Premise: 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.

Methods: Seedlings of Arabidopsis thaliana wild type and two mutants of the essential nucleolar protein nucleolin (nuc1, nuc2) 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 1g control.

Results: 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 1g values than those obtained from samples grown in darkness. Differential results were obtained in the two nucleolin mutants.

Conclusions: 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 nuc1 and nuc2 mutants to gravitational stress points to new strategies to produce more resilient plants to travel with humans in new extraterrestrial endeavors.

Seedlings of Arabidopsis thaliana wild type and two mutants of the essential nucleolar protein nucleolin (nuc1, nuc2) 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 1g control.

Statistical analyses of data were performed using SPSS 22.0 software (IBM). First, we checked the population's normal distribution (Shapiro-Wilk Test for n<20 or Kolmogorov-Smirnov Test for n>20) and their homoscedasticity (Levene Test).

Quantitative variables description was performed using mean and standard deviation values. Mean values were compared using the Student`s t-test for independent samples (normal distribution and homoscedasticity) or by non-parametric U Mann-Whitney-Wilcoxon Test (non-normal distribution). All statistical analyses were carried out by comparing, for each lighting regime, the simulated microgravity experimental condition with respect to its corresponding 1g control, in plants of the same genotype. As a general rule, differences between samples were considered significant for a bilateral probability value, p, lower than 0.05 (p-value<0.05); however, a more stringent probability value (p<0,0125) has been also applied, with the purpose of reducing the chance of false positives in the four comparisons (Appendix S6). In addition, a two-way ANOVA test is provided as supplementary material (Appendix S7). This factorial variance analysis was carried out following a univariate (general linear model) procedure, defining as dependent variables each measured parameter and as fixed factors (categorical independent variables) the light and the gravity.