How plants control fleshy and woody tissue growth
Scientists have identified a crucial mechanism that allows plants to shape their vascular systems, determining whether they grow soft edible storage organs or develop the rigid woody tissue characteristic of trees.
Published in Science, research led by the University of Cambridge and University of Helsinki, reveals the regulatory dynamics that guide xylem formation, offering new insights into how plants build both structural and storage tissues.
Understanding how plants fine-tune their vascular development offers a promising path for future work aimed at optimising growth traits that are critical to agriculture and forestry, including the production of commercially valuable materials such as wood, paper, edible roots and bioproducts.
Plants have evolved multiple signalling molecules that work together to build a vascular system architecture that extends throughout their body.
This vascular development is carefully fine-tuned by adjusting signalling inputs at various steps, helping determine whether the plant produces more water-conducting cells or more storage cells.
Plants can switch between these growth priorities because their water-conducting xylem tissue consists of two distinct cell types:
- Vessel cells with thickened secondary cell walls enriched with lignin that allow rapid movement of water, and
- Parenchyma cells with thin primary cell walls that store water and nutrients like starch.
Cross section of Arabidopsis thaliana hypocotyl showing xylem tissue patterning formed by two different cell types – xylem vessels (red) and xylem parenchyma cells. SR2200 and Fuchsin staining of vessels. Imaged by Donghwi Ko.
To uncover how plants fine-tune the balance between these cell types, researchers from Professor Ykä Helariutta’s labs at Cambridge and Helsinki studied a single-copy gene mutant in the model plant Arabidopsis thaliana, called overachiever(ovac), which produces excess vessel cells at the expense of parenchyma cells.
The team identified that the OVAC gene is an rRNA methyltransferase responsible for the m3U2952 base modification specifically in the peptidyl transferase centre of 25S rRNA in Arabidopsis. They then focused on thermospermine, a small positively charged polyamine molecule already known to regulate vessel differentiation by increasing the translation of SAC51 transcription factors, which in turn inhibit xylem vessel initiation.
“It was unclear how this translational regulation works and how it was adjusted, said equal first author Dr Donghwi Ko, from the Sainsbury Laboratory Cambridge University. “We also knew that polyamines interact with ribosomes by binding to ribosomal RNA (rRNA), and so we wanted to explore this aspect too.”
Cross section of hypocotyl of 8-week-old Arabidopsis thaliana. The ovac mutant displays enhanced secondary vessel differentiation (red) at the expense of parchenchyma cells. Images A2 and B2 show the vessel cells only. SR2200 and Fuchsin staining. Imaged by Donghwi Ko.
Ribosomes as signalling sensors
The researchers found that thermospermine binds to methylated ribosomes, allowing the ribosome to act as a signalling sensor that promotes the translation of SAC51.
Ribosomes are protein factories and they often interact and bind with polyamines like thermospermine.
However, no specific cellular functions have yet been attributed to ribosome-bound polyamines despite their roles in diverse biological processes, including the regulation of cell proliferation, differentiation, autophagy and ageing.
Researchers from Professor Alan Warren’s lab at the Cambridge Institute for Medical Research and Associate Professor Finn Kirpekar’s lab at the University of Southern Denmark took a deep-dive into analysing chemical modifications and the structural biology of the Arabidopsis ribosome to visualise what was happening.
“The location of the methylation is in the ribosome’s peptidyl transferase centre where the peptide bond can be catalysed. This modification enables the thermospermine to bind at the same site, bridging key components of translational regulation,” said equal first author Dr Alexandre Faille, of the Cambridge Institute for Medical Research.
Location of the methylation is in the ribosome’s peptidyl transferase centre (PTC) where the peptide bond can be catalysed. Visualisation of the binding of thermospermine (tSpm) to the Arabidopisis ribosome using a cryo-EM map of Arabidopsis WT 80S ribosome with P-site bound tRNA (cyan). Schematic of the PTC, with the CCA end of the P-site tRNA, tSpm, and residue m3U2952 indicated. Ribosome structure visualisation by Alexandre Faill.
As a result of further investigations, they discovered thermospermine binding to the methylated ribosomes affects a second transcription factor that regulates vascular development called LHW, which increase vessel initiation. Thermospermine inhibited LHW.
“When SAC51 translation and that of LHW are regulated by thermospermine in wild type plants, the normal balance between vessel and parenchyma cells is established,” said equal first author Dr Eva Hellmann, from Sainsbury Laboratory Cambridge University. “In the mutant, however, thermospermine cannot bind stably to non-methylated ribosomes, preventing this regulation and allowing high levels of LHW translation, which disrupts normal vascular patterning.
The research concludes that thermospermine regulates both transcription factors in different ways. It is a bifunctional translational regulator for SAC51 and LHW, but requires a specific methylation, m3U2952, for its bifunctional regulation. It acts by promoting SACLs translation while inhibiting that of LHW.
Thermospermine requires m3U2952 modification of ribosomes for its bifunctional translation regulation. In WT plants the OVAC gene can methylate ribosomes (m3U2952). With this specific methylation and the presence of themospermine, SACLs can be translated but LHW cannot. This mechanism is impaired in the ovac mutant where SACLs are not translated, but LHW can still be translated. Graphic and images by Donghwi Ko.
“This research sheds light on how plants fine-tune the vascular development to determine the fate of their vascular cells,” said equal first author Dr Raili Ruonala, of the University of Helsinki. “These findings have potential to influence plant traits ranging from drought resilience to root/tuber growth in food crops, as well as wood formation.”
While the research was undertaken using the model plant Arabidopsis, it indicates the same signalling may be happening in other plants. For example, in trees these signals could potentially be tuned to produce large numbers of water-conducting vessels to support tall growth. In radishes, the same signals could be adjusted to favour storage cells in the root, allowing the plant to store more energy.
Reference
Donghwi Ko†, Raili Ruonala†, Alexandre Faille†, Eva Hellmann†, Hanna Help, Huili Liu, Ronni Nielsen, Anders Haakonsson, Nuria De Diego, Anja Paatero, Mariia V. Shcherbii, Karolina Stefanowicz, Sanja Ćavar Zeljković, Tine Drud Lundager Rasmussen, Ondrej Novak, Zsuzsanna Bodi, Gugan Eswaran, Brecht Wybouw, Matthieu Bourdon, Cristina Urbez, Xiaonan Liu, Kari Salokas, Tiina Öhman, Tanya Waldie, Petri Törönen, Sedeer el-Showk, Martin Balcerowicz, Fabrice Besnard, Xiaomin Liu, Patrick Perkins, Serina Mazzoni-Putman, Julia P. Vainonen, Maija Sierla, Mikko J. Frilander, Susanne Mandrup, Teva Vernoux, Karin Ljung, Alejandro Ferrando, Miguel A. Blazquez, Liisa Holm, Rupert Fray, Markku Varjosalo, Ottoline Leyser, Ville O. Paavilainen, Ari Pekka Mähönen, Anna Stepanova, Jose Alonso, Steffen Heber, Robert Malinowski, Finn Kirpekar*, Alan J. Warren*, Ykä Helariutta* (2026) Recruitment of bifunctional regulator thermospermine to methylated ribosomes directs xylem fate. Science.
†Equal first authors
*Corresponding authors