNA through a series of steps¹¹.

They also show that florigen does more than just trigger the start of flowering; it later takes on additional, independent roles during the formation of flowers¹².

Together, these findings describe a new mechanism for how the FAC assembles and reveal that its functions differ between the shoot meristem and the developing flower. Given the strong conservation of florigen and the FAC across seed plants, these discoveries also advance our understanding of flowering and floral development in major crops.

Sadly, Chailakhyan passed away in the early 1990s and never got to witness the remarkable progress made in understanding flowering. Turns out that even after a century of research, there are still exciting discoveries to be made!

Edited by Laura Turchi

References:

1.         Bar-On, Y. M., Phillips, R. & Milo, R. The biomass distribution on Earth. Proc. Natl. Acad. Sci. U. S. A. 115, 6506–6511 (2018).

2.         Strother, P. K. & Foster, C. A fossil record of land plant origins from charophyte algae. Science 373, 792–796 (2021).

3.         Zuntini, A. R. et al. Phylogenomics and the rise of the angiosperms. Nature 629, 843–850 (2024).

4.         Darwin, C. (1903). More letters of Charles Darwin: a record of his work in a series of hitherto unpublished letters (Vol. 2). D. Appleton.

5.         Somssich, M. A Short History of Vernalization. Preprint at https://doi.org/10.5281/zenodo.3708478 (2020).

6.         Zeevaart, J. A. D. Florigen Coming of Age after 70 Years. Plant Cell 18, 1783–1789 (2006).

7.         Mathieu, J., Warthmann, N., Küttner, F. & Schmid, M. Export of FT Protein from Phloem Companion Cells Is Sufficient for Floral Induction in Arabidopsis. Curr. Biol. 17, 1055–1060 (2007).

8.         Corbesier, L. et al. FT Protein Movement Contributes to Long-Distance Signaling in Floral Induction of Arabidopsis. Science 316, 1030–1033 (2007).

9.         Jaeger, K. E. & Wigge, P. A. FT Protein Acts as a Long-Range Signal in Arabidopsis. Curr. Biol. 17, 1050–1054 (2007).

10.      Lin, M.-K. et al. FLOWERING LOCUS T Protein May Act as the Long-Distance Florigenic Signal in the Cucurbits. Plant Cell 19, 1488–1506 (2007).

11.      Gao, H. et al. Florigen activation complex forms via multifaceted assembly in Arabidopsis. Nature 1–10 (2025).

12.      Romera-Branchat, M. et al. FD and FDP bZIP transcription factors and FT florigen regulate floral development and control homeotic gene expression in Arabidopsis floral meristems. Development 152, dev204241 (2025).

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