Functional alterations of root meristematic cells of Arabidopsis thaliana induced by a simulated microgravity environment

Abstract

Environmental gravity modulates plant growth and development, and these processes are influenced by the balance between cell proliferation and differentiation in meristems. Meristematic cells are characterized by the coordination between cell proliferation and cell growth, that is, by the accurate regulation of cell cycle progression and the optimal production of biomass for the viability of daughter cells after division. Thus, cell growth is correlated with the rate of ribosome biogenesis and protein synthesis. We investigated the effects of simulated microgravity on cellular functions of the root meristem in a sequential study. Seedlings were grown in a clinostat, a device producing simulated microgravity, for periods between 3 and 10days. In a complementary study, seedlings were grown in a Random Positioning Machine (RPM) and sampled sequentially after similar periods of growth. Under these conditions, the cell proliferation rate and the regulation of cell cycle progression showed significant alterations, accompanied by a reduction of cell growth. However, the overall size of the root meristem did not change. Analysis of cell cycle phases by flow cytometry showed changes in their proportion and duration, and the expression of the cyclin B1 gene, a marker of entry in mitosis, was decreased, indicating altered cell cycle regulation. With respect to cell growth, the rate of ribosome biogenesis was reduced under simulated microgravity, as shown by morphological and morphometric nucleolar changes and variations in the levels of the nucleolar protein nucleolin. Furthermore, in a nucleolin mutant characterized by disorganized nucleolar structure, the microgravity treatment intensified disorganization. These results show that, regardless of the simulated microgravity device used, a great disruption of meristematic competence was the first response to the environmental alteration detected at early developmental stages. However, longer periods of exposure to simulated microgravity do not produce an intensification of the cellular damages or a detectable developmental alteration in seedlings analyzed at further stages of their growth. This suggests that the secondary response to the gravity alteration is a process of adaptation, whose mechanism is still unknown, which eventually results in viable adult plants.

Interaction of light and gravity signals as a mechanism of counteracting alterations caused by simulated microgravity in proliferating plant cells

“…In other words, phototropism can counteract the lack of gravitropism, at least to a certain extent. These results are in agreement with a previous experiment carried out in a clinostat [46]. In general, there is increasing evidence that light is an important regulator of stress acclimation processes [47].…”

“…The incorporation of an illumination regime, in this case photoperiod, has been su cient to attenuate or suppress the effects caused by gravitational stress, and that plants can grow and develop correctly (no alterations at the cellular level) in microgravity, showing only alterations at the molecular level. Therefore, and, as shown by other experiments carried out at real [51][52][53] and simulated microgravity [46,54], if the growth conditions are optimal (nutrients, light, temperature, humidity) the plants present a correct growth in their rst stages of development. Despite that, molecular alterations are still compatible with the expected adaptation mechanisms involving the modulation of duplicated and specialized genes, expressed under environmental stress, such as NUC2, that should be promoted to increase the plant acclimation to a new extraterrestrial environment.…”

“…This suggests an effect on the rate of production of ribosomes. Alterations of ribosome biogenesis induced by both real and simulated microgravity in root meristematic cells and in vitro cell cultures have been reported in previous experiments from our laboratory [25,[44][45][46]. Different indirect markers of the process have been used in these studies, such as the protein levels and gene expression regulation of nucleolin and brillarin, nucleolar proteins known to play a key regulatory role.…”

“…The results obtained in the nuc1 mutant are of particular interest. In a previous paper, the effects of simulated microgravity on this mutant line were described by looking at the ultrastructure of the nucleolus, which was shown severely damaged, containing the so-called "nucleolar peri-chromatin-like granules", which are the structural expression of a bad processing of pre-rRNA [46]. This effect actually represents an intensi cation, caused by simulated microgravity, of the defective pre-rRNA processing that occurs naturally in this mutant under control 1 g gravity conditions [16].…”

Root Skewing-Associated Genes Impact the Spaceflight Response of Arabidopsis thaliana

“…SKU5 could act to modulate growth regulation through its aforementioned association with the ABP1 apoplastic signaling pathway upstream of TOR, and both SKU5 and SPR1 can affect microtubule reorganizing pathways. This would enable SKU5 to impact the spaceflight response via the regulation of meristematic competence and cell cycle progression, processes that are affected in spaceflight (Matía et al, 2010), clinorotation, RPM exposure, and magnetic levitation Boucheron-Dubuisson et al, 2016). This is supported by the M-phase peak of SKU5 expression and its previously hypothesized role in regulating the cell cycle (Menges et al, 2002;Sedbrook et al, 2002).…”