HMN 2026: How plants balance woody xylem and fleshy tissues: Thermospermine targets methylated ribosomes

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 optimizing 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 signaling molecules that work together to build a vascular system architecture that extends throughout their body.

This vascular development is carefully fine-tuned by adjusting signaling 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:

1. Vessel cells with thickened secondary cell walls enriched with lignin that allow rapid movement of water, and
2. Parenchyma cells with thin primary cell walls that store water and nutrients like starch.

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 center 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.”

Ribosomes as signaling sensors

The researchers found that thermospermine binds to methylated ribosomes, allowing the ribosome to act as a signaling 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 aging.

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 analyzing chemical modifications and the structural biology of the Arabidopsis ribosome to visualize what was happening.

“The location of the methylation is in the ribosome’s peptidyl transferase center where the peptide bond can be catalyzed. 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.

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 increases 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.

“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 signaling 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 favor storage cells in the root, allowing the plant to store more energy.

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