Light is sufficient to compensate for random positioning machine-simulated microgravity in plant roots

Growing food crops for space missions requires significant improvements in technical competence. Many issues remain, including ensuring that roots grow vertically in low gravity. Although plant roots grow towards gravity, they also perceive and bend away from light, allowing for light to substitute for gravity. To explore this issue, we designed a 3D-printed mini-phytotron with adjustable light-emitting diodes to use with a random positioning machine (RPM). Simulated microgravity in the RPM, together with darkness, caused Arabidopsis roots to lose vertical perception, resulting in significantly altered root morphology parameters consistent with gravity loss. This validated the method as an Earth-based analogue and allowed us to test the addition of light. White light as low as 10 μmol m^-2 s^-1 compensated for simulated microgravity in the RPM. Red light was less effective than white, and white light at 1 μmol m^-2 s^-1 was much less effective. A dwarf variant of Arabidopsis responded similarly to the wild type, and lettuce roots also responded to light. Food plants in space will require much higher levels than 10 μmol m^-2 s^-1 for photosynthesis, so there are good prospects that light in growth facilities in space will replace gravity for normal root growth, as long as roots can be exposed to some light. The RPM combined with the mini-phytotron was developed here as an inexpensive Earth-based analogue to analyse root growth behaviour to changing light levels under varying gravity conditions and will serve as a valuable experimental platform for further dissection of light responses in roots.