22 June 2023
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If humans are going to live on Mars one day, we need to work out how to grow plants there. Plants cultivated on Mars will not only provide future inhabitants with food, they will also generate cheap oxygen, and reduce the carbon dioxide content of the Martian environment.

While Mars is our closest planetary neighbour, it can hardly be described as hospitable. Future explorers (including those of the plant variety) will have to contend with high levels of solar ultraviolet radiation, night temperatures well below freezing and soil devoid of nutrients. While a summer day at the Martian equator may be as warm as 20 degrees Celsius, at night temperatures fall to around -73 degrees Celsius. Any plants we bring to Mars would likely need to be grown in a greenhouse where conditions can be kept within more hospitable parameters.

If all that wasn't enough, the soil on Mars contains perchlorate salts which are toxic to plants as well as humans. The high concentration (5%) found in Martian soil severely impacts the growth of plants, significantly depletes the level of chlorophyll found in their leaves and leads to an accumulation of perchlorates in the leaves. Some desalination is possible, but complete removal of the salts would be very costly, and the expense of space travel makes shipping more plant friendly soil from Earth highly impractical. We therefore need to take plants that are capable of withstanding the harsh Martian conditions, and at least some amount of perchlorate salt.

Designer plants for Mars

This is where genetic engineering could provide a solution. NIAC (NASA Institute for Advanced Concept) funds projects that could assist future space exploration efforts and have contributed towards genetically modified plant research.

Efforts thus far to genetically engineer a Mars-friendly plant have focussed on the plant stress response. When plants encounter unfavourable conditions, such as temperature extremes or a lack of water, various mechanisms help the plant withstand the stressor. However, these mechanisms tend to inhibit growth, and produce toxic superoxides. For a plant experiencing a short period of unfavourable conditions, this is not a problem, but for a plant living in a constantly unfavourable environment, such as Mars, the prolonged stress state can harm the plant and permanently stunt growth. Most plants have evolved mechanisms to deal with the toxic effect of superoxides, but these are likely to be insufficient in an environment like Mars, where extreme temperature swings can be expected regularly, even in a greenhouse.

Researchers at North Carolina State University sought to solve this problem by taking genes from organisms used to dealing with extreme temperatures, referred to as "extremophiles". These extremophiles have developed better ways to cope with the larger number of superoxides they encounter. One such extremophile is a bacteria called Pyrococcus furiosus, which lives near deep-sea vents and encounters both very hot temperatures from hot water jets, as well as colder ocean currents.

Researchers hoped incorporating the superoxide reductase gene from P. furiosus into plants would allow them to cope better with temperature swings. North Carolina State University researchers have already successfully incorporated this gene into tobacco plant cells without any deleterious effects.

More recently, researchers from the University of Alberta created genetically modified rice variants. As before, the aim was to modify the stress response to promote better growth in the Martian environment. Rice is a crucial crop plant; around 520 million metric tons of rice are consumed annually. If it could be successfully adapted for Mars, it could provide a vital food source for future Martian inhabitants.

The researchers modified genes which encode signalling molecules involved in the regulation of the rice plant stress response. They are involved in the response to sugar starvation, salinity, drought, and scarcity of soil nutrients. Normal rice plants grown in model Martian soil with 0.1% perchlorate salt showed stunted growth, poor root morphology and lower photosynthetic activity than those grown in normal soil. In contrast, the mutant rice plants performed much better, developing good roots and a shoot. However, there is still some way to go, as none of the rice plants were capable of growing in the model Martian soil with 0.3% perchlorate salt.

The future

There are many challenges facing the first human inhabitants of Mars. However, advances in genetic engineering could provide the answer to one of them and pave the way to the development of Mars-friendly plants capable of feeding the first Martian explorers.

Isobel is a trainee patent attorney in the life sciences team. She has a BSc in Biochemistry from Imperial College London. Her final year project involved comparing the genomic response of human and rats post burn to help understand the evolution of the burn response in humans.
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