Light counteracts microgravity alterations in plant proliferating cells

<p><b>Premise:</b> Light and gravity are fundamental cues for plant development. In space, without gravity, understanding the role of a light stimulus is key for enabling plant acclimation to extraterrestrial environment. Here we tested the hypothesis that the alterations caused by the absence of gravity in root meristematic cells can be counteracted by light.</p> <p><b>Methods: </b>Seedlings of <i>Arabidopsis thaliana</i> wild type and two mutants of the essential nucleolar protein nucleolin <i>(nuc1, nuc2)</i> were grown in simulated microgravity, either under a white light photoperiod, or under continuous darkness. Key parameters of cell proliferation (cell cycle regulation) and cell growth (ribosome biogenesis), as well as of auxin transport, were measured in the root meristem using in situ cellular markers and transcriptomic methods, compared with a 1<i>g</i> control.</p> <p><b>Results:</b> The incorporation of a photoperiod regime has been sufficient to attenuate or suppress the effects caused by gravitational stress at the cellular level in the root meristem. In all cases, parameters recorded from samples receiving light stimuli in simulated microgravity were closer to 1<i>g </i>values than those obtained from samples grown in darkness. Differential results were obtained in the two nucleolin mutants.</p> <p><b>Conclusions: </b>Light signals may totally or partially replace gravity signals, significantly improving plant growth and development in microgravity. Despite that, molecular alterations are still compatible with the expected acclimation mechanisms that should be better understood. The differential sensitivity of <i>nuc1</i> and <i>nuc2</i> mutants to gravitational stress points to new strategies to produce more resilient plants to travel with humans in new extraterrestrial endeavors.</p>