Gravitropism

Plants can sense and respond to gravity. This is a fundamental concept in astrobotany.

Arabidopsis thaliana demonstrating root gravitropism. | Image Credit: Anna Gobova

Summary

As we move away from Earth, the acceleration of gravity imparted on objects no longer holds steady at 9.8 m/s². While in space or on other planets, plants will experience microgravity, hypergravity, and everything in between. This is an important consideration for astrobotany because plants sense and respond to gravity. Their response to gravity is called gravitropism and it is a critical aspect of astrobotany research. Gravitropism is also sometimes also referred to as geotropism– this term is outdated.

Plant gravitropism may be more intuitive than it seems. Let’s break it down. You can think of plants as simple systems of roots and shoots. Roots take up water and nutrients and shoots transport them. Roots grow down and shoots grow up. The question on our minds? How do plants know where up and down are?

Plants must sense the influence of gravity to maximize their growth. The key to plant gravity sensing lies in the plant’s root tips, specifically in organelles called amyloplasts. Amyloplasts contain starch and are the main player in gravity response.

Gravitropic Assays

To further explore gravitropism, a gravitropic assay is a common experiment performed by astrobotanists.  Using a 2-d or 3-d clinostat, a plant (typically Arabidopsis thaliana) is stressed in a way that mimics components of gravity by rotation.  The analysis is performed using RootTrace software.  Usually, gravitropic assays cause abnormalities in root growth, typically resulting in a skewing of roots.

QTL Analysis

Arabidopsis thaliana has 5 chromosomes- each of which contains genes that allow the plant to adapt to its environment. Quantitive Trait Loci (QTL) analysis performed in the Spalding Lab at the University of Wisconsin-Madison identified regions that are likely to contain genes that are involved in plant response to a gravitropic assay. The use of time-lapse photography and high-throughput computing the researchers to identify chromosomal regions linked to different stages of the gravitropic response.  
 

Plant Gravitropism in Microgravity

So how does this affect plants grown in space microgravity?

Here are some takeaways from research that’s been performed:

  • plants grown in microgravity have statistically shorter roots
  • plants grown in microgravity have shorter root hairs
  • microgravity can weaken cell walls, affecting root systems
  • root systems in space experience more skewing and waving
  • the literature suggests that directional light can direct root growth in a microgravity environment

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