How Do Plants Change Under Stronger Gravity? — Mechanism of Enhanced Photosynthesis in Moss Under 6–10 G Conditions

Key Points

• Physcomitrium patens was cultivated under 1, 3, 6, and 10 G for 8 weeks.
• At 6–10 G, both total community photosynthesis and CO₂ diffusivity increased.
• The mechanism involves an increase in the number of gametophores and an enlargement of chloroplasts, expanding the “CO₂ reception area.”
• The AP2/ERF transcription factor (IBSH1) is partially involved in the molecular mechanism.

Introduction: What Changes First in Plants When Gravity Increases?

Can you imagine which mechanism in plants changes first when placed in a world heavier than Earth?
The study introduced here cultivated the model moss Physcomitrium patens under 1, 3, 6, and 10 G for 8 weeks and carefully tracked changes in photosynthesis at the community level—a “miniature forest” of clustered tiny gametophores (shoots with leaves).
Systematic evaluations were conducted on what happens as gravity increases, based on “total community photosynthesis,” “ease of CO₂ entry into leaves (diffusivity),” and morphological factors such as chloroplast size/number and gametophore count.

Reference: First contact with greater gravity: Moss plants adapted via enhanced photosynthesis mediated by AP2/ERF transcription factors

Overview of the Study: What Was Discovered Through the Experiment?

The results under hypergravity were clear.
Under 6 G and 10 G, moss communities shifted to forms that facilitated easier CO₂ uptake, leading to increased total photosynthesis.
Specifically, the number of gametophores increased, expanding the total leaf area of the community, and chloroplasts within each leaf enlarged, increasing the surface area for CO₂ exposure—the “reception area.”
In contrast, the number of chloroplasts and the thickness of cell walls did not change.
This indicates that the system optimized not by increasing quantity but by revising arrangement and size.

Key Experimental Results: How Much Did It Change?

In numerical terms, total community photosynthesis increased by +36–52%, and CO₂ diffusivity rose by +35–56% under 6–10 G.
At 3 G, the difference was minimal, suggesting a threshold effect beginning at 6 G.

The total “chloroplast surface area” of the community increased by +79–118%—expanding the CO₂ reception area—improving CO₂ uptake and ultimately boosting photosynthesis.
This increase was due to two factors: the expansion of total leaf area by increasing gametophore number and the enlargement of individual chloroplasts.
There were no major changes in chloroplast number, cell wall thickness, or average leaf area/number per shoot.

Why It Happens: Gene Switches and Morphological Reorganization

At the molecular level, a group of AP2/ERF transcription factors was upregulated under 10 G.
Among them, one called ISSUNBOSHI1 (IBSH1) stood out.
When overexpressed under normal 1 G conditions, it reproduced the changes seen under 10 G—increased chloroplast size, gametophore count, and total photosynthesis.
Conversely, mutants with suppressed IBSH1 activity showed weakened responses even under 10 G.

A pathway within the cell was revealed:

“Gravity increase → activation of AP2/ERF (IBSH1) → regulation of chloroplast size and shoot number → improved CO₂ uptake → increased photosynthesis!”

This underlies the physiological changes observed in moss under high gravity.

Unresolved Issues: Threshold, Generalizability, and Conditional Dependence

First, the nature of the “threshold” effect—minimal response at 3 G but clear effects at 6–10 G—remains unknown.
What happens at 4 G or 5 G? Is it due to sensitivity in transcriptional regulation or the minimum mechanical force needed to trigger morphological changes?
Further investigation is awaited.

Second, generalizing to flowering plants requires caution.
For example, in wheat, photosynthesis was reported to decrease under ultra-high gravity (500–2000 G), opposite to moss’s response under 6–10 G.

Moreover, interactions with other environmental factors—light, CO₂ concentration, water availability—as well as comparative mapping with low gravity (space or lunar environments) are still under development.
Clarifying what mechanisms are universal and what are moss-specific or condition-dependent is a key future task.

Conclusion: Significance and Potential Applications in the Space Age

This study showed a concrete adaptation pathway in which changes in gravity—a normally constant factor—induce plants to adjust “chloroplast size” and “community structure (shoot number)” to enhance CO₂ uptake and boost photosynthesis.
The gene-level mechanism was also elucidated.
However, since this pathway is inactive under Earth’s usual 1 G, questions remain about its role in normal environments.

As an application, transcription factors like IBSH1 could be used to manipulate chloroplast exposure and community structure in crop species, increasing the CO₂ reception area and enhancing photosynthesis.
Moreover, humanity may one day migrate to planets with higher gravity.
In that case, such research will be foundational, with both low-gravity space cultivation and high-gravity optimization forming the basis of survival and prosperity on new worlds.
It’s a romantic thought, isn’t it?

Terminology Notes (for article appendix)

• G (gee)
  Symbol indicating a multiple of gravitational acceleration.
  1 G = Earth’s gravity; 10 G = ten times that.
  It refers to perceived acceleration (felt weight), not mass itself.
• Community (canopy)
  A colony composed of many individuals.
  In this article, it refers to a unit of clustered moss gametophores.
  “Community photosynthesis” means CO₂ uptake by this whole unit.
• Gametophore
  The leafy shoot of moss, visible as a small “individual.”
  More gametophores result in larger effective leaf area per community.
• Community-level photosynthesis
  The rate of CO₂ uptake measured at the scale of the colony, not individual leaves or shoots.
• CO₂ diffusivity (ease of entry)
  The ease with which CO₂ moves from air → leaf interior → chloroplasts.
  Higher values indicate easier CO₂ access at the community level.
• Chloroplast exposure area (CO₂ “reception area”)
  Refers to the total visible surface area of chloroplasts in the whole community.
  Enlargement of chloroplasts and increase in shoot number expand this “reception area,” facilitating CO₂ capture.
  Note: “Exposure” is metaphorical; CO₂ passes through the cell wall and chloroplast membrane.
• Chloroplast size / number
  In this study, size increased while number remained largely unchanged.
  Enlargement directly contributes to greater CO₂ reception.
• Cell wall thickness
  No significant changes under 6–10 G.
  Maintaining thin walls aids CO₂ permeability and boosts photosynthesis.
• Threshold
  The critical point where response becomes noticeable—minimal effects at 3 G, pronounced effects at 6–10 G.
  The underlying stage (genetic regulation vs. morphological change) remains unknown.
• AP2/ERF transcription factor family
  A widespread group of genetic switches in plants.
  They regulate growth, stress responses, and metabolism.
  Several were upregulated under 10 G in this study.
• IBSH1 (ISSUNBOSHI1)
  A key AP2/ERF-type transcription factor in this study.
  Overexpression at 1 G mimicked the changes seen under 10 G.
  Suppression dampened those changes even under 10 G.
• High gravity / Low gravity
  High gravity = greater than 1 G (6–10 G in this study).
  Low gravity = less than 1 G (e.g., lunar or space environments).
  “Ultra-high gravity” often refers to levels of hundreds to thousands of Gs generated by centrifugation.
• Model moss Physcomitrium patens
  A widely used model organism in plant research.
  Sometimes referred to by its former name Physcomitrella patens.