Journal article Open Access
The coming enterprises of space exploration by humans, e.g. the future colonization of
the Moon and Mars, will require the utilization of plants as key components of
Bioregenerative Life Support Systems. The space environment is very different from the
Earth environment in many factors. Many of these adverse factors can be counteracted
in spaceships, or in Martian or Lunar settlements, but the living beings must adapt to
grow and survive in microgravity. The Earth gravity has
remained constant in magnitude and direction throughout the entire history of our
planet, including biological evolution. Gravity establishes the direction of plant growth
through the process called gravitropism and this orientation is essential for the normal
function of roots, stems and leaves, assuring the adequate nutrition of the plant.
The phytohormone auxin is known to play a main role in gravitropism. Auxin is also a major
regulator of plant development, since this hormone is ultimately responsible of the
maintenance of meristematic cells, which are totipotent cells, continuously engaged in
the cell cycle, and are the suppliers of differentiated cells for plant development.
Meristematic competence is the balance between cell growth and cell proliferation
occurring in meristematic cells. A major effect of the microgravity environment is the
disruption of meristematic competence, comprising the increase of the proliferation rate
and the decrease of the growth rate, estimated through the rate of production of
ribosomes, the cellular factories of proteins. Microgravity also induces a noticeable
reprogramming of gene expression. In meristematic cells, genes driving the cell cycle
regulation are affected. In general, the systems responsible of the plant defense against
abiotic stresses and the energy/redox systems are major targets of the gene
reprogramming. Noticeably, no specific genes related to gravity alteration have been
identified, although a significant proportion of altered genes encode unknown functions.
Despite these cellular and molecular alterations, plants are capable of surviving,
developing until the adult stage, and even reproducing under microgravity conditions.
This means that plants indeed adapt to this environment, although the mechanisms of
adaptation are currently unknown.
A major challenge of current research is to identify
environmental cues that may replace gravity in driving growth and development. Light
can be one of these cues and understanding the role of light, as a countermeasure for
gravitational stress, will contribute to the success of the culture of plants in