Growing Plants in Human Space Exploration Enterprises

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

extraterrestrial habitats.