DOI: 10.5061/dryad.63xsj3v23
title: Light counteracts microgravity alterations in plant proliferating cells

description: 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.
description: 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. 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&lt;20
 or Kolmogorov-Smirnov Test for n&gt;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 1<i>g</i> control, in plants of the same
 genotype. As a general rule, differences between samples were considered
 significant for a bilateral probability value, <i>p</i>, lower
 than 0.05 (<i>p-value</i>&lt;0.05); however, a more
 stringent probability value (p&lt;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.