Seeds include a plant embryo with a root and a shoot already developed. When the seed germinates, the root and the shoot each elongate through tissue growth at the tip (meristem). Without environmental cues, the root and shoot will each continue to grow in the same general direction they had inside the seed. I emphasize "general" because the root or shoot can branch, and the roots have also been shown to form a helix; in both cases, the overall growth is still in the same general direction.
To prevent plants from growing the "wrong way" into the ground, they have responses called tropisms. These cause the root or shoot to bend as it grows, as much as 180 degrees! The three major tropisms are gravity (geotropism or gravitropism), light (phototropism), and water (hydrotropism). As described below, each has been studied on the ISS, and sometimes in combination.
Although tropism experiments have been hosted in several different facilities aboard the ISS, the EMCS was specifically built for these types of experiments:
The European Modular Cultivation System (EMCS) is an ESA experiment facility that is dedicated to studying plant biology in a reduced gravity environment. It supports the cultivation, stimulation, and crew-assisted operation of biological experiments under controlled conditions (e.g. temperature, atmospheric composition, water supply, illumination, observation, and gravity). The facility has performed multi-generation (seed-to-seed) experiments and studies the effects of gravity and light on early development and growth, signal perception and transduction in plant tropisms.
An experiment in microgravity and darkness:
Biological Research In Canisters - 16: Investigations of the plant cytoskeleton in microgravity with gene profiling and cytochemistry (BRIC-16-Cytoskeleton) studies the effects of microgravity on the structure and organization of the actin cytoskeleton in plants using the model plant Arabidopsis.
Seedlings grown in total darkness in microgravity showed more skewing at the roots and more roots forming from shoot tissues than ground control plants.
The results of the BRIC-16 study showed fewer seeds germinating, roots growing out of "above-ground" parts of the plant, and the roots corkscrewing as they grew:
The first major physical difference was observed at the root apex and proximal root where extreme screwing occurred in the flight seedlings compared to slightly skewed roots with the ground controls. Another major difference was the greater amount of adventitious roots (roots formed from the stem or leaves) found on the flight samples. Other effects included the percentage of plants arose from seeds were significantly lower in samples grown in flight hardware (FL, GC) compared to the growing seedlings in just the petri dishes. In addition, the endodermal cells (the deepest cells in the outer layer) were significantly smaller in seedlings grown in the BRIC-PDFU system compared to those in the HC. This change in the shape of endodermal cells indicates alterations in the cell wall and appears to be a true microgravity effect.
Another experiment in microgravity, with and without light:
The Characterizing Arabidopsis Root Attractions (CARA) experiment looks at mechanisms at the molecular and genetic level that influence the growth of a plant’s roots in the absence of gravity, and how those change with or without light. Researchers expose one set of seedlings to light, keep another set in the dark, and then examine how each environment influences the patterns of root growth. Some of the plants are also imaged with the Light Microscopy Module on orbit, and at the end of the experiment, all plants are harvested by the astronaut, and preserved for their return to Earth in order to evaluate genes associated with plant responses on orbit.
Gravity, water, and nutrients each examined separately:
Multiple-Tropism: Gravity, Nutrient and Water Interaction of Stimuli for Root Orientation in Microgravity (MULTI-TROP) separately evaluates the role of three stimuli – gravity, water and nutrients – on plant growth. Tropism refers to an organism directional response to an external stimulus, such as plant roots growing downward into soil in response to gravity on Earth. Previous research shows that plant roots grow randomly without specific direction in microgravity, presenting a challenge when developing facilities to cultivate plants in space.
The interaction of gravity and water:
The Hydrotropism and Auxin‐Inducible Gene expression in Roots Grown Under Microgravity Conditions (HydroTropi) experiment has three specific aims:
- First, it demonstrates that gravitropism (a plant's ability to change its direction of growth in response to gravity) interferes with hydrotropism (a directional growth response in which the direction is determined by a stimuli in water concentration).